V. Flood Hydrology And Floodplain Hydraulics

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V. Flood Hydrology And Floodplain Hydraulics

IntroductionA. Hydrology

DefinitionsMethods for Estimating DischargesHydrologic Models

B. HydraulicsDefinitionsTypes of FloodplainsMethods for Delineating FloodplainsHydraulic Models

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A.Hydrology1. Definitions

2. Methods for Estimating Discharges

3. Hydrologic Models

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A.1. Hydrology: Definitionsa) Recurrence Interval

How often (statistically) does the event occur? Once every 10 years, once every 50 years?

Small floods happen more frequently, large floods happen less frequently

Storm Based: x-year storm causes x-year flood

It’s a statistical estimate; it may not happen that often or it may happen more often

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A.1. Hydrology: Definitionsb) The Base (100-year) Flood:

The Base Flood is FEMA’s standard for flood insurance mapping

The Base Flood has a recurrence interval of 100-years.

It is the flood having a one percent chance of being equalled or exceeded in any given year.

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A.1. Hydrology: Definitionsc) Hydrograph

The hydrograph for a given flood event is a plot of time vs. discharge.

The “Peak Flow Rate” is the maximum amount of flow (in unit volume per unit time).

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0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00

Time (minutes)

Dis

char

ge

(cfs

)

Example Flood Hydrograph


A.1. Hydrology: DefinitionsExample Flood Hydrograph

Peak Flow = 90 cfs @ Time = 15 minutes

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A.2. Hydrology: Methods for Estimating Discharge

a) Existing InformationFlood Insurance StudiesOther state or local floodplain studiesSubdivision reportsHighway/roadway reports

Check to make sure existing data includes evaluation of needed return interval

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b) Flood Frequency Estimates from Gage DataUSGS and others maintain gages that

measure stream flowMany gages have established flood

frequency relationsUSGS Water Resources Investigations

Report 98-4225 documents USGS gage data for Arizona through 1996

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b) Flood Frequency Estimates from Gage Data

Typical Stream Gage

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A.2. Hydrology: Methods forEstimating Discharge

c) Regional Regression EquationsBased on gaged data from actual watersheds

within the region of applicationRecent (1997) equations developed by USGSShould not be used for:

Urbanized watersheds, and Watersheds with characteristics that vary from

those of watersheds used to develop the equations

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c) Regional Regression Equations

Cover and Figure 41 from USGS WSP 2433

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d) Watershed ModelingMathmatical approach based on specific

watershed characteristics including; Catchment subarea delineation Rainfall relationship input Rainfall-runoff relation description Flood routing

Lacks the benefit of calibration with gaged data

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Example Watershed from HEC-1 Manual

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e) State Standard 2-96Outlines three level approach

Level 1 – Most conservative – applies envelope curve from USGS WSP 2433

Level 2 – Applies individual regional equations from USGS WSP 2433

Level 3 – Recommends: Computer Models HEC-1, TR-20, TR-55, others Approved Local Methodologies Flood Frequency from Gage Data

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f) Issues Selection of Method; Agency requirements, availability

of existing information and/or gage data Design Frequency; Selecting the proper return interval Calibration; Comparison of method results with real

world data such as gage data or high water marks. Arid Region Environments; Application of methods to

distributary flow areas, alluvial fans, high-permeability surfaces, etc.

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A.3. Hydrology: Hydrologic Modelsa) FEMA Requirements

b) ADOT Manual; Rational Method and HEC-1

c) Local Models/ProceduresMaricopa County; Rational Method and HEC-1Pima County; Modified Rational Method

d) Computer ModelsHEC-1/HEC-HMS most commonly usedSee FEMA requirements list for others

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PROGRAM DEVELOPED BY AVAILABLE FROM COMMENTS

HEC-1 4.0.1 and up2 (May 1991)

U.S. Army Corps of Engineers Water Resources Support Center3 Corps of Engineers Hydrologic Engineering Center (HEC) 609 Second Street Davis, CA 95616-4687

Flood hydrographs at different locations along streams. Calibration runs preferred to determine model parameters.

HEC-HMS 1.1 and up (March 1998)

U.S. Army Corps of Engineers U.S. Army Corps of Engineers Hydrologic Engineering Center 609 Second Street Davis, CA 95616-4687 http://www.hec.usace.army.mil/

The Hydrologic Modeling System provides a variety of options for simulating precipitation-runoff processes. It has a capability to use gridded rainfall data to simulate runoff. It does not provide snowmelt and snowfall functions; it cannot be used for areas where snowmelt is an important flood hazard source and must be considered in estimation of flood discharges.

TR-20 (February 1992) U.S. Department of Agriculture, Natural Resources Conservation Service

U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161


TR-55 (June 1986) U.S. Department of Agriculture, Natural Resources Conservation Service

U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 http://www.wcc.nrcs.usda.gov/water/quality/common/tr55/tr55.html

Peak discharges and flood hydrographs at a single location.

SWMM (RUNOFF) 4.30 (May 1994), and 4.31 (January 1997)

U.S. Environmental Protection Agency and Oregon State University

Center for Exposure Assessment Modeling U.S. Environmental Protection Agency Office of Research and Development Environmental Research Laboratory 960 College Station Road Athens, GA 30605-2720

Calibration or verification to the actual flood events highly recommended.

MIKE 11 UHM (June 1999)

DHI Water and Environment DHI Inc. 301 South State Street Newton, PA 18940

Simulates flood hydrographs at different locations along streams using unit hydrograph techniques. Three methods are available for calculating infiltration losses and three methods for converting rainfall excess to runoff. The web page is at: http://www.dhi.dk

DBRM 3.0 (1993)

Bernard L. Golding, P.E. Consulting Water Resources Engineer Orlando, FL

Center for Microcomputers in Transportation (McTrans) University of Florida 512 Weil Hall Gainesville, FL 32611-6585


HYMO U.S. Department of Agriculture, Natural Resources Conservation Service

U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161


PondPack v.8 (May 2002)

Haestad Methods, Inc. Haestad Methods, Inc. 37 Brookside Road Waterbury, CT 06708-1499 http://www.haestad.com

The program is for analyzing watershed networks and aiding in sizing detention or retention ponds. Only the NRCS Unit Hydrograph method and NRCS Tc calculation formulas are acceptable. Other hydrograph generation methods or Tc formulas approved by State agencies in charge of flood control or floodplain management are acceptable for use within the subject State.

FEMA Accepted Hydrologic Models


A.3. Hydrology: Hydrologic Models d) Computer Models

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B.Hydraulics1. Definitions

2. Types of Floodplains

3. Methods for Floodplain Delineation

4. Hydraulic Models

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B.1. Hydraulics: Definitions Open Channel Flow – Flow with a free surface Energy – The total energy (ft-lbs/ft) of the flow,

equal to the flow depth (d) plus the velocity head (V2/2g), or E = d + V2/2g

Energy Gradeline – Slope of the energy of the flow measured along the direction of flow.

Uniform Flow – Flow at constant or gradually changing depth along the channel.

Normal Depth – Open channel flow depth under uniform flow conditions.

V. 20Definition Sketch for Hydraulic Grade Line & Energy Grade Line

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B.1. Hydraulics: Definitions (continued) Backwater – Flow condition accounting for

obstructions and other non-uniformities within the floodplain.

Critical Depth – A condition where the energy is at a minimum for a given discharge.

Subcritical Flow – A condition where the flow depth is greater than critical depth and flow is influenced by downstream conditions.

Supercritical Flow – A condition where the flow depth is less than critical depth and flow is influenced by upstream conditions.

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B.2. Hydraulics: Types of Floodplainsa) Riverine

Includes any flow of runoff in a well-defined, tributary flow pattern

Evaluation generally assumes one-dimensional flow

Includes all major and most smaller streams in Arizona

V. 23Aerial View of Typical Riverine Condition

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B.2. Hydraulics: Types of Floodplainsb) Sheet Flooding

Low topographic relief across flow path. Very poorly defined channels (or none). No channel banks readily identified from aerial. Very uniform vegetative characteristics over area affected by

sheet flow. Soil characteristics may not be visible on aerials due to

vegetation density. Soils characteristics are usually very uniform within the sheet

flow area. Soil units mapped by the Natural Resources Conservation

Service (NRCS, formerly the Soil Conservation Service) as floodplain soils.

V. 25Aerial View of Sheet Flow Condition

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B.2. Hydraulics: Types of Floodplainsc) Ponding

Flooding condition that occurs behind floodplain obstructions such as roadways, railroads, agricultural berms, etc.

Little or no flow velocity.Can occur at isolated locations along other flow

systems (e.g, riverine, sheet flow, etc.)Often shown as Zone AH on flood insurance

maps

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B.2. Hydraulics: Types of Floodplainsd) Uncertain Flow Path; includes

Distributary flowBraided flowAnastomosing FlowAlluvial Fans (discussed in Section VI)

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B.2. Hydraulics: Types of Floodplainsd) Uncertain Flow Path

Distributary Flow Low, but distinguishable topographic relief across flow path. Topographic relief is sufficient to create isolated islands during

flooding. Channels divide in the downstream direction so that the

number of flow paths conveying floodwaters increases in the downstream direction.

Higher vegetative density along flow paths with upland vegetation between.

Soils units mapped by the NRCS as alluvial fan terraces, inactive alluvial fans, or alluvial fans.

V. 29Aerial View of Distributary Flow Condition

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Braided Flow; occurs where flow within a well-defined channel or floodplain is divided into separate flow paths created by shifting patterns of sediment deposition.

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Anastomosing Flow Branching, interlacing, and interconnecting flow paths, which produce a

net-like or braided appearance. Slight topographic relief across flow path. Anastomosing flow areas have poorly defined channels downstream of a

relatively large drainage area. Channel banks may not be visible on aerial photos for large portions of

the anastomosing alluvial surface. May occur on the lowest portion of alluvial fans. Higher vegetative density may occur along flow lines, with uniform

vegetative characteristics between flow lines. Soils mapped by the Soil Conservation Service as floodplain soils.

V. 32Aerial View of Anastomosing Flow Condition

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Alluvial Fans; Per FEMA (Feb. 2002) an active alluvial fan flood hazard is indicated by the following three related criteria. Flow path uncertainty below the hydrographic apex; Abrupt deposition and ensuing erosion of sediment as a

stream or debris flow lose its ability to carry material eroded from a steeper, upstream source area; and

An environment where the combination of sediment availability, slope, and topography creates an ultrahazardous condition for which elevation on fill will not reliably mitigate the risk.

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Figure G-1 of FEMA Guidelines and Specifications for Flood Hazard Mapping Partners (Feb. 2002)

Alluvial Fans Illustration

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B.3. Hydraulics: Methods for FloodplainDelineation

a) Approximate Methods

b) Backwater Modeling

c) Two-Dimensional Modeling

d) State Standards

e) Information Required

f) Issues

g) Assumptions

h) Encroachment

i) Floodways

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a) Approximate MethodsManning’s equation for uniform flow commonly used

for approximate delineations: V = 1.49 Rh2/3 S1/2, Q = VA

Generally limited to areas where flood profile slope is equal to channel bed slope (i.e., no backwater).

Guidelines for application available in State Standard SS 2-96, Level 2 procedures

n

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b) Backwater Modeling Based on measurement of energy losses along floodplain

reach One-dimensional model approach, i.e., flow profile varies in

one direction only. Generally limited to riverine flooding conditions. Assumes steady flow (i.e., no hydrograph) HEC-2 and HEC-RAS are most common computer

applications State Standard SS 9-02 provides application guidelines

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c) Two-dimensional modelingBased on assumption of variation in flow in two directions

(upstream to downstream and perpendicular to flow path)Generally based on routing of complete event

hydrograph rather than peak flow rateAccounts for floodplain storage through hydrograph

routingGenerally more applicable to broad shallow flow,

distributary or alluvial fan flow conditions.

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B.3. Hydraulics: Methods for Floodplain

Delineationd) State Standards

SS 2-96 for riverine systems using approximate methods

SS 9-02 for riverine systems using one-dimensional backwater methods.

SS 4-95 for sheet flow areas.SS 3-94 for supercritical flow conditions

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e) Information RequiredHydrology; Peak flow rate and/or event hydrograph

needed.Topography; ground elevation data needed for

cross-section developmentField Data; information on channel and floodplain

roughness, structures (bridges, culverts, levees, etc.)

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f) IssuesRoughness; References for guidance include USGS and

Maricopa County.Flow Regime; Subcritical vs. Supercritical, FEMA requires

subcritical for flood hazard delineation, but supercritical can predict higher velocities and associated scour.

Event Selection; Roadway design level vs. flood hazard delineation.

Multiple Events; Flood Insurance Studies (FIS) generally evaluate multiple events.

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g) Assumptions Horizontal Water Surface; not always true in bend reaches Divided Flow; may require separate analysis of each flow path Composite Flow; most models assume flow within section is entirely

subcritical or supercritical Steady vs. Unsteady Flow; most models are a “snap-shot” and to not

measure change in flow over time. Gradually vs. Rapidly Varied Flow; most models assume either uniform

flow depth or gradually varied depth over short distances. Stable Bed Geometry; most models assume the channel and floodplain

ground surface does not change during the flood.

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B.3. Hydraulics: Methods for FloodplainDelineationh) Encroachment

Placement of fill or obstructions within the floodplain is an encroachment

Hydraulic Response Flood depths usually increase (if depths decrease check flow regime) Flow velocities usually increase Flow quantity distribution can be shifted from one part of floodplain to

anotherFEMA/Local Requirements

FEMA prohibits increased flood depths within the “floodway” Local jurisdictions can have stricter encroachment limits

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B.4. Hydraulics: Hydraulic Modelsa) Approximate Methods

b) Backwater (one-dimensional) Models

c) Two-Dimensional Models

d) FEMA Requirements

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B.4. Hydraulics: Hydraulic Modelsa) Approximate Method

(* = not formally accepted by FEMA)Check model applicability to design situation

Quick 2 (FEMA)XSPRO (US Forest Service)*Simplified Floodway Determination (SFD),

(U.S. Army Corps of Engineers)Customized Spreadsheet Applications*

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B.4. Hydraulics: Hydraulic Modelsb) Backwater (1-D) Models

HEC-2 (USACOE, 1991)HEC-RAS (USACOE, 2002), replaces HEC-2WSPRO (USGS, 1988)

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B.4. Hydraulics: Hydraulic Modelsc) Two-Dimensional Models

TABS RMA4 (USACOE, 2000)FESWMS 2DH (USGS, 1995)FLO-2D (Jimmy S. O’Brien, 2000)

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B.4. Hydraulics: Hydraulic Models Bridge/Culvert Models

HY-8 (FHWA, 1992)WSPGW (Joseph E. Bonadiman & Associates,

Inc., 2000)Culvert Master (Haestad Methods Inc., 2000)

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B.4. Hydraulics: Hydraulic Modelsd) FEMA Accepted Models

All of the previous models (except as noted)Others as listed in the reference CD

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V. Flood Hydrology And Floodplain Hydraulics

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