City of Norwalk, Connecticut Drainage Manual

City of Norwalk, Connecticut Drainage Manual

EFFECTIVE DATE JUNE 1, 2017

MMI #1383‐37‐7

CITY OF NORWALK, CONNECTICUT | 2017

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Table of Contents ACKNOWLEDGEMENTS

ACRONYMS DEFINITIONS SECTION 1 ‐ INTRODUCTION AND SUBMITTAL REQUIREMENTS 1.0 INTRODUCTION ......................................................................................................................................... 1‐1 1.1 PURPOSE OF MANUAL ................................................................................................................................. 1‐1 1.2 STORMWATER MANAGEMENT POLICY OBJECTIVES .................................................................................... 1‐1 1.2.1 Policy Statements ........................................................................................................................... 1‐2 1.3 REGULATORY FRAMEWORK AND STANDARDS ............................................................................................ 1‐4 1.3.1 Permits – Jurisdictional Control and Contact Information ............................................................. 1‐5 1.4 STORMWATER MANAGEMENT STANDARDS ............................................................................................... 1‐5 1.4.1 Floodplains ..................................................................................................................................... 1‐6 1.4.2 LID Site Planning and Design Strategies ......................................................................................... 1‐6 1.4.2.1 Protecting Riparian Buffers .............................................................................................. 1‐6 1.4.3 Stormwater Quantity Detention .................................................................................................... 1‐8 1.4.4 Groundwater Recharge .................................................................................................................. 1‐8 1.4.5 Stormwater Quality Management Practices ................................................................................ 1‐11 1.5 ILLICIT DISCHARGES .................................................................................................................................... 1‐12 1.6 SUBMITTAL REQUIREMENTS ...................................................................................................................... 1‐12

1.6.1 Stormwater System Design Plan and Report ............................................................................... 1‐12 1.6.2 Floodplain Management Plan ...................................................................................................... 1‐12 1.6.3 Bridge or Culvert Hydraulics Report ............................................................................................. 1‐13 1.6.4 Landscaping Plan .......................................................................................................................... 1‐13 1.6.5 Erosion and Sediment Control Plan ............................................................................................. 1‐13 1.6.6 Stormwater Management System Operation and Maintenance Plan ......................................... 1‐14

1.7 APPLICABILITY AND EXCLUSIONS ............................................................................................................... 1‐14 SECTION 2 ‐ HYDROLOGY 2.0 HYDROLOGY INTRODUCTION ....................................................................................................................... 2‐2 2.1 DESIGN STORM FREQUENCY ........................................................................................................................ 2‐2 2.2 RAINFALL DEPTHS ......................................................................................................................................... 2‐2 2.3 RAINFALL INTENSITY ..................................................................................................................................... 2‐3 2.4 STREAM FLOWS ......................................................................................................................................... 2‐3

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2.5 PROCEDURE SELECTION ............................................................................................................................... 2‐3 2.6 RATIONAL METHOD ..................................................................................................................................... 2‐3

2.6.1 Drainage Area (A) ........................................................................................................................... 2‐4 2.6.2 Time of Concentration (Tc) ............................................................................................................ 2‐5 2.6.3 Rational Method Runoff Coefficient (C) ......................................................................................... 2‐6

2.7 NRCS METHOD ......................................................................................................................................... 2‐7 2.8 HYDROLOGIC ANALYSIS FOR LID .................................................................................................................. 2‐8

SECTION 3 ‐ STORMWATER QUANTITY/QUALITY, DETENTION, AND RETENTION 3.0 DETENTION AND RETENTION FACILITIES...................................................................................................... 3‐1 3.1 STORMWATER QUANTITY ............................................................................................................................ 3‐1 3.2 STORMWATER QUALITY ............................................................................................................................... 3‐1 3.3 GENERAL STORMWATER QUANTITY AND QUALITY POLICES ....................................................................... 3‐2 3.4 DESIGN CRITERIA ......................................................................................................................................... 3‐3

3.4.1 Required Design Analysis ............................................................................................................... 3‐6 3.4.2 General Analysis Procedures .......................................................................................................... 3‐7

3.5 DESIGN STANDARDS FOR STORMWATER FACILITIES ................................................................................... 3‐8

3.5.1 Grading and Depth of Detention/Retention Basins ....................................................................... 3‐8 3.5.2 Underground Detention ................................................................................................................. 3‐9 3.5.3 Stormwater Quality Basin Design Criteria .................................................................................... 3‐10

3.6 OUTLET HYDRAULICS .................................................................................................................................. 3‐11 3.7 OUTLET PROTECTION ................................................................................................................................. 3‐11

3.7.1 Trash Racks ................................................................................................................................... 3‐12 3.8 EMERGENCY SPILLWAY/OVERFLOW DESIGN ............................................................................................. 3‐12

3.9 CONSTRUCTION AND MAINTENANCE CONSIDERATIONS .......................................................................... 3‐13

SECTION 4 ‐ STORM SEWERS AND GUTTER FLOW

4.0 STORM SEWER AND GUTTER FLOW INTRODUCTION .................................................................................. 4‐1

4.1 GENERAL DESIGN GUIDELINES ..................................................................................................................... 4‐1 4.1.1 Design Flow Frequency Criteria ..................................................................................................... 4‐3 4.1.2 Computation Requirements ........................................................................................................... 4‐3 4.1.3 Tailwater Conditions ...................................................................................................................... 4‐4 4.1.4 Material and Construction Standards ............................................................................................ 4‐4

4.2 GENERAL DESIGN METHODOLOGY .............................................................................................................. 4‐7 4.2.1 Predevelopment and Postdevelopment Hydraulic Analysis .......................................................... 4‐8 4.2.2 Gutter Flow Analysis ...................................................................................................................... 4‐8 4.2.3 Stormwater System Layout ............................................................................................................ 4‐8 4.2.4 Submittal Requirements ................................................................................................................ 4‐9

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SECTION 5 ‐ OPEN CHANNEL HYDRAULICS

5.0 OPEN CHANNEL HYDRAULICS INTRODUCTION ............................................................................................ 5‐1 5.1 GENERAL DESIGN PARAMETERS ................................................................................................................... 5‐1 5.2 DESIGN STANDARDS ..................................................................................................................................... 5‐1

5.2.1 Design Storm .................................................................................................................................. 5‐2 5.2.2 Erosion Control .............................................................................................................................. 5‐2 5.2.3 Maintenance Requirements .......................................................................................................... 5‐4

5.3 OPEN‐CHANNEL FLOW METHODS AND EQUATIONS ................................................................................... 5‐4

5.3.1 Manning's n Value .......................................................................................................................... 5‐4 5.4 CHANNEL LININGS ........................................................................................................................................ 5‐5

5.4.1 Vegetation ...................................................................................................................................... 5‐5 5.4.2 Vegetative Design .......................................................................................................................... 5‐6 5.4.3 Flexible Linings ............................................................................................................................... 5‐6 5.4.4 Riprap Design ................................................................................................................................. 5‐6 5.4.5 Rigid Linings ................................................................................................................................... 5‐6

SECTION 6 ‐ CULVERTS AND BRIDGES

6.0 CULVERTS AND BRIDGES INTRODUCTION .................................................................................................... 6‐1

6.1 CULVERTS ‐ GENERAL DESIGN CRITERIA ...................................................................................................... 6‐1 6.1.1 Design Frequency ........................................................................................................................... 6‐2 6.1.2 Allowable Headwater ..................................................................................................................... 6‐2 6.1.3 Tailwater Conditions ...................................................................................................................... 6‐3 6.1.4 Inlet and Outlet Control ................................................................................................................. 6‐3 6.1.5 Culvert Velocity .............................................................................................................................. 6‐4

6.2 STRUCTURAL DESIGN AND MATERIALS ........................................................................................................ 6‐4 6.2.1 Culvert Sizes ................................................................................................................................... 6‐5 6.2.2 End Conditions ............................................................................................................................... 6‐5 6.2.3 Multiple Barrel Culverts ................................................................................................................. 6‐6 6.2.4 Culvert Skew .................................................................................................................................. 6‐6 6.2.5 Buoyancy ........................................................................................................................................ 6‐6 6.2.6 Debris and Trash Racks .................................................................................................................. 6‐6

6.3 ENVIRONMENTAL CONSIDERATIONS AND FISHERY PROTECTION ............................................................... 6‐7 6.3.1 Embedded and Open Bottom Culverts .......................................................................................... 6‐7

6.4 MAINTENANCE REQUIREMENTS .................................................................................................................. 6‐8

6.5 BRIDGES ......................................................................................................................................... 6‐9 6.5.1 Bridge Design Considerations ........................................................................................................ 6‐9

6.5.1.1 Piers and Abutments ....................................................................................................... 6‐9 6.5.1.2 Erosion Protection ........................................................................................................... 6‐9

6.5.2 Acceptable Methods ...................................................................................................................... 6‐9 6.5.3 Design Standards ......................................................................................................................... 6‐10

6.5.3.1 FEMA Standards ............................................................................................................. 6‐10 6.5.4 Procedures for Coastal Structures ............................................................................................... 6‐11

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SECTION 7 ‐ OUTLET PROTECTION

7.0 OUTLET PROTECTION INTRODUCTION ......................................................................................................... 7‐1 7.1 TYPES OF OUTLET PROTECTION ................................................................................................................... 7‐1 7.2 DESIGN CRITERIA ......................................................................................................................................... 7‐2 7.3 RIPRAP OUTLET PROTECTION ....................................................................................................................... 7‐3

7.3.1 Tailwater Depth ............................................................................................................................. 7‐3 7.3.2 Apron Dimensions .......................................................................................................................... 7‐3 7.3.3 Materials ........................................................................................................................................ 7‐5 7.3.4 Thickness ........................................................................................................................................ 7‐5 7.3.5 Stone Quality .................................................................................................................................. 7‐5 7.3.6 Filter .............................................................................................................................................. 7‐6 7.3.7 Construction Guidelines ................................................................................................................. 7‐6 7.3.8 Maintenance .................................................................................................................................. 7‐6

7.4 PREFORMED SCOUR HOLE ........................................................................................................................... 7‐7 SECTION 8 ‐ EROSION AND SEDIMENT CONTROL 8.0 EROSION PREVENTION AND SEDIMENT CONTROL ..................................................................................... 8‐1 8.1 BASIC PRINCIPLES ......................................................................................................................................... 8‐1 8.2 APPLYING BMPs ............................................................................................................................................ 8‐2

8.2.1 Construction versus Postconstruction BMPs ................................................................................. 8‐2 8.2.2 Identify Objectives ......................................................................................................................... 8‐2 8.2.3 Select BMPs .................................................................................................................................... 8‐4 8.2.4 Factors for Construction Sites ........................................................................................................ 8‐4

8.3 EROSION PREVENTION ................................................................................................................................. 8‐5

8.3.1 Temporary and Permanent Considerations ................................................................................... 8‐5 8.3.2 Surface Stabilization ...................................................................................................................... 8‐5 8.3.3 Slope and Channel Protection ........................................................................................................ 8‐6 8.3.4 Outlet Protection ........................................................................................................................... 8‐6

8.4 SEDIMENT CONTROL .................................................................................................................................... 8‐6

8.4.1 Temporary and Permanent Considerations ................................................................................... 8‐6 8.4.2 Sediment Barriers .......................................................................................................................... 8‐6 8.4.3 Sediment Traps and Basins ............................................................................................................ 8‐7 8.4.4 Construction Road and Entrance Management ............................................................................. 8‐7

8.5 EROSION AND SEDIMENT CONTROL PLANS ................................................................................................. 8‐7

8.5.1 Overall Requirements .................................................................................................................... 8‐7 8.5.2 Minimize Disturbed Areas .............................................................................................................. 8‐7 8.5.3 Site Perimeter Controls .................................................................................................................. 8‐8 8.5.4 Internal Erosion and Drainage Design ............................................................................................ 8‐9 8.5.5 Inspection and Maintenance ......................................................................................................... 8‐9 8.5.6 ESCP Preparation Guidance ......................................................................................................... 8‐10

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APPENDIX Appendix A: Checklists Stormwater System Design Plan and Report Checklist for Submission ........................................... Checklist A‐1 to A‐3 Typical Checklist for Preparing Erosion and Sediment Control Plans and Narratives ................................ Checklist A‐4 Policy on Construction Inspection in the City of Norwalk .......................................................................... Checklist A‐5 Erosion and Sedimentation Prevention Guidelines ................................................................................................... A‐6 Appendix B: Time of Concentration Figure B‐1: Nomograph for the Determination of Time of Concentration, Overland Flow ....................................... B‐1 Figure B‐2: Nomograph for Time of Concentration, Small Drainage Basins .............................................................. B‐2 Figure B‐3: Nomograph for Flow in Triangular Channels ........................................................................................... B‐3 Appendix C: Storm Sewer and Gutter Flow Analysis Storm Sewer System Design Worksheet .........................................................................................................Figure C‐1 Circular Channel Ratios ..... ..............................................................................................................................Figure C‐2 Flow Characteristic Curves ..............................................................................................................................Figure C‐3 Nomograph for Solution of Manning's Formula for Pipes Flowing Full ...........................................................Figure C‐4 Gutter Flow Velocity ......... ..............................................................................................................................Figure C‐5 Gutter Flow Analysis Computations, Inlet Spacing Sheet ................................................................................Figure C‐6 Gutter Flow Analysis Computations, Low Point Analysis.................................................................................Figure C‐7 Appendix D: Pipes and Culverts Table D‐1 Conversion of Nominal Gauge to Thickness .............................................................................................. D‐1 Table D‐2 Corrugated Galvanized Steel Pipe Recommended Gages ......................................................................... D‐1 Table D‐3 Asphalt Coated Corrugated Galvanized Steel Pipe Recommended Gauges .............................................. D‐2 Table D‐4 Culvert Pipe Schedules .............................................................................................................................. D‐2 Culvert Data Worksheet ... ............................................................................................................................. Figure D‐3 Culvert Computations Worksheet .................................................................................................................. Figure D‐4 Appendix E: Outlet Control Structures, Weir and Orifice Flow EQ. E1 Sharp‐Crested Weir Equation, No End Contractions .................................................................................... E‐1 EQ. E2 Sharp‐Crested Weir Equation, Two End Contractions.................................................................................. E‐1 EQ. E3 Sharp‐Crested Weir Equation, Submerged Discharge .................................................................................. E‐1 EQ. E4 Broad‐Crested Weir Equation ...................................................................................................................... E‐2 EQ. E5 V‐Notch Weir Equation ................................................................................................................................ E‐2 Figure E‐1: Sharp‐Crested "V" Notch Weir Discharge Coefficients ............................................................................. E‐2 EQ. E6 Proportional Weir Equation ......................................................................................................................... E‐3 Figure E‐2: Dimensions Used for Design of a Proportional Weir ................................................................................ E‐3 EQ. E7 Orifice Equation .. ......................................................................................................................................... E‐4 Appendix F: Hydraulic Analysis, Energy Loss Equations EQ. F1 Head Losses at Manholes ............................................................................................................................. F‐1 EQ. F2 Head Losses from Multiple Entering Flows .................................................................................................. F‐1 EQ. F3 Entrance Losses .. ......................................................................................................................................... F‐2 Table F‐1 Coefficients for Entrance Losses ................................................................................................................. F‐2 Appendix G: Open Channel Hydraulics EQ. G1 Continuity Equation ..................................................................................................................................... G‐1 EQ. G2 Flow Energy Equation .................................................................................................................................. G‐2 EQ. G3 Uniform Flow: Velocity Equation ................................................................................................................ G‐2 EQ. G4 Uniform Flow: Flow Rate Equation ............................................................................................................. G‐2 EQ. G5 Uniform Flow: Energy Grade Line Equation ................................................................................................ G‐3

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Table G‐1 Recommended Manning's Roughness Coefficients for Channels ............................................................. G‐4 Figure G‐1: vR, Product of Velocity and Hydraulic Radius ......................................................................................... G‐5 Figure G‐2: Solution of Manning's Equation for Trapezoidal Channels ..................................................................... G‐6 EQ. G6 Manning's Equation for Trial‐and‐Error Solutions ....................................................................................... G‐7 EQ. G7 Trapezoidal Conveyance Factor Equation ................................................................................................... G‐7 Figure G‐3: Normal Depth of Flow in a Uniform Channel .......................................................................................... G‐8 EQ. G8 Critical Depth Equation ............................................................................................................................... G‐9 EQ. G9 Froude Number Equation ............................................................................................................................ G‐9 EQ. G10 Hydraulic Radius Equation ........................................................................................................................ G‐10 EQ. G11 Manning's Equation, Solving for vR .......................................................................................................... G‐10 Figure G‐4: Drainage Channel Lining Design Worksheet ......................................................................................... G‐12 Appendix H: Trash Rack Design EQ. H1 Head Loss through a Trash Rack Equation .................................................................................................. H‐1 Appendix I: Riprap Design EQ. H1 Manning's n value for Riprap Equation ......................................................................................................... I‐1 EQ. H2 Minimum d50 value Equation ......................................................................................................................... I‐1 EQ. H3 Stone Weight Equation ................................................................................................................................. I‐2 Appendix J: City of Norwalk Ordinances and Standards

Standards for Removal of Oil, Grit, and Other Material from Stormwater

Chapter 93. Stormwater, Illicit Discharges, and Connections Ordinance

City of Norwalk Drainage Manual

ACKNOWLEDGEMENTS This manual relies significantly on work previously completed by others in several existing storm drainage manuals including the following:

2002 Soil Erosion and Sediment Control Manual 2004 Connecticut Stormwater Quality Manual and 2011 Appendices "A Guide for the Design of Storm Drainage Facilities in the Town of Plymouth, Massachusetts" March 2009 American Society of Civil Engineers, 1993. Design and Construction of Urban Stormwater Management Systems Manual of Practice #77 City of Memphis/Shelby County Storm Water Management Manual, Version 1, August 2006 City of Norwalk Ordinances: Chapter No. 93. Stormwater, Illicit Discharges and Connections, adopted 2010 City of Norwalk Planning & Zoning Regulations, Effective October 16, 1929, Amended to April 24, 2015 City of Norwalk Subdivision and Resubdivision Regulations, Adopted December 8, 1948, Amended to January 20, 2012 City of Norwalk Inland Wetlands & Watercourses Regulations, Adopted February 12, 1974, Amended to December 9, 2008 City of Norwalk Connecticut Department of Public Works Roadway Standards, May 1982, Amended to January 2017 City of Philadelphia, Philadelphia Water Department, Planning & Environmental Services Division "Stormwater Management Guidance Manual, Version 2.0, Revise: April 29, 2011" Connecticut Department of Transportation (CTDOT) "Drainage Manual" October 2000 Final Brazoria County (TX) Drainage Criteria Manual, Prepared by Klotz Associates, Inc., November 2003 Georgia Coastal Stormwater Supplement, April 2009 Milone & MacBroom, Inc. "Shoreline Management Manual: A Homeowner's Guide to Shoreline Stabilization and Vegetated Buffer Zones" New Jersey Stormwater Best Management Practices Manual, February 2004, as amended


New York City Department of Environmental Protection. "NYC green infrastructure plan: A sustainable strategy for clean waters" Office of Facilities, Utilities & Engineering, "Yale University Sustainable Stormwater Management Plan" September 2013 Prince George's County, Maryland Department of Environmental Resources, Programs and Planning Division, "Low‐Impact Development Design Strategies An Integrated Design Approach" June 1999 State of Florida Department of Transportation Drainage Handbook Bridge Hydraulics, July 2012 Stormwater Management Manual, Metropolitan Government of Nashville and Davidson County, Nashville, Tennessee, April 2012 Town of Greenwich Drainage Manual, Low Impact Development and Stormwater Management, February 2012 Town of South Windsor Public Improvement Specifications, June 30, 2006 Washington State Department of Transportation, 2016 Standard Specifications


ACRONYMS AASHTO American Association of State Highway and Transportation Officials ASTM American Society for Testing and Materials BMP Best Management Practice C Coefficient CTDOT Connecticut Department of Transportation CTDEEP Connecticut Department of Energy & Environmental Protection cfs Cubic Feet per Second CMP Corrugated Metal Pipe CPP Corrugated Polyethylene Pipe CWA Clean Water Act DPW Department of Public Works ESCP Erosion and Sediment Control Plan FEMA Federal Emergency Management Agency FHWA Federal Highway Administration ft Feet ft/sec Feet per Second Fr Froude Number GIS Geographic Information System GRV Groundwater Recharge Volume HEC‐HMS Hydrologic Engineering Center – Hydrologic Modeling System HDPE High‐Density Polyethylene HGL Hydraulic Grade Line I Intensity IMP Integrated Management Practice La Length of an Apron LID Low Impact Development MS4 Municipal Separate Storm Sewer System NAD83 North American Datum of 1983 NAVD88 North American Vertical Datum of 1988 NFIP National Flood Insurance Program NOAA National Oceanic and Atmospheric Administration NPDES National Pollutant Discharge Elimination System NRCS Natural Resources Conservation Service O&M Operation and Maintenance OSHA Occupational Safety & Health Administration PFDS Precipitation Frequency Data Server PVC Polyvinyl Chloride RCP Reinforced Concrete Pipe RECP Rolled Erosion Control Product SCS Soil Conservation Service SQM Stormwater Quality Manual SWMM Stormwater Management Model Tc Time of Concentration TN Total Nitrogen TP Total Phosphorus


TSS Total Suspended Solids USACE United States Army Corps of Engineers USEPA United States Environmental Protection Agency USGS United States Geological Survey WQF Water Quality Flow WQv Water Quality Volume


DEFINITIONS

For clarity, the following definitions shall apply to the text in this document. Aquatic Buffer: An aquatic buffer is an undisturbed natural area located immediately adjacent to a river,

stream, tidal creek, coastal marshland, or other aquatic resource where land‐disturbing activities are significantly restricted or prohibited. While they function primarily to preserve the integrity of streams, wetlands, and other aquatic resources and protect them from the direct impacts of the land development process, they also provide a number of other important ecological services including pollutant removal, erosion control, and flood attenuation.

Best Management Practice (BMP): "Best Management Practice" means a structural or nonstructural

practice designed to temporarily store or treat stormwater runoff in order to mitigate flooding, reduce pollution, and provide other amenities.

Bypass Flow (for Inlet Design): Flow that bypasses an inlet and flows along the gutter line or other flow

path to the next downstream inlet. Common Area: A parcel or part of a parcel the benefits of which are shared by the owners or occupants

of other parcels or buildings within the parcel. Examples include sidewalks within an industrial park, parking areas, swimming pools, and clubhouses.

Construction: Any construction or demolition activity, clearing, grading, grubbing, or excavation or any

other activity that results in land disturbance. Construction does not include emergency construction activities required to immediately protect public health and safety or routing maintenance activities required to maintain the integrity of structures by performing minor repair and restoration work.

Development: Development means the division of a parcel of land into two or more parcels; the

construction, reconstruction, conversion, structural alteration, relocation, or enlargement of any building or structure; any mining excavation or landfill; and any use or change in the use of any building or other structure or land or extension of use of land by any person for which a permit is required under Norwalk City Code and/or Zoning Regulations.

Detention: The temporary storage of floodwater that is usually released by a measured but

uncontrolled outlet. Detention facilities typically flatten and spread the inflow hydrograph, lowering the peak. Structures that release storage over a period of 12 to 36 (or more) hours may also serve water quality purposes.

Disturbance: Disturbance means the addition of impervious surface (e.g., pavement); exposure or

movement of soil or bedrock (e.g., grading, excavation); or clearing, cutting, or removing vegetation.

FEMA 100‐Year Floodplain: The area that will be inundated by the flood event having a 1‐percent

chance of being equaled or exceeded in any given year. The 1‐percent‐annual‐chance flood is also referred to as the base flood or 100‐year flood.


FEMA 100‐year Floodway: The channel of a river or other watercourse and the adjacent land areas that

must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height. Communities must regulate development in these floodways to ensure that there are no increases in upstream flood elevations.

Green Infrastructure: All potential practices, landscapes, and storage devices that can be used to slow

the flow of stormwater, reduce stormwater volume, and improve stormwater quality before it enters the sewer system. Green infrastructure can include, but is not limited to, bioretention areas, green roofs, permeable pavement, and increased urban tree planting.

Gutter: Portion of the roadway along the curb or edge of pavement (in the case of curbless roadways). Hydraulic grade line: A theoretical line whose height can be defined as the height above the invert of a

pipe to which pressure would cause water to rise if a tube were inserted into the pipe during peak flow. If the hydraulic grade line is calculated to be above the elevation of the top of a catch basin or manhole, then water will flow from the system out of the structure and into the roadway. Therefore, for design purposes, maintaining a hydraulic grade line below the elevation of the top of the structure is imperative to eliminate flooding.

Impervious Area: A surface composed of any material that impedes or prevents natural infiltration of

water into the soil. Impervious surfaces shall include but are not limited to eaves, roofs (including overhangs), solid decks, driveways, patios, sidewalks, parking area, tennis courts, concrete or asphalt streets, or compacted gravel surfaces. Slatted decks, porous paving with a runoff coefficient of less than 25 percent including grass pavers, ponds, streams, and other water surfaces, including the water area of swimming pools shall be considered to be pervious. Calculations of impervious surfaces for streets shall include the area compacted for pavements of gravel base. New gravel is not considered impervious under the Stormwater Management rules. However, if an applicant can demonstrate that under existing conditions on a previously developed site the gravel material performs as an impervious surface, it may be considered impervious for the purposes of redevelopment. Any pavers with a void area of 20 percent or less will be considered completely impervious for the purposes of the Stormwater Management rules. In pavers with greater than 20 percent void area, the applicant may count only the nonvoid area as impervious provided the void areas are not grouted or made impermeable in any way.

Impoundment: A large body of water that is impounded (i.e., formed) by a structure (i.e., dam, dike,

levee, or other structures) and is surrounded by land. Infiltration: The process of percolating precipitation into the subsoil. Inlets: Inlets are entry points into the system, generally grates that are set on top of catch basins. For

some structures, an opening may exist along the curb line. Inlets are said to be curbed (Type C) if an inlet exists along the curb line. Inlets are said to be curbless (Type C‐L) if no such opening exists. Manholes are structures in a drainage system but usually do not serve as inlets.

Lake: A large body of water surrounded by land.


Low Impact Development (LID): A site design strategy intended to maintain or replicate predevelopment hydrology through the use of small‐scale controls integrated throughout the site to manage runoff as close to its source as possible.

Low‐impact stormwater management strategies: Include natural features, landscapes, and green

infrastructure systems. Implement stormwater management strategies following a fundamental order or priority: first infiltration of stormwater where it falls, then storage for infiltration or reuse, and finally temporary detention and gradual release of stormwater to the city's storm sewer systems.

Outfall: The place where a river, drain, or sewer empties into the sea, a river, or a lake. Permeable Paving/Pervious Concrete: Materials that are alternatives to conventional

pavement/sidewalk surfaces and that are designed to increase infiltration and reduce stormwater runoff and pollutant loads.

Rain Gardens/Biofiltration: A practice to manage and treat stormwater runoff by using a specially

designed planting soil bed and planting materials to filter runoff stored in a shallow depression. Redevelopment: Redevelopment is defined as any construction, alteration, or improvement that

disturbs a total of 10,000 square feet or more of existing impervious area where the existing land use is commercial, industrial, institutional, governmental, recreational, or multifamily residential. Previously developed areas are those portions of a site covered by paved, gravel, or dirt driveways, streets, roads, and parking areas; gravel; pavement; buildings; impervious surfaces; lawns; or structures. Areas that simply have been (or have once been) cleared of vegetation are not considered "previously developed" if woody vegetation has been reestablished. Building demolition is included as an activity defined as "redevelopment," but building renovation is not. Similarly, removal of roadway materials down to the erodible soil surface is an activity defined as "redevelopment," but simply resurfacing of a roadway surface is not. Pavement excavation and patching that are incidental to the primary project purpose, such as replacement of a collapsed storm drain, are not classified as redevelopment. Any creation of new impervious area over portions of the site that are currently pervious is required to comply fully with the requirements of this manual. In no case on a redevelopment shall the levels of stormwater treatment and recharge be less than the levels prior to initiation of the proposed project. BMP strategies for redevelopment include green roofs, microdetention, alternative pavers and/or porous pavement, infrastructure upgrade (such as storm sewer overflows and leaking older pipes), and in‐pipe or small structural devices.

Retention: Storage provided in a facility without a positive outlet, or with a specially regulated outlet,

where all or a portion of the inflow is stored for a prolonged period. Infiltration basins are a common type of retention facility. Ponds that maintain water permanently, with freeboard provided for flood storage, are probably the most common type retention facility.

Retrofit: Stormwater retrofitting is providing stormwater treatment on existing development that is

currently untreated by any BMP or is inadequately treated by an existing BMP. Reservoir: A large natural or artificial lake used for collecting and storing water for human consumption

or agricultural use.


Stormwater Management Plan: Plan describing the potential water quality and quantity impacts

associated with a development project both during and after construction. It also identifies selected source controls and treatment practices to address those potential impacts, the engineering design of the treatment practices, and maintenance requirements for proper performance of the selected practices.

Stormwater Runoff: Aboveground water flow resulting from precipitation or snowmelt. Stormwater Treatment Practice: Devices constructed for primary treatment, pretreatment, or

supplemental treatment of stormwater. Tailwater: The elevation of water in the receiving stream at the drainage system outfall. Vegetated Buffer Zone: An area within the project boundary adjacent to the water intended to provide

for the establishment of native vegetated cover plants over time to the extent reasonable and practicable. The area may or may not currently support native vegetated cover.

Vegetated Buffer: An area of native vegetation created either through natural succession (i.e., stop

mowing grass) or the planting of native trees, shrubs, and herbaceous or ground covers. Water Quality Swales: Vegetated open channels designed to treat and attenuate the water quality

volume and convey excess stormwater runoff. Water Quality Volume: The volume of runoff generated by 1 inch of rainfall on a site. Wet Basins and Infiltration Basins: Wet basins are detention basins designed to maintain a permanent

pool of water. Wet basins are used for aesthetic or water quality enhancement. All outlets are above the normal level of the pool. They may retain flood flows for a prolonged period of time for the purpose of encouraging infiltration into the groundwater. Wet ponds generally provide more pollutant removal.

Section 1 Introduction and Submittal Requirements


City of Norwalk Drainage Manual 1‐1

1.0 INTRODUCTION 1.1 PURPOSE OF MANUAL

In May 1983, the City of Norwalk Drainage Manual was published to establish guidelines for the design of drainage facilities within the city. Many new stormwater management techniques that can enhance water quality and minimize flooding have been developed since the original manual was created. This 2017 Drainage Manual (the "Drainage Manual") provides updated design guidelines and criteria for new and redevelopment projects within the city.

The objectives of good drainage design are the following:

1. Provide for the safety of the public and adjacent property owners. 2. Eliminate or minimize undesirable upstream or downstream flooding effects that may result

from development and redevelopment. 3. Reduce the risk of flood damage resulting from improper design and construction of drainage

facilities. 4. Produce reliable and long‐lasting drainage structures and facilities, capable of being

satisfactorily operated and maintained. 5. Anticipate climate change to design facilities that will protect the public throughout their entire

design life.

The intent of this manual is to supplement existing available design documents at the federal and state level rather than provide a complete design basis document that restates existing published information. Nothing in this document relieves the designer of the responsibility to exercise professional judgment, follow prudent stormwater design principles, and develop accurate assessments of the existing conditions. 1.2 STORMWATER MANAGEMENT POLICY OBJECTIVES The objectives of these policies are the following:

1. Protect public health, safety, and general welfare. 2. Eliminate any unallowable discharges to Norwalk's stormwater system that adversely impact

water quality. 3. Provide for the sound use and development of flood‐prone areas in such a manner as to

maximize beneficial use without increasing flood hazard potential or diminishing the quality of the natural stormwater resources.

4. Increase the awareness of the public, property owners, and potential home buyers regarding stormwater impacts (i.e., flooding, erosion).

5. Minimize prolonged business interruptions. 6. Minimize damage to public facilities and utilities such as water and gas mains; electric,

telephone, storm, and sanitary sewer lines; and streets and bridges. 7. Promote functional public and private stormwater management systems that will not result in

excessive maintenance costs. 8. Encourage the use of natural and aesthetically pleasing design that maximizes preservation of

natural areas. 9. Promote the use of comprehensive watershed management plans.


10. Encourage preservation of floodplains, floodways, and open spaces. 11. Minimize the impacts from new development or areas of significant redevelopment. 12. Enable Norwalk to comply with the National Pollutant Discharge Elimination System (NPDES)

Municipal Separate Storm Sewer System (MS4) permits and applicable regulations (40 CFR 122.32‐35) for stormwater discharges.

13. Minimize the pollution of air, streams, and ponds; determine the adequacy of drainage facilities; safeguard the water table; and encourage the wise use of natural resources.

14. Ensure that facilities are designed to function under a variety of rainfall scenarios and tide ranges.

1.2.1 Policy Statements

Minimum standards and procedures for the design, construction, operation, and maintenance of the stormwater management infrastructure are set forth in the 2004 Connecticut Stormwater Quality Manual (SQM), as amended. The SQM is available in its most updated form at www.ct.gov/deep/stormwater. To implement the objectives presented in the SQM, the following general policy statements shall apply: 1. All development and land disturbance activities including capital improvement projects

conducted by the City of Norwalk shall be in compliance with the SQM. However, linear disturbances shall be exempt from detention requirements.

2. No construction, whether by private or public action, shall be performed in a manner that will

negatively impact stormwater quantity or quality in its vicinity or in other areas whether by flow restrictions, increased runoff, or diminishing channel or overbank storage capacity.

3. New construction or redevelopment shall not aggravate upstream or downstream flooding. A

developer may be required to correct existing upstream or downstream problems as part of his new development.

4. Grading of lots must result in positive drainage away from buildings. Minimum finished floor

elevations and elevation of the 1 percent annual chance (100‐year) storm events as mapped by the Federal Emergency Management Agency (FEMA) must be shown on all recorded lots where appropriate. Final grading shall not result in an increase in rate or volume of flow onto adjacent properties.

5. New development must minimize the impact on stormwater quality by applying structural

and/or nonstructural management practices selected to address site‐specific conditions during construction.

6. The city reserves the right to require more stringent erosion prevention and sedimentation

control practices on properties within sensitive (or impaired) watersheds. 7. Proposed plans for construction shall be stamped by a professional engineer licensed in the

State of Connecticut. This shall include all proposed improvements or modifications of the existing or new stormwater infrastructure and other related improvements or modifications. Plans and specifications that include the design of sediment basins or other sediment controls


involving structural, hydraulic, hydrologic, or other engineering calculations must be prepared by a licensed professional engineer.

8. When bonds are required, before they will be released for new construction, a postconstruction

runoff control Operation and Maintenance Agreement (with as‐built plan) must be provided. The as‐built must be submitted as a georeferenced electronic file certified by a licensed professional engineer or other suitable licensed professional as appropriate. The licensed professional shall certify on the as‐built plan that the facilities have been constructed in substantial and essential conformance to the design plan. As‐built capacities must be verified with a survey. It is understood that as‐builts will include final grading by home builders on individual lots; the stormwater infrastructure and direction to home builders and home buyers regarding use of areas acting as passive paths conveying stormwater flows must be included.

9. The city reserves the right to require maintenance or modification of stormwater management

practices that are not operating properly as determined by the city. 10. For properties where stormwater quantity management practices are either not feasible or are

not necessary, the city reserves the right to require on‐site controls for stormwater quality. 11. The city encourages regional and shared watershed management practices and facilities,

especially for redevelopment areas. These practices will be encouraged in order to replace or reduce the implementation of on‐site stormwater management facilities.

12. Construction in floodplains and floodways should be conducted in a way that protects or

enhances stormwater quality and promotes land and tree conservation, greenways, floodplain preservation, and hazard mitigation. Furthermore, development within a floodplain shall be consistent with the requirements of Article 110: Flood Hazard Zone of the City's Zoning Regulations and standards of FEMA.

13. The city reserves the right to require a registered professional engineer to be on site for

inspection and enforcement of proper construction and maintenance of erosion prevention and sediment control management practices at a specific construction site.

14. All new and redevelopment will have detention of stormwater unless supported by appropriate

drainage calculations and approval of the city. a. Underground detention is allowed for commercial/industrial sites whose owner shall be

responsible for the maintenance of the infrastructure. Additional water quality features will be required for underground detention systems (e.g., oil‐water separators).

b. There will be no detention permitted in the floodplain; however, Low Impact Development (LID) strategies and water quality ponds in the floodplain are encouraged.

15. Engineers must provide all calculations for review by the city in a format designated in this

Drainage Manual.


1.3 REGULATORY FRAMEWORK AND STANDARDS Stormwater discharges have been regulated since the passage of the Clean Water Act (CWA) by Congress in 1972. In implementing the requirements of the CWA, the Environmental Protection Agency developed standards and regulations regarding stormwater management throughout the United States. In Connecticut, CWA implementation ultimately led to development of the following permit programs:

General Permit for the Discharge of Stormwater Associated with Industrial Activities

General Permit for the Discharge of Stormwater and Dewatering Wastewaters from Construction Activities

General Permit for the Discharge of Stormwater Associated with Commercial Activities

For any development in the city of Norwalk, one or more of these permits may be required in addition to local permit requirements. Applicants are responsible for securing all state and federal permits for their own projects, and the list above should not be considered comprehensive.

Historically, development and redevelopment projects were designed to maximize use of the property with stormwater management being an afterthought. Management strategies focused on end‐of‐pipe treatment systems such as detention to mitigate increases in peak rates of runoff. Little thought was given to the quality of runoff and its impact on receiving rivers and streams. In recent years, the design focus has trended toward LID techniques whereby impervious areas are minimized, and efforts are made to preserve the existing site runoff patterns. Currently, the following documents are available to provide standards and guidance on stormwater design:

2000 Drainage Manual, as amended, prepared by the Connecticut Department of Transportation

2004 Connecticut Stormwater Quality Manual (with August 2011 Appendix) prepared by the Connecticut Department of Energy & Environmental Protection

2002 Connecticut Guidelines for Soil Erosion and Sediment Control (with August 2011 Appendix) prepared by the Connecticut Department of Energy & Environmental Protection

Low Impact Development Hydrologic Analysis, as amended, prepared by Prince Georges' County Maryland, July 1999

The Norwalk River Watershed Action Plan, 2011 Update to the 2008 Plan prepared by HydroQual and SWRPA

These existing documents will be referenced throughout this Drainage Manual, and the designer is encouraged to obtain copies of these.

At the local level, the City of Norwalk regulates stormwater management through its Planning and Zoning Commissions, Inland Wetlands Commission, and the Department of Public Works. Recognizing that every site is different with respect to potential impacts, designers are encouraged to meet with city staff early during the design process to discuss site‐specific plans and design standards.

The criteria set forth herein shall be used in the design of drainage facilities in projects carried out by the Department of Public Works as well as in subdivisions, site development, and other projects designed by engineers over which the city has a responsibility for review and approval.


All design plans submitted for review by the city shall be referenced to the North American Datum of 1983 (NAD83) horizontal datum and the North American Vertical Datum of 1988 (NAVD88) vertical datum. 1.3.1 Permits – Jurisdictional Control and Contact Information Permits to construct drainage facilities shall be obtained from all agencies that have jurisdictional control. The following may be used as a guide: Area of Control Agency Freshwater Streams 1. Norwalk Inland Wetlands & Watercourse Agency

(203) 854‐7744 2. U.S. Army Corps of Engineers New England District, (978) 318‐8238 3. Connecticut Department of Energy & Environmental Protection,

when the watershed area falls under the CT Water Diversion Policy Act P.A. 82‐402

Land and Water Resources Division, (860) 424‐3019 Inland Wetland Areas 1. Norwalk Inland Wetlands & Watercourses Agency

2. U.S. Army Corps of Engineers

Coastal Area 1. Connecticut Department of Energy & Environmental Protection Land and Water Resources Division, (860) 424‐3019 2. Norwalk Planning and Zoning Commission (203) 854‐7780 3. Norwalk Harbor Management Commission (203) 854‐7780 4. U.S. Army Corps of Engineers 5. U.S. Coast Guard, when work is in navigable waters Coast Guard District One Commander, (617) 223‐8555

City Highways, Sidewalks, and/or Drainage Facilities

1. Norwalk Department of Public Works (encroachment permit, for storm drainage connections)

(203) 854‐7791 2. Connecticut Department of Transportation – Approval to

Construct on or Near Railroad State‐Maintained Highways & Drainage Facilities

1. Connecticut Department of Transportation District 3 Permit Office, (203) 389‐3000

Archaeologically and/or Historically Important Sites

1. State Archaeologist (860) 486‐5248 2. State Historic Preservation Officer (860) 256‐2755 3. Norwalk Historical Society (203) 846‐0525

1.4 STORMWATER MANAGEMENT STANDARDS The stormwater management standards address water quality (pollutants) and water quantity (flooding, low base flow and recharge) by establishing standards that require the implementation of a wide variety


of stormwater management strategies. These strategies include environmentally sensitive site design and LID techniques to minimize impervious surface and land disturbance, source control and pollution prevention, structural BMPs, construction period erosion and sedimentation control, and the long‐term operation and maintenance of stormwater management systems. Table 1‐1 summarizes the stormwater treatment practice criteria for all site development plans.

1.4.1 Floodplains Development of property located within the floodplain must comply with guidelines established in Article 110: Flood Hazard Zone of the City of Norwalk Zoning Regulations. Wise use of the floodplain is encouraged to minimize adverse effects on flood heights, flow velocities, and stormwater quality as well as to maximize land conservation, greenways, floodplain preservation, hazard mitigation, and quality of life.

1.4.2 LID Site Planning and Design Strategies LID site planning and design strategies must be used to the maximum extent practicable to reduce the generation of the water runoff volume for both new and redevelopment projects. All development proposals must include a completed Stormwater Management Plan checklist (see Appendix A) and appropriate computations and plans that show compliance with this standard. If full compliance is not provided, an applicant must document why key steps in the process could not be met and what is proposed as mitigation. The objective of the LID Site Planning and Design Strategies standard is to provide a process by which LID is considered at an early stage in the planning process such that stormwater impacts are prevented rather than mitigated for. 1.4.2.1 Protecting Riparian Buffers Riparian buffers are areas of planted or preserved vegetation between developed land and surface water. Riparian buffer areas protect water quality by cooling water, stabilizing banks, mitigating flow rates, and providing for pollution and sediment removal by filtering overland sheet runoff before it enters the water. An example of a three‐zone riparian buffer is show in Figure 1‐1.

Figure 1‐1: Three‐Zone Riparian Buffer Source: Adapted from Atlanta Regional Commission, 2001


The inner zone, also termed the "streamside zone," begins at the edge of the stream bank of the active channel and extends a minimum distance of 30 feet measured horizontally on a line perpendicular to the water body. Undisturbed vegetated area aims to protect the physical and ecological integrity of the stream ecosystem. The vegetative target for the streamside zone is undisturbed native woody species with native plants forming canopy, understory, and duff layer. Where such forest does not grow naturally, then native vegetative cover appropriate for the area (such as grasses, forbs, or shrubs) is the vegetative target. Generally speaking, structural BMPs are not allowed in the inner zone.

The middle zone extends immediately from the outer edge of the inner zone for a minimum distance of 50 feet. This managed area of native vegetation protects key components of the stream ecosystem and provides distance between upland development and the streamside zone. The vegetative target for the middle zone is either undisturbed or managed native woody species, or, in their absence, native vegetative cover of shrubs, grasses, or forbs. Undisturbed forest is encouraged strongly to protect future water quality and the stream ecosystem. Forested riparian buffers should be maintained, and reforestation should be encouraged where no wooded buffer exists. Proper restoration should include all layers of the forest plant community, including understory, shrubs, and groundcover (not just trees).

The outer zone extends a minimum of 20 feet immediately from the outer edge of the middle

zone. This zone prevents encroachment into the riparian buffer area, filters runoff from adjacent land, and encourages sheet flow of runoff into the buffer. The vegetative target for the outer zone is native woody and herbaceous vegetation to increase the total width of the buffer; native grasses and forbs are acceptable.

Generally, all three zones of the riparian buffer should remain in their natural state. However, some maintenance is periodically necessary, such as planting to minimize concentrated flow, the removal of exotic plant species when these species are detrimental to the vegetated buffer, and the removal of diseased or damaged trees. Effective treatment of stormwater runoff is achieved when pervious and impervious area runoff is discharged to a grass or forested buffer via overland flow. The use of a filter strip is recommended to treat overland flow in the green space of a development site. The area draining by sheet flow to a buffer can be subtracted from the total area in the Water Quality Volume (WQv) calculation, and the impervious area draining to the buffer by sheet flow can be subtracted from the impervious area in the Groundwater Recharge Volume (GRV) calculation and the postdevelopment impervious area in the Runoff Reduction Volume calculation, provided the following conditions are met:

The minimum stream buffer width (i.e., perpendicular to the stream flow path) shall be 50 feet as measured from the top bank elevation of a stream or the boundary of a wetland.

The maximum contributing path shall be 150 feet for pervious surfaces and 75 feet for impervious surfaces.

The average contributing overland slope to and across the buffer shall be less than or equal to 5 percent.

Runoff shall enter the buffer as sheet flow. A level spreader shall be utilized where local site conditions prevent sheet flow from being maintained.

The stream buffer remains unmanaged other than routine debris removal.


The buffer is protected by an acceptable conservation easement or other enforceable instrument that provides perpetual protection of the area. The easement must clearly specify how the natural area vegetation shall be managed and boundaries will be marked. [Note: managed turf (e.g., playgrounds, regularly maintained open areas) is not an acceptable form of vegetation management.]

Source: Town of Greenwich Drainage Manual, February 2014, Section 4.4.4

1.4.3 Stormwater Quantity Detention This policy is primarily concerned with maintaining predevelopment conditions for stormwater quality, flood storage, flow, and velocity. It should also be applied under certain conditions for the purpose of maintaining adequate capacity of an existing outfall or combining public and private efforts to correct existing deficiencies for flooding, erosion, and stormwater quality. In some cases, controlling the total volume of runoff to predevelopment levels may also be required. In accordance with the design storm frequencies listed in Table 2‐1 (page 2‐2) to prevent storm damage and downstream and off‐site flooding, stormwater management systems shall be designed so that postdevelopment peak discharge rates do not exceed predevelopment peak discharge rates except in coastal areas. Exceptions to this can be granted with written approval of the City Engineer if in the opinion of the City Engineer the design engineer has presented detailed technical analysis supporting the lack of need for detention.

Applicants must calculate runoff rates from preexisting and postdevelopment conditions and shall include flow from the entire watershed to the site including off‐site sources. Measurement of peak discharge rates is calculated at a design point, typically the lowest point of discharge at the downgradient property boundary. The topography of the site may require evaluation at more than one design point if flow leaves the property in more than one direction. An applicant may demonstrate that a feature beyond the property boundary (e.g., culvert) is more appropriate as a design point. Section 2 of this Drainage Manual describes hydrologic analysis methods, and Section 8 describes detention/retention design methods.

BMPs that slow runoff rates through storage and gradual release, such as LID techniques, extended dry detention basins, and wet basins, must be provided. Where an area is within the 1 percent annual chance coastal floodplain, the control of peak discharge rates may be waived by written authorization from the City Engineer. Note that state and federal agencies may require retention of freshwater flows to tidal systems for habitat protection.

Green infrastructure practices in the form of better site planning and design techniques and LID practices should be used to the maximum extent practical during the creation of a stormwater management design plan for a proposed development project. 1.4.4 Groundwater Recharge All site development plans shall include a groundwater recharge analysis in accordance with Natural Resources Conservation Service (NRCS) soil classification and recharge rates as described in Section 7.5 of the 2004 Connecticut SQM as amended. The GRV is the postdevelopment recharge volume (on a storm event basis) required to minimize the loss of annual predevelopment groundwater recharge. The


required GRV equals a depth of runoff corresponding to the soil type times the impervious areas covering that soil type at the postdevelopment site.

GRV = F x I Where: GRV = required Groundwater Recharge Volume (cubic feet or acre‐feet)

F = target depth factor associated with each Hydrologic Soil Group (inches) (see Table 1‐1) Convert to feet when calculating GRV.

I = impervious area on the postdevelopment site for new development projects or the net increase in impervious area for redevelopment projects.

TABLE 1‐1

Recharge Target Depth by Hydrologic Soil Group

NRCS Hydrologic Soil Group Approximate Soil Texture Target Depth Factor (F)

A Gravels, sand, loamy sand, or sandy loam 0.60 inch

B Silty loam 0.35 inch

C Sandy clay loam 0.25 inch

D Clay, silty clay loam, sandy clay, silty clay 0.10 inch

The discharge of groundwater from sump pumps, subdrains, etc. onto sidewalks and roadways is prohibited. This also includes surface discharge from properties that would be deemed to cause a public safety hazard.

Infiltration BMPs must be designed, constructed, operated, and maintained in accordance with the specifications and procedures set forth in the 2004 Connecticut SQM. Pretreatment of runoff shall be required in accordance with the design standards set forth in the manual. Roof runoff may be infiltrated without pretreatment unless the roof is deemed to have a higher potential pollution load (e.g., industrial buildings with pollutant exhaust vent fallout or with highly erodible roofing materials). Recharged roof runoff can be subtracted from WQV but not from larger storm calculations unless the applicant verifies that the drywells are sized for the 1 percent annual chance (100‐year) 24‐hour Type III storm event.

When designing infiltration BMPs, adequate subsurface information must be obtained. All infiltration systems must be designed and installed in accordance with the 2004 SQM. Infiltration systems must be installed in soils capable of absorbing the recharge volume (i.e., not D soils). Infiltration structures must be able to drain fully within 72 hours.

Stormwater must be recharged within the same subwatershed to maintain base flow at predevelopment recharge levels to the maximum extent practicable. The objective of the groundwater recharge standard is to protect water table levels, stream base flow, wetlands, and soil moisture levels. Infiltrating stormwater may also provide significant water quality benefits such as reduction of bacteria, nutrients, and metals when infiltrated into the soil profile.

The stormwater recharge requirement may be waived if an applicant can demonstrate a physical limitation that would make implementation impracticable or where unusual geological or soil features may exist such as significant clay deposits or ledge where recharge does not currently occur; fill soils; or areas of documented slope failure.


TABLE 1‐2

Summary of Stormwater Treatment Practice Criteria Criteria Description Post‐Development

Storm Magnitude

Groundwater Recharge

Groundwater Recharge Volume (Rev)* Maintain predevelopment annual groundwater recharge volume to the maximum extent practicable through the use of infiltration measures. See equation in Section 7.5.1 of the 2004 SQM.

First 1 inch of runoff

Pollutant Reduction

Water Quality Volume (WQv)* Volume generated by1 inch of runoff on the site See equation in Section 7.4.1 of the 2004 SQM. Water Quality Flow (WQf)* Peak flow associated with the WQv See Section 7.4.2 of the 2004 SQM.

First 1 inch of runoff

Overbank Flood Protection

Peak Runoff Attenuation (Qp)* Control the postdevelopment peak discharge rates from the 4 percent and 1 percent annual chance (25‐ and 100‐year) storms to the corresponding predevelopment peak discharge rates. Calculations must be provided that show how runoff from the 4 percent and 1 percent annual chance (25‐ and 100‐year) storms reaches the proposed facilities. Emergency Outlet Sizing Size the emergency outlet to safety pass the postdevelopment peak runoff from, at a minimum, the 1 percent annual chance (100‐year) storm in a controlled manner without eroding the outlet works and downstream drainages. Downstream Analysis Downstream analysis of the overbank and extreme flood 4 percent and 1 percent annual chance (25‐year and 100‐year, respectively) shall be conducted to identify potential detrimental effects of proposed stormwater treatment practices and detention facilities on downstream areas.

4 Percent (25‐year) and 1 Percent Annual Chance (100‐year) 24‐hour Type III rainfall 1 Percent Annual Chance (100‐year) 24‐HourType III rainfall 4 Percent (25‐year) and 1 Percent (100‐year) Annual Chance 24‐hour Type III rainfall

*Note that the Rational Formula is not allowed for determining required volumes to meet the stormwater criteria. The Rational Formula is appropriate for calculating peak discharge rates and thus for sizing pipes but not for volume‐based requirements.


For sites comprised solely of NRCS Type C and D soils and/or bedrock at the land surface, proponents are required to infiltrate the required recharge volume only to the maximum extent practicable, meaning the following:

1. The applicant has made all reasonable efforts to meet this Drainage Manual's requirements. 2. The applicant has made a complete evaluation of all possible applicable infiltration measures,

including environmentally sensitive site design that minimizes land disturbance and impervious surfaces, LID techniques, and structural stormwater BMPs.

3. If the postdevelopment recharge does not at least approximate the annual recharge from predevelopment conditions, the applicant has demonstrated that she/he is implementing the highest practicable method for infiltrating stormwater.

1.4.5 Stormwater Quality Management Practices Use of proprietary oil and grit separators shall be allowed provided they meet the requirements set forth in the City of Norwalk's Department of Public Works Water Quality Subcommittee on Oil & Grit Separators' "Standards for Removal of Oil, Grit, and Other Material from Stormwater." A copy of these standards is provided in Attachment J of this Drainage Manual.

Increased pollutant concentrations and loads impact the ability of the major systems and Waters of the State to meet designated use goals. To minimize stormwater quality impacts, on‐site stormwater quality management practices are encouraged as detailed in Section 7.0. The city maintains the right to require stormwater quality management practices on a case‐by‐case basis. The extent and type of management practices implemented should be proportionate to the land use, pollutant discharge potential, and proximity to regional stormwater quality management practices. The city encourages that a series of BMPs be implemented that optimize the use of required green and open spaces especially along buildings and within or along parking lots. The stormwater controls should be designed to limit the discharge of stormwater pollutants off site to predevelopment levels to the maximum extent practicable.

Stormwater runoff must be treated before discharge. The WQv is the amount of stormwater runoff that must be captured and treated in order to remove a significant fraction of stormwater pollutants on an average annual basis. The required WQv, which results in the capture and treatment of the entire runoff volume for 90 percent of the average annual storm events, is equivalent to the runoff associated with the first 1.2 inches of rainfall over the impervious surface (i.e., 1 inch of runoff). This can be accomplished through the use of oil/grit separators. Shop drawings shall be submitted for review and approval. The WQv is calculated as described in Section 7.4.1 of the 2004 Connecticut SQM.

The WQf is the peak flow rate associated with the water quality design storm or WQv. Although most stormwater treatment practices are sized based on WQv, flow diversion structures for off‐line stormwater treatment practices must be designed to bypass flows greater than the WQf. The WQf shall be calculated using the method described in Section 7.4.2 of the 2004 Connecticut SQM.

Structural BMPs shall be incorporated into site designs to provide adequate treatment of stormwater, including the following:

80 percent removal of total suspended solids (TSS) 60 percent removal of pathogens


30 percent removal of total phosphorus (TP) for discharges to freshwater systems 30 percent removal of total nitrogen (TN) for discharges to saltwater or tidal systems

1.5 ILLICIT DISCHARGES All illicit discharges to stormwater management systems are prohibited, including discharges of industrial wastes and/or effluent from sanitary sewer, subdrains and French drains near subsurface sewage disposal systems, cesspools, septic tanks, etc. Illicit discharges to the stormwater management system are discharges not entirely comprised of stormwater; uncontaminated groundwater; irrigation water; street wash water; or naturally occurring discharges associated with groundwater, springs, streams, etc. An Illicit Discharge Ordinance was adopted by the City of Norwalk Common Council on May 25, 2010. A copy of this ordinance is provided in Appendix J of this Drainage Manual. 1.6 SUBMITTAL REQUIREMENTS This section is provided to guide developers through the necessary submittals required for review and approval of a new stormwater management system in Norwalk. It is the responsibility of the design engineer to develop the required hydrologic and hydraulic data for a project. Field investigation, data collection, data analysis, and sound judgment are all fundamentals of good drainage design and must be used by the engineer while employing the criteria set forth in this Drainage Manual.

All land use applications submitted to the city shall include the following unless otherwise directed by the city:

Stormwater System Design Plan and Report

Floodplain Management Report (if applicable)

Bridge or Culvert Hydraulics Report (if applicable)

Landscaping Plan (if required)

Erosion and Sediment Control Plan

Stormwater Management System Operation and Maintenance Plan

Predrainage and Postdrainage maps listed in Section 2.6.1

1.6.1 Stormwater System Design Plan and Report A Stormwater System Design Plan and Report must be prepared for all proposed development projects where piped drainage systems are proposed and submitted for review and approval. A checklist for all required items has been provided in Appendix A (pages A‐1 to A‐2). 1.6.2 Floodplain Management Plan For any site located partially or entirely within a floodplain or floodway as designated on the Fairfield County Flood Insurance Rate Maps (https://msc.fema.gov/portal ), the applicant shall prepare a report describing the existing conditions of the floodplain or floodway, proposed activities within the floodplain or floodway, and the potential impacts to flooding that may result from the project. In cases where floodplain or floodway fill is proposed, the applicant shall include hydraulic analysis in sufficient detail to document compliance with FEMA standards. Note that for floodway fill detailed hydraulic modeling will be required.


1.6.3 Bridge or Culvert Hydraulics Report The Bridge or Culvert Hydraulics Report should clearly demonstrate that the proposed structure length, dimensions, and configuration are justified for the crossing. At a minimum, the following information must be provided in the report:

1. Design flow rates including back‐up computations used to derive the flow 2. FEMA flow rates (for streams with defined regulatory floodplains) 3. Existing and proposed conditions hydraulic analysis to support the design 4. Scour analysis and countermeasure design 5. Habitat considerations incorporated into the design such as those imposed by the United States

Army Corps of Engineers (USACE)with respect to fish and amphibian habitat passage at structures

1.6.4 Landscaping Plan The landscaping plan should be included with the submittal of the stormwater management design plan if requested. All landscaping plans should illustrate the layout of the proposed development project and should identify any landscaping features that will be installed on the development site. Consequently, it is recommended that all landscaping plans include the following information:

Existing trees and other vegetation

Existing and proposed roads, buildings, parking areas, and other impervious surfaces

Existing and proposed primary and secondary conservation areas (e.g., aquatic buffers, trees, and other existing vegetation)

Proposed limits of clearing and grading

Existing and proposed LID and stormwater management practices

Other landscaping features and areas

Proposed plantings

Information about the landscaping methods and materials that will be used during construction 1.6.5 Erosion and Sediment Control Plan An erosion and sediment control plan should be prepared for all proposed development projects. All erosion and sediment control plans should be prepared in accordance with requirements of the 2002 Connecticut Sediment and Erosion Control Manual and the state's NPDES General Permit for Stormwater Discharges Associated with Construction Activities and should include erosion and sediment control practices that will help minimize the negative impacts of construction stormwater runoff on coastal Norwalk's valuable aquatic and terrestrial resources. The erosion and sediment control plan should be included with the submittal of the Stormwater System Design Plan and Report and shall include computations for sizing all proposed sediment and erosion control measures. A checklist for items required in the erosion and sediment control plans and narratives is provided in Appendix A (page A‐4).


1.6.6 Stormwater Management System Operation and Maintenance Plan The stormwater management system, including all structural stormwater controls and conveyances, must have an Operation and Maintenance Plan (O&M Plan) to ensure it will operate as designed. The O&M Plan should be included with the submittal of the Stormwater System Design Plan and Report and shall outline all routine inspection and maintenance tasks that will be completed on all components of the postconstruction stormwater management system including: (1) green infrastructure practices; (2) stormwater management practices; (3) stormwater conveyance features; and (4) storm drain infrastructure.

At a minimum, the O&M Plan shall include the following information:

Time line indicating, in general, when routine inspection and maintenance activities will occur

The routine and nonroutine inspection and maintenance tasks for each BMP to be undertaken after construction is complete and a schedule for implementing those tasks

A plan that is drawn to scale and shows the location of all stormwater BMPs in each treatment train along with the discharge point

Proper procedures for good housekeeping, including storing materials and waste products inside or under cover; vehicle washing; spill prevention and response; maintenance of lawns, gardens, and other landscaped areas; and storage and use of fertilizers, herbicides, and pesticides

Name of the person or party responsible for completing routine inspection and maintenance activities; pet waste management; operation and management of septic systems; and proper management of deicing chemicals and snow

Signed statement confirming that responsibility for the inspection and maintenance of the postconstruction stormwater management system will remain with the property owner

Signed statement confirming that if portions of the property are sold or otherwise transferred arrangements will be made to pass the inspection and maintenance responsibilities to the successive owners

Signed statement providing city representatives with permission to enter the property at reasonable times and in a reasonable manner to inspect the postconstruction stormwater management system

1.7 APPLICABILITY AND EXCLUSIONS Except as expressly provided herein, stormwater runoff from all industrial, commercial, institutional, office, residential, and transportation projects including site preparation, emergency repairs to roads or their drainage systems, construction and redevelopment, and all point source stormwater discharges from said projects shall be managed according to the Stormwater Management Standards.

The Stormwater Management Standards shall apply to all land use development applications.

Section 2 Hydrology



2.0 HYDROLOGY INTRODUCTION

In evaluating predeveloment and postdevelopment runoff rates, the designer is encouraged to follow best practice within the engineering community. At a minimum, for all projects within the city of Norwalk the applicant shall submit the required plans described in Section 1 of this Drainage Manual.

2.1 DESIGN STORM FREQUENCY

All design storm events shall be 24 hours in duration and utilize NRCS Type III precipitation distribution. Table 2‐1 summarizes the various return frequencies that shall be used for design. Under exceptional circumstances, with the written approval of the Director of the Department of Public Works, the rainfall intensity may be reduced on the basis of an engineering study.

TABLE 2‐1

Design Storm Frequencies

Structure Type Design Frequency

Curb Inlet/Storm Drainage System 4 Percent Annual Chance Storm (25‐year design storm)

Channels and Ditches not conducting a watercourse

4 Percent Annual Chance Storm (25‐year design storm)

Cross Culverts and Channels serving less than 1 mi2 that do not convey a watercourse


Culverts, Watercourses serving less than 1 mi2


Culverts, Watercourses serving over 1 mi2


Bridges 1 Percent Annual Chance Storm (100‐year design storm)

2.2 RAINFALL DEPTHS All rainfall depth values shall be obtained from the National Oceanic and Atmospheric Administration (NOAA) National Weather Service Hydrometeorological Design Studies Center Precipitation Frequency Data Server (PFDS) available at http://hdsc.nws.noaa.gov/hdsc/pfds/. The PFDS is a point‐and‐click interface developed to deliver NOAA Atlas 14 precipitation frequency estimates and associated information. Applicants shall use site‐specific data derived from this web tool. Include the downloaded NOAA Atlas 14 table for rainfall depth with the submittal package.


2.3 RAINFALL INTENSITY Rainfall intensity (I) is the amount of rainfall in inches per hour based on a duration that equals the time of concentration and the design storm frequency. All rainfall intensity values shall be obtained from the NOAA PFDS available at http://hdsc.nws.noaa.gov/hdsc/pfds/. Include the downloaded NOAA Atlas 14 table for rainfall intensity with the submittal package. 2.4 STREAM FLOWS Applicants for site developments occurring on parcels with existing watercourses shall determine flow rates in the watercourse by accessing the United States Geological Survey (USGS) StreamStats web application. The application utilizes regional regression equations and gauge data for regional streams to estimate flow rates at a chosen point on a watercourse. The application can be accessed via http://water.usgs.gov/osw/streamstats/connecticut.html. StreamStats flow rates should be compared to other available sources (if they exist) such as Flood Insurance Rate Studies or other published information. Ultimately, design flows should be determined by considering a variety of sources and using engineering judgment as to the most appropriate flow for design purposes. This may include considering future stream flows taking into account historic increases in rainfall and base flows. 2.5 PROCEDURE SELECTION Although there are a number of methods that can be used to evaluate site hydrology, the hydrologic methods presented in this section were selected because of their accuracy in predicting stormwater runoff rates and volumes and because there are a variety of guidance materials and computer programs that support their use.

The Rational Method shall be used for all drainage system designs with a watershed up to 200 acres

in size and without storage. Propriety software systems such as Hydraflow or HydroCAD can be utilized for analysis as long as all supporting computations are submitted to the city for review.

The NRCS Method shall be used for overall site hydrology and peak flow comparison including LID

design. Programs such as TR‐20 and HEC‐HMS or proprietary software such as Hydaflow or HydroCAD can be utilized for analysis as long as all supporting computations are submitted to the city for review.

2.6 RATIONAL METHOD

The Rational Method is to be used for storm sewer design and represents an accepted method for determining peak storm runoff rates for small watersheds (<200 acres) that have a drainage system unaffected by complex hydrologic situations such as ponding areas, storage basins, and watershed transfers (overflows) of storm runoff. The Rational Method can be used in the analysis of drainage inlets, stormwater systems, small ditches, swales, and culverts. This method is not to be used for storage design (i.e., detention basins) and LID. The analysis shall include the delineation of all subwatersheds to each catch basin or inlet.


The Rational Method is based on a direct relationship between rainfall intensity and runoff and is expressed by the following equation:

Q=CiA

In which: Q = the peak rate of runoff in cubic feet per second (cfs) C = the dimensionless coefficient of runoff representing the ratio of peak discharge per acre to

rainfall intensity (i) i = the average intensity of rainfall in inches per hour for a period of time equal to the time of

concentration for the drainage area at the point of interest A = the area in acres contributing runoff to the point of interest during the critical storm

duration The procedure for using the Rational Method is as follows: Step 1: Obtain the following information for each design segment:

a. Drainage area – See Section 2.6.1 b. Land use (percentage of impermeable area such as pavement, sidewalks, roof, etc.) c. Soil types (highly impermeable or impermeable soils) d. The hydraulically most distant point of the drainage area to the point of discharge

with significant contribution e. Difference in elevation from the farthest point of the drainage area to the point of

discharge Step 2: Determine the Time of Concentration (Tc) – See Section 2.6.2 Step 3: Determine the rainfall intensity (I) – See Section 2.3 Step 4: Select the appropriate runoff coefficient (C) – See Section 2.6.3 Step 5: Compute the design peak flow (Q=CiA)

2.6.1 Drainage Area (A)

Drainage area maps prepared in accordance with the following guidelines need to accompany all engineering drainage analysis calculations for the determination of existing (predevelopment) condition and proposed (postdevelopment) condition peak runoff discharge rates.

• Separate drainage area maps should be submitted for existing (preproject) and proposed

(postdevelopment) conditions. The Department of Public Works (DPW) can provide topographic or aerial maps.

• Drainage area boundaries or limits need to be complete. If drainage areas include upgradient areas that extend beyond the subject property, provide adequate drainage area mapping of off‐site areas so as to depict the entirety of each drainage area.

• Provide a suitable scale for existing and proposed condition drainage area maps, such as 1"=40', 1"=50', or 1"=100'. If the drainage area(s) is/are very large, the on‐site map scale must be no


smaller than 1"=100'. Smaller scale mapping (such as 1"=2,000' USGS topography map scale) may be used for off‐site areas.

• Common analysis points must be used for comparison of predevelopment and postdevelopment runoff discharge. Be sure to account for any postdevelopment drainage areas that do not drain to a stormwater facility.

• Provide sufficient off‐site detail, either on the drainage area maps or on accompanying maps of appropriate scale (such as USGS quadrangle maps), to provide the downstream destination of watercourses that leave the site. (For example, two streams leaving the site may converge to discharge to the same downstream river or may diverge to flow to separate watersheds.)

• Provide existing (predevelopment) condition and proposed (postdevelopment) condition topography with a minimum of a 2' contour interval. Topography for upgradient, off‐site areas may utilize a 10'‐contour interval (e.g., USGS topography) if coverage is not available from the city Geographic Information System (GIS).

• Indicate the property lines of the subject site and the proposed project limits on all submitted drainage area maps.

• Indicate the limits of wetland areas on site and depict approximate limits of off‐site wetlands in the project vicinity (USGS quadrangle map information may be used for off‐site areas).

• The drainage area maps need to map and indicate the various cover types in each drainage area that are used to calculate the weighted curve number for each drainage area. Map and label the cover types (e.g., woods, brush, grass, impervious, etc.) along with pertinent hydrologic soil groups (A, B, C, or D) and applicable hydrologic conditions (i.e., good) within each subarea. Number each existing and proposed condition subarea. Please ensure that the labeling of the subareas is consistent with the submitted analysis.

• Indicate the Tc flow path of each drainage area. • Indicate all existing and proposed stormwater conveyance features (swales, pipes, catch basins,

drainage inlets, ditches, culverts, etc.) and stormwater management facilities and/or practices used for detention and/or water quality management purposes. Clearly indicate which areas these items serve. Label all proposed stormwater management facilities and/or BMPs. Please ensure that the labeling of the proposed stormwater facilities/BMPs is consistent with the submitted analysis.

• Provide a north arrow on each drainage map. • For all proposed buildings, indicate the areas which drain to each respective drainage area. Also

indicate roof leaders and the drainage areas to which these roof leaders are directed.

2.6.2 Time of Concentration (TC) Use of the rational formula requires the Tc for each design point within the drainage basin. The duration of rainfall is then set equal to the Tc and is used to estimate the design average rainfall intensity (I).

The Tc is the time it takes rainwater to flow from the hydraulically most remote point of the drainage area to the point under investigation. The path of runoff that governs the Tc is the longest hydraulic length of the drainage area. It may be necessary to check several watercourses to determine the longest flow path with significant contribution. A field investigation of the drainage basin should be made in all cases to verify assumed runoff patterns.

The type and length of flow vary over the hydraulic length. The characteristics of these flow types divide them into three flow patterns: overland (sheet) flow, shallow concentrated flow, and open channel


flow. The Tc is the summation of the travel times for each type of flow over each of their hydraulic lengths. The minimum value of Tc is 5 minutes (0.1 hour), and the maximum is 600 minutes (10 hours). The minimum Tc to be used for a mostly paved or impervious area shall be 5 minutes. For mostly unpaved areas (greater than 50 percent of the total area unpaved), the minimum Tc shall be 10 minutes.

The Tc shall be determined using either the calculation tables provided in Appendix C of Section 6.0 Hydrology of the Connecticut Department of Transportation (CTDOT) Drainage Manual, as amended, or using nomographs provided in Appendix B of this Drainage Manual. Total Tc shall be computed by adding together overland flow time and appropriate allowances for ditch, channel, and/or pipe time and for travel time through reservoirs, lakes, and swamps. 2.6.3 Rational Method Runoff Coefficient (C) The Rational Method uses a runoff coefficient that is representative of the drainage area being studied. After drainage areas are divided into predeveloped and postdeveloped subareas, the runoff coefficient is assigned, and a weighted coefficient is calculated based on the coefficients summarized in Table 2‐3.

The runoff coefficient shall be based on ultimate development of the watershed in accordance with the Building Zone Map, a copy of which may be obtained from the Planning and Zoning Commission or from city GIS mapping.

TABLE 2‐2 Rational Method Runoff Coefficients for the City of Norwalk

(for Use in the Rational Method)

Zone Steep Slope Average Slope Flat Slope

AAA Residence 0.40 0.30 0.25

AA Residence 0.45 0.35 0.30

A Residence 0.50 0.40 0.35

B Residence 0.55 0.50 0.45

C Residence 0.57 0.52 0.47

In other zones, a composite runoff coefficient based on the percentage of different types of surface in the drainage area may be developed utilizing the following coefficients:


Character of Surface Runoff Coefficients Streets: Asphaltic................... 0.70 to 0.95 Concrete................... 0.80 to 0.95 Brick.......................... 0.70 to 0.85 Drives and Walks...... 0.75 to 0.85 Roofs........................ 0.75 to 0.95 Lawns: Sandy Soil: Flat, 2%.................... 0.05 to 0.10 Average, 2% to 7%... 0.10 to 0.15 Steep, 7%................. 0.15 to 0.20 Lawns: Heavy Soil: Flat, 2%.................... 0.13 to 0.17 Average, 2% to 7%... 0.18 to 0.22 Steep, 7%................. 0.25 to 0.35

Ref: ASCE Manual No. 37 2.7 NRCS METHOD Postdevelopment storm drainage systems shall be sized to provide for zero increase in runoff up to the 4 percent annual chance (25‐year) rainfall event using the TR‐20 or TR‐55 method. The NRCS method shall be used for zero increase computations, LID feature design, and groundwater recharge analysis.

The NRCS Soil Conservation Service (SCS) method estimates direct runoff from rainfall using the concept of water balance. Softwares such as TR‐20, TR‐55, and HEC‐HMS are typically used to compute these values. Many proprietary software programs such as Hydaflow or HydroCAD are essentially running the NRCS method and can be utilized for zero increase design and LID analysis in Norwalk. Applicants shall submit supportive documentation and program output from any of these software programs for city review in accordance with submission requirements detailed in Section 1.0 of this Drainage Manual.

Groundwater Recharge All site development plans, except those exempted in Section 1.7: Applicability and Exclusions, of this Drainage Manual, shall include a groundwater recharge analysis in accordance with NRCS soil classification and recharge rates as described in the SQM. Measures to incorporate groundwater recharge in site development or redevelopment shall be implemented only when needed.

The NRCS classifies soils into four hydrologic groups, A through D, indicative of the minimum infiltration obtained for a soil after prolonged wetting. Group A soils have the lowest runoff potential and the highest infiltration rates while Group D soils have the highest runoff potential and the lowest infiltration rates. The required recharge volume shall be determined using existing site conditions and the infiltration rates set forth below.


TABLE 2‐3 Hydrologic Group Volume to Recharge

Hydrologic Group Volume to Recharge (x Total Impervious Area)

Hydrologic Group Volume to Recharge x Total Impervious Area

A 0.60 inches of runoff

B 0.35 inches of runoff

C 0.25 inches of runoff

D 0.10 inches of runoff

For each NRCS Hydrologic Group on the site, the required recharge volume equals the recharge volume set forth above multiplied by the total area within that NRCS Hydrologic Group that is impervious. Infiltration of these volumes must be accomplished using appropriate BMPs. The following BMPs may be used to infiltrate stormwater: dry wells, infiltration basins, infiltration trenches, subsurface structures, leaching catch basins, exfiltrating bioretention areas, and porous pavement. Some proprietary BMPs can also be used to infiltrate stormwater. 2.8 HYDROLOGIC ANALYSIS FOR LID Traditional site design techniques aim to quickly convey runoff to a centrally located management device, usually at the end of pipe system, using roadways, roofs, gutters, downspouts, driveways, curbs, pipes, and drainage swales. In concept, the principal goal of LID is to ensure protection of ecological integrity and maintain the watershed's hydrologic regime. This is achieved via the incorporation of hydrologic functions within a site design. LID techniques focus on four fundamental measures of site hydrology: runoff volume control, peak runoff rate control, flow frequency/duration control, and water quality control. Tools to preserve predevelopment hydrologic site characteristics include the following:

Reduce/minimize imperviousness.

Disconnect unavoidable impervious surfaces.

Preserve and protect environmentally sensitive site features.

Maintain Tc.

Mitigate for impervious surfaces with Integrated Management Practices (IMPs) (i.e., bioretention facilities, grassed swales, filter strips, rain barrels, etc.).

Locate the impervious areas on less pervious (e.g., Type C or D) soil types. The hydrologic evaluation can be performed using various approaches and analytical techniques. Typically, hydrologic evaluation follows a series of steps resulting in defining the needs for hydrologic control and management. The hydrologic evaluation steps are performed using an iterative process. Numerous site planning and management configurations may need to be evaluated to identify the optimum solutions. The concept of LID is to emphasize the simple and cost‐effective solutions. Use of hydrologic evaluations can assist in identifying these solutions prior to detailed design and construction costs. Meeting zero increase through LID is possible but must be documented through the following method as outlined below.

Step 1. Delineate the watershed and microwatershed areas. Hydrologic evaluation requires delineation of the drainage area for the overall study area or site and the subwatersheds contributing to key


portions of the site. Delineation may need to consider previously modified drainage patterns, roads, or stormwater conveyance systems.

Step 2. Determine design storm(s). The 1‐ and 2‐year storm events are usually selected to protect receiving channels from sedimentation and erosion. The 5‐ and 10‐year storm events are selected to provide adequate flow conveyance design and minor flooding considerations. The 100‐year event is used to define the limits of floodplains and for consideration of the impacts of major floods.

Step 3. Compile information for predevelopment conditions. Typical information needed includes area, soils, slopes, land use, and imperviousness (connected and disconnected).

Step 4. Evaluate predevelopment conditions and develop baseline measures using NRCS Methodology. The results of the modeling analysis are used to develop the base line conditions using the four evaluation measures.

Step 5. Evaluate site planning benefits and compare with base line. The site planning tools provide the first level of mitigation of the hydrologic impacts. The modeling analysis is used to evaluate the cumulative hydrologic benefit of the site planning process in terms of the four evaluation measures. The comparison is used to identify the remaining hydrologic control needs.

Step 6. Evaluate IMPs. The hydrologic control needs may be addressed through the use of IMPs. Examples of IMPs include bioretention facilities, grassed swales, filter strips, and rain barrels. Further design details on these practices can be found in Chapter 11 of the Connecticut SQM (Primary and Secondary Treatment Practices). Chapter 2, Low Impact Development Techniques, of the February 2004 New Jersey Stormwater Best management Practices Manual and the June 1999 Low‐Impact Development Design Strategies document prepared by Prince George's County, Maryland, also serve as helpful references.

After IMPs are identified for the site, a second‐level hydrologic evaluation that combines the controls provided by the planning techniques with the IMPs can be conducted. Results of this hydrologic evaluation are compared with the predevelopment conditions to verify that the discharge volume and peak discharge objectives have been achieved. If not, additional IMPs are located on the site to achieve the optimal condition.

Step 7. Evaluate supplemental needs. If after use of IMPs supplemental control for either volume or peak flow is still needed, selection and listing of additional management techniques should be considered. For example, where flood control or flooding problems are a key design objective, or where site conditions such as poor soils or high water table limit the use of IMPs, additional conventional end‐of‐pipe methods such as large detention ponds or constructed wetlands should be considered. In some cases, these controls can be sized much smaller than normal due to use of LID as part of the management system. The hydrologic evaluation is used to compare the supplemental management techniques and identify the preferred solutions.

Section 3 Stormwater Quantity/Quality,

Detention, and Retention



3.0 DETENTION AND RETENTION FACILITIES 3.1 STORMWATER QUANTITY Land development activities often alter the hydrologic characteristics of a watershed, which may in turn affect the timing, velocity, magnitude, and quality of runoff. Stormwater detention/retention to address quantity and quality is required by the City of Norwalk to control and mitigate adverse impacts on existing downstream drainage systems caused by development upstream.

This chapter provides general design criteria for detention/retention basins as well as procedures for performing preliminary sizing and acceptable methods of making final reservoir routing calculations. Hydrologic Engineering Center – Hydrologic Modeling System (HEC‐HMS) from the USACE Hydrologic Engineering Center and United States Environmental Protection Agency's (USEPA) Stormwater Management Model (SWMM) are acceptable public domain softwares for performing detention routing calculations. Other proprietary software already being used by a designer may be allowed on a case‐by‐case basis; however, the city may require that it be run with a "standard" set of input parameters to demonstrate that it produces output values comparable to the accepted methods. Flood‐prone drainage areas may require retention/detention storage areas designed to address special conditions to protect public safety from flooding and/or facility failure. Wet detention facilities with a permanent pool also require special design considerations to protect the public health and safety. Detention facilities offer temporary stormwater storage accompanied by controlled release of the stored water while retention refers to stormwater storage without access to a positive outlet below a certain elevation. Since retention facilities will impound a permanent pool of water below the lowest outlet elevation, water budget calculations may be required at the discretion of the city if the facility is intended to serve as an aesthetic element for the development of which it is a part. Examples of detention facilities are natural swales with berms; subsurface tanks, pipes, or reservoirs; rooftop storage; and parking lots. Examples of retention facilities include ponds, lakes, streams, and rivers. The design considerations for detention/retention facilities should include the following: 1. Multiple systems and BMPs 2. Maximum allowable outflow rate for design storm 3. Detention volumes ("live" pool and permanent pool, if applicable) 4. Grading, depth, and geometry requirements 5. Outlet structure(s) performance throughout its expected range of operation Detention and retention can be used separately or together in series or parallel to other stormwater BMPs to offer a cumulative benefit to stormwater quantity and/or quality.

3.2 STORMWATER QUALITY

As per the Connecticut SQM and as required by the MS4 and local ordinance, stormwater quality management practices are required for all new development and redevelopment. When installed, these practices are intended to benefit stormwater quality by controling frequent storm event flooding, erosion, and nonpoint pollutant loading.


The stormwater quality systems are to be designed to "treat" the small frequent stormwater quality events and to bypass all larger storm (flood) events unless the facility is designed to serve as both a stormwater quantity and quality management practice. Refer to Table 1‐1 for required water quality site design. 3.3 GENERAL STORMWATER QUANTITY AND QUALITY POLICIES 1. It is the policy of the city that whenever the calculated stormwater runoff, considering the fully

developed basin at proposed zoning for the 4 percent annual chance (25‐year), 24‐hour storm, exceeds the capacity of the downstream pipes or improved channel lining or where the calculated runoff from the site exceeds the existing runoff from the site, detention facilities shall be used. In determining the capacity of the downstream system, the engineer shall examine all the pipes, culverts, channel linings, and streams that serve the site under consideration.

2. Any underground systems used shall be designed to meet the same storage and discharge

requirements as aboveground detention facilities under conditions acceptable to the city. 3. Provisions shall be made to permit access and use of auxiliary equipment to facilitate emptying,

cleaning, and maintenance or for emergency purposes. 4. The developer shall provide a bond, in a form acceptable to the city, to ensure the proper

construction of detention facilities. The bond will be held by the city for a minimum of 1 year after completion of the project. Prior to release of the bond, the city shall inspect the detention facility to ensure the following:

a. The outlet control structure has been constructed in accordance with the approved

plans. b. The detention basin is substantially in conformance with the approved plans as to

location, depth, storage volume, height of any berm built as a part of the basin, and the location and size of the emergency spillway.

c. The basin sides and bottom have been adequately stabilized to prevent erosion. d. The basin and the outlet structure have been maintained and are free from trash,

debris, sedimentation, or other problems that could impair the proper function of the basin.

e. A legally identifiable, stable ownership has been established to ensure the continued proper maintenance of the basin.

f. The city will make this inspection upon written request for release of the bond by the developer.

5. Detention systems must be constructed during the first phase of major developments to

eliminate damage to adjacent properties during construction. In this regard, the detention systems shall be designed to function as sediment traps and be cleaned out to proper volumes before completion. If siltation has occurred, detention systems must be restored to their design dimensions after construction is complete and certified as part of the as‐built submittal. Maintenance of the portion of the detention facilities not located in a public drainage easement is the responsibility of the individual property owners or property owners' association.


6. The responsibility of all maintenance of the detention facilities and subdivision projects shall remain with the developer until the project has been accepted by the city. Upon acceptance of the subdivision by the city, maintenance responsibility shall transfer to the city for all components located in the public drainage easements and to the property owner or owners' association for all components of the detention system located in the private easement.

7. The following note shall be clearly placed on the final plat of any development requiring on‐site

stormwater detention facilities: The areas denoted by "Reserved for Stormwater Detention" shall not be used as a building site or filled without first obtaining written permission from the city engineer as applicable. The stormwater detention systems located in these areas shall be owned and maintained by the property owner and/or owners' association. Such maintenance shall be performed to ensure that the system operates in accordance with the approved plan located in the city engineer's office. Such maintenance shall include, but not be limited to, removal of sedimentation, fallen objects, debris, and trash; mowing; outlet cleaning; and repair of drainage structures. The city engineer shall have a "right of access" to use the drives, parking areas, and yards of this property to make inspections of the stormwater detention facility to ensure that said maintenance has been properly performed. In the event that the property owner or owners' association has not properly performed maintenance on the facility to the extent that the facility poses a threat to public health, safety, or welfare, the city shall retain the right to perform emergency repairs to the facility. The cost of any such repairs will remain the responsibility of the property owner or owners' association and may be added as a lien on the next year's tax bill.

8. Detention basins that require the construction of a levee must be evaluated with regard to the

Connecticut Department of Energy & Environmental Protection (CTDEEP) Division of Dam Safety state regulations that define a dam as an artificial barrier that does or may impound water and that is 25 feet or greater in height or has a maximum storage volume of 15 acre‐feet or more. Exceptions are that dams less than 6 feet in height (regardless of potential storage) or impounding less than 15 acre‐feet (regardless of height) are excluded. Dams subject to regulation under state statutes must meet certain design requirements as detailed in State Regulations Section 22a‐409‐1.

9. Permanent pool facilities may be designed for use as detention/retention structures. Wet detention ponds may be preferable over dry detention ponds because of the added sediment storage flexibility provided by the permanent pool easing some maintenance activities (namely sediment removal is required less frequently). Provisions for safe slopes, safety benches (grading), access restriction to dangerous areas (fencing), weed control, mosquito control shelf, and aeration for prevention of anaerobic conditions should be considered. The city may reject facility designs with the potential for becoming nuisances or health hazards.

3.4 DESIGN CRITERIA Stormwater management practices (also known as structural stormwater controls, structural stormwater best management practices, or structural stormwater BMPs) are engineered facilities designed to intercept and manage postconstruction stormwater runoff rates, volumes, and pollutant loads. Together with green infrastructure practices, which can be used to help prevent increases in postconstruction stormwater runoff rates, volumes, and pollutant loads, stormwater management practices can be used to help control and minimize the negative impacts of land development and


nonpoint source pollution. Stormwater management practices can be used whenever green infrastructure practices cannot, on their own, be used to completely satisfy the postconstruction stormwater management criteria required per this Drainage Manual.

Stormwater Runoff Reduction: Reduce the stormwater runoff volume generated by the 85th percentile storm event (and the "first flush" of the stormwater runoff volume generated by all larger storm events) on a development site through the use of appropriate green infrastructure practices. In Norwalk, this equates to reducing the stormwater runoff volume generated by the 1‐inch rainfall event (and the stormwater runoff generated by the first inch of all larger rainfall events).

Overbank Flood Protection: Prevent an increase in the duration, frequency, and magnitude of damaging overbank flooding by controlling (attenuating) the peak discharge generated by the 4 percent annual chance (25‐year), 24‐hour storm event under postdevelopment conditions.

Extreme Flood Protection: Prevent an increase in the duration, frequency, and magnitude of dangerous extreme flooding. Stormwater detention shall be provided on a development site to ensure that the peak discharge generated by the 1 percent annual chance (100‐year), 24‐hour storm event under postdevelopment conditions does not exceed the peak discharge generated by the same storm event under predevelopment conditions. Detention facilities shall have a primary discharge structure capable of accommodating the 24‐hour storms up through the 10 percent annual chance (10‐year) storm with an emergency overflow capable of handling at least the 1 percent annual chance (100‐year), 24‐hour postdevelopment discharge unless waived or altered by the city.

Water Quality Protection: Adequately treat postconstruction stormwater runoff before it is discharged from a development site. In Norwalk, these criteria can be met simply by satisfying the stormwater runoff reduction criteria. However, if any of the stormwater runoff generated by the 1‐inch storm event (and the first inch of all larger rainfall events) cannot be reduced on a development site due to site characteristics or constraints, it should be intercepted and treated in one or more stormwater management practices that: (1) provide for at least an 80 percent reduction in TSS loads; and (2) reduce nitrogen and bacteria loads to the maximum extent practical.

General application practices can be used to treat stormwater runoff and manage the postconstruction stormwater runoff rates and volumes generated by larger, less frequent rainfall events (e.g., 1 percent annual chance [100‐year], 24‐hour event). Several of these practices, namely bioretention areas, infiltration practices, and dry swales, can also be used to reduce postconstruction stormwater runoff volumes and consequently are also classified as runoff‐reducing LID practices. Since they can be used to both treat and manage postconstruction stormwater runoff, it is recommended that general application practices be used whenever green infrastructure practices cannot, on their own, be used to completely satisfy the stormwater runoff reduction, stormwater quality protection, aquatic resource protection, overbank flood protection, and extreme flood protection criteria presented in this Drainage Manual. The general application practices recommended for use in Norwalk include stormwater ponds, stormwater wetlands, bioretention areas, filtration practices, infiltration practices, and swales. Each of these facilities is described in detail in the 2004 Connecticut SQM.


Water quantity management practices can only be used to manage the postconstruction stormwater runoff rates and volumes generated by larger, less frequent rainfall events (e.g., 1 percent annual chance [100‐year], 24‐hour event). They provide little, if any, stormwater runoff reduction or stormwater treatment. Consequently, it is recommended that they be used only on a limited basis and only when green infrastructure practices and general application stormwater management practices cannot be used to completely satisfy the aquatic resource protection, overbank flood protection, and extreme flood protection criteria presented in this Drainage Manual. The water quantity management practices that may be used in Norwalk include the following:

Dry Detention Basins

Dry Extended Detention Basins

Multipurpose Detention Areas

Underground Detention Systems Water quality management practices can only be used to treat postconstruction stormwater runoff. They typically have high or special maintenance requirements, provide little if any stormwater runoff reduction, and cannot be used to manage the postconstruction stormwater runoff rates and volumes generated by larger, less frequent rainfall events (e.g., 1 percent annual chance [100‐year], 24‐hour event. Consequently, it is recommended that they be used only on a limited basis and only when green infrastructure practices and general stormwater management application practices cannot be used to completely satisfy the stormwater runoff reduction and stormwater quality protection criteria presented in this Drainage Manual. The water quality management practices that may be used in Norwalk include the following:

Organic Filters

Underground Filters

Gravity (Oil‐Grit) Separators

Proprietary Systems Supportive data must be submitted to justify the type of stormwater facility selected. If the facility is designed to retain (volume control) all or a significant portion of runoff, then appropriate soil analyses findings shall be submitted to the city. The facility may be designed to infiltrate runoff to groundwater rather than transmit it downstream under conditions up to a 10 percent annual chance (10‐year), 24‐hour storm event. It must be able to bypass all other storms in compliance with the detention requirements of this Drainage Manual. Filter strips, biofiltration swales, bioretention areas, and infiltration ditches shall be designed based on hydrologic analysis in combination with hydraulic analysis. All detention and retention facilities shall provide for WQv in accordance with the Connecticut SQM. A written exemption to the detention requirement request shall include a brief report detailing the existing and proposed conditions, the reason the exemption should be granted, and any supporting engineering calculations and drawings necessary for the City Engineer to examine the request. Where detention would be required by the preceding applicability section, if one of the following conditions applies, then the City Engineer may grant an exemption to the detention requirement where the following apply:


1. The runoff of a 10 percent annual chance (10‐year) storm from the proposed projects exceeds

the maximum release rates by less than 15 percent, or 1.5 cubic feet per second (whichever is greater).

2. There is no downstream restriction, and the project is located in the lower 25 percent of the

basin above the downstream end of the restriction as determined by travel times or where the design engineer can show that detaining the stormwater from the project would increase the 10 percent annual chance (10‐year storm) peak flows at all downstream restrictions.

3. Detention is deemed impractical by the city.

3.4.1 Required Design Analysis A hydrological study of the impact of proposed changes in runoff on the entire watershed shall be prepared by a registered professional engineer. The study should include the following:

1. An investigation of the adequacy of the existing downstream drainage system 2. A review of existing flooding problems 3. An estimate of the impact of additional runoff from development 4. A review of the probability and impact of further development within the watershed, which may

cause increased runoff 5. An analysis of the effect of time lag between drainage systems of developments and the

surrounding watershed at critical points 6. All calculations shall be done to one decimal place. 7. All reports shall be submitted with flow rates computed in cfs and volumes computed in cubic feet

(not acre‐feet).

The study should be based on various storm durations and frequencies to ensure that the retention or detention facility will function properly and prevent increased downstream flooding for the design storm as well as more frequent storms.

Detailed engineering computations should be made although with the approval of the Director, DPW, qualitative judgments will be accepted where conclusions are obvious or where an inordinate amount of computations would be required to demonstrate impacts known to be insignificant and not accumulative in the watershed under consideration. A report of the results shall be submitted to and approved by the Director, DPW. Detention basins will be designed with an emergency spillway to direct overflow safely over the basins when storms exceed the design storage capacity. Release rate will be equivalent to what existed before development took place and shall be capable of reducing peak flow for storms with return frequencies of 2, 10, 25, and 100 years after development to what the runoff would be before development occurred for a rainfall of 24 hour duration, Type III distribution and antecedent moisture condition II (TR‐20).

Retention and detention facilities shall not be provided where discharge is directly into Long Island Sound or in areas where the engineering study indicates retention or detention is not required such as in the lower reaches of large streams or rivers.


3.4.2 General Analysis Procedures

The use of computerized hydraulic routing models such as HEC‐HMS from the USACE Hydrologic Engineering Center or SWMM from USEPA are required for final reservoir routing calculations for detention facility final design. Other proprietary software already being used by a designer may be allowed on a case‐by‐case basis. Whatever method is used must be capable of providing detailed time series data for the hydrographs it generates so that compliance with volume detention requirements can be verified.

The following procedure shall be used to perform the required calculations:

1. Develop an in‐flow hydrograph (or calculate the peak flow) for the area(s) proposed to be developed

representing the resultant runoff from the 10 percent (10‐year), 4 percent (25‐year), 2 percent (50‐year), and 1 percent (100‐year) annual chance 24‐hour storms for the area in its existing condition. These values then become the maximum allowable outflow value for the same storms occurring on the area(s) after installation of the proposed improvements. Utilize the Rational Method, as described in Section 2.0 of this Drainage Manual, for all watersheds smaller than 50 acres. For watersheds in excess of 50 acres, the engineer shall utilize the NRCS Unit Hydrograph Method to determine in‐flow hydrograph(s).

2. Develop an outflow hydrograph for the area(s) proposed to be developed representing the resultant

runoff from the same design storms used in the analysis of existing conditions. Input parameters should accurately reflect all factors affecting the amount and timing of the stormwater runoff such as increased percentage of impervious surface and resulting decreases in rainfall abstractions, shortened basin response times due to increased flow velocities in both overland and concentrated flow regimes, and so forth.

3. Compute the storage/stage curve for the proposed detention or retention facility indicating the

volume of storage at various water surface elevations. 4. Select a specific outlet control structure and compute the outflow discharge rates for various water

surface elevations to produce the outflow/stage curve. 5. The time interval selected shall be no greater than 5 minutes for a Tc less than 1 hour and no less

than 10 minutes for a Tc equal to or greater than 1 hour. 6. Route the proposed conditions hydrograph through a structure designed to provide adequate

temporary storage of stormwater and appropriate outlet works so that the peak discharge from the structure does not exceed existing peak flow values. Consider emergency overflow from the 1 percent annual chance design storm and establish freeboard requirements. The minimum crown elevation of the detention structure embankment or finished grade of all areas adjacent to the structure should be set at least 1 foot above the peak elevation calculated for water surface in the routing of the 1 percent annual chance, 24‐hour storm. This requirement is not to be construed to supersede any minimum elevation requirements established by FEMA or other regulatory agencies.

7. Evaluate the downstream effects of detention outflow to ensure that the recession limb of the

outflow hydrograph does not cause downstream flooding problems. The potential significance of


downstream effects from detention can be evaluated by comparing the recession limbs of the predevelopment and routed postdevelopment hydrographs. When the maximum difference in discharge rates and the hydrograph time base both increase by more than 20 percent for the routed postdevelopment hydrograph, then watershed modeling or information from a watershed master plan may be required to show that downstream impacts can be controlled.

8. Evaluate the control structure outlet velocity and provide stabilization if velocity is greater than 4

feet per second (ft/s) for any storm event. 9. The Hydrological Report shall document the method of generating the in‐flow hydrograph and all

assumptions for the routing. The routing curves and tables shall be included. The routing output shall provide, but not be limited to, the time, the in‐flow rate, the outflow rate, and the maximum storage volume.

10. The input parameters, equations, output results, and backup data assimilated in the design process

are required to be submitted with a proposed detention/retention basin design to allow the city to review the design. All field permeability tests must utilize half the tested rate for the design of infiltration facilities to provide a safety factor. Furthermore, for any field‐tested rate of 20 inches per hour or greater, the maximum allowable design soil permeability rate shall be 10 inches per hour.

3.5 DESIGN STANDARDS FOR STORMWATER FACILITIES 3.5.1 Grading and Depth of Detention/Retention Basins The construction of detention/retention facilities usually requires excavation or placement of earthen embankments to obtain sufficient storage volume. Vegetated embankments should be less than 10 feet in height and should have side slopes no steeper than 3:1 (horizontal to vertical). Stormwater quality facilities with a littoral zone should be graded at a 6:1 (horizontal to vertical) slope in those areas. The remainder of the grading should be no steeper than 4:1 (horizontal to vertical). Riprap‐protected embankments should be no steeper than 2:1. Geotechnical slope stability analysis is recommended and may be required for embankments greater than 6 feet in height and for embankment slopes steeper than those given above.

Areas above the normal high‐water elevation of detention/retention facilities should be sloped at a minimum of 5 percent toward the facilities to allow drainage and to prevent standing water except in areas designed to control that flow such as landscape swales and biofilters. The bottom area of dry detention facilities should be graded toward the outlet to prevent standing water conditions. A minimum 2 percent bottom slope is recommended. Concrete‐lined low‐flow or pilot channels constructed across the facility bottom from the inlet to the outlet are not preferred. Low flows should be distributed evenly into sheet flow across the bottom of the facility.

The maximum depth of stormwater detention/retention facilities will normally be determined during the permitting process. In general, if the facility provides a permanent pool of water, a depth sufficient to discourage growth of emergent aquatic vegetation (without creating undue potential for anaerobic bottom conditions) should be considered. A maximum depth of 6 to 12 feet is generally reasonable. Aeration may be required in permanent pools deeper than 12 feet to prevent thermal stratification and conditions that could result in anaerobic conditions and odor problems.


Depending upon the depth of the interior of the basin and the existing conditions adjacent to the basin, a fence may be required for public safety purposes at the discretion of the director. An access drive 7 feet to 10 feet wide around the exterior of the basin may be required for maintenance purposes at the discretion of the director. Other considerations when setting depths include regulatory flood elevation requirements, public safety, land availability, land value, present and future land use, water table fluctuations, soil characteristics, maintenance requirements, and required freeboard. Aesthetically pleasing features are also important. A minimum freeboard of 1 foot above the 100‐year design storm high‐water elevation should be provided for impoundment depths of less than 20 feet. Embankments greater than 25 feet in height (above the streambed elevation at the downstream toe) or impounding more than 15 acre‐feet of water are generally subject to the requirements of the state regulations.

3.5.2 Underground Detention The following minimum requirements must be followed in the design and construction of an underground detention facility: 1. Water quality treatment must be provided before the collected stormwater is discharged to the

underground facility. Treatment mechanisms shall be in conformance with those described in the 2004 Connecticut Stormwater Quality Manual and can include propriety treatment structures.

2. The underground detention structure must be constructed of durable materials with a typical

100‐year lifetime. Detention storage volume shall not include the porous space within a stone or gravel bed.

3. The underground detention structure shall be designed to have positive drainage into the

receiving channel assuming that there is a 10 percent annual chance (10‐year) flood in the receiving channel. This ensures that the design volume is used for on‐site detention rather than containing off‐site floodwaters.

4. The underground detention structure shall not receive surface runoff directly from parking lots

through the top opening. Surface runoff shall be directed to a BMP that improves stormwater quality such as an oil‐water separator or grass filter strips. The underground structure will usually have a curb or other barrier around the perimeter to prevent this. In addition, trash and debris must be prevented from clogging inlets to the structure.

5. Design measures must be taken to trap and store sediments in locations where cleanout and

maintenance can be easily performed. This generally requires that some type of water quality inlet or other stormwater treatment BMP be installed upstream from the underground detention facility.

6. Good design practices also require that structural measures be in place to prevent blockages.

Floatable waste materials shall be collected by trash racks for periodic removal. The underground detention structure shall have a positive means of being dewatered for inspection and maintenance purposes. The outlet structure shall include a trash screen or trash rack with a


minimum bar spacing (center to center) equal to one‐half the minimum dimension of the outlet control unless the outlet control is 12 inches or greater in diameter. The trash screen shall be designed to allow removal for easy cleaning. The outlet structure shall include a manhole and an inlet at grade or other means of access to permit inspection, cleaning, and other maintenance. In the event that the orifice or other control device becomes clogged, the outlet structure shall provide for bypass flow without overflow entering a public right‐of‐way.

7. A postconstruction maintenance plan, including inspection schedules and guidelines, must be

submitted. Evidence of responsibility and financial budgeting must be presented in addition to the usual bonds and agreements necessary for all detention structures.

8. The pipes shall be a minimum of 24 inches in diameter. Arch or elliptical pipe is acceptable with

the same minimum and maximum equivalent cross‐sectional area. 9. The system shall not be part of the conveyance system for adjacent properties except in cases

where runoff from small portions of the adjacent property enters the site as sheet flow. The system shall not be in line with any portion of the public drainage system.

10. The system shall be in an easily accessible location. No permanent structure shall be

constructed over the detention basin. 11. The owner or his/her representative shall inspect the system at least annually, and any

deficiencies noted in the inspection shall be repaired within 60 days of the date of the inspection. The owner and his/her representatives should be aware that inspection of an underground detention system is considered a confined space entry and take all necessary precautions in accordance with Occupational Health & Safety Administration (OSHA) regulations. Exemptions to this policy require the prior site‐specific approval of the city.

3.5.3 Stormwater Quality Basin Design Criteria All stormwater quality dry or wet detention facilities designs shall consider the following criteria:

1. Bleed down or "live" storage volume of the first 0.5 to 1.0 inch of runoff and residence time of

12 to 24 hours for dry ponds and 24 to 60 hours for wet ponds 2. Permanent ponds must be designed to incorporate flushing of collected stormwater a minimum

of every 2 to 4 weeks (hydraulic residence time) to prevent algae growth. 3. Sediment forebay, baffle boxes, or equivalent pretreatment device with high flow (10 percent

annual chance storm frequency or greater) bypass 4. Length to width ratio of at least 4:1 5. Energy‐dissipating inlet structures 6. Safe side slopes ‐ ≤ 3:1 on the exterior and ≤ 2:1 on the interior 7. Oil and grease traps or floatable debris skimmers 8. Maximum permanent pool depth of no more than 12 feet unless recirculated with an aerator or

fountain 9. For wet ponds, a littoral zone of 10 to 30 percent pond coverage within 3 years of planting 10. The city strongly encourages that major stormwater quality controls, especially detention

facilities, be designed as off‐line devices.


3.6 OUTLET HYDRAULICS Outlet structures selected for detention/retention facilities should typically include a principal outlet structure (riser, weir, orifice, etc.) and an emergency overflow. Outlet structures can take the form of drop inlets or any combination of pipes, weirs, orifices, and other geometric structures. The principal outlet is intended to convey the 50 percent annual chance (2‐year), 20 percent annual chance (5‐year), and 10 percent annual chance (10‐year) postdevelopment design storms while effecting the required attenuation specified above without allowing flow to enter an emergency outlet. Selecting a magnitude for sizing the emergency outlet should be consistent with the potential threat to downstream life and property if the basin embankment were to fail. The sizing of a particular outlet structure should be based on results of hydrologic routing calculations.

Runoff volume and peak outflow rate detention requirements herein will normally dictate the use of a well‐designed outlet structure for the release of excess stormwater. For the convenience of the designers using this document, accepted equations for calculating the theoretical discharge of various configurations of outlet devices are given below. These include sharp‐crested weir flow equations for no‐end contractions, two‐end contractions, and submerged discharge conditions, followed by equations for broad‐crested weirs, v‐notch weirs, proportional weirs, and orifices, or combinations of these facilities. If culverts are used as outlet works, procedures presented in Section 5 should be used to develop stage‐discharge data. Slotted riser pipe outlet facilities should be avoided, and all outlet structure designs should incorporate measures to prevent debris accumulations from damaging the structure or affecting its hydraulic capacity. Outlet control structure hydraulics should be computed using standard weir and orifice equations as summarized in this Drainage Manual's Appendix E. Designers may also choose to use computer programs to evaluate storage options and hydraulics.

3.7 OUTLET PROTECTION Small low‐flow orifices such as those used for extended detention applications can easily clog, preventing the structural control from meeting its design purpose(s) and potentially causing adverse impacts. Therefore, extended detention orifices need to be adequately protected from clogging. Suggested anticlogging mechanisms include the following:

The use of a reverse slope pipe attached to a rise for a stormwater pond or wetland with a permanent pool. The inlet is submerged 1 foot below the elevation of the permanent pool to prevent floatables from clogging the pipe and to avoid discharging warmer water at the surface of the pond.

The use of a hooded outlet for a stormwater pond or wetland with a permanent pool

Internal orifice protection through the use of an overperforated vertical stand pipe with ½‐inch orifices or slots that are protected by wire cloth and a stone filtering jacket

Internal orifice protection through the use of adjustable gate valves can achieve an equivalent orifice diameter.


3.7.1 Trash Racks The susceptibility of larger inlets to clogging by debris and trash needs to be considered when estimating their hydraulic capacities. In most instances, trash racks will be needed. The inclined vertical bar rack is most effective for lower stage outlets.

The surface area of all trash racks should be maximized, and the trash racks should be located a suitable distance from the protected outlet to avoid interference with the hydraulic capacity of the outlet. The spacing of trash rack bars must be proportioned to the size of the smallest outlet protected. However, where a small orifice is provided, a separate trash rack for that outlet should be used so that a simpler, sturdier trash rack with more widely spaced members can be used for the other outlets.

To facilitate removal of accumulated debris and sediment from around the outlet structure, the racks should have hinged connections. The inside of the outlet structure for a dry basin should be depressed below the ground level to minimize clogging due to sedimentation. Depressing the outlet bottom to a depth below the ground surface at least equal to the diameter of the outlet is recommended.

Trash racks at entrances to pipes and conduits should be sloped at about 3H:1V to 5H:1V to allow trash to slide up the rack with flow pressure and rising water level – the slower the approach of flow, the flatter the trash rack angle.

The bar opening space for small pipes should be less than the pipe diameter. For larger‐diameter pipes, openings should be 6 inches or less. Collapsible racks can be used in applications where clogging can become excessive. Alternately, debris for culvert openings can be caught upstream from the opening by using pipes placed in the ground or a chain safety net. Racks can be hinged on top to allow for easy opening and cleaning.

The control for the outlet should not shift to the grate, nor should the grate cause the headwater to rise above planned levels. Therefore, head losses through the grate should be calculated in accordance with Eq. H1 in Appendix H.

3.8 EMERGENCY SPILLWAY/OVERFLOW DESIGN Emergency spillway designs are open channels, usually trapezoidal in cross section, and consist of an inlet channel, a control section, and an exit channel. The emergency spillway is proportioned to pass flows in excess of the 1 percent annual chance (100‐year) flood without allowing excessive velocities and without overtopping the embankment. Flow in the emergency spillway is open‐channel flow. Normally, it is assumed that critical depth occurs at the control section. Refer to Section 4 for open channel hydraulics.

Both the inlet and exit channels should have a straight alignment and grade. Spillway side slopes should be no steeper than 3:1 (horizontal to vertical). The most common type of emergency spillway used is a broad‐crested overflow weir cut through original ground next to the embankment. The transverse cross section of the weir cut is typically trapezoidal in shape for ease of construction.

The release rate from any stormwater quality detention facility should be as described above with emergency overflow as described in Section 6 of this Drainage Manual except where waived or altered by the city. An adequate alternate stormwater management system must be provided to accommodate


major storm flows. Major stormwater quality controls must be constructed during the first phase of developments to eliminate damage to adjacent properties during construction. 3.9 CONSTRUCTION AND MAINTENANCE CONSIDERATIONS To assure acceptable performance and function, the city discourages the design of stormwater detention/retention facilities that may require excessive maintenance. The following maintenance activities should be considered:

1. Weed growth 2. Grass maintenance 3. Sediment removal 4. Slope deterioration 5. Mosquito control

Proper design may eliminate or reduce maintenance requirements by addressing the potential for problems to develop. Both weed growth and grass maintenance may be addressed by constructing side slopes that can be maintained using available power‐driven equipment such as tractor mowers. Sediment removal may be facilitated by constructing forebays or baffle boxes at the inlets to contain sediment for easy removal. Bank deterioration can be controlled with protective soil bioengineering techniques or lining or by limiting bank slopes. Mosquito control will not be a major problem if the permanent pool is designed with a 12‐inch shelf at the edge. The property owner or property owners' association shall maintain the detention, retention, and other drainage facilities to the extent necessary to achieve the intended drainage, retention, and detention functions. Maintenance shall include repair of appurtenances and removal of obstructions and siltation. The property owner or property owners' association shall provide customary grounds maintenance within the detention easement area in accordance with the following standards: 1. Grass areas shall be mowed (in season) at regular intervals not exceeding 4 weeks. Grass

clippings shall be mulched or removed so as not to impair the function of the facility. 2. Leaves, litter, and any other material that may impair the function of the facilities shall be

promptly removed and properly disposed of so as not to create a property hazard. 3. The facilities shall be kept free of brush and trees or other undesired growing material that may

impair their function. No trees or shrubs shall be planted in the facilities without the approval of the city. Any plantings shall be of types that are tolerant of periodic flooding.

4. Concrete and other appurtenances shall be maintained in good condition and replaced if

damaged. 5. The property owner or property owners' association shall complete an annual inspection of all

stormwater. The annual inspection shall include verifying that sufficient temporary storage volume exists for the facility to continue to operate properly. The outlet control structure shall be inspected to ensure that it is in proper repair and that debris or siltation does not clog the outlet control structure. Any deficiencies noted in the inspection shall be promptly repaired.


6. Installation of a sedimentation chamber on the upstream side of any detention facility is encouraged. Sediment chambers should be inspected at least twice each year and built‐up sediment removed when necessary. Sediment removed from detention facilities or sediment chambers shall be disposed of in accordance with the laws of the State of Connecticut.

7. Underground detention facilities shall be inspected at least twice per year. This inspection will

determine the need for cleaning, sediment removal, or other maintenance. Any required maintenance shall be performed promptly to ensure the continued proper operation of the detention facility.

Section 4 Storm Sewers and Gutter Flow


City of Norwalk Storm Drainage Manual 4‐1

4.0 STORM SEWERS AND GUTTER FLOW INTRODUCTION This section provides standards and guidelines for sizing piped drainage systems in roadways, parking lots, driveways, and other developed areas.

Analysis shall consider the following:

1. Storm sewer infrastructure spacing and sizing 2. Storm sewer infrastructure materials 3. Pipe capacity based on both flow and hydraulic grade line (HGL) 4. Gutter flow (i.e., inlet capacity) 5. System tailwater 6. Hydrology as defined in Chapter 2 of this Drainage Manual When questions arise with respect to analysis procedures, designers should refer to the Chapter 11 of the CTDOT Drainage Manual.

4.1 GENERAL DESIGN GUIDELINES The following requirements for storm drainage systems shall be incorporated into all drainage projects. All subsurface storm drainage systems shall be designed in accordance with the methods and procedures defined in this Drainage Manual and the CTDOT Drainage Manual, as amended, and shall meet the following requirements: 1. Use of curbing shall be minimized in order to encourage overland‐disbursed flow through stable

vegetated areas. 2. The hydrology and hydraulic design of catch basins, gutters, and storm drain pipes shall comply with

this Drainage Manual. 3. The foundation drains and floor drains of buildings connected into storm drainage systems shall be

designed to prevent backflow for the 1 percent annual chance (100‐year) flood into the building. 4. Surface runoff shall be directed through vegetated filter strips or grass swales whenever possible

prior to storm drain inlets. 5. The design of the storm drainage system should be coordinated with the soil erosion and sediment

control plan. 6. Storm drainage discharges shall be coordinated with the NPDES permit program administered by the

Water Compliance Unit of the CTDEEP. 7. Storm drainage systems discharging into watercourses tributary to public water supply reservoirs

shall be in compliance with the Public Health Code. 8. Storm drains shall be extended to a suitable discharge point into a watercourse or public drainage

system or to where drainage rights have been secured. Where a suitable outlet is not available within a developer's property or the adjacent street right‐of‐way, it will be necessary to obtain


appropriate drainage easements and to construct new and/or improve existing drainage facilities. No new stormwater conveyances (e.g., outfalls) may discharge untreated stormwater directly to or cause erosion in wetlands or waters of the City of Norwalk.

Where there is no suitable outlet, dry wells may be used for drainage from very small sites that have frontage along city streets; drainage may be permitted to discharge into roadway gutters. The design of dry wells and the discharge into roadway gutters shall be subject to the review and approval of the Director of the DPW.

9. A capacity analysis of an existing storm sewer system shall be required when any proposed changes

to a site will result in changes to the timing of predicted runoff. 10. For any residential, industrial, and/or commercial site plan, the number and direction of all roof

drains must be addressed. 11. Lot‐to‐lot drainage must be addressed. The grading of all lots in a subdivision or an

industrial/commercial site plan must be such that one lot or site does not adversely affect or cause a storm drainage problem on another lot. The grading must be an integral part of the overall storm drainage plan for the subdivision/site plan. Yard drains shall be included to catch stormwater in back yards or front yards as directed by the City Engineer.

12. Where a development street joins an existing city street, the developer must provide drainage at

the intersection as necessary or as directed by the city. 13. Along roadways having curbs or shoulders raised above the roadway, catch basins shall be located

to prevent gutter and/or catch basin and gutter capacities from being exceeded and to prevent ice from building up in the gutter. Proper inlet analysis and design are required to define the hydraulic design capacity of the drainage system.

14. On roadways that do not have curbs or shoulders raised above the edge of pavement, no provision

will ordinarily be required to provide drainage of the pavement other than the cross slopes of the pavement and shoulder, except at low points which shall be drained by catch basins located within the area normally clear after snow removal. In such cases, the curb head of the catch basin shall be set back 2.5' from the normal edge of pavement in order to keep clear of snow.

15. In residential areas where the design flow across a roadway intersection is 1 cfs or more, a catch

basin shall be provided upstream of the intersection to intercept gutter flow. In business districts, catch basins shall be provided upstream of all intersections irrespective of design flow.

16. At the downgrade end of all curbed or cut sections, catch basins shall be provided to prevent

erosion of the adjacent shoulder or slope. 17. Within parking lots, catch basins shall be located: (a) at low points; (b) to prevent gutter and/or

catch basin capacities from being exceeded; and (c) to drain gutters located along the high side of parking lots to prevent hazardous freezing of snowmelt.

18. In business districts, catch basins shall be provided within the business drive at driveway entrances,

where the design flow would significantly inconvenience pedestrians, or where existing gutter


capacity along the public road is already exceeded or would become exceeded. Catch basins at driveway entrances shall be located outside the street right‐of‐way.

19. Catch basins are required at intersections where grades may cause pockets of ponding water. 4.1.1 Design Flow Frequency Criteria

The recommended design flow frequency criteria to be used for continuous closed‐circuit systems are as follows:

1. Pipe conveyance systems for parking lots, driveways, and roads must be designed to provide

adequate passage for flows leading to, from, and through stormwater management facilities for at least the peak flow from the 4 percent annual chance (25‐year), 24‐hour Type III design storm event. Refer to the Hydrology section for design rainfall events.

2. Pipes shall be sized to convey the flow from a 4 percent annual chance (25‐year) event with a HGL 1

foot below the catch basin or manhole frame. When discharging to existing piped drainage systems, calculations should begin at the downstream end of the existing system and be carried back to calculate the HGL of the proposed system.

3. For systems serving an area larger than 200 acres, the 1 percent annual chance (100‐year) flow for

fully developed conditions shall be used for design. Conduits shall be sized to ensure that the 1 percent annual chance (100‐year) HGL will be below the natural ground elevation at all points along this portion of the closed system.

4. All storm drainage facilities shall be designed based on the following storm frequency criteria:

TABLE 4‐1

Design Criteria Frequency (Return Period in Years)

Normal Sags* Culverts

Local Streets and Parking Lots 25 25 50**

Collector and Arterial Streets 25 50 100

Detention and Retention Facilities (Business Districts) 50

Detention and Retention Facilities (Residential Districts)

25

*Sags include any low point without a reliable means of overland flood relief where ponding will occur if the drainage system cannot carry off storm flows. ** Crossings of the following rivers and streams will require 100‐year design frequency: Norwalk River, Betts Pond Brook, Silvermine River, Five Mile River, Keelers Brook, Stony Brook, Noroton River, and Indian River.

4.1.2 Computation Requirements Use of the Rational Method and the rainfall intensities and depths outlined in the hydrology section is acceptable for design of enclosed drainage systems. For all storm sewer systems or for enclosing an open channel, the hydraulic calculations and hydraulic profiles along with the construction plans of the closed‐circuit system must be submitted to the city for review.


Storm drainage systems shall be designed on the basis of ultimate development of the tributary watershed. The latest edition of the Planning and Zoning Commission's "Building Zone Map" shall be used as the basis for the future development to be expected.

1. The storm drainage system shall be designed to effectively transport stormwater for the required

frequency, assuring a minimum self‐cleansing velocity of 3 ft/sec yet not attaining destructive velocities over 15 ft/sec. At no time will the slope be less than 0.50 percent (.005 ft/ft).

2. Runoff from all undeveloped areas that contribute runoff to the site must be accommodated in the

design. 3. Rainfall intensity, Tc, and runoff coefficients based on weighted "c" value shall be determined in

accordance with the methodology and references detailed in Section 2.6 of this Drainage Manual. 4. Hydraulic design for culverts shall be based on the Manning Formula and in accordance with the

CTDOT Drainage Manual, as amended.

4.1.3 Tailwater Conditions

Designs must consider system tailwater and its potential effect on pipe capacity. When discharging to a receiving stream or open water body, design should consider the joint probability of events happening simultaneously in the drainage system and receiving stream as outlined by CTDOT.

For discharges to inland, nontidal watercourses, tailwater should be taken as the elevation of the 10 percent annual chance (10‐year) water surface using published FEMA profiles in the Fairfield County Flood Insurance Study or appropriate backwater calculations if the watercourse has not been mapped by FEMA.

For discharges to tidal systems, tailwater shall be equal to coastal storm tide elevations. Consideration should be given to increasing the design tailwater in consideration of future sea level rise scenarios. Design elevations for common design frequencies are as follows:

TABLE 4‐2 Design Elevation of Tidal Waters

Design Storm Frequency Design Elevation of Tidal Waters*

10% annual chance (10–year) 6.5

4% annual chance (25‐year) 6.9



*Mean sea level datum in feet, NAVD88

4.1.4 Material and Construction Standards 1. The minimum diameter of a pipe in a storm sewer line in a city street shall be 15" except that lines

carrying flow from a single inlet may be 12". Elliptical pipe of equal hydraulic characteristics to the


circular conduit may be substituted. Appendix D of this manual includes reference tables for pipe gauges.

2. All storm sewers shall be constructed with high‐density polyethylene (HDPE) pipe, reinforced

concrete pipe (RCP), Class IV, or approved equal. When the grade is 10 percent or greater, Class V concrete pipe is to be used; HDPE pipe shall not be used at these grades. Materials must meet CTDOT Form 817 Specifications, as amended. Pipe joints for RCP shall be of the rubber gasket type. Values for roughness coefficient "n" to be used in Manning formula are as follows:

TABLE 4‐3

Manning's Coefficients for Pipe Types

Type Roughness Coefficient "n"

Reinforced Concrete Pipe 0.011

Polyvinyl Chloride (PVC) Plastic Pipe ‐ SDR35

0.010

Corrugated Polyethylene Pipe (CPP) Type S (smooth wall)

0.010

Corrugated Metal or Polyethylene (corrugated wall)

0.024

Source: U.S. Department of Transportation Federal Highway Administration Hydraulic Engineering Circular No. 22, Third Edition, Urban Drainage Design Manual, September 2009, Revised August 2013

3. All storm sewers and appurtenant construction shall conform to the City of Norwalk Construction

Specifications, City of Norwalk Standard Details, CTDOT Standard Details, and CTDOT Standard Specifications for Roads, Bridges and Incidental Construction, Form 817, as amended.

4. All cast‐in‐place concrete storm sewers shall follow the alignment of the right‐of‐way or easement. 5. All precast concrete pipe storm sewers shall be typically designed in a straight line. All storm sewer

inlet leads shall be designed in a straight line alignment. 6. Two types of inlets are recommended for use in Norwalk: the Type C, Type II curbed inlet and the

Type C‐L, Type II curbless inlet. All inlets shall be constructed as specified in Section 11.8.2 of the CTDOT Drainage Manual.

7. Easements: Storm sewers shall be located in public street rights‐of‐way or in easements that will

not prohibit future maintenance access. In most cases where easements are restricted to storm sewers, the pipe should be centered within the limits of the easement. Where drainage facilities are located within private property, city maintenance easements shall be provided and registered on a plan filed with the Town Clerk's office. Easements shall generally be at least 20 feet in width. A swale, channel, or ditch that passes drainage runoff from adjacent upstream public or private property shall be considered a drainage facility requiring an easement.

Maintenance roads and easements shall be provided for all permanent facilities. The road shall be at a minimum, 10 feet wide, having filter fabric under at least 8 inches of rolled gravel subbase and 4 inches of topsoil seeded and fertilized on top of the gravel. The gradient shall not exceed 8 percent. When a raised manhole is located within the easement access, the roadway shall be widened to


provide sufficient room for vehicles to pass on both sides of the structure. This may require additional easement beyond the 20‐foot requirement.

In subdivisions, where roadway drainage discharges into private property, a note designating the private property owner as being responsible for properly passing drainage through his property shall be shown on the record plan.

8. For all storm sewers having a cross‐sectional area equivalent to a 42" inside diameter pipe or larger,

soil borings with logs shall be made along the alignment of the storm sewer at intervals not to exceed 500' and to a depth not less than 3' below the flow line of the sewer. The required bedding of the storm sewer as determined from these soil borings shall be shown in the profile of each respective storm sewer. The design engineer shall inspect the open trench and may authorize changes in the bedding indicated on the plans. Such changes shall be shown on the record drawings and, along with soil boring logs, submitted to the city for review. All bedding shall be constructed as specified in CTDOT Form 817, as amended and all subsequent revisions, or approved equal.

9. All storm sewer outfalls shall conform to the requirements and specifications defined in Section 5.0,

Open Channel Hydraulics. 10. Within roadways, a minimum cover of 2 feet over the bell of the pipe shall be provided for all storm

drains. If PVC (SDR35) is used, a minimum cover of 3 feet is required. Pipes with less cover shall be class V RCP or encased in concrete.

11. Underdrain outlets shall be connected to drainage structures whenever practical. When impractical,

they shall be terminated in an approved manner. Underdrain pipes shall be a minimum of 6" inside diameter for roadway system. There shall be no connection to underdrains from footing drains.

12. All private drains that connect to the city storm drain system shall be connected at a structure (CB or

MH) or with a cleanout so they are accessible for inspection and maintenance. Check valves shall be provided near a residence or on the property line in a small manhole with cleanout end. No valves shall be installed within city structures.

13. Pipe backfill and support conditions shall be in accordance with Standard Construction Details and

Specifications, DPW. All trenches on slopes greater than 6 percent shall have cutoff walls. 14. Where drainage pipes cross sanitary sewers, water mains, gas mains, or other utilities, minimum

clearance shall be 12" properly cushioned. Where it is not possible to provide the minimum 12" of clearance, provision shall be made for the protection of the pipes in accordance with the requirements of the utility company and subject to the written approval of the Director of the DPW.

15. Conduits, except for underdrains, shall be installed on straight alignments, both horizontally and

vertically, with manholes providing access to all deflection points and to all junctions of two or more lines. Long radius bend sections shall be allowed when using pipes 48 inches in diameter and larger. Such curvature shall not exceed the pipe manufacturer's recommendations.

16. Where a change in pipe size is required at a structure, the crowns of the pipe are to be aligned on

grade.


17. It shall be the responsibility of the engineer to obtain all data concerning the structural design of pipes and culverts to be installed in the City of Norwalk. This data shall include, but shall not be limited to, height of overfill, probable future changes in roadway grade, foundation conditions, permissible or expected live loads during and after construction, and the expectable corrosive or erosive characteristics of the discharge.

18. The gauges of metal of pipes or of pipe arches fabricated from corrugated steel shall not be less

than those shown in the Gauge and Fill Height Tables shown in Appendix B for the types of pipes and with the depth of fill described. For different types of corrugations or metals, the manufacturer's recommendations shall be followed.

19. RCP and reinforced concrete elliptical pipe shall meet load‐strength criteria as shown in the "Design

Manual‐Concrete Pipe" published by the American Concrete Pipe Association. 20. Except where indicated by an engineering study submitted to and approved in writing by the

Director, DPW, all RCP shall be Class IV. A unit weight of 125 lbs/CF for fill or backfill shall be used to determine the loading.

21. All pipes shall be installed in accordance with the City of Norwalk Roadway and Drainage Standards

and Specifications. Class "C" bedding (Ref: ASCE Manual No. 37) will be used unless otherwise detailed and specified.

22. Conduits placed under railroad roadbeds shall be designed to conform to the pertinent provisions of

the current American Railway Engineering Association Specifications for the type of conduit provided.

4.2 GENERAL DESIGN METHODOLOGY It is recommended that the design of a storm sewer system proceed as follows. For reference, a Storm Sewer System Design Worksheet can be found in Appendix C, page C‐1, and additional reference materials for storm sewer analysis can be found on pages C‐2 to C‐3. 1. For freshwater systems, determine the 10 percent annual chance (10‐year) water surface elevation

in the channel at the storm sewer outfall using published FEMA profiles in the Fairfield County Flood Insurance Study or appropriate backwater calculations if the watercourse has not been mapped by FEMA.

2. For systems discharging to tidally influenced waters, determine the storm elevation per Table 4‐2.

3. Determine the design flow rates for all sections of storm sewer based on drainage area size. 4. Assuming storm sewer pipes are full at design flows, determine the appropriate sizes for all sections

of storm sewer using Manning's equation and assuming uniform flow conditions. 5. Begin calculation at the 10 percent annual chance (10‐year) water surface elevation in the outfall

channel (or coastal storm elevation per Table 4‐2) and plot the hydraulic gradient for the design storm. Include all relevant energy losses. The hydraulic gradient (HGL) must meet the 1 foot of freeboard requirement to the top of grate.


4.2.1 Predeveloment and Postdevelopment Hydraulic Analysis Storm sewers shall be designed as open channel where there is a free water surface (just full or less than full) or for pressure or pipe flow under surcharged conditions. The hydraulic design of a storm sewer system shall take into account the effect of tailwater and shall allow for all energy losses in the system. Appendix F of this Drainage Manual contains reference data for computing energy losses in the system.

A hydraulic analysis shall be performed to determine whether the pipe will operate as an open channel or as a pressure system. Backwater calculations shall be made for each run of conduit or pipe. Friction losses as well as form losses shall be calculated. Form losses include losses due to manholes, bends, contractions, enlargements, and transitions. The Manning formula shall be used for computing flow characteristics of conduits operating as open channels or under pressure. Computer modeling programs can be used in place of hand computations provided that the designer submit all input and output data for the city's review. 4.2.2 Gutter Flow Analysis Catch basin spacing shall be determined in accordance with the gutter flow analysis based on the following design limits for the extent of flooding or ponding on streets and highways for a 4 percent annual chance (25‐year) storm: 1. The shoulders of all roads may be flooded. 2. Two‐lane bidirectional roads may have one‐half of a lane in each direction flooded or if

superelevated one‐half of the lane on the low side of superelevation. 3. Four‐lane undivided roads may have a lane in each direction or one lane on the low side of a

superelevation flooded. 4. Depressed roads shall have a minimum of 12 feet of traveled way free of flooding.

The capacity of a catch basin in a sag location shall be determined by the hydraulic capacity of the grate inlet and shall include all flows associated with upstream bypassing of catch basins as determined by the gutter flow analysis.

Gutter flow analysis and inlet design shall be carried out using the procedures defined in Section 11.9 of the 2000 CTDOT Drainage Manual, as amended. Reference materials for a gutter flow analysis can be found in Appendix C of this Drainage Manual. Computerization of gutter flow analysis is acceptable. A detailed explanation of the method of analysis employed by the program shall be provided. 4.2.3 Stormwater System Layout

Manholes: Manholes shall be placed at the location of all pipe size or cross‐section changes, pipe sewer intersections or P.I.s, pipe sewer grade changes, and street intersections, at maximum intervals of 300 feet measured along the centerline of the pipe sewer and at all inlet lead intersections with the pipe sewer where precast concrete pipe sewers are designed.

Inlets: Curb inlets must be spaced to handle the design storm discharge so that the

hydraulic gradient does not exceed the roadway gutter elevation. Inlets shall be


spaced so that the maximum travel distance of water in the gutter will not exceed 300'.

Curb inlets shall be located on intersecting side streets to major thoroughfares for all original designs or developments. Special conditions warranting other locations of inlets shall be determined on a case‐by‐case basis.

There shall be a minimum of two catch basins in a cul‐de‐sac.

Undesirable sheet flow patterns: a) Cul‐de‐sac streets sloping downhill designed so that sheet flow can only escape through

building lots b) The placing of a curve or turn in a roadway in a low area so that sheet flow into that curve or

turn can escape only through existing building lots c) Many streets "T"‐ing into one street that is lower than the intercepting streets so that sheet

flow down the streets can escape only through existing building lots

4.2.4 Submittal Requirements Storm sewer hydraulic analysis shall be completed using the procedures defined in Section 1 of this Drainage Manual, as amended. Computer programs such as Hydraflow or HydroCAD can be utilized for system design. The computer printout of all results shall be in accordance with the submission requirements detailed in Section 1 of this Drainage Manual. Drainage reports shall be submitted for review as outlined in Section 1 of this Drainage Manual and shall follow the checklist provided in Appendix A.

Section 5 Open Channel Hydraulics



5.0 OPEN CHANNEL HYDRAULICS INTRODUCTION This chapter contains criteria and methods for open‐channel hydraulics design for projects with artificial and natural channel improvements. In general, this chapter provides criteria for the geometric and hydraulic design of open channels as well as guidance on providing them with linings that will serve to minimize erosion and promote channel stability. This chapter is intended to provide guidance for drainage channels only. For design of stabilization and/or relocation of existing channels, designers should consult appropriate geomorphic design standards and guidelines. Example reference sources include the Federal Stream Corridor Restoration Handbook (NEH‐653) by NRCS/United States Department of Agriculture; Hydraulic Design of Stream Restoration Projects, September 2001, by USACE; Environmental Considerations for Vegetation in Flood Control Channels, December 2001, by USACE; and Channel Restoration Design for Meandering Rivers, September 2001, by USACE.

As discussed in this section, channels are considered to provide conveyance only, in contrast to water quality swales that provide conveyance and treatment. 5.1 GENERAL DESIGN PARAMETERS There are several types of open channels used in drainage design including drainage or water quality swales, toe drains, and also gutter flow on curb streets. In general, a nonstructural approach to drainage improvements should be given preference primarily through good land use planning and management. Design principles and the level of analysis performed should be based on the intended purpose of the channel at the discretion of the Director of Public Works.

Considerations should be given in the design to maintenance requirements. The design should reduce maintenance to a minimum.

Appropriate outlet protection such as culvert ends or headwalls shall be provided wherever an open drainage system discharges into or receives discharge from an enclosed system. Culvert ends are preferred to endwalls. Headwalls shall conform to City of Norwalk Roadway and Drainage Standard Details. Safety of the structure location must be considered in relation to pedestrian and vehicular traffic. Safe recovery areas shall be provided where vehicles may impact the structure if guide rail is not used.

5.2 DESIGN STANDARDS Channels shall be designed to convey the design discharge within their banks unless an engineering study is submitted and approved by the Director of the DPW that demonstrates that an alternative solution is acceptable. Freeboard should be provided where overflow would cause considerable damage by providing extra height of lining above the design depth of flow. The following requirements must also be met in the design of a surface swale: 1. The design flow shall be determined based on project land use using a 4 percent (25‐year) annual

chance design storm and the Rational Method. 2. Minimum acceptable swale section shall have a side slope no steeper than 3 horizontal to 1 vertical. 3. The minimum bottom width for surface swales shall be 3 feet.


4. The "n" coefficient for the swale calculations shall be a minimum of 0.04. All values must be justified.

5. The minimum grade or slope of the swales shall be 0.10 percent. 6. Hydraulic design computations must be submitted for each drainage ditch system. Computations

shall include the effect of future driveway culverts, which shall be sized taking into account design flow and swale depth.

7. The computed water surface of the swales shall be a minimum of 0.5 foot below finished grade along the street right‐of‐way lines.

8. The entire swale must be revegetated immediately after construction to minimize erosion. 9. Erosion control methods shall be utilized in the swale designs where velocities of flow are calculated

to be greater than 5 feet per second or where soil conditions dictate their need. 10. The minimum depth of the swales shall be 18 inches, and the maximum depth shall be 4 feet. 5.2.1 Design Storm Open drainage conveyance systems must be designed to provide adequate passage for flows leading to, from, and through stormwater management facilities for at least the peak flow from the 4 percent annual chance (25‐year), 24‐hour Type III design storm event. In general, the following criteria shall be considered for open channels:

1. Channels with bottom widths greater than 10 feet shall be designed with a minimum bottom cross

slope of 10 to 1. 2. Low‐flow sections shall be considered for channels designed to carry flows greater than 50 cfs. 3. Channel side slopes shall be stable throughout the entire length and shall consider the channel

material. A maximum of 3:1 is allowed for vegetated slopes and 2:1 for flexible lined slopes unless otherwise justified by calculations.

4. Superelevation of the water surface at horizontal curves shall be accounted for by increased freeboard.

Minor drainage systems shall be sized to handle a 4 percent annual chance (25‐year) design storm within drainage easement; major systems shall be sized to handle a 1 percent (100‐year) annual chance design storm without flooding structures.

Sediment transport requirements must be considered for conditions of flow below the design frequency. A low‐flow channel component within a larger channel can reduce maintenance by improving sediment transport in the channel. 5.2.2 Erosion Control

All channels shall be designed so as to minimize erosion. Erosive velocities may be reduced by flattening channel grades where uniform flow conditions exist; otherwise, an appropriate channel lining shall be used to prevent erosion. All channels, ditches, or swales shall be lined in accordance with their design as soon as they are excavated.

The final design of artificial open channels should be consistent with the velocity limitations for the selected channel lining. Refer to Table 5‐1 for permissible velocities for various channel linings. Native vegetation should be used wherever possible.


TABLE 5‐1

Maximum Velocities for Channel Linings

Vegetation Type Slope Range (%) Maximum Velocity1 (feet per second)

Bermuda grass 0‐5 5

5‐10 4

Kentucky bluegrass Buffalo grass

0‐5 5

5‐10 4

Grass mixture 0‐5 4

5‐10 3

Lespedeza Sericea Kudzu, alfalfa

0‐5 2.5

Annuals 0‐5 2.5

Sod 4.0

Lapped sod 5.5

Riprap (6"‐24") 10‐18

Rigid2 10

Vegetated coir mat 9

Coconut fiber with net 4

Nondegradable RECPs3

15

1. Based on erosive soils 2. Higher velocities may be acceptable for rigid linings if appropriate protection is provided. 3. RECPs = Rolled Erosion Control Products

Source: United States Department of Agriculture, Natural Resources Conservation Service Part 654 Stream Restoration Design National Engineering Handbook, Chapter 8 Threshold Channel Design

In addition to stormwater conveyance, vegetated open channel systems can be designed to capture and treat the full WQv within dry or wet cells formed by check dams or other means. Design variants include dry swales and wet swales.

In general, the maximum longitudinal slope of any open drainage channel should not exceed 2 percent without check dams. The bottom width of the swales shall be no greater than 8 feet but no less than 2 feet.

Swales intended to provide pretreatment can provide up to 10 percent of the WQv by using check dams at culverts or driveway crossings. Swales intended to provide full treatment shall be sized to store the full WQv in the facility (wet swale) or through properly sized filter media/bioretention soil (dry swale).


5.2.3 Maintenance Requirements Considerations should be given in the design to maintenance requirements. The design should reduce maintenance to a minimum. However, open channels rapidly lose hydraulic capacity without adequate maintenance. Examples of maintenance considerations include the following:

Legally binding maintenance agreement

Maintain an average grass height of 6" in dry swales.

Correct erosion gullies

Maintain healthy stand of vegetation. 5.3 OPEN‐CHANNEL FLOW METHODS AND EQUATIONS The most common equations used in designing and analyzing open‐channel flow can be found in Appendix G of this Drainage Manual. The basic principles of fluid mechanics can be applied to open‐channel flow analysis (i.e., continuity, momentum, and energy). For design of drainage swales, hydraulic conditions are assumed to be steady and uniform. The capacity or discharge of an open channel shall be determined by using the modified Manning's equation. Locations, linings, and cross slope of banks are important safety considerations. Along streets and highways, motorists' safety generally improves with increasing distance between the traveled way and the channel. For nonuniform flow where the depth of flow, velocity, or cross section changes, the principles and equations for unsteady, nonuniform flow shall be applied, and the computer program HEC‐RAS shall be used.

5.3.1 Manning's n Value Recommended Manning's n values for artificial channels with rigid, unlined, temporary, and riprap linings are given in Table 5‐2.


TABLE 5‐2 Recommended Manning's n Values for Artificial Channels

Lining

Category Lining Type

n Value

Rigid Concrete (Broom or Float Finish) 0.015

Gunite 0.020

Grouted Riprap 0.030

Stone Masonry 0.032

Soil Cement 0.022

Asphalt 0.016

Unlined Bare Soil 0.020

Rock Cut 0.035

Temporary Woven Paper Net 0.015

Jute Net 0.022

Fiberglass Roving 0.021

Straw with Net 0.033

Curled Wood Mat 0.035

Synthetic Mat 0.025

Gravel Riprap 1‐inch d50 0.033

2‐inch d50 0.041

Rock Riprap N/A n=0.0395 (d50)1/6

d50=Diameter of stone for which 50 percent, by weight, of the gradation is finer, in feet

Reference: USDOT, FHWA, HEC‐15 (1986)

5.4 CHANNEL LININGS The three main classifications of open‐channel linings are vegetative, flexible, and rigid. Vegetative linings include grass with mulch, sod, and lapped sod. Flexible linings include rock riprap and soil bioengineering techniques that generally include vegetative lining. Rigid linings are generally concrete. 5.4.1 Vegetation Vegetation is the most desirable lining for an artificial channel. It stabilizes the channel body, consolidates the soil mass of the bed, checks erosion on the channel surface, and controls the movement of soil particles along the channel bottom. Conditions under which vegetation may not be acceptable, however, include but are not limited to the following:

1. Flow conditions in excess of the maximum shear velocity for bare soils 2. Standing or continuous flowing water 3. Lack of regular maintenance necessary to prevent domination by taller vegetation 4. Excessive shade (under a small bridge or large culvert)

Proper seeding, mulching, and soil preparation are required during construction to assure establishment of a healthy growth of grass. If soils do not support grass growth, soil testing should be performed and the results evaluated by an agronomist to determine soil treatment requirements for pH, nitrogen, phosphorus, potassium, and other factors. In many cases, temporary erosion control measures are


required to provide time for the seeding to establish a viable vegetative lining. Sodding should be staggered to avoid seams in the direction of flow. Lapped or shingle sod should be staggered and overlapped by approximately 25 percent. Sod should be staked on steeper slopes to prevent sliding (4:1 or steeper). 5.4.2 Vegetative Design A two‐part procedure, adapted from Chow (1959) and presented below, shall be used for final design of temporary and vegetative channel linings.

Part 1, the design stability component, involves determining channel dimensions for low vegetative retardance conditions.

Part 2, the design capacity component, involves determining the depth increase necessary to maintain capacity for higher vegetative retardance conditions.

If temporary lining is to be used during construction, vegetative retardance should be used for the design stability calculations.

If the channel slope exceeds 10 percent, or a combination of channel linings will be used, additional procedures not presented below are required. References include HEC‐15 (USDOT, FHWA, 1986) and HEC‐14 (USDOT, FHWA, 1983). Detailed equations for this method can be found in Appendix I of this Drainage Manual. 5.4.3 Flexible Linings

Rock riprap including rubble is the most common (while not the most preferred) type of flexible lining. It presents a rough surface that can dissipate energy and mitigate erosive velocity. Filter fabric is required beneath the riprap to allow the infiltration and exfiltration of water without allowing the migration of underlying soils. These linings are usually less expensive than other rigid linings and have self‐healing qualities that reduce maintenance.

5.4.4 Riprap Design

Riprap should only be utilized when "green," "soft," geotextiles and soil bioengineering techniques have been explored and thoroughly considered. Riprap may be used provided calculations are presented to the city that illustrate that soil bioengineering or other techniques are not cost effective for the site or are not feasible. Refer to the procedure detailed in Appendix I for riprap placement in both natural and prismatic channels.

5.4.5 Rigid Linings

Rigid linings are generally constructed of concrete and used where smoothness offers a higher capacity for a given cross‐sectional area. They should only be applied when vegetative and flexible lining techniques have been thoroughly considered. Higher velocities, however, create the potential for scour at channel lining transitions. A rigid lining can be destroyed by flow undercutting the lining, channel headcutting, or the buildup of hydrostatic pressure behind the rigid surfaces. When properly designed, rigid linings may be appropriate where the channel width is restricted. Filter fabric may be required to prevent soil loss through pavement cracks. Under continuous base conditions when a vegetative lining alone would be inappropriate, a small concrete pilot channel could be used to handle the continuous low flows. Vegetation could then be maintained for handling larger flows.

Section 6 Culverts and Bridges



6.0 CULVERTS AND BRIDGES INTRODUCTION Culverts and bridges both provide for management of flow through roadway systems. The designer will need to determine which structure is best for their particular project based on hydraulics, aesthetics, and economics. For small drainage areas, the most economical means of moving open channel flow beneath a road or railroad is generally with culverts. The design procedures for the culverts referenced in this section pertain only to those in the main channels and not those in roadside ditches. This section considers all design to be completed for ultimate development. Where appropriate, the actual construction of a crossing may be phased as development occurs. In this case, both the ultimate and the interim phases must be shown on the construction plans. Calculations for each must be submitted for approval.

Culvert and bridge design data presented herein should be supplemented and considered in conjunction with the CTDOT 2002 Drainage Manual and the various Federal Highway Administration publications that guide culvert and bridge design.

Culverts and bridges to be installed on waterways with FEMA‐defined floodplains and/or floodways should be designed consistent with FEMA standards. 6.1 CULVERTS ‐ GENERAL DESIGN CRITERIA A culvert is a single run of storm drain pipe that conveys water or stormwater under a road, railway, embankment, sidewalk, or other obstruction. Proper culvert design must consider many factors including the following:

Design flow

Inlet conditions (flow approach conditions, allowable headwater, culvert inlet configuration)

Culvert conditions (material roughness, pipe slope, and length)

Tailwater depth

Buoyancy potential

Environmental considerations and effects on aquatic life

Design loads and service life of the pipe material

Culvert computation worksheets are provided in Appendix D of this Drainage Manual.

Refer to Section 4.0 of the CTDOT 2002 Drainage Manual for a more thorough discussion of these items. Design review of development or redevelopment projects that include existing drainage facilities should include an inspection of all structures and documentation thereof. Appendix B to Section 4.0 of the CTDOT Drainage Manual outlines the necessary inspections and provides forms that should be completed and submitted with any project application.

Computer programs such as HEC‐RAS, NFF, and HY‐8 automate the design methods and can be utilized in project design. When using any approved procedure, the results must be summarized to include the projected flow capacity, the pipe size, the required slope, discharge velocity, and a comparison of the calculated values with the design criteria.


Culverts should be designed with the aid of the "Hydraulic Charts for the Selection of Highway Culverts" as found in Hydraulic Engineering Circulars Nos. 5 and 10 and should be checked for inlet and outlet control.

6.1.1 Design Frequency

All culverts should be designed to handle the flow equal to the capacity of the channel carried without causing significant backwater effects. Culverts shall be designed to convey the design storm in accordance with street classification as summarized in Table 4‐1 of this Drainage Manual. The culvert check should consider the potential effects of upstream and downstream flooding that may occur as a result of a proposed culvert. Where a culvert will carry a discharge reduced by detention, the DPW reserves the right to require that the design be based on zero storage. In that case, the culvert will be designed as if the detention facility or facilities did not exist. Compliance with the National Flood Insurance Program (NFIP) is necessary for all locations where construction will encroach on a 1 percent annual chance (100‐year) floodplain. 6.1.2 Allowable Headwater The allowable headwater is the depth of water that can be ponded at the upstream end of a culvert during the design condition as measured from the culvert inlet invert. The allowable headwater depth shall be limited by the following conditions:

Headwater does not cause upstream property damage.

Headwater does not increase the 1 percent annual chance (100‐year) flood elevation as mapped by NFIP.

During a design storm event, the water surface shall be a minimum of 18 inches below the shoulder of the road at the point where the culvert crosses under or the low point of the road grade where the water would overtop the road.

Headwater depth shall not exceed 1.5 times the diameter or height of the culvert barrel.

Headwater depth shall not be such that stormwater flows to other ditches or terrain, which permits the flow to divert around the culvert.

The maximum overtopping depths during the 1 percent annual chance (100‐year) storm event for various street classifications are as follows:

Classification Maximum Depth at Crown

Maximum Velocity

Local 1 foot 6 ft/sec

Collector 1 foot 6 ft/sec

Arterial No Overflow No Overflow

Highway No Overflow No Overflow


At all culvert inlets, the approximate limits of the flooded area covered by the headwater should be shown on the plan submitted for review. Where successive culverts are utilized and the flow in upper culverts is affected by headwaters in the lower culverts, a water surface profile and appropriate computations should be submitted for review.

6.1.3 Tailwater Conditions Tailwater is the water into which a culvert outfall discharges. Culvert design should be based on tailwater conditions that could reasonably be anticipated during the design condition.

If an upstream culvert outlet is located near a downstream culvert inlet, the headwater elevation of the downstream culvert may establish the design tailwater depth at the upstream culvert.

If the culvert discharges into a lake, pond, stream, or other body of water, the maximum water elevation of the body of water during the design storm will establish the design tailwater elevation at the upstream culvert.

If flow in the channel downstream of the culvert is subcritical, a computer‐aided backwater analysis or calculation of normal depth is warranted to determine the tailwater elevation. If the downstream flow is supercritical, tailwater is inconsequential to the culvert's hydraulic capacity. The effects of tidal action should be investigated to ensure that scour or erosion resulting from high velocities due to the movement of the tidal prism will not endanger the structure, roadway, or adjacent property. 6.1.4 Inlet and Outlet Control Culvert hydraulic design should consider both inlet and outlet control conditions. Under inlet control, the culvert entrance may or may not be submerged. In all cases, inlet‐controlled flow through the culvert barrel is free surface flow. Inlet control suggests that the culvert will see high velocity supercritical flow within the barrel. In outlet control, culvert flow through the structure tends to be deeper with lower velocities. When the culvert inlet is submerged, the most reliable means for determining discharge is with standard empirical relationships.

The following factors should be considered when calculating inlet control headwater:

Inlet Area – cross‐sectional area of the culvert entrance face

Inlet Edge – Projecting, mitered, headwall, or beveled edges are common.

Inlet Shape – Rectangular, circular, elliptical, or arch are common.

The following factors should be considered when calculating outlet control headwater:

Manning's Roughness (n) based on barrel material

Barrel Area – cross section perpendicular to the flow

Barrel Length

Barrel Slope

Tailwater Elevation


Outlet control affects the HGL of the flow through the culvert. To calculate the hydraulic grade, reference the equations for velocity, velocity head, entrance losses, friction losses, and exit losses contained in Section 8.6 of the CTDOT Drainage Manual. 6.1.5 Culvert Velocity Outlet velocity must be checked to assure that excessive erosion and scour problems will not occur. Culvert outlet protection must be provided in accordance with the standards and specifications for Outlet Protection in Chapter 5 of the 2002 Connecticut Guidelines for Soil Erosion and Sediment Control.

Culverts under roads shall be provided with end sections or endwalls in accordance with the outlet protection requirements summarized in Section 11.13 of the CTDOT Drainage Manual.

The minimum velocity in a culvert barrel must be adequate to prevent siltation at low flow rates. At a minimum, this velocity shall be 3 ft/sec for a 1 percent annual chance (100‐year) storm event. 6.2 STRUCTURAL DESIGN AND MATERIALS All culverts shall be designed to withstand an HS‐20 highway loading unless it crosses under a railroad, in which case the culvert shall be designed for railroad tracks. The structural design shall consider the depth of cover, trench width and condition, bedding type, backfill material, and compaction.

Culverts, both public and private, shall be constructed of materials as follows:

1. All precast RCP shall be American Society for Testing and Materials (ASTM) C‐76 (minimum). 2. All precast reinforced box culverts with more than 2 feet of earth cover shall be ASTM C789‐

79. 3. All precast reinforced concrete box culverts with less than 2 feet of cover shall be ASTM 850‐

79. 4. All corrugated metal pipes (CMP) shall be ASTM A‐760 and shall be allowed only for private

systems with 36‐inch diameter or smaller. Minimum gauge thickness for CMP culverts shall be 16 gauge for 30‐inch diameter and smaller. Minimum gauge thickness for 36‐inch diameter CMP culverts shall be 14 gauge. Trench design for CMP culverts shall meet ASTM or American Association of State Highway and Transportation Officials (AASHTO) standards.

5. Special CMP culverts including diameters greater than 36‐inch, elliptical, and arch designs will be considered on a case‐by‐case basis by the city for use as culvert pipe material in private storm drain systems.

6. Corrugated HDPE with an integrally formed smooth interior is allowed for sizes 48‐inch diameter or smaller.

6. Guardrails are suggested at all roadway culvert crossings. The approach ends of the guardrail should be flared away from the roadway and properly anchored.

7. Joint sealing material for precast concrete culverts should comply with "AASHTO Designation M‐198 74 I, Type B, Flexible Plastic Gasket (Bitumen)" specification.

8. Two‐sack‐per‐ton cement‐stabilized sand should be used for backfill around culverts. 9. A 6‐inch bedding of two‐sack‐per‐ton cement‐stabilized sand is required for all precast

concrete box culverts.


6.2.1 Culvert Sizes The minimum culvert size shall be 24‐inch diameter except that culverts under private entrance roads or driveways may be 15‐inch diameter if it meets all design flow conditions. These restrictions are made to guard against flow obstruction. Sizes less than these should be considered on a case‐by‐case basis.

Culverts shall meet all cover conditions required. Where the site conditions preclude the use of a single culvert barrel to meet the design flow conditions, multiple barrel culverts are acceptable.

The maximum length of a culvert shall be 300 feet. A culvert longer than 300 feet shall have manholes or junction boxes. 6.2.2 End Conditions

Headwalls and endwalls should be utilized for culverts greater than 3 feet wide or velocities exceeding 5 ft/s to control erosion and scour, to anchor the culvert against lateral pressures, and to ensure bank stability. End sections and headwalls shall be required on inlets and outlets as described below.

A. Prefabricated End Sections: Also known as flared end sections, these structures provide for

a better flow path, improving the design flow and headwater conditions. Flared end sections shall be provided for culverts 18‐inch to 36‐inch diameter except for the following:

No end section is required for 15‐inch or 18‐inch diameter driveway culverts.

Where culvert alignment exceeds 20 feet in vertical elevation change or culvert slope exceeds a 2:1 slope, a standard concrete headwall shall be provided instead of a prefabricated end section.

Where a concrete headwall is provided

B. Concrete Headwalls and Structures: Precast concrete headwalls shall be provided at all culvert inlets and outlets. Precast concrete headwalls shall meet the requirements of the CTDOT Standards and Specifications. Protective guardrails may be required along culvert headwalls depending on site conditions.

Wingwalls may be required in conjunction with headwalls. Culvert pipes 48" or larger in diameter shall have concrete wingwalls. Wingwalls are generally used where the culvert is skewed to the normal channel flow or where the side slopes of the channel or roadway are unstable. Wingwalls shall meet the requirements of the CTDOT Standards and Specifications. Wingwalls shall be set at an angle between 30 degrees and 60 degrees from the headwall.

Riprap should be used as headwall protection to prevent scouring around the inlet and outlet of the culvert. High flows can erode the soil surrounding the inlet and the soil underneath the outlet of a culvert. Both instances can cause culvert undermining and can adversely affect the structural integrity of the road crossing.


6.2.3 Multiple Barrel Culverts Multiple barrel culverts shall be allowed where single culverts cannot handle the design flow while meeting the required cover or headwater condition requirements. The design of multiple barrels should avoid the need for excessive widening of the upstream or downstream receiving channels.

The minimum spacing between culverts in a multiple barrel culvert design shall be that required to provide adequate lateral support and allow proper compaction of bedding material under the pipe haunches. 6.2.4 Culvert Skew Where possible, culverts shall be installed parallel to the flow path. The maximum allowable skew shall be 45 degrees as measured from the line perpendicular to the roadway centerline. 6.2.5 Buoyancy Verify that culvert pipe, end sections, concrete endwall structures, and stormwater treatment units will not fail under hydrostatic uplift conditions.

Buoyancy force consists of the weight of water displaced by the pipe and fill material that is over the pipe (below the headwater depth). The force resisting buoyancy includes the weight of the pipe, weight of the water within the pipe, and the weight of fill material over the pipe.

Buoyancy is more likely to be a problem where the following conditions exist:

• Lightweight pipe is used. • Pipe is on a steep slope (usually inlet control with the pipe flowing partially full). • There is little weight on the end of the pipe (flat embankment slopes, minimum cover, and/or no endwalls). • High headwater depths (HW/D>1.0)

Suitable cover, footings, or anchor blocks may be required to ensure the culvert's integrity during design conditions. 6.2.6 Debris and Trash Racks In general, trash racks or debris deflectors shall not be used where other site modifications may be made to prevent excessive trash or debris from entering the culvert. However, they may be required at specific locations, by the city, where large amounts of storm debris may be anticipated. The following criteria shall be used for the design of trash racks to be installed on concrete headwalls.

Materials All trash racks should be constructed with a rectangular smooth steel tube with a minimum 2‐inch x 4‐inch x ¼‐inch cross section. The tube steel should be A36 and should meet the ASTM A500 Grade B requirements.


The trash rack end bracing should be constructed with a steel channel section 2‐inch x 8‐inch x ¼‐inch and should be A36 steel. The headwall connection plate should be a ½‐inch x 6‐inch plate and should be A36 steel. The headwall connection bolts should be 5/8‐inch Red Head wedge anchor bolts and should be driven to a minimum depth of 4 inches. All trash rack components should have a corrosion‐protective finish. All welds should be ¼‐inch welds.

Design The trash racks should be constructed without cross‐braces (if possible) in order to minimize debris clogging. The trash rack should be designed to withstand the full hydraulic load of a completely plugged trash rack based on the highest anticipated depth of ponding at the trash rack. The trash rack should be removable for maintenance purposes. Bar Spacing The steel tube runner bars should be spaced with a maximum clear opening of 6‐inches. In addition, the entire trash rack should have a minimum clear opening area (normal to the rack) at the design flow depth of four times the culvert opening area.

Trash Rack Slope The trash rack should have a longitudinal slope of no steeper than 3 horizontal to 1 vertical for maintenance purposes. Hydraulics Hydraulic losses through trash racks should be accounted for by increasing the entrance and/or the exit loss coefficients by 0.1. This adjustment should apply to all trash racks constructed normal to the approach flow direction. 6.3 ENVIRONMENTAL CONSIDERATIONS AND FISHERY PROTECTION Where compatible with good hydraulic engineering, a culvert shall be located in "dry" conditions. Where this is not possible, the culvert shall be located to minimize impacts to streams or wetlands.

When a culvert is set in a perennial stream, the invert of the culvert shall be set below the normal flow line of the stream. The grade of the culvert shall not exceed the grade of the natural stream in the area. 6.3.1 Embedded and Open Bottom Culverts Stream crossing structures in small streams should be designed and installed so that the natural stream flow and bottom substrate are mimicked throughout the crossing and so that the structure does not constrict or fragment the stream. Bridges and open‐bottom box culverts are the preferred crossing method.


Open‐bottom culverts are similar to bridge structures, generally spanning the entire streambed and minimizing impacts to the natural stream channel. They are differentiated from bridges in that the fill placed over these structures is an integral structural element.

An embedded structure should be designed and installed at the same slope as the stream and should retain the same preconstruction stream gradient and substrate characteristics within the culvert. Riprap should be used to provide scour protection. For cylindrical culverts, the embedment should make up at least 40 percent of the culvert diameter. For pipe‐arch or box culverts, embedment depth should be at least 20 percent of the vertical rise of the arch. The streambed should consist of sufficient layers of unconsolidated gravel, sand, cobble, and other sediment to allow for proper embedment without sinking. A thalweg (low‐flow channel) should be established through the culvert to enable fish passage at low flow.

The width of embedded and open‐bottom structures should be 1.25 times the normal width of the streambed. The overall culvert capacity should be able to accommodate expected high flows. The figures below illustrate sizing design for open‐bottom culverts.

6.4 MAINTENANCE REQUIREMENTS The permittee is responsible for maintenance of culverts until construction is complete, including final cleanup and site stabilization to the satisfaction of the city. After the completion of construction, the property owner or responsible party is responsible for maintenance of all culverts not located in public easements. No one shall modify culverts in any way that impairs or restricts flow. The property owner shall periodically remove silt and sediment from the pipe and prune vegetation around the pipe entrance to avoid restricting flow capacity and shall correct erosion damage as necessary. All removed silt and sediment shall be properly disposed of away from storm drainage pipes and open channels and shall be properly stabilized with vegetation.


6.5 BRIDGES 6.5.1 Bridge Design Considerations Bridges differ from culverts and should be defined by having the following characteristics:

a) Greater than 20‐foot span b) Independent deck c) No pressure flow

At a minimum, bridges must be designed to pass the fully developed 1 percent annual chance (100‐year) design flow without causing backwater problems, structural damage, or erosion. The low chord of all bridges must be located at least 1 foot above the 1 percent annual chance (100‐year) flood elevation or at or above the level of natural ground, whichever is higher.

Newly constructed bridges must be designed to completely span the existing or proposed channel such that the channel will pass under the bridge without modification. Energy losses due to flow transitions should be minimized. In addition, provision must be made for future channel enlargements should they become necessary. 6.5.1.1 Piers and Abutments Piers and abutments must be aligned parallel to the longitudinal axis of the channel so as to minimize obstruction of the flow. Piers should be placed as far away from the channel centerline as possible and if possible should be eliminated entirely from the channel bottom. 6.5.1.2 Erosion Protection Increased turbulence and velocities associated with flow in the vicinity of bridges require the use of erosion protection in affected areas. See Section 8 of this Drainage Manual for minimum erosion protection requirements. 6.5.2 Acceptable Methods Before beginning any hydraulic analysis of a bridge, one must first determine the mode of flow for the waterway. The mode of flow determines the analysis required for the design of the bridge.

1. Riverine Flow – crossings with no tidal influence during the design storm. Bridges identified as

riverine dominated require only examination of design runoff conditions. 2. Tidally Dominated Flow – crossings where the tidal influences are dominated by the design

hurricane surge. Tidally dominated bridges with negligible upland influx require only examination of design storm surge conditions.

3. Tidally Influenced Flow – Flows in tidally influenced crossings, such as tidal creeks and rivers opening

to tidally dominated waterways, are affected by both river flow and tidal fluctuations. Tidally


influenced bridges require examination of both design runoff and surge conditions to determine which hydraulic and scour parameter will dictate design.

6.5.3 Design Standards Chapter 9 of the CTDOT Drainage Manual details the required analyses for the design of new bridge structures. This should be supplemented by appropriate Federal Highway Administration (FHWA) manuals and documents including those for scour analysis and countermeasure design. The FHWA has published the April 2012 Hydraulic Engineering Circular No. 18 entitled "Evaluating Scour at Bridges," which presents the state of knowledge and practice for the design, evaluation, and inspection of bridges for scour. There are two companion documents, HEC‐20 entitled "Stream Stability at Highway Structures" and HEC‐23 entitled "Bridge Scour and Stream Instability Countermeasures."

The most commonly used one‐dimensional models are HEC‐RAS and WSPRO. HEC‐RAS was developed by the USACE Hydrologic Engineering Center for a number of river hydraulic modeling applications, including the hydraulic design of waterway bridges. The drainage engineer should always ensure that the latest version is being used and document the version in the Bridge Hydraulics Report.

The FHWA requires that safety assessments be completed for various culvert design sizes. Example measures include safety barriers or grates. Small culverts (30‐inch diameter or less) can use an end section or slope paving to inhibit access to

the culvert. Culverts greater than 30 inches in diameter should receive one of the following:

Be extended to the appropriate "clear zone" distance

Safety treated with a grate if the consequences of clogging and causing a potential flooding hazard are less than the hazard of vehicles impacting an unprotected end. If a grate is used, the net area of the grate (excluding bars) should be 1.5 to 3.0 times the culvert entrance area.

Shielded with a traffic barrier if the culvert is very large, cannot be extended, has a channel that cannot be safety traversed by a vehicle, or has a significant flooding hazard with a grate

6.5.3.1 FEMA Standards

All bridge crossings mapped by FEMA must be consistent with the NFIP. The simplest way to be consistent with the NFIP standards for an established floodway is to design the bridge and approach roadways such that their components are excluded from the floodway. If a project element encroaches on the floodway but has a very minor effect on the floodway water surface elevation (such as piers in the floodway), the project may normally be considered as being consistent with the standards as long as hydraulic conditions can be improved so that no water surface elevation increase is reflected in the computer printout for the new conditions. A No‐Rise Certification will need to be prepared and supported by technical data. The data should be based on the original model used to establish the floodway. Regardless of the return period checked in the culvert design for a particular roadway, the 1 percent annual chance (100‐year) flood must be checked to evaluate the effects of the culvert on the base flood elevations.


For some rivers and streams, a detailed study was performed, but a floodway was not established. The bridge and roadway approaches should be designed to allow no more than a 1 foot increase in the base flood elevation.

If the encroachment is in an area without a detailed study (Zone A), then technical data should be generated for the project. Base flood information should be given to the local community and coordination carried out with FEMA where the increase in base flood elevations exceeds 1 foot in the vicinity of insurable buildings. 6.5.4 Procedures for Coastal Structures For coastal bridges, the applicable hydrology and hydraulics are influenced by waves, tides, storm surges, longshore sand transport, inlet dynamics and stability, and other coastal processes.

Smaller bridges at a single inlet or river mouths can be analyzed with one‐dimensional unsteady flow models. The use of a tidal prism approach is suitable only for smaller bridges or low average daily traffic bridges in well‐protected tidal arms and embayments. The range used in the analysis should be a combination of the highest daily astronomical tidal elevation (mean higher high water) and design event storm surge still‐water‐level. The use of this approach is not recommended for bridges at inlets or causeway bridges.

Causeway bridges, bridges with unusual configurations, and larger and more complicated bridges require the use of two‐dimensional unsteady flow models. Scour analyses of complex piers and bridges necessitate application of two‐dimensional numerical hydraulic models. The use of a HEC‐18 based qualitative approach is never suitable for coastal bridge hydraulic design or scour estimates on its own. Chapter 9 of the April 2012 FHWA Manual details scour analysis for tidal waterways. Coastal bridges should be elevated 1 foot above the design wave crest. If the clearance is less than 1 foot, which often occurs near the bridge approaches, the bridge must be designed according to AASHTO's Guide Specifications for Bridges Vulnerable to Coastal Storms.

The reference USACE (1986) Engineering and Design Storm Surge Analysis EM 1110‐2‐1412 provides a methodology for estimating rainfall associated with land‐falling hurricanes. The methodology is applicable for the area within 25 miles of the coast. The reference provides techniques for estimating rainfall associated with hurricanes traveling at high, moderate, and slow speeds by multiplying the rainfall from the graphs by a ratio coefficient that is a function of area. Alternatively, a steady 10 percent annual chance discharge may be assumed over the duration of the surge.

Section 7 Outlet Protection



7.0 OUTLET PROTECTION INTRODUCTION Velocity control and energy dissipation measures shall be installed at all new and existing stormwater outfalls and discharges from detention/retention/water quality facilities. Implementing these erosion control practices will help prevent localized erosion in coastal Norwalk's freshwater, estuarine, and marine resources. This chapter provides a general procedure to identify cases when outlet protection may be required as well as selection criteria and design details for protection facilities.

Outlet protection for a pipe or an open drain tributary is required where the flow velocity of an outlet will exceed the permissible velocity of the receiving channel. Protection is usually achieved with a structurally lined apron downstream from the outlet where discharge changes from pipe flow to channel flow or from one open drain to another. The purpose of the structure is to prevent scour at the discharge point and to minimize the potential for downstream erosion by reducing the velocity of concentrated stormwater flows.

For pipe, the depth of the tailwater immediately below the pipe outlet must be determined to calculate the design capacity of the pipe as well as the design discharge velocity. Fluctuations in the assumed tailwater depth must be considered. The design of the outlet protection must determine the apron length and width, considering the bottom grade, side slopes, alignment, and materials. Applicants shall utilize the design methods described in Section 11.13 of the CTDOT Drainage Manual, as amended. 7.1 TYPES OF OUTLET PROTECTION The most commonly used device for outlet protection is a structurally lined apron. These aprons are generally lined with riprap, grouted riprap, or concrete. Where practical, they are constructed at a zero grade of minimum slope to slow the outlet velocity. The type and length of the riprap‐lined apron is related to the outlet flow rate and the tailwater level and whether there is a defined channel downstream. A flat riprap apron can be used to prevent erosion at the transition from a pipe or box culvert outlet to a natural channel. Riprap aprons are appropriate when the culvert outlet Froude Number (Fr) is less than or equal to 2.5. If the tailwater depth is less than half the outlet pipe rise, it shall be classified as a Minimum Tailwater Condition. If the tailwater depth is greater than or equal to half the outlet pipe rise, it shall be classified as a Maximum Tailwater Condition.

There are three types of riprap aprons to be used for outlet protection. They are designated as Types A, B, and C. Type A riprap aprons would be used under minimum tailwater conditions while Type B riprap aprons would be used for maximum tailwater conditions as defined above, where the pipe outlets overland with no defined channel. Type C riprap aprons would be used when there is a well‐defined channel downstream of the outlet. The use of a Type C riprap apron on channels that are designated as watercourses or wetlands is discouraged due to potential wetland and fisheries impacts.

Where the flow rate proves to be excessive for the economical or practical use of an apron, preformed scour holes may be used. There are two types of preformed scour holes. Type 1 preformed scour holes are depressed one‐half the pipe rise, and Type 2 preformed scour holes are depressed the full pipe rise.


7.2 DESIGN CRITERIA The design of riprap outlet protection applies to the immediate area or reach downstream of the pipe outlet and does not apply to continuous rock linings of channels or streams. For pipe outlets at the top of exit slopes or on slopes greater than 10 percent, the designer should assure that suitable safeguards are provided beyond the limits of the localized outlet protection to counter the highly erosive velocities caused by the reconcentration of flow beyond the initial riprap apron. Outlet protection shall be designed according to the following criteria: • Outlet protection should be installed at the outlets of all pipes, culverts, catch basins, sediment

basins, ponds, interceptor dikes, and swales or channel sections where the velocity of flow may cause erosion in the receiving channel. Outlet protection should also be used at outlets where the velocity of flow at the design capacity may result in plunge pools (small, permanent pools located at an inlet or outfall). Outlet protection should be installed early during construction activities but may be added at any time as necessary.

• In situations not covered by the above‐noted criteria and where the exit velocity is <14 ft/sec, a

riprap apron shall also be used. For Type A and Type B riprap aprons, the type of riprap specified is dependent on the outlet velocity and can be determined from Table 8‐1. For Type C aprons, the type of riprap specified is determined by the procedures in HEC‐15 and HEC‐11 depending on the design discharge.

• When the outlet velocity is >14 ft/sec, the designer should first investigate methods to reduce the

outlet velocity. This may be accomplished by any one or combination of the following: increasing the pipe roughness, increasing the pipe size, and/or decreasing the culvert slope. When this is not possible or economical, a number of outlet protection or energy dissipation design options are available. These are presented in detail in HEC‐14. In most instances, however, a preformed scour hole design should be used as it generally can provide the necessary degree of protection at an economical cost.

The design criteria of this section should be applicable to most outlet situations. However, recognizing that design and site conditions can vary significantly depending on the project or location on a particular project, it is the responsibility of the designer to ensure that the criteria are suitable to the site or to provide an alternate design that will adequately protect the outlet area from scour and erosion. These situations should be documented in the drainage design report.

TABLE 7‐1

Allowable Outlet Velocities for Type A and Type B Riprap Aprons

Outlet Velocity (fps) Riprap Specification

0‐8 Modified

8‐10 Intermediate

10‐14 Standard

Source: CTDOT Drainage Manual, Table 11.11


7.3 RIPRAP OUTLET PROTECTION Limitations include the following:

Drainage area 5 acres

Maximum slope of 40 percent

The minimum particle size of the rock should be sized for the maximum expected velocity of flow out of the outlet and the soil conditions where the outlet will be located.

The design of rock outlet protection depends entirely on the location. Pipe outlets at the top of cuts or on slopes steeper than 10 percent cannot be protected by rock aprons or riprap sections due to reconcentration of flows and high velocities encountered after the flow leaves the apron. 7.3.1 Tailwater Depth Tailwater depth immediately below the pipe outlet should be determined for the design capacity of the pipe. If the tailwater depth is less than half the diameter of the outlet pipe and the receiving stream is wide enough to accept divergence of the flow, it should be classified as a minimum tailwater condition. If the tailwater depth is greater than half the pipe diameter and the receiving stream will continue to confine the flow, it should be classified as a maximum tailwater condition. Pipes that outlet onto flat areas with no defined channel may be assumed to have a minimum tailwater condition. 7.3.2 Apron Dimensions

The apron length and width should be determined according to the tailwater condition. If the pipe discharges directly into a well‐defined channel, the apron should extend across the channel bottom and up the channel banks to an elevation 1 foot above the maximum tailwater depth or to the top of the bank, whichever is less. The upstream end of the apron, adjacent to the pipe should have a width two times the diameter of the outlet pipe or conform to pipe end section if used.

The outlet protection apron should be constructed with no slope along its length. There should be no overfall at the end of the apron. The elevation of the downstream end of the apron should be equal to the elevation of the receiving channel or adjacent ground.

The outlet protection apron should be located so that there are no bends in the horizontal alignment. Length

The length of an apron (La) is determined using the following empirical relationships that were developed for the USEPA (1976) and modified by CTDOT for use in Connecticut. Tables 11‐12 and 11‐13 show the various lengths of Types A, B, and C riprap aprons based on discharge and pipe size. The tables also show the minimum and maximum lengths of aprons to be computed using these equations. When the table indicates that the required apron length would exceed the maximum shown, a preformed scour hole should be used in lieu of the riprap apron.


Type A Riprap Apron (Minimum Tailwater Condition) TW < 0.5 Rp

Type B Apron (Maximum Tailwater Condition) TW ≥ 0.5 Rp

Type C Riprap Apron ‐ The length of a Type C riprap apron shall be determined using the formula for a Type B riprap apron.

La = length of apron, m (ft) Sp = inside diameter for circular sections or maximum inside pipe span for noncircular sections,

(ft) Q = pipe (design) discharge, (cfs) TW = tailwater depth, (ft) Rp = maximum inside pipe rise, (ft) Note: Sp = Rp = inside diameter for circular sections

Width

For Type A or B riprap aprons, when there is no well‐defined channel downstream of the apron, the width of the apron at the pipe outlet, W1, should be at least three times the maximum inside pipe span, and the width, W2, of the outlet end of the apron as shown in Figure 11‐13 should be as follows: Type A Riprap Apron (Minimum Tailwater Condition)

W1= 3Sp (min.) W2 = 3Sp + 0.7La for TW < 0.5 Rp Type B Riprap Apron (Maximum Tailwater Condition)

W1 = 3Sp (min.) W2 = 3Sp + 0.4La for TW ≥ 0.5 Rp

W1 =width of apron at pipe outlet or upstream apron limit W2 =width of apron at terminus or downstream apron limit Type C Riprap Apron For a Type C riprap apron when there is a well‐defined channel downstream of the outlet, the bottom width of the apron should be at least equal to the bottom width of the channel, and the lining should extend on the channel side slopes at least 1 foot above the tailwater depth (TW) or at least two‐thirds of the vertical conduit dimension (0.7 Rp) above the invert, whichever is greater. (In all cases, the overall width of the apron shall be a minimum of 3Sp).


7.3.3 Materials The outlet protection shall be done using rock riprap. Riprap size should be based on calculated shear stress. It should be composed of a well‐graded mixture of stone size so that 50 percent of the pieces by weight should be larger than the d50 size determined by using the charts. A well‐graded mixture as used herein is defined as a mixture composed primarily of larger stone sizes but with a sufficient mixture of other sizes to fill the smaller voids between the stones. The diameter of the largest stone size in such a mixture should be 1.5 times the d50 size. Grouted riprap shall not be used as outlet protection due to freeze/thaw conditions that will cause the grout to break. Hydrostatic pressure can also build in areas without adequate drainage, causing further grout breakage and heaving of the riprap apron.

7.3.4 Thickness The minimum thickness of the riprap layer should be 1.5 times the maximum tone diameter for d50 of 15 inches or less and 1.2 times the maximum tone size for d50 greater than 15 inches. Suggested thicknesses for predicted shear stresses are shown in Table 7‐2.

TABLE 7‐2

Rock Riprap Sizes and Thickness

Unit shear stress (lb/ft2)

D50 (in.) dmax (in.) Minimum blanket thickness (in.)

0.67 2 4 6

2.00 6 9 14

3.00 9 14 20

4.00 12 18 27

5.00 15 22 32

6.00 18 27 32

7.80 21 32 38

8.00 24 36 43

Unit shear stress calculated as T = yds Where: T = shear stress in lb/ft2 y = unit weight of water, 62.4 lb/ft3 d = flow depth in ft. S = channel gradient in ft/ft

Source: IDEQ Storm Water Best Management Practices Catalog, September 2005, Table 30‐1.

7.3.5 Stone Quality Stone for riprap should consist of fieldstone or rough unhewn quarry stone. The stone should be hard and angular and of a quality that will not disintegrate on exposure to water or weathering. The specific gravity of the individual stones should be at least 2.5. Recycled concrete equivalent may be used provided it has a density of at least 150 pounds per cubic foot and does not have any exposed steel or reinforcing bars.


7.3.6 Filter A filter is a layer of material placed between the riprap and the underlying soil surface to prevent soil movement into and through the riprap. Riprap should have a filter placed under it in all cases. A filter can be of two general forms: a gravel layer or a plastic filter cloth. The plastic filter cloth can be woven or nonwoven monofilament yarns and should meet these base requirements: thickness 10‐60 mils, grab strength 90‐120 lbs; and should conform to ASTM D‐1777 and ASTM D‐1682. Gravel filter blanket, when used, should be designed by comparing particle sizes of the overlying material and the base material. Design criteria are available in any soils or civil engineering reference or from the NRCS.

Additional guidelines:

• The side slopes of the Type C riprap apron should be 2H:1V or flatter. • The location of outlets and outlet protection should be carefully considered to minimize rights‐of‐

way and wetland impacts. 7.3.7 Construction Guidelines The subgrade for the filter or riprap should be prepared to the required lines and grades. Any fill required in the subgrade should be compacted to a density of approximately that of the surrounding undisturbed material. The rock or gravel should conform to the specified grading limits when installed respectively in the riprap or filter.

Filter cloth should be protected from punching, cutting, or tearing. Any damage other than an occasional small hole should be repaired by placing another piece of cloth over the damaged part or by completely replacing the cloth. All overlaps whether for repairs or for joining two pieces of cloth should be a minimum of 1 foot.

Stone for the riprap outlets may be placed by equipment. Both should be constructed to the full course thickness in one operation and in such a manner as to avoid displacement of underlying materials. The stone for riprap outlets should be delivered and placed in a manner that will ensure that it is reasonably homogenous with the smaller stones and spalls filling the voids between the larger stones. Riprap should be placed in a manner to prevent damage to the filter blanket or filter cloth. Hand placement will be required to the extent necessary to prevent damage to the permanent works.

Construction of the outlet protection should be complete before allowing erosive flows to pass through the outlet. 7.3.8 Maintenance Once a riprap outlet has been installed, the maintenance needs are relatively low. Inspect after heavy storms and high flows for scouring under the outlet and dislodged stones, and repair damage promptly. For dikes, maintain the area upstream of the outlet structure so that accumulated sediments can be removed when they reach a depth of one‐third the height of the dike, or 12 inches, whichever is less.


7.4 PREFORMED SCOUR HOLE The preformed scour hole is an excavated hole or depression that is lined with rock riprap of a stable size to prevent scouring. The depression (F) provides both vertical and lateral expansion downstream of the culvert outlet to permit dissipation of excessive energy and turbulence. The first type, Type 1, is depressed one‐half the pipe rise, and the second type, Type 2, is depressed the full pipe rise. A significant reduction in stone size is achieved by the excavation. Therefore, the scour hole depressed the full pipe rise would require a smaller stone size; however, the dimensions of the hole would be larger. The type that provides the most economical and practical design given the site conditions should be selected. Sizing and reference tables for preformed scour holes can be found in the 2002 CTDOT Drainage Manual, Section 11.13.6.

Section 8 Erosion and Sediment Control



8.0 EROSION PREVENTION AND SEDIMENT CONTROL Construction and land development activities that impact existing topography, vegetative cover, and hydrologic characteristics often increase the potential for soil erosion and sediment transport. Erosion control measures and practices are actions that are taken to inhibit the dislodging and transporting of soil particles by water or wind, including actions that limit the area of exposed soil and minimize the time the soil is exposed.

All projects to be reviewed and approved by the DPW, City of Norwalk, shall include methods to adequately minimize erosion or contamination in streams, ponds, rivers, and reservoirs. The design engineer shall submit with the drainage plans the proposed erosion and sedimentation control measures. The erosion and sediment plan must be prepared concurrently with the drainage plan and in accordance with the 2002 Connecticut Sediment and Erosion Control Manual. The plan should provide information for each of the following items:

1. Existing and proposed contours 2. A construction activity schedule with a plan for implementing erosion prevention and

sediment control measures 3. Erosion and sediment control measures that will be implemented 4. Removal of temporary measures when appropriate and establishment of permanent

stabilization 5. Postconstruction runoff control measures that will be implemented 6. Maintenance requirements for temporary and permanent control measures 7. Measures to protect adjacent areas 8. Contingency measures in the event that planned controls are not effective 9. Permanent stormwater conveyance facilities

8.1 BASIC PRINCIPLES

The design of erosion prevention and sediment controls involves the application of common sense planning, scheduling, and control actions that will minimize the adverse impacts of soil erosion, transport, and deposition. The following five basic principles govern the development and implementation of a sound erosion prevention and sediment control plan:

1. The project should be planned to take advantage of the topography, soils, waterways, buffers, and natural vegetation at the site.

2. The smallest practical area should be exposed for the shortest possible time. 3. On‐site erosion prevention measures should be applied to reduce the suspension of soil

particles. 4. Sediment control measures should be used to prevent suspended soil from leaving the site. 5. A thorough inspection and maintenance program should be implemented.

These principles should be tied together in the planning process, which identifies potential erosion and sediment transport problems before construction begins.

Vegetative control measures are required for all disturbed areas and generally include practices such as filter strips, temporary seeding, permanent seeding, sodding, and mulching. Structural control


measures are required when runoff leaves a disturbed site and generally include practices such as sediment barriers, flow diversions, sediment traps, sediment basins, and permanent detention ponds.

The erosion prevention and sediment control plan must include appropriate construction specifications for all control measures. These specifications must be developed and/or implemented by the design engineer as required for site‐specific conditions.

8.2 APPLYING BMPs

Both erosion prevention practices and sediment control practices must be employed at all sites. 8.2.1 Construction versus Postconstruction BMPs The same level of care should be taken to select, install, and maintain construction (temporary) BMPs as is taken with postconstruction (permanent) BMPs. The only difference is in the intended life span of the BMP. In the construction industry, a temporary solution may be in place for a number of years due to oversight, neglect, good performance, etc. In general, temporary BMPs are intended to address construction activities while permanent BMPs address long‐term stormwater management objectives. A final Certificate of Occupancy will not be issued unless all site work is complete, and the site is stable and fully landscaped.

Temporary BMPs may include a variety of "good housekeeping" measures and short‐term erosion and sediment control activities. The contractor is responsible for properly constructing, implementing, and maintaining the temporary practices and/or seeking guidance when the measures do not appear to be meeting the stormwater management objectives (namely that sediment and other pollutants do not leave the construction site).

Permanent BMPs, which are designed to control long‐term stormwater pollution, are the final improvements to and configuration of the project. Permanent BMPs are selected by licensed professional engineers and incorporated into the plans and specifications for the project and have long‐term maintenance responsibilities identified. The contractor is responsible for properly constructing the permanent controls. Permanent BMPs are normally selected in the planning phase in conjunction with the approval of the tentative map designed during the design phase of a project and completed to the satisfaction of the city.

8.2.2 Identify Objectives The objectives in pollution prevention for each property can vary widely. Therefore, a specific understanding of pollution risks for each activity is essential for selecting and implementing BMPs. Defining these risks requires review of the characteristics of the site and the nature of the construction process or industrial activity. This information should be carefully assembled and reviewed early in the design process. Once these pollution risks are defined, then BMP objectives are developed, and specific BMPs can be selected. The BMP objectives for a typical construction project are as follows: • Practice Good Housekeeping: Perform activities in a manner that keeps potential pollutants from

either draining or being transported off site by managing pollutant sources and modifying construction activities. Dispose of waste materials in designated areas and in designated containers away from rainfall and stormwater runoff.


• Minimize Disturbed Areas: Determine project scheduling and only clear land that will be actively

under construction in the near term (within the next 3 months). Minimize new land disturbance, and do not clear or disturb sensitive areas (e.g., steep slopes, buffers, and natural watercourses).

• Stabilize Disturbed Areas: Provide temporary stabilization of disturbed soils whenever active

construction is not occurring on that portion of the site. Provide permanent stabilization during the final grading process and carefully landscape the site.

• Protect Slopes and Channels: Avoid disturbing steep or unstable slopes. Safely convey runoff from

the top of the slope and stabilize disturbed slopes as quickly as possible. Avoid disturbing natural channels. Stabilize temporary and permanent channel crossings as quickly as possible and ensure that increases in runoff velocity caused by the project do not erode the channel.

• Control Site Perimeter: Upstream runoff should be diverted around or safely conveyed through the

construction project and must not cause downstream property damage. Runoff from the project site should be free of excessive sediment and other constituents.

• Control Internal Erosion and Drainage: Detain sediment‐laden waters from actively disturbed areas

within the site to minimize the risk of sediment leaving the site.

BMP objectives for an industrial or commercial facility already in operation will basically have all of the same BMP objectives. However, there will be greater emphasis placed on good housekeeping, institutional controls and procedures, good training methods and regular refresher classes, and using the best available technology. BMP objectives in this chapter are generally discussed from a construction point of view but are applicable to all types of land uses. Site characteristics and proposed contractor activities will affect the potential for site erosion and contamination by other constituents used on the construction site. It is important to plan the project to fit the topography and drainage patterns of the site. Before defining BMP objectives, these factors should be carefully considered:

1. Site conditions that affect erosion and sedimentation, which include the following:

a. Soil type, including underlying soil strata that are likely to be exposed b. Natural terrain and slope c. Final slopes and grades d. Location of concentrated flows, storm drains, and streams e. Existing vegetation and ground cover

2. Climatic factors, which include the following: a. Seasonal rainfall patterns b. Appropriate design storm (quantity, intensity, duration)

3. Type of construction activity 4. Construction schedules, construction sequencing, and phasing of construction 5. Size of construction project and area to be graded 6. Location of the construction activity relative to adjacent uses and public improvements 7. Cost effectiveness 8. Types of construction materials and potential pollutants present or that will be brought on site 9. Floodplain, floodway, and buffer requirements


8.2.3 Select BMPs Once the BMP objectives are defined, it is necessary to identify the BMPs that are best suited to meet each objective. To determine where to place BMPs, a map of the project site can be prepared with sufficient topographic detail to show existing and proposed drainage patterns and existing and proposed permanent stormwater control structures. The project site map should identify the following: • Locations where stormwater enters and exits the site. Include both sheet and channel flow for the

existing and final grading contours. • Identify locations subject to high rates of erosion such as steep slopes and unlined channels. Long,

steep slopes over 100 feet in length are considered as areas of moderate to high erosion potential. • Categorize slopes as low erosion potential (0 to 5 percent slope), moderate erosion potential (5 to

10 percent slope), or high erosion potential (slope greater than 10 percent). • Identify wetlands, springs, sinkholes, floodplains, floodways, sensitive areas, or buffers that must

not be disturbed as well as other areas where site improvements will not be constructed. Establish clearing limits around these areas to prevent disturbance by the construction activity.

• Identify the boundaries of tributary areas for each outfall location. Then calculate the approximate area of each tributary area. Define areas where various contractor activities have a probable risk of causing a runoff or pollutant discharge.

With this site map in hand, categories of BMPs can be selected and located. Detailed planning before construction begins and phasing construction activities achieve erosion and pollution prevention most cost effectively. It is more cost effective to prevent erosion and pollution than it is to remove sediment and pollutants.

BMPs that can achieve multiple BMP objectives should be used to achieve cost‐effective solutions. For instance, it is not always necessary to install extensive sediment trapping controls during initial grading. In fact, sediment trapping should be used only as a short‐term measure for active construction areas and replaced by permanent stabilization measures as soon as possible. A permanent detention pond may be built first and used as temporary sediment control by placing a filter on the outlet. After construction is complete and the tributary area is stabilized, the permanent outlet configuration can be reestablished. 8.2.4 Factors for Construction Sites Certain contractor activities may cause pollution if not properly managed. Not all BMPs will apply to every construction site; however, all of the suggested BMPs should be evaluated. Considerations for selecting BMPs for contractor activities include the following:

• Is it expected to rain? BMPs may be different on rainy days versus dry days, winter versus summer,

etc. For instance, a material storage area may be covered with a tarp during the rainy season but not in the summer. However, it should be noted that plans should be made for some amount of rain even if it is not expected to generate a flooding event.

• How much material is used? Less‐intensive BMP implementation may be necessary if a "small"

amount of pollutant‐containing material is used. However, remember that even small amounts of some materials may be dangerous or have the potential to cause widespread pollution.


• How much water is used? The more water used and wastewater generated the more likely that

pollutants transported by this water will reach the stormwater system or be transported off site. Washing out one concrete truck on a flat area of the site may be sufficient (as long as the concrete is safely removed later), but a pit should be constructed if several trucks will be washed out at the same site.

• What are the site conditions? BMPs selected will differ depending on whether the activity is

conducted on a slope or flat ground, near a stormwater structure or watercourse, etc. Anticipating problems and conducting activities away from environmentally sensitive areas will reduce the cost and inconvenience of performing certain BMPs.

• In general, establishing a BMP for each conceivable pollutant discharge may be very costly and

significantly disrupt construction. As a rule of thumb, establish controls for common (daily or weekly) activities and be prepared to respond quickly to accidents. This rule of thumb only applies to contractors handling unusual materials that are not usually at the project site. Industries and commercial facilities are expected to have contingency plans and spill measures for every material that is used regularly. Therefore, keep in mind that the BMPs for contractor activities are suggested practices, which may or may not apply in every case. Construction personnel should be instructed to develop additional or alternative BMPs that are more cost effective for a particular project.

8.3 EROSION PREVENTION Erosion prevention is generally the easiest and least costly way to prevent sediment from leaving the site. It is important to note that if erosion is prevented then controlling sediment is not necessary.

8.3.1 Temporary and Permanent Considerations To the maximum extent possible, surface stabilization measures should provide permanent protection once construction is complete. In addition, the layout for temporary runoff control measures should be consistent with the layout of permanent drainage facilities. 8.3.2 Surface Stabilization Soil stabilization includes mulches, seeding and vegetation, chemical binders, and tacks. The principal types of mulching material are straw, hay, and wood chips. Suitable vegetative cover plants and plant mixtures should be chosen for a site along with appropriate planting dates and application rates. If construction occurs at a time when conventional vegetative measures are not feasible, or immediate protection is required under adverse conditions, chemical binders and tacks may be suitable. Other stabilization practices include buffer zones, filter strips, topsoil management, surface roughening, nets, mats, geotextiles, soil bioengineering, and terracing.


8.3.3 Slope and Channel Protection Steep slopes, both natural and cut and fill, have the potential for severe erosion. As a result, slope protection is often required to safely convey upland stormwater runoff to the toe of slopes. Slope and channel protection practices intended to reduce the potential for slope and gully erosion include temporary seeding, surface roughening, mulching, nets, mats, geotextiles, terracing, check dams, diversions (drains, swales, and berms), and bank stabilization. 8.3.4 Outlet Protection Design procedures for outlet protection should be consistent with the erosion prevention information for open channels and energy dissipation methods. The design should include a plan view, profile, and cross section for each unique channel reach between the storm sewer outlet and the existing publicly maintained system or natural stream channel. The velocity should be indicated for the outlet (pipe, structure, or reinforced channel), riprap or paved apron section, and each successive channel reach from the end of the apron to the point of entry into the existing drainage system or natural stream channel. The plan should indicate the proposed method of stabilizing each channel reach consistent with computed velocities. The velocity at the end of a structure or channel reach must not exceed the allowable velocity of the next downstream reach. 8.4 SEDIMENT CONTROL Sediment control measures that can prevent the transport of detached soil from a site include sediment barriers, sediment traps, sediment basins, construction entrance stabilization, and related activities.

8.4.1 Temporary and Permanent Considerations To the maximum extent possible, permanent facilities should be phased/scheduled to be used as temporary (construction‐phase) sediment control facilities. This is a more cost‐effective approach than implementing many more small sediment control devices sitewide as the generally larger permanent facilities must be graded and eventually constructed. It must be noted that it may still be necessary to implement some sediment controls in other areas of the site to prevent the permanent facility from being overloaded with sediment. Furthermore, the permanent facility will generally need to be overexcavated to account for the trapped sediment. The outlet structure will need to be reconfigured to perform under the construction‐phase runoff sediment loadings that generally are significantly higher than postconstruction (stabilized site) runoff. 8.4.2 Sediment Barriers Sediment barriers are intended to intercept and/or filter small volumes of sediment resulting mainly from sheet flow and rill erosion. Typical sediment barrier applications include continuous berms, brush barriers, sand bag barriers, silt fences, straw bale barriers, and inlet barriers. Check dams are similar to sediment barriers in that they slow water in small channels to the point that sediment can settle out of runoff. In general, sediment barriers have a useful life expectancy of 3 to 6 months depending on the construction technique. Continuous berms are strongly encouraged because of their installation ease and minimal maintenance requirements. Straw bales are the least preferred because of the inconsistent material qualities and very high maintenance considerations. Extreme care should be used when locating sediment barriers, and application limitations must be carefully considered. Improper location


and installation may result in failure of the barrier, which can cause more damage than the erosion the barrier was intended to prevent. 8.4.3 Sediment Traps and Basins Temporary sediment traps are generally formed by constructing a small ponding area behind an embankment and/or gravel outlet. The tributary drainage area and required service life will dictate the sizing of a small trap or temporary basin. However, it should be noted that designers are strongly encouraged to use permanent facilities with outlet structures configured to manage temporary (construction‐phase) sediment control. Temporary sediment traps and basins are often constructed in combination with temporary diversion berms or barriers. A method for designing sedimentation pools is found in the CTDOT Drainage Manual (Section 11.13).

8.4.4 Construction Road and Entrance Management Soil tracked off the construction site by delivery and other vehicles is a significant problem. Road and entrance management is required to reduce the amount of soil transported from a construction site. At a minimum, stone‐stabilized entrance pads should be constructed at vehicular traffic entrances and exits to a public road or paved area. When a stabilized pad proves inadequate, a wash rack or additional road stabilization will be required. Washwater runoff should be conveyed to a sediment basin or trap.

8.5 EROSION AND SEDIMENT CONTROL PLANS 8.5.1 Overall Requirements The use of source control BMPs to control erosion before it starts is the preferred method of long‐term sediment control. However, the best protection on active construction sites is generally obtained through simultaneous application of both source control and sediment containment BMPs. This combination of controls is effective because it prevents most erosion before it starts and has the ability to capture sediments that become suspended before the transporting flows leave the construction site. BMPs for erosion and sediment control are selected to meet the BMP objectives based on specific site conditions, construction activities, and cost effectiveness.

Since construction site conditions are constantly changing, different BMPs may be needed at different times during construction. In most cases, permanent BMPs can be implemented most effectively when they can be integrated into other aspects of the project design. This requires that stormwater control be considered early in the design process. Stormwater detention is required for most types of development and redevelopment within Norwalk. Some BMPs can be incorporated into stormwater detention facilities with modest design refinements and limited increases in land area and cost. Some areas can be used for more than one purpose, usually in a cost‐effective way. Landscaped open space, which is relatively flat, may be combined with stormwater quality/quantity facilities. Vegetated swales and buffer areas may be used as roadside corridors or along parking lots.

8.5.2 Minimize Disturbed Areas The first step for selecting BMPs is to compare the project layout and schedule with on‐site management measures that, where appropriate, can limit the exposure of the project site to erosion and sedimentation. Scheduling and planning considerations are the least expensive way to limit the


need for erosion and sediment control measures. Consider the following procedures to minimize disturbed areas: 1. Do not disturb any portion of the site unless an improvement is to be constructed there. Retain

existing vegetation and ground cover where feasible, especially along streams and watercourses and along the downstream perimeter of the site.

2. Minimize the size of disturbed areas and time of exposure by careful phasing of construction.

Minimize the amount of denuded areas and any new grading activities during the wet months of December through May. Do not clear any portion of the site until active construction begins. Use temporary cover (such as seeding or straw) whenever construction is halted or delayed.

3. Phased grading operations should limit the areas exposed to the process of erosion at any one time.

Only areas that are actively involved in cut and fill operations or are otherwise being graded should be exposed. Exposed areas should be stabilized as soon as grading is complete in that area.

4. Construct permanent stormwater control facilities such as detention basins and perimeter channels

early in the project and use these BMPs for sediment trapping, slope stabilization, and runoff velocity reduction throughout the construction period.

5. Quickly complete construction on each portion of the site. Install landscaping features and other

improvements that permanently stabilize each part of the site immediately after the land has been graded to its final contour. The purpose of site stabilization BMPs is to prevent erosion by covering disturbed soil. This covering may be vegetative, chemical, or physical. Any exposed soil is subject to erosion — either by rainfall striking the ground, runoff flowing over the soil, wind blowing across the soil, or vehicles driving on the soil. Thus, all exposed soils should be stabilized except where active construction is in progress. Locations on a construction site that are particularly subject to erosion and should be stabilized as soon as possible include the following:

• Slopes • Highly erosive soils • Construction entrances • Swales/wet weather conveyances • Soil stockpiles

8.5.3 Site Perimeter Controls The purpose of site perimeter controls is to protect downstream areas, including properties, environmentally sensitive areas, and public rights‐of‐way, from erosion, sediment, flooding problems, and excessive runoff. By doing this, the contractor will not only be obeying the laws but also making good neighbors. If construction phasing will allow, consider installing permanent stormwater control facilities (detention basins and perimeter channels) early in the project and use these BMPs for sediment trapping, slope stabilization, and runoff velocity reduction throughout the construction period. • Disturbed areas or slopes that drain toward adjacent properties, storm drain inlets, or receiving

waters should be protected with continuous berms, silt fences, sandbags, straw bales, etc. to prevent sediment discharge. The contractor should be prepared to stabilize those soils with additional protective measures prior to the onset of rain. When grading has been completed, the


areas should be protected with vegetative measures such as mulching, seeding, planting, emulsifiers, or a combination of these methods. The combination of erosion protection measures and sediment control devices should remain in place until the area is permanently stabilized.

• Significant off‐site flows (especially concentrated flows) that drain onto disturbed areas or slopes should be controlled through use of continuous berms, earth dikes, drainage swales, check dams, and lined ditches that will allow for controlled passage or containment of flows.

• Concentrated flows that are discharged off site should be controlled through outlet protection and velocity dissipation devices to prevent erosion of downstream areas.

• Perimeter controls should be placed everywhere runoff enters or leaves the site, before clearing and grubbing begin. Both runoff and sediment typically overload perimeter controls so that constant monitoring and maintenance are required. Additional controls within the interior of the construction site (such as check dams and sediment traps, etc.) should supplement perimeter controls once rough grading is complete.

8.5.4 Internal Erosion and Drainage Design When perimeter controls and outfall devices have been installed, internal erosion and drainage design can be addressed. Internal design elements are generally more time intensive. The middle of a project site is where construction phasing and sequencing becomes important. Until the permanent facilities are constructed, temporary stormwater facilities will be subjected to erosion from concentrated flows. • These facilities should be stabilized through temporary check dams and geotextile mats and under

extreme erosive conditions by lining with concrete. • Long or steep slopes should be terraced at regular intervals. Terraces will slow down the runoff and

provide a place for small amounts of sediment to settle out. • Slope benches may be constructed either with ditches along them or back sloped at a gentle angle

toward the hill. These benches and ditches intercept runoff before it can reach an erosive velocity and divert it to a stable outlet.

• Creating a rough surface for runoff to cross (such as tall grass) can reduce overland flow velocities. 8.5.5 Inspection and Maintenance Inspection and maintenance are the key elements in controlling erosion and sediment. Erosion and sediment control devices are installed as necessary and moved around the project site. Inspection should be performed after each rainfall and at least weekly. Maintenance must be performed immediately whenever deficiencies are noted. Checklists can help to document the inspection and maintenance process. A sample Erosion and Sediment Control Plan (ESCP) Review Checklist and a sample Site Inspection Checklist are included at the end of this section.

Larger projects should generally arrange for a preconstruction assistance meeting in order to coordinate erosion control objectives and methods with the city or county inspector prior to commencing clearing and grading. The city or county inspector will visit the project periodically and will also be involved in the final inspection checklists and approval.

Many BMP controls work on the same principle — the velocity of sediment‐laden runoff is slowed by temporary barriers or traps, which pond the stormwater, allowing the sediments to settle out. Therefore, sediment removal is an important activity in maintaining several BMPs.


Excessive sediment should be removed from the stormwater both within and along the perimeter of the project site. Appropriate strategies for the inspection and maintenance of erosion and sediment control features include the following: 1. Verify that sediment‐laden stormwater is directed to temporary sediment traps or basins. Verify

that sediment basins and traps are at low points below disturbed areas. 2. Protect all existing or newly installed storm drainage structures from sediment clogging by providing

inlet protection for area drains and curb inlets. Stormwater inlet protection can utilize sandbags, sediment traps, or other similar devices.

3. Excavate permanent stormwater detention ponds early in the project, use them as sedimentation

ponds during construction, remove accumulated sediment, and landscape the ponds when the upstream drainage area is stabilized.

4. Inspect temporary sediment barriers such as silt fences, straw bale barriers, rock filters, and

continuous berms after each rainfall. These barriers should only be used in areas where sheet flow runoff occurs. They are ineffective if the runoff is concentrated into rill or gully flow.

5. Internal outfalls must also be protected to reduce scour from high‐velocity flows leaving pipes or

other drainage facilities. 8.5.6 ESCP Preparation Guidance In general, the ESCP will consist of a narrative and a drawing. The project designer may choose to have the narrative included on a drawing or issued as a report. The owner of the land being developed has the ultimate responsibility for the ESCP preparation and submittal to the city. The owner may designate someone to prepare the plan, but the owner still has the ultimate responsibility.

A narrative (limited to one paragraph on the plans submittal) is required to explain the decisions concerning erosion and sediment control. The narrative should be adequate to allow a reviewer to make intelligent judgments concerning the effectiveness of the controls. It should contain complete and concise information regarding information that is not usually shown on drawings, such as construction schedules, existing soils, calculations and computations, types of vegetation, inspection and maintenance of controls, etc.

The level of detail should be appropriate to the project size and complexity. Projects near streams and other sensitive areas will require a high level of detail. In general, properties with a high percentage of impervious area will need a higher level of detail than properties with a low percentage of impervious area. Steep slopes may also affect the amount of effort in preparing an ESCP. A typical checklist for preparing an ESCP is included in Appendix A. Many engineering firms and construction contractors have their own checklists or templates for preparing an ESCP.

Many engineering firms and construction contractors have a standard set of general notes, which are generally sufficient. The following list contains general notes for a typical small project.


Typical General Notes for ESCPs: 1. As a minimum, all erosion and sediment control practices will be constructed and

maintained according to the standards located in the 2004 Connecticut SQM and as required by state and federal laws.

2. A copy of the approved ESCP shall be maintained at the project site at all times. This copy

shall be made available to the city upon request. 3. Prior to commencing land‐disturbing activities in any area not on the approved ESCP, the

contractor shall submit a supplementary erosion control plan to the city for review and approval.

4. All erosion and sediment control measures are to be placed prior to or as the first step in

clearing and grading. The contractor is responsible for any additional erosion control measures necessary to prevent erosion and sedimentation.

5. The city must be notified prior to dewatering operations. Water must be pumped through

an approved filtering device. The city may suspend dewatering operations if pollution is observed.

6. The contractor shall inspect all erosion and sediment control devices. The contractor shall

perform any repairs or maintenance immediately in order to ensure effective erosion and sediment control.

7. The contractor shall maintain a record of all inspections and maintenance activities at the

project site. This record shall be made available to the city upon request. 1383‐37‐7‐a717‐rpt

Appendix


City of Norwalk Drainage Manual Appendix A

APPENDIX A

CHECKLISTS AND GUIDELINES

Stormwater System Design Plan and Report ..................................................... Checklist A‐1 to A‐3 Erosion and Sediment Control Plan Narrative .............................................................. Checklist A‐4 Policy on Construction Inspection in the City of Norwalk ........................................................... A‐5

Erosion and Sedimentation Prevention Guidelines ..................................................................... A‐6

* All checklists are required to be completed and submitted along with the project at the time

of submission.

Checklists adapted from lists found in the following references:

Appendix A. Proposed Guidance and LID Checklists for UConn and Town of Mansfield, June 2011

Town of Greenwich Drainage Manual, Low Impact Development and Stormwater Management, February 2012

Rhode Island Stormwater Design and Installation Standards Manual, December 2010, Appendix A: Stormwater Management Checklist

Town of South Windsor Public Improvements Specifications

The Borough of Naugatuck Engineering Department Subdivision/Site Plan Checklist for Drainage Designs, November 2008

City of Norwalk Drainage Manual Checklist A‐1

Stormwater System Design Plan and Report Checklist for Submission

Project narrative including: [Provide page number where located] _____ Common address of site _____ Project name _____ Legal description of site _____ Vicinity map _____ Description of past, present, and proposed uses of the site _____ Description of the proposed drainage system, including:

o LID elements incorporated in the design o water quality treatments o how impervious surfaces have been minimized to the extent practical o outfall or discharge location (open watercourse versus existing downstream drainage system)

_____ Description of how zero increase in rate of runoff is met through the design, including a table documenting existing and proposed peak flows from the site for the 50, 10, 4, 2, and 1 percent annual chance events

Cover sheet with drawing index: (only required if not submitted with the Site Plan review) _____ Title block ‐ Title shall include type of submittal from Site Development Review Request. _____ Legend _____ North arrow _____ Property boundary of subject property (including parcels or portions thereof, of abutting land in

roadways within 100 feet of the property boundary) _____ Site location map (recommended scale 1" = 1,000') with north arrow Existing conditions map(s) detailing: [Provide page number where located] _____ Base mapping for the Storm Drainage Plan shall consist of an existing conditions survey prepared in

accordance with the Minimum Standards for Surveys and Maps in the State of Connecticut. The class of survey shall be A‐2 and T‐2 and shall be represented as such on the map. The base map shall be sealed and signed by a Professional Land Surveyor licensed in the State of Connecticut.

_____ The plan shall indicate the scale of the drawing and if possible be at a scale of 1" = 20' or 1" = 40'. _____ The plan shall depict the existing topography at contour intervals of 2 feet for the site and at a

minimum of 100 feet beyond the property limits of the subject property. _____ Topography that is flatter than 2 percent requires additional spot elevations, and the contour

interval shall be a 1‐foot contour interval instead of a 2‐foot contour interval. _____ The plan shall show all required spot elevations. _____ The plan shall indicate the referenced or assumed elevation datum (the FEMA datum shall be used

for sites located within a Flood Hazard Zone). _____ The plan shall have one permanent benchmark on the site within 100 feet of the proposed

construction work area on the site. _____ The plan shall include roads, buildings, driveways, parking areas, patios, walks, walls, other

structures on the property, storm drainage, sanitary sewers, subsurface sewage disposal systems (septic systems), curbs, sidewalks, trees, retaining walls, utilities such as water, gas, and electric etc.

_____ The plan shall show the locations of stormwater discharges. _____ The plan shall show wetlands, perennial, intermittent streams. _____ The plan shall show vegetation and the proposed limits of clearing and disturbance. _____ The plan shall show the locations of deep test and infiltration tests.


_____ The plan shall show resource protection areas such as wetlands, lakes, ponds, and other setbacks (stream buffer zones, drinking water well setbacks, septic system setbacks, etc.).

_____ The plan shall show utilities and easements. _____ The plan shall show the location of FEMA floodplain and floodway limits and the relationship of the

site to upstream and downstream properties and drainage systems. _____ The plan shall show existing channel modifications (e.g., bridge or culvert installations). _____ The plan shall show existing peak‐flow attenuation facilities. Proposed conditions map(s) detailing: [Provide page number where located] _____ The plan shall indicate the scale of the drawing and if possible be at a scale of 1" = 20' or 1" = 40'. _____ The plan shall depict the proposed topography at contour intervals of 2 feet for the site and at a

minimum of 100 feet beyond the property limits of the subject property. _____ Topography that is flatter than 2 percent requires additional spot elevations, and the contour

interval shall be a 1‐foot contour interval instead of a 2‐foot contour interval. _____ The plan shall show all required spot elevations. _____ The plan shall show proposed roads, buildings, driveways, parking areas, and other impervious

surfaces. _____ The plan shall show proposed utilities (e.g., water, sewer, gas, electric) and utility easements. _____ The plan shall show proposed storm drain infrastructure (e.g., inlets, manholes, storm drains). _____ The plan shall show proposed channel modifications (e.g., bridge or culvert installations). _____ The plan shall show temporary and permanent conveyance systems (grass channels, swales, ditches,

storm drains, etc.) building grades, dimensions, and directions of flow. _____ The plan shall show the location, size, maintenance access, and limits of disturbance of proposed

structural stormwater management practices and low impact development (treatment practices, flood control facilities, stormwater diversion structures, etc.).

_____ The plan shall show final landscaping plans for structural stormwater management practices and site revegetation (if no structural items are proposed, this portion of the plan may be signed by a licensed landscape architect or other environmental professional).

_____ The plan shall show locations of nonstructural stormwater management practices (i.e., source controls).

_____ The plan shall show the excavation and fill quantities in a table if required by the Director. _____ The plan shall be sealed and signed by a Professional Engineer licensed in the State of Connecticut.


Existing conditions hydrologic analysis including: [Provide page number where located] _____ Existing conditions map _____ Delineation of each of the existing drainage areas and subwatersheds found on the development site

(e.g., size, soil types, land cover characteristics) _____ Delineation of any off‐site drainage areas that contribute stormwater runoff to the development site

(e.g., size, soil types, land cover characteristics) _____ Calculation of the stormwater runoff rates (cfs) and volumes (ft3) generated, under existing

conditions, in each of the drainage areas found on the development site _____ Calculation of the stormwater runoff rates (cfs) and volumes (ft3) generated, under existing

conditions, in each of the off‐site drainage areas that contribute stormwater runoff to the development site

_____ Documentation (e.g., model diagram) and calculations showing how the existing conditions hydrologic analysis was completed

Proposed conditions hydrologic analysis, which includes: [Provide page number where located] _____ Proposed conditions map _____ Delineation of each of the proposed drainage areas found on the development site (e.g., size, soil

types, land cover characteristics) _____ Delineation of the proposed conditions of any off‐site drainage areas that contribute stormwater

runoff to the development site (e.g., size, soil types, land cover characteristics) _____ Calculation of the stormwater runoff rates (cfs) and volumes (ft3) generated, under proposed

conditions, in each of the drainage areas found on the development site _____ Calculation of the stormwater runoff rates (cfs) and volumes (ft3) generated, under proposed

conditions, in each of the off‐site drainage areas that contribute stormwater runoff to the development site

_____ Design computations of any detention or retention systems proposed as part of the project _____ Documentation (e.g., model diagram) and calculations showing how the proposed conditions

hydrologic analysis was completed Postconstruction drainage system design computations including: [Provide page number where located] _____ Drainage diagrams indicating design/analysis points and all stormwater features _____ Subwatershed mapping to each inlet structure _____ Tailwater used in the drainage system analysis _____ Analysis of the receiving drainage system if discharge is to an existing system _____ Computations supporting design of the drainage system including pipe capacity and hydraulic grade

line determinations _____ Table of watershed areas, times of concentration, and runoff coefficients for each subwatershed

area _____ Profiles of drainage conveyance systems showing proposed structures, proposed grade, flow depth,

hydraulic grade line, pipe and channel slopes, soil profiles in swales, bedding details, subdrain details, and types of construction materials

_____ Indicate the discharge location of all building roof leaders. If a building will discharge to more than one discharge location, these locations need to be indicated along with the pertinent area of contributing rooftop.

_____ Gutter flow analysis for applications that propose new roads or improvements to existing public roads


Typical Checklist for Preparing Erosion and Sediment Control Plans and Narratives Erosion and Sediment Control Plans shall comply with the requirements of the current version of the Connecticut Guidelines for Soil Erosion and Sediment Control. Provide the following information on a minimum size 24"x36" plan. Make sure the site name and address are included in the paragraph and check the submittal for legibility, page numbers, and correct spelling. Narrative _____ Describe existing site conditions (vegetation, drainage patterns, topography) and proposed site

conditions (ground cover, drainage patterns, and site grading). _____ Project description — purpose of grading or construction activity, total area to be disturbed _____ Adjacent property and uses _____ Critical or sensitive areas — steep slopes, streams, wetlands, sinkholes, etc. _____ Construction scheduling — duration of clearing, open grading, installation of permanent

stormwater controls _____ Inspection and maintenance schedule for BMPs and erosion control devices _____ Briefly describe the design standards used for any detention structures Erosion and Sediment Control Plan ______ The plan shall be based on the Site Plan and show the Sediment and Soil Erosion Controls. ______ Indicate the scale of the drawing and if possible be at a scale of 1" = 20' or 1" = 40'. ______ Include a north arrow and drawing legend. _____ Include a vicinity map: a small map showing the surrounding area, including landmarks, streams,

and roads. ______ Show existing and proposed conditions so that the proper location for the installation of the

erosion and sediment control measures can be determined. ______ Show construction fencing delineating the limits of disturbance and areas not to be disturbed. ______ Show construction fence delineating areas of the BMPs to be protected from compaction (i.e.,

area of root zones for trees to remain). ______ Show construction phasing and erosion and sediment control sequencing. ______ Include details for BMPs and erosion control devices. ______ Show any required computations. ______ Show the operations and maintenance of the erosion and sedimentation controls. _____ Show existing and proposed contours at an appropriate interval. _____ Show all buildings, roads, parking lots, and other structures. _____ Show all construction access routes, borrow areas, and spoil areas. _____ Existing and proposed drainage structures: sizes, materials, slopes, other important dimensions _____ Indicate drainage patterns and watershed boundaries; include drainage area for each

watershed. _____ Include property boundaries and easements. _____ Show all critical or sensitive areas on the site, including existing vegetation and all trees 6" or

greater caliper in diameter. _____ The plan shall be sealed and signed by a Professional Engineer licensed in the State of

Connecticut.

City of Norwalk Drainage Manual A‐5

Policy on Construction Inspection in the City of Norwalk

The Planning and Zoning staff is to be notified by the developer, contractor, or the property owner prior to commencing the following stages of development: 1. Site clearing 2. Rough grading 3. Installation of storm drainage 4. Installation of sanitary sewers 5. Placing of road base and final grading 6. Installation of bituminous concrete paving 7. Installation of landscaping 8. Installation of curbs and sidewalks 9. Receipt of certification of foundation location (as per Section 118‐1420G of the Planning and Zoning

Regulations) 10. Receipt of Certificate of Zoning Compliance A set of as‐built plans must be submitted along with a certificate from a professional engineer stating that the construction has been built according to city standards. Please contact the engineer of record so that an acceptable inspection program can be instituted in order to enable the engineer to certify the project. All development proposals approved by the Planning & Zoning Commissions are subject to this policy on construction. Responsibility for compliance with this policy shall be upon the property owner. Any violation of this procedure could mean loss of time and money to the developer or contractor through delay in release of the surety or delay in the issuance of a Certificate of Zoning Compliance. Messages concerning these matters will be received at the Planning & Zoning Commissions' office at City Hall, 125 East Avenue, Norwalk, CT; Telephone 854‐7780, between the hours of 8:30 a.m. and 5:00 p.m., Monday through Friday. If any changes in the construction plans are proposed, the Planning Engineer's approval shall be obtained before proceeding. If the change is substantial, a revised Utility Plan shall be submitted for approval before the work is done. If the construction plans call for excavation in an existing public street for any reason, a permit for such work shall be obtained from the Department of Public Works prior to the beginning of work. No additional bond from the Planning or Zoning Commissions shall be required.

City of Norwalk Drainage Manual A‐6

Erosion and Sedimentation Prevention Guidelines

The following factors shall be considered in a plan to minimize erosion and sedimentation caused by earth‐disturbing activities: 1. Erosion and sedimentation controls are to be installed prior to construction, where possible. 2. Land disturbances are to be kept to a minimum; restabilization is to be scheduled as soon as

possible. 3. All control measures are to be maintained in effective condition throughout the construction period

and until all disturbed areas are thoroughly stabilized. 4. Additional control measures are to be installed during construction if required by the Planning

Engineer. 5. Sediment removed from control structures is to be disposed of in a manner consistent with the

intent of the control plan. 6. Hay bale filters are to be installed at all culverts, catch basins, and along the toe of all critical cut/fill

slopes. 7. Culvert discharge areas are to be riprapped, and energy dissipaters are to be used where necessary. 8. All erosion and sedimentation controls are to be constructed in accordance with the standards and

specifications of the Erosion and Sedimentation Control Handbook. 9. The responsibility for implementing the Erosion and Sedimentation Control Plan must be assigned to

an individual. This individual is responsible for informing all concerned of the requirements of the plan and for seeing that a copy of such plan is transferred to any successor in interest to the title of the land or any portion thereof. This individual must be identified to the Commission prior to the start of any construction.

City of Norwalk Drainage Manual Appendix B

APPENDIX B

TIME OF CONCENTRATION

Figure B‐1: Nomograph for the Determination of Time of Concentration, Overland Flow ........ B‐1 Figure B‐2: Nomograph for Time of Concentration for Small Drainage Basins ........................... B‐2 Figure B‐3: Nomograph for Flow in Triangular Channels ............................................................ B‐3

City of Norwalk Drainage Manual B‐1

Figure B‐1 Nomograph for the Determination of Time of Concentration, Overland Flow

Source: CTDOT Drainage Manual


Figure B‐2 Nomograph for Time of Concentration for Small Drainage Basins

Source: Civil Engineering, Vol. 10, June 1940 by P.Z. Kirpich


Figure B‐3 Nomograph for Flow in Triangular Channels

Source: AASHTO, Highway Drainage Guidelines, 1990

City of Norwalk Drainage Manual Appendix C

APPENDIX C

STORM SEWER AND GUTTER FLOW ANALYSIS

Storm Sewer System Design Worksheet .......................................................................... Figure C‐1 Circular Channel Ratios ..................................................................................................... Figure C‐2 Flow Characteristic Curves ................................................................................................ Figure C‐3 Nomograph for Solution of Manning's Formula for Pipes Flowing Full ........................... Figure C‐4

Gutter Flow Velocity ......................................................................................................... Figure C‐5 Gutter Flow Analysis Computations, Inlet Spacing Sheet ................................................. Figure C‐6 Gutter Flow Analysis Computations, Low Point Analysis ................................................. Figure C‐7

City of Norwalk Drainage Manual Figure C‐1


Circular Channel Ratios

Experiments have shown that n varies slightly with depth. This figure gives velocity and flow rate ratios

for varying n (solid line) and constant n (broken line) assumptions.

Source: Professional Publications, Inc., Appendix 19.C



Nomograph for solution of Manning's formula for pipes flowing full



Source: CTDOT Drainage Manual, October 2000


Source: CTDOT Drainage Manual, October 2000

City of Norwalk Drainage Manual Appendix D

APPENDIX D

PIPES AND CULVERTS

Table D‐1 Conversion of Nominal Gauge to Thickness ................................................................ D‐1 Table D‐2 Corrugated Galvanized Steel Pipe Recommended Gauges ......................................... D‐1 Table D‐3 Asphalt Coated Corrugated Galvanized Steel Pipe Recommended Gauges ............... D‐2 Table D‐4 Culvert Pipe Schedules ................................................................................................ D‐2 Culvert Data Worksheet.................................................................................................... Figure D‐3 Culvert Computations Worksheet .................................................................................... Figure D‐4

City of Norwalk Drainage Manual D‐1

APPENDIX D: PIPES AND CULVERTS

The gauge and fill height tables to follow can be converted from gauge thickness as below:

Table D‐1 Conversion of Nominal Gauge to Thickness

Gauge No. 22 20 18 16 14 12 Galvanized Thickness * (in.)

0.034 0.040 0.052 0.04 0.079 0.109

Gauge No. 10 8 7 5 3 1 Galvanized Thickness * (in.)

0.138 0.168 0.188 0.218 0.249 0.280

*Also referred to as "specified thickness" for corrugated steel pipe products Ref.: CTDOT Drainage Manual

Table D‐2 Corrugated Galvanized Steel Pipe (2 2/3" x ½" Corrugations)

Recommended Gauges

Diam. (in.) Fill Height (ft) 1

Min. – 10 11‐15 16‐20 21‐25 26‐30 31‐35

15 16 16 16 16 16 16

18 16 16 16 16 16 16

21 16 16 16 16 16 16

24 16 16 16 14 14 14

30 14 14 14 14 12 12

36 14 14 12 12 10 10

42 12 12 12 10 10 10

48 12 12 12 10 8 8

54 10 10 8 8 10 10

60 8 10 10 10 8 8

66 10 10 10 8 8 8

72 8 8 8 8 8 8

Notes: 1. Fill height above top of pipe in feet 2. All pipe below heavy line to be shop strutted or elongated in accordance with CTDOT specs. 3. For pipe or fill height larger than shown in this table, use Structural Plate. 4. Minimum cover of 18" is the distance from the top of pipe to the finished grade of the road. 5. Unit weight dead load is 125 lb/c.f. 6. Ref. CTDOT Drainage Manual.


Table D‐3 Asphalt Coated Corrugated Galvanized Steel Pipe (2 2/3" x ½" Corrugation)

Recommended Gauges

Span (in.) Rise (in.) Fill Height (ft) 1

1 ½ ‐ 3 4‐5 6‐10 11‐15

18" Minimum Cover

18 11 16 16 16 16

22 13 16 16 16 16

29 18 14 14 14 14

36 22 14 14 14 14

43 27 12 12 12 12

50 31 12 12 12 12

58 36 10 12 12 10

65 40 10 10 10 8

72 44 8 8 8

Notes: 1. Fill height above top of pipe in feet 2. For larger structures use Structural Plate. 3. Fill limited to 15 ft. in order to hold corner bearing pressures to 4 ton maximum 4. Unit weight dead load is 125 lb/c.f. 5. Minimum cover is the distance from the top of the pipe arch to the finish grade of the road. 6. Ref. CTDOT Drainage Manual.

Table D‐4 Culvert Pipe Schedules

Schedule (Fill Height)

Diameter (in) Concrete Steel 2 2/3" x ½"

Aluminum 2 2/3" x ½"

Thermoplastic PE1, PVC2, or PP3

A 2'‐15'

12, 18, 24 30, 36 42, 48

Plain or CI. IV Class III Class III

.064" (16 Ga.)

.064" (16 Ga.)

.064" (16 Ga.)

.060" (16 Ga.)

.075" (14 Ga.)

.105" (12 Ga.)

PE, PVC, or PP PE, PVC, or PP PE, PVC, or PP

B 15'‐25'

12, 18, 24 30, 36 42, 48

Class V Class V Class V

.064" (16 Ga.)

.064" (16 Ga.)

.064" (16 Ga.)

.060" (16 Ga.)

.075" (14 Ga.)

.105" (12 Ga.)

PE, PVC, or PP PE, PVC, or PP PE, PVC, or PP

C 25'‐40'

12, 18, 24 30, 36 42, 48

None None None

.064" (16 Ga.)

.064" (16 Ga.)

.064" (16 Ga.)

.060" (16 Ga.)

.075" (14 Ga.)

.105" (12 Ga.)

None None None

D

40'‐60'

12, 18 24

30, 36 42, 48

None None None None

.064" (16 Ga.)

.064" (16 Ga.)

.064" (16 Ga.)

.079" (14 Ga.)

.060" (16 Ga.)

.075" (14 Ga.)

.105" (12 Ga.)

.135" (10 Ga.)

None None None None

Source: 2016 Standard Specifications, WA DOT

Notes: 1. Corrugated polyethylene pipe 2. Polyvinyl chloride pipe. Solid wall or profile for diameters through 27 inches Profile wall for diameters

larger than 27 inches. 3. Polypropylene pipe, 12 inch to 30‐inch diameters for Schedule A and Schedule B, and 36 inch to 60‐inch

diameters approved for Schedule A only.

City of Norwalk Drainage Manual Figure D‐3

CULVERT DATA WORKSHEET

Prep. By: _____________________ Project No. ___________________ Date: ________________________ Town ________________________ Route ________________________ Checked: _____________________ Location ______________________ Date: ________________________ 1. DRAINAGE AREA (a) Total Area Acres (b) Above Storage Acres (c) Effective Area Acres (d) Special Considerations______________________________ (e) Existing Culverts 2. DESIGN DISCHARGE C.F.S. for year frequency (a) Rational Formula, Time of Concentration Minutes (b) Other Methods used with data (c) Remarks 3. HYDRAULIC DATA (a) Size Type (b) Maximum permissible headwater elevation and description (c) Elevation of channel bed at outlet (d) Length Ft. and Slope Ft./Ft. of Culvert (e) Entrance invert elevation (f) Type and Location of Hydraulic Control 4. MISCELLANEOUS DATA (a) Height of Overfill Ft. (b) Strength Requirements Gage Metal Pipe or

Class RCP (submit computations) (c) End Treatment (d) Entrance Channel Protection (submit design) (e) Outlet Channel Protection (submit design) (f) Bank protection (submit design) (g) Bedload (heavy, medium, or light) Note: Forms using an alternative format will be accepted as long as they contain all of the

information specified above.


City of Norwalk Drainage Manual Appendix E

APPENDIX E

OUTLET CONTROL STRUCTURES, WEIR AND ORIFICE FLOW

EQ. E1 Sharp‐Crested Weir Equation, No End Contractions ....................................................... E‐1 EQ. E2 Sharp‐Crested Weir Equation, Two End Contractions ..................................................... E‐1 EQ. E3 Sharp‐Crested Weir Equation, Submerged Discharge ..................................................... E‐1 EQ. E4 Broad‐Crested Weir Equation .......................................................................................... E‐2 EQ. E5 V‐Notch Weir Equation .................................................................................................... E‐2 Figure E‐1: Sharp‐Crested "V" Notch Weir Discharge Coefficients ............................................. E‐2 EQ. E6 Proportional Weir Equation ............................................................................................. E‐3 Figure E‐2: Dimensions used for Design of a Proportional Weir ................................................. E‐3 EQ. E7 Orifice Equation ............................................................................................................... E‐4

City of Norwalk Drainage Manual E‐1

APPENDIX E: OUTLET CONTROL STRUCTURES, WEIR AND ORIFICE FLOW

Sharp‐Crested Weirs – No End Contractions The discharge equation for a sharp‐crested weir with no end contractions is expressed as (Chow, 1959):

EQ. E1

Where: Q = Discharge, in cfs H = Head above the weir crest excluding velocity head, in feet Hc = Height of weir crest above channel bottom, in feet L = Horizontal weir length, in feet Sharp‐Crested Weirs – Two End Contractions The discharge equation for a sharp‐crested weir with two end contractions is expressed as (Chow, 1959):

EQ. E2

Where: Q = Discharge, in cfs H = Head above the weir crest excluding velocity head, in feet Hc = Height of weir crest above channel bottom, in feet L = Horizontal weir length, in feet Sharp‐Crested Weirs—Submerged Discharge The effect of submergence on a sharp‐crested weir should be considered. When the tailwater rises above the weir crest elevation, the discharge over the weir will be reduced. To account for this submergence effect, the free discharge obtained by equations in section 6.4.1 or 6.4.2 should be modified using the following equation (Brater and King, 1976):

EQ. E3

Where: Qs = Submergence flow, in cfs Qf = Free flow, in cfs H1 = Upstream head above crest, in feet H2 = Downstream head above crest, in feet


Broad‐Crested Weirs The general form of the broad‐crested weir equation (Brater and King, 1976) is expressed as: EQ. E4

Where: Q = Discharge, in cfs C = Broad‐crested weir coefficient L = Broad‐crested weir length, in feet H = Head above weir crest, in feet If the upstream edge of a broad‐crested weir is so rounded as to prevent contraction and if the slope of the crest is as great as the loss of head due to friction, flow will pass through critical depth at the weir crest; this gives the maximum C value of 3.087 for a broad‐crested weir. For sharp corners on the broad‐crested weir, a minimum C value of 2.6 should be used. V‐Notch Weirs The discharge through a v‐notch weir can be evaluated using the equation (Merritt, et al, 1995):

EQ. E5

Where: C1 = discharge coefficient (See Figure 6‐1) Q = Discharge, in cfs θ = Angle of v‐notch, in degrees H = Head on vortex of notch, in feet

Figure E‐1 Sharp‐Crested "V" Notch Weir Discharge Coefficients


Proportional Weirs Although more complex to design and construct, a proportional weir may reduce the required detention/retention volume for a given site. The proportional weir is distinguished from other control devices by having a linear head‐discharge relationship achieved by allowing the discharge area to vary nonlinearly with head. Design equations for proportional weirs from Sandvik (1985) are as follows: EQ. E6

where Q is the weir discharge, in cfs, and the dimensions a, b, H, x, and y are shown in Figure E‐2.

Figure E‐2 Dimensions Used for Design of a Proportional Weir


Orifices The discharge through an orifice can be evaluated using the equation: EQ. E7

Where: Q = Discharge, in cfs C = Orifice coefficient, a value of 0.6 is usually appropriate, but if further refinement is desired, a hydraulics handbook (such as Brater and King, 1976) can be consulted. A = Area of orifice, in square feet g = Acceleration due to gravity, 32.174 feet/second2 H = Head above orifice centroid, in feet

City of Norwalk Drainage Manual Appendix F

APPENDIX F

HYDRAULIC ANALYSIS, ENERGY LOSS EQUATIONS

EQ. F1 Head Losses at Manholes ................................................................................................ F‐1 EQ. F2 Head Losses from Multiple Entering Flows ..................................................................... F‐1 EQ. F3 Entrance Losses ................................................................................................................ F‐2 Table F‐1: Coefficients for Entrance Losses ................................................................................. F‐2

City of Norwalk Drainage Manual F‐1

APPENDIX F: HYDRAULIC ANALYSIS, ENERGY LOSS EQUATIONS

Head Losses Head losses at structures shall be determined for inlets and manholes in the design of closed conduits. The design engineer should determine the relative significance of the minor losses and their applicability to the design. If they are insignificant, they may be omitted. Head Losses at Manholes The equation for the head loss (in feet) at a straight flow through manhole with no change in pipe sizes is as follows:

EQ. F1

Where, V = velocity in the pipe (fps) g = acceleration due to gravity At locations where there are one or more incoming lateral storm sewer pipes entering the main line, the head loss equation for the junction is: Head Losses from Multiple Entering Flows

EQ. F2

Where, D1 = the diameter of the upstream pipe (feet) D2 = the diameter of the downstream pipe (feet) A1 = the flow area of the upstream pipe

(square feet) A2 = the flow area of the downstream pipe

(square feet) A3 = the flow area of the lateral pipe (square feet) Q1 = the discharge in the upstream pipe (cfs) Q2 = the discharge in the downstream pipe (cfs) Q3 = the discharge in the lateral pipe (cfs) Ө = the angle between the upstream pipe and the

lateral pipe (degrees) g = acceleration due to gravity

City of Norwalk Drainage Manual F‐2

Entrance Losses A special case of sudden contraction is the entrance loss for pipes. The equation for head loss at the entrance to a pipe is given as follows:

EQ. F3

Where, K = entrance loss coefficient (see Table 3‐3) V = flow velocity in pipe (fps)

Table F‐1

Coefficients for Entrance Losses

Type of Entrance Coefficient (K)

Projecting from fill, socket end (groove‐end) 0.2

Projecting from fill, sq. cut end 0.5

Headwall or headwall and wingwalls

Socket end of pipe (groove‐end) 0.2

Square‐edge 0.5

Rounded (radius = 1/12D) 0.2

Mitered to conform to fill slope 0.7

Source: Hydraulic Charts for Selection of Highway Culverts, U.S. Dept. of Commerce, Dec. 1965

City of Norwalk Drainage Manual Appendix G

APPENDIX G

OPEN CHANNEL HYDRAULICS

EQ. G1 Continuity Equation ......................................................................................................... G‐1 EQ. G2 Flow Energy Equation ...................................................................................................... G‐2 EQ. G3 Uniform Flow: Velocity Equation ..................................................................................... G‐2 EQ. G4 Uniform Flow: Flow Rate Equation .................................................................................. G‐2 EQ. G5 Uniform Flow: Energy Grade Line Equation .................................................................... G‐3 Table G‐1: Recommended Manning's Roughness Coefficients for Channels .............................. G‐4 Figure G‐1: vR, Product of Velocity and Hydraulic Radius ........................................................... G‐5 Figure G‐2: Solution of Manning's Equation for Trapezoidal Channels ....................................... G‐6 EQ. G6 Manning's Equation for Trial and Error Solutions ........................................................... G‐7 EQ. G7 Trapezoidal Conveyance Factor Equation ....................................................................... G‐7 Figure G‐3: Normal Depth of Flow in a Uniform Channel ............................................................ G‐8 EQ. G8 Critical Depth Equation .................................................................................................... G‐9 EQ. G9 Froude Number Equation ................................................................................................ G‐9 EQ. G10 Hydraulic Radius Equation .......................................................................................... G‐10 EQ. G11 Manning's Equation, Solving for vR ............................................................................ G‐10 Figure G‐4: Drainage Channel Lining Design Worksheet ........................................................... G‐12

City of Norwalk Drainage Manual G‐1

APPENDIX G: OPEN CHANNEL HYDRAULICS

Geometric Relationships Mathematical expressions for calculating the area, wetted perimeter, hydraulic radius, and channel top width for selected open‐channel cross sections are presented in equations G1 through G3. These cross sections include trapezoidal, rectangular, triangular, parabolic, and circular shapes. Geometric properties of trapezoidal channels also can be evaluated using the chart presented in Figure G‐2. Irregular channel cross sections (i.e., those with a narrow, deep main channel and a wide, shallow overbank channel) must be subdivided into segments so that the flow can be computed separately for the main channel and overbank portions. This same process of subdivision may be used when different parts of the channel cross section have different roughness coefficients. When computing the hydraulic radius of the subsections, the water depth common to the two adjacent subsections is not counted as wetted perimeter (USGS, 1976a,b). Direct Solutions When the hydraulic radius, cross‐sectional area, and roughness coefficient and slope are known, discharge can be calculated directly from the equations G1 through G3 found in this appendix. Likewise, the velocity in the channel can be calculated using known values of slope, hydraulic radius, and Manning's coefficient. The trapezoidal channel nomograph in Figure G‐2 of this appendix can be used to find the depth of flow if the design discharge is known or the design discharge if the depth of flow is known. Continuity Equation The continuity equation is used for a one‐dimensional, steady‐flow system of water. The continuity equation can be used to compute the unknown variable if the other two variables are known. The continuity equation is expressed as follows: EQ. G1 Q = VA Where: Q = discharge, (ft3/s) A = cross sectional area of flow, (ft2) V = mean cross sectional velocity, (ft/s)


Flow Energy The energy head relative to the channel bottom is defined as the specific energy, E. It is the sum of the pressure head (or depth, d) and the velocity head (V2/2g). If the channel is not too steep (generally slopes less than 10 percent) and the streamlines are straight and parallel, the specific energy is expressed as: EQ. G2 E = d + V2/2g Where: d = depth of flow, (ft) V = mean cross‐sectional velocity, (ft/s) g = acceleration due to gravity, (32.2 ft/sec2) Uniform Flow Equations Manning's equation is used to compute the mean velocity in an open channel with steady uniform flow as follows:

EQ. G3

Where: V = mean cross sectional velocity, (ft/s) n = Manning's coefficient of channel roughness, (dimensionless) A = cross sectional area of flow, (ft2) P = wetted perimeter (the cross section length touched by water), (ft) R = hydraulic radius, (ft), (R = A/P) S = energy grade line slope, (ft/ft) The flow rate of an open‐channel flow can be determined by combining Manning's equation with the continuity equation. The open‐channel flow rate equation is as follows:

EQ. G4

Where: Q = Flow Rate (ft3/sec) n = Manning's coefficient of channel roughness, (dimensionless) A = cross sectional area of flow, (ft2) P = wetted perimeter (the cross section length touched by water), (ft) R = hydraulic radius, (ft), (R = A/P) S = energy grade line slope, (ft/ft)


The energy grade line slope of an open channel can be defined by rearranging the flow rate equations and computed as follows:

EQ. G5

Where: S = energy grade line slope, (ft/ft) Q = Flow Rate (ft3/sec) n = Manning's coefficient of channel roughness, (dimensionless) A = cross sectional area of flow, (ft2) P = wetted perimeter (the cross section length touched by water), (ft) R = hydraulic radius, (ft), (R = A/P) For prismatic channels, in the absence of backwater conditions, the slope of the energy grade line and channel bottom can be assumed to be the same. The following general factors should be considered when selecting the value of Manning's n: 1. The physical roughness of the bottom and sides of the channel should be taken into account. Fine

particle soils on smooth, uniform surfaces result in relatively low values of n. Coarse materials, such as gravel or boulders, and pronounced surface irregularity cause higher values of n.

2. The value of n will be affected by the height, density, and type of vegetation. Consideration should be given to density and distribution of the vegetation along the reach and the wetted perimeter, the degree to which the vegetation occupies or blocks the cross section of flow at different depths, and the degree to which the vegetation may be bent or "shingled" by flows of different depths. The n value will increase in the spring and summer as vegetation grows and foliage develops and diminish in the fall as the dormant season approaches.

3. Channel shape variations, such as abrupt changes in channel cross sections or alternating small and large cross sections, will require somewhat larger n values than normal. These variations in channel cross section become particularly important if they cause the flow to meander from side to side.

4. A significant increase in the value of n is possible if severe meandering occurs in the alignment of a channel. Meandering becomes particularly important when frequent changes in the direction of curvature occur with relatively small radii of curvature.

5. Active channel erosion or sedimentation will tend to increase the value of n since these processes may cause variations in the shape of a channel. The potential for future erosion or sedimentation in the channel should also be considered.

6. Obstructions such as log jams or deposits of debris will increase the value of n. The level of this increase will depend on the number, type, and size of obstructions.

7. To be conservative, it is better to use a higher resistance for capacity calculations and a lower resistance for stability calculations.

8. Proper assessment of natural channel n values requires field observations and experience. Special attention is required in the field to identify floodplain vegetation and evaluate possible variations in roughness with depth of flow.


Table G‐1

Recommended Manning's Roughness Coefficients for Channels

Type of Channel and Lining Design n

Rigid Lined Channels

Asphalt 0.015

Concrete 0.017

Concrete rubble 0.024

Gabions 0.027

Metal, smooth (flumes) 0.013

Metal, corrugated 0.027

Plastic lined 0.013

Reno mattress 0.025

Shotcrete 0.016

Wood, (flumes) 0.013

Earth Lined Channels

Firm loam, fine sand, sandy loam, silt loam 0.02

Stiff clay, alluvial silts, colloidal 0.025

Shales, hardpans, coarse gravels 0.025

Graded silt or loam 0.03

Alluvial silt 0.02

Earth, straight and uniform 0.023

Earth bottom, rubble sides 0.030

Coarse gravel 0.030

Rock cuts, shale and hardpan 0.030

Durable rock cuts, jagged and irregular 0.040

Cobbles and shingles 0.035

Stony bed, weeds on bank 0.035

Straight, uniform 0.0225

Winding, sluggish 0.025

Natural Stream Channels

Clean, straight, full stage, deep pools 0.03

Clean, straight, full stage, weeds and stones 0.035

Winding, some pools and shoals 0.039

Sluggish river reaches, weedy, w/deep pools 0.065

Summarized and adapted from Schwab et al., 1966, USDA‐SCS, 1972b, and PA‐DER, 1990.


Figure G‐1 vR, Product of Velocity and Hydraulic Radius


Figure G‐2 Solution of Manning's Equation for Trapezoidal Channels


Trial‐and‐Error Solutions A trial‐and‐error procedure for solving Manning's Equation is used to compute the normal depth of flow in a uniform channel when the channel shape, slope, roughness, and design discharge are known. For purposes of the trial‐and‐error process, Manning's Equation can be arranged as:

EQ. G6

Where: A = Cross‐sectional area, (ft2) R = Hydraulic radius, (ft) Q = Discharge rate for design conditions, (ft3/sec) n = Manning's roughness coefficient, (dimensionless) S = Slope of the energy grade line, (ft/ft) Graphical procedures for simplifying trial‐and‐error solutions are presented in Figure G‐3 for trapezoidal channels, which is described below. 1. Determine input data, including design discharge, Q, Manning's n value, channel bottom width, b,

channel slope, S, and channel side slope, z. 2. Calculate the trapezoidal conveyance factor using the equation:

EQ. G7

Where: KT = Trapezoidal open channel conveyance factor Q = Discharge rate for design conditions, (ft3/sec) n = Manning's roughness coefficient, (dimensionless) b = Bottom width, (ft) S = Slope of the energy grade line, (ft/ft) 1. Enter the x‐axis of Figure G‐3 with the value of KT calculated in Step 2 and draw a line vertically to

the curve corresponding to the appropriate z value from Step 1. 2. From the point of intersection obtained in Step 3, draw a horizontal line to the y‐axis and read the

value of the normal depth of flow over the bottom width, d/b. 3. Multiply the d/b value from Step 4 by b to obtain the normal depth of flow.


Figure G‐3 Normal Depth of Flow in a Uniform Channel


Critical Flow Calculations

Critical depth depends only on the discharge rate and channel geometry. The general equation for determining critical depth is expressed as:

EQ. G8

Where: Q = Discharge rate for design conditions, (ft3/sec) g = Acceleration due to gravity, (32.2 ft/sec2) A = Cross‐sectional area, (ft2) T = Top width of water surface, (ft) A trial‐and‐error procedure is needed to solve this equation. The following guidelines are presented for evaluating critical flow conditions of open‐channel flow: 1. A normal depth of uniform flow within about 10 percent of critical depth is unstable and should be

avoided in design, if possible. 2. If the velocity head is less than one‐half the mean depth of flow, the flow is subcritical. 3. If the velocity head is equal to one‐half the mean depth of flow, the flow is critical. 4. If the velocity head is greater than one‐half the mean depth of flow, the flow is supercritical. 5. If an unstable critical depth cannot be avoided in design, the least‐favorable type of flow should be

assumed for the design. The Froude Number is a dimensionless number representing the ratio of the inertial forces to gravitational forces. The Froude Number, Fr, calculated by the following equation, is useful for evaluating the type of flow conditions in an open channel: EQ. G9

Where: Fr = Froude Number V = mean cross sectional velocity, (ft/s) g = acceleration due to gravity, (32.2 ft/sec2) D = hydraulic depth*, (ft), (D= A/T) A = cross sectional area of flow, (ft2) T = channel top width at the water surface, (ft) * For rectangular channels, hydraulic depth equals depth of flow. The Froude Number can be used to determine if the flow regime is subcritical or supercritical and can be applied to channel flow at any cross section. When the Froude Number is less than one, the flow is considered subcritical. The flow is supercritical where the Froude Number is greater than one. The Froude Number is equal to one for critical flow conditions.


Vegetative Design A two‐part procedure, adapted from Chow (1959) and presented below, shall be used for final design of temporary and vegetative channel linings. Design Stability 1. Determine appropriate design variables, including discharge, Q, bottom slope, S, cross section

parameters, and vegetation type. 2. Use Table G‐1 to assign a maximum velocity, Vm, based on vegetation type and slope range. 3. Assume a value of n and determine the corresponding value of vR from the n versus vR curves in

Figure G‐1. 4. Calculate the hydraulic radius using the equation:

EQ. G10

Where: R = Hydraulic radius of flow, in feet vR = Value obtained from Figure G‐1 in Step 3 Vm = Maximum velocity from Step 2 5. Use the following form of Manning's Equation to calculate the value of vR:

EQ. G11

Where: vR = Calculated value of vR product R = Hydraulic radius value from Step 4, in feet S = Channel bottom slope, in feet/foot n = Manning's n value assumed in Step 3 6. Compare the vR product value obtained in Step 5 to the value obtained from Figure G‐1 for the

assumed n value in Step 3. If the values are not reasonably close, return to Step 3 and repeat the calculations using a new assumed n value.

7. For trapezoidal channels, find the flow depth. Design Capacity 1. Assume a depth of flow greater than the value from Step 7 above and compute the waterway area

and hydraulic radius. 2. Divide the design flow rate by the waterway area from Step 1 to find the velocity. 3. Multiply the velocity from Step 2 by the hydraulic radius from Step 1 to find the value of vR. 4. Use Figure G‐1 to find a Manning's n value based on the vR value from Step 3.


5. Use Manning's Equation to find the velocity using the hydraulic radius from Step 1, Manning's n value from Step 4, and appropriate bottom slope.

6. Compare the velocity values from Steps 2 and 5. If the values are not reasonably close, return to Step 1 and repeat the calculations.

7. Add an appropriate freeboard to the final depth from Step 6. Generally, 20 percent is adequate.


Figure G‐4 Drainage Channel Lining Design Worksheet

City of Norwalk Drainage Manual Appendix H

APPENDIX H

TRASH RACK DESIGN

EQ. H1 Head Loss through a Trash Rack Equation ....................................................................... H‐1

City of Norwalk Drainage Manual H‐1

APPENDIX H: TRASH RACK DESIGN

The following equation can be used to determine the head loss through a trash rack. Grate openings should be calculated assuming a certain percentage blockage as a worst case to determine losses and upstream head. Often 40 percent to 50 percent blockage is chosen as a working assumption.

EQ. H1

Where: Hg = head loss through the grate (ft) Kg1 = bar shape factor: 2.42 – sharp‐edged rectangular 1.83 – rectangular bars with semicircular upstream faces 1.79 – circular bars 1.67 – rectangular bars with semicircular up‐ and downstream faces w = maximum cross‐sectional bar width facing the flow (in) x = minimum clear spacing between bars (in) Vu = approach velocity (ft/s) ϴg = angle of the grate with respect to the horizontal (degrees) g = acceleration of gravity (32.2 ft/s2)

City of Norwalk Drainage Manual Appendix I

APPENDIX I

RIPRAP DESIGN

EQ. I1 Manning's n value for Riprap Equation ............................................................................ I‐1 EQ. I2 Minimum d50 value Equation ............................................................................................ I‐1 EQ. I3 Stone Weight Equation ..................................................................................................... I‐2

City of Norwalk Drainage Manual I‐1

APPENDIX I: RIPRAP DESIGN

This procedure has the following assumptions and limitations: 1. Minimum riprap thickness equal to d100 2. The value of d85/d15 less than 4.6 3. Froude Number less than 1.2 4. Side slopes up to 2:1 5. A safety factor of 1.2 6. Maximum velocity less than 18 feet per second If significant turbulence is caused by boundary irregularities, such as installations near obstructions or structures, this procedure is not applicable. 1. Determine the average velocity in the main channel for the design condition. Use the higher value

of velocity calculated both with and without riprap in place. Manning's n values for riprap can be calculated from the equation:

EQ. I1

Where: n = Manning's roughness coefficient for stone riprap d50 = Diameter of stone for which 50 percent, by weight, of the gradation is finer, in feet 2. Determine the required minimum d50 value from the following equation:

EQ. I2

Where: d50 = The median riprap particle size, in feet (ft) Va = The average velocity in the channel, in feet/second (ft/s) davg = The average flow depth in the channel, in feet (ft)

K1 =

= the bank angle with the horizontal

= the riprap material's angle of repose

City of Norwalk Drainage Manual I‐2

3. Determine available riprap gradations. A well‐graded riprap is preferable to uniform size or gap

graded. The diameter of the largest stone, d100, should not be more than 1.5 times the d50 size. Blanket thickness should be greater than or equal to d100 except as noted below. Sufficient fines (below d15) should be available to fill the voids in the larger rock sizes. The stone weight for a selected stone size can be calculated from the equation:

EQ. I3 W = 0.5236 γs d3 Where: W = Stone weight, in pounds d = Selected stone diameter, in feet γs = Specific weight of stone, in pounds/cubic foot Filter fabric or a filter stone layer should be used to prevent turbulence or groundwater seepage from removing bank material through the stone or to serve as a foundation for unconsolidated material. Layer thickness should be increased by 50 percent for underwater placement. 4. If d85/d15 is between 2.0 and 2.3 and a smaller d30 size is desired, a thickness greater than d100 can be

used to offset the smaller d30 size. Other minor gradation deficiencies may be compensated for by increasing the stone blanket thickness.

5. Perform preliminary design, ensuring that adequate transition is provided to natural materials both

upstream and downstream to avoid flanking and that toe protection is provided to avoid riprap undermining.

City of Norwalk Drainage Manual Appendix J

APPENDIX J

Standards for Removal of Oil, Grit, and Other Material from Stormwater

Chapter 93. Stormwater, Illicit Discharges, and Connections Ordinance

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