Stormwater Design for Redevelopment in Urban

Watersheds

June 10, 2010Brought to you by the Chesapeake Bay Trust and

the U of MD Mid-Atlantic Water Program

Speaker Info

Tom SchuelerChesapeake Stormwater

Network117 Ingleside AvenueBaltimore, MD 21228

watershedguy@hotmail.comwww.chesapeakestormwater.net

Bill StackCenter for Watershed

Protection8390 Main Street

Ellicott City, MD 21043

bps@cwp.orgwww.cwp.org

Webcast Agenda

• The Skinny on the MDE Redevelopment Rules

• Why Managing Stormwater at Redevelopment Sites is So Hard (and So Important)

• Design Strategy for Redevelopment Sites

• Review of Effective Practices for Redevelopment Sites

• Municipal Role in Green Streets

• Setting a Mitigation Fee when full compliance is not possible

The skinny on redevelopment in Maryland

A copy of May 2010 emergency regs can be found in Resource 1

Redevelopment & Stormwater in Maryland

PREVIOUSLY (2000-2009)

• Redevelopment “loosely” defined.

• Must treat or reduce existing impervious area by at least 20%.

• “Green technology” encouraged but not required

• Offset fees if criteria cannot be met on site

• Recharge waived.

Redevelopment and Stormwater Now

• Redevelopment is defined as a site with at least 40% impervious area.

• Applies to disturbed area of project site, not the entire site area

• Must treat or reduce existing imperviousness by 50%

• New development criteria if IC is increased

• “Green technology” referred to as Environmental Site Design required

• Offset fees only as last resort

Redevelopment Math

Case 1: Proposed IC <= Existing IC/2

•Met your requirement

•Get out of jail

•Get keys to the City

Example: Existing IC = 10 acres

Proposed IC= 5 acres

5 acres<= 10 acres/2

5 acres IC goal is met

•

Redevelopment Math

Case 2: Existing IC/2< Proposed IC <= Existing IC

•WQV = 1.0 inch * (Proposed IC-Existing IC/2)

•No Rev And CPV

•Minor but nagging headache

Example: Existing IC = 10 acres

Proposed IC= 6 acres

WQv = 1.0 inch*6ac-(10ac/2)

Redevelopment Math

Case 3 Proposed IC > Existing IC

•WQV = 1.0 inch * Existing IC/2

•WQV and REv required for (Proposed IC – Existing IC)

•CPv (1.0 yr) required for (Proposed IC – Existing IC)

•Time for a miracle

Note: (WQv and Rev are inclusive of CPv)

Redevelopment Math

Case 3: Example

Existing IC = 10 acres Proposed IC = 11 acres

WQV = (1.0 inch * 10 acres/2) + (1.0 inch * 1 acre)

REv required for (11 acres IC – 10 acres IC )

CPv (1.0 yr) required for (11 acres IC – 10 acres IC)

(WQv and Rev are inclusive of CPv)

Design Implications

Redevelopment sites with less than 40% IC are sized using the full new development criteria

Strong incentive to sharply reduce IC at redevelopment sites, although this may be at odds with urban density objectives

Strong stormwater “penalty” for large increases in IC at redevelopment sites

What Else Changed in May of 2010?

• Grandfathering

• Quantity control waivers

• More local flexibility to provide options in the event of non-compliance at a redevelopment site (although many of these existed before)

Local Options in the Event Full Site Compliance is Not Possible

• Combo of ESD and on-site or off-site structural BMP

•Retrofitting (BMP upgrades, filtering practices and off-site ESD)

• Participation in stream restoration project

• Pollution trading

• Payment of a fee-in-lieu

• Partial waiver

Step 2: Calculate Site Imperviousness and Water Quality Volume, WQv

Site Area, A (acres) 4

Existing Impervious Surface Area (acres) 2.5

Proposed Impervious Surface Area (acres) 3

Existing Imperviousness, Ipre 62.5%

Proposed Imperviousness, Ipost 75.0%

Development Category Redevelopment

Rainfall Depth, P (in) 1.0

Runoff Coefficient, Rv 0.73

Water Quality Volume, WQv (ac-in) 2.90

Water Quality Volume, WQv (cf) 10,527

Step 4: Calculate Environmental Site Design (ESD) Rainfall Target, PE

% Soil Type A 0%

% Soil Type B 60%

% Soil Type C 40%

% Soil Type D 0%

Pre-Developed Condition, RCNwoods 61

New Development

Soil Type A ESD Rainfall Target, PE (in) 0.00

Soil Type B ESD Rainfall Target, PE (in) 1.32

Soil Type C ESD Rainfall Target, PE (in) 0.80

Soil Type D ESD Rainfall Target, PE (in) 0.00

Site ESD Rainfall Target, PE (in) 1.80

ESD Runoff Depth, QE (in) 1.31

ESD Runoff Volume, ESDv (cf) 18,949

A revised version of the ESD to the MEP spreadsheet is provided in Resource 2

Several bugs were fixed including an error in ESD sizing for redevelopment

Throw out your old version!

Updated users guide to be released next week

Many Bay States and Cities are Enhancing Stormwater Requirements at Redevelopment Sites

For a comparative review, check out Resource No. 3

Why is Stormwater Management So Hard for Redevelopment Projects in Highly Urban Watersheds?

Why Redevelopment is So Hard

• Many projects are quite small

• Many cities traditionally waive redevelopment projects

• Lack of space and/or high cost of land

• Constrained by inverts of existing storm drains

• Conflicts with existing underground utilities

• Compacted and polluted soils

• Traditional and even some new stormwater ESD practices developed in suburban areas don’t work in our cities

• Designers have little or no experience in designing the practices that do

Why Redevelopment is So Hard – 2

• Most sites discharge to impaired waters subject to TMDLs

• Natural stream network altered or eliminated

• Underground treatment is very expensive

• Full compliance can not be achieved at many sites

• Higher cost of compliance than in greenfield settings*

• Conflicts with Smart Growth objectives of land use efficiency

• Surface practices could result in loss of development intensity

• OTHERS?

The Degree of Difficulty Redevelopment Intensity (Post Development IC)

Less than 40%

40 to 65% 66 to 85% 85 to 100%

Alternate Surfaces

Alternate Surfaces

Alternate Surfaces

Alternate Surfaces

Landscaping ESDLandscaping ESD

Landscaping ESD

Landscaping ESD

IC Reduction IC Reduction

Micro ESD Micro ESD

Disconnections

Why Redevelopment is So Important

• Incrementally Reduces Untreated Pollution from Existing Development

• Green Building and Green Infrastructure Movement

• Sustainable Cities

• Combined Sewer Overflow Abatement

Redevelopment expected to increase as a share of total development in the future

• About 2 million acres of existing IC in Bay watershed

• 42% of urban land expected to be redeveloped by 2030

• Sharp increase in growth in core cities and inner suburbs in bay cities in last 5 years

• Sprawl seems to be slowing a bit in this economy

Street Dirt Contains Many Harmful Pollutants

Stormwater

Pollutant

BALTIMORE National

Average

Fecal Coliform Bacteria 36,025 5,091

Total Copper 28 ug/l 16 ug/l

Total Lead 64 ug/l 16 ug/l

Total Nitrogen 2.8 mg/l 2.0 mg/l

Total Phosphorus 0.32 mg/l 0.27 mg/l

Oxygen Demand 19.3 mg/l 8.6 mg/l

Baltimore Data from Diblasi (2008)

City Runoff Can Be More Polluted than Suburban Runoff

City Runoff Has a Very High load of Trash and Floatables

Stormwater Runoff is a Leading Cause of Water Quality Impairment

Stormwater Compounding ( 5 to 59% IC treated in 25 years)

Source: Philadelphia OW

Ten Strategies to Integrate Redevelopment with Stormwater

1. Understand the Urban Watershed Context

• Pollutant of Concern

• Combined or Separate Sewers

• Age of watershed development

• Habitat condition of streams

• Hydraulic capacity of existing stormwater conveyance and floodplain

• Historical flooding capacity

• Existence of watershed plans

• Other stormwater retrofit and restoration opportunities

2. Investigate Site History

• Most redevelopment projects require an environmental site assessment to determine if they are subject to “brownfield” remediation

• Site history investigation, soil testing and groundwater analysis

• These data are critical in stormwater design to determine whether:

• Soils need to be capped

• Infiltration should be encouraged or discouraged

• Historical drainage paths can be used to route stormwater

• Existing utilities will constrain design

3. Better Site Design in the Urban Context

• Land Use Efficiency (density is encouraged)

• Unique and Attractive Street-Scapes

• Integration of Stormwater & Landscaping

• Reduce Parking Demand

• Shared or Structured Parking

Several useful guides can be found in the weblinks found in Resource 4

4. Identify Potential Hotspot Generating Areas (HGAs)

• Review future site operations and activities

• Common areas include loading/unloading, fueling, outdoor storage, dumpsters, compactors and maintenance

• Identify areas of high pedestrian and vehicular traffic

• HGAs usually only a fraction of site area

• Isolate HGA in design and cover or filter runoff

• Integrate pollution prevention into design

5. Really Reduce Impervious Cover at the Site

• Strong incentive to make token change in site footprint to reduce IC to comply

• The “reduced IC” should perform hydrologically as if it were un-compacted grass, and ideally should be used to filter some runoff from remaining hard surfaces

• Deed or covenant that the area cannot be rebuilt in the future

6. Decompose Site Into Smaller Drainage Units

Source: COE,(2005)

Source: COE,(2005)

7. “Roof to Street” Design Approach

8. Maximize Forest Canopy and Restore Natural Area Remnants

9. Careful Urban Infiltration and Recharge

• Past development has destroyed soil structure and porosity

• Urban soil infiltration rates are very low

• Increased risk to foundations, infrastructure and landscaping

• Avoid infiltrating at hotspot & brown-field sites

• Infiltrate a fraction of WQv and rely on extended filtration (storage and underdrains)

Urban Fill Soils and Recharge

• Most redevelopment sites will be on fill soils

• Fill soils cannot be classified into any hydrological soil group

• Infiltration into fill soils is not desirable

• Most redevelopment sites are not subject to the recharge volume requirement

• For new IC, assume D soils for Pe comps

10. Establish Offset Fee

• Set Offset Fee when compliance cannot be achieved

• Fee based on fraction of untreated runoff volume, impervious cover or phosphorus load generated by site

• Must demonstrate that some ESD has been utilized on the site

• Fees are used for retrofit or restoration projects in the same watershed

Photo

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Questions and Answers

Green Roofs *

Cisterns and Rain Tanks *

Permeable Pavers *

Bioretention *

Expanded Tree Pits

Urban Tree Planting

Foundation Planters

Green Streets

Sand Filters

Sustainable Stormwater Practices for the City

After: Courtyard bioretention

Green Roof

• Extensive green roof can be major element of compliance at many redevelopment sites

• High installation cost is compensated by long term energy savings and roof longevity

• 70% of Bay engineers have never designed one

• New design specification available from CSN

Rain Tanks and Cisterns

• Redevelopment intensity means more internal demand for non-potable water

• Outdoor water demand for landscape irrigation

• Moderate cost of $15 cubic foot

• 60% of Bay Engineers have never designed one

• Design spec and sizing spreadsheet available from CSN

Permeable Pavers

• Limited Infiltration at most redevelopment sites

• Design with underdrains for extended filtration

• Good for pedestrian space and plazas

• Design as enhanced filter rather than as an alternative surface

Foundation Planters

Proposed Design Guidelines for Foundation Planters

• Not specifically described in MDE Chapter 5, but is certainly an acceptable micro-ESD practice

• Pe = 15” x Surface Area/CIDA

• Rapid flow through design

• Use hi sand media recipe (80%)

• More details can be found in Urban Bioretention Spec and Portland Guide (see Resources 5 and 6)

Impervious Cover Removal

Proposed Design Guidelines for IC Removal

• No specific MDE guidelines yet impervious cover reduction

• Plans should show the specific areas where concrete or asphalt will be removed

• Underlying soils should be deep tilled and amended with compost to restore porosity

• Areas should be graded to accept runoff from adjacent hard surfaces

•Planting plan should reflect landscaping objectives

Courtesy S. Schwartz

Reforestation and Street Tree Credit

Proposed Design Guidelines for Reforestation

Not specifically addressed in MDE manual

It can be treated as a disconnection

• 1 Street Tree = 100 sf Imp Area• Soil Restoration and Reforestation = 200 sf

Or as a Micro-ESD Practice

• Expanded Tree Pit• Pe = 12” x Surface Area/CIDA

• Must combine adequate root volume and water drainage in either case

Expanded Tree Pits

Source: City of Baltimore, MD

Design and Construction Issues

Underground Sand Filter and Proprietary Practices

Green Street Bioretention:

Lessons Learned in Green Streets in Baltimore

Figure 9: Sidewalk enhancements on Collington between Lombard and Baltimore

Streets

Figure 10: Bump out at Collington and Lombard Street

Green Streets are Popular in Neighborhoods

Tree Box Inlet with Curb Extension and Bioretention

B-15: Tree Box Inlet with Curb Extension and Bioretention

Post- Construction

Pre-Construction

During Construction

Project Selection and Design Issues

• Initially High Design Costs

• Initial High Interagency Coordination

Right-of-WayHighway DesignStreet Lighting Traffic EngineeringESCWastewater Engineering Stormwater EngineeringTraffic Control MOT

Construction Issues

• Neighborhood Disruption: – 10 to 30 day construction period

• Maintenance of traffic– Every project requires closing at least two travel lanes

• Coordination with utilities and other City agencies– street lights, parking meters, changes in traffic patterns and

parking restrictions)

• Equipment and materials staging locations

– Not a lot of extra space in an urban area

– Contractor has to haul excess material off-site

Making Green Streets Happen

• Strong Grass Roots support

•Visual aides showing outcomes

•Cost benefit numbers (expanded community benefit)

•Requires municipal leadership

• Interagency coordination to get consensus

• Initial demonstration and testing to convince the skeptics

• Local Green street design manuals

The Price of Stormwater

• Variation Due to: – IC and CDA

– New vs Redevelopment

– Design Era

– Your Agenda

The Stormwater Version of the Price is Right Show

For the technical assumptions for the following cost projections, please consult resources 7 and 8

$120.00 $10.00

$225.00 $25.00

What is the Price of Treating a Cubic Foot of Stormwater?

What is the Cost to Treat:

One Acre of IC of Urban Redevelopment One acre of IC at Pre-ESD Greenfield Development

One acre of IC at Greenfield Development to ESD

One Acre of IC with Storage Retrofits

$31,700

$46,500

$191,000

$32,500

What is the Cost to Treat:

One acre IC with Green Streets Stream Restoration in length equivalent to one acre IC, expressed in terms on nutrient load

$167,120 $35,600$167,100

Setting up a stormwateroffset fee

• Fully recover all public costs for stormwater mitigation (D-E-P, Contracting, Maintenance)

• Municipally-driven

• Watershed-based

• Equitable (the price)

• Easy to administer

• Indexed for construction inflation

• Must do some on-site ESD

• Reflect the development intensity in your community

Why a Locality Should Avoid Waivers

• You will get some nutrient liability in the Bay TMDL and the MS4 permit in the future

• Mitigation fee creates a revenue stream to support retrofitting and watershed restoration

• Environmental community is watching this issue very more closely

•

Recommended Accountability Elements in Local Stormwater Offset Fee

• Define qualifying public sector projects (e.g., retrofits, stream restoration, green streets)

• Conduct watershed restoration inventory to ID candidate projects

• Identify priority projects in watershed plan

• Track funds collected and disbursed

• Keep a retrofit registry that tracks project implementation and IC areas treated

Suggested Stormwater Offset Fees for Maryland

Fee Should be expressed in unit terms such as:

Per pound of phosphorus to be removedPer acre of untreated impervious cover Per cubic feet of untreated water quality volume

To be equitable, the fee should be set at the cost of effectively retrofitting development by the public sector.

Funds collected should be used to provide equivalent runoff reduction and/or pollutant removal in the same watershed

Example of Offset Fee

Assume a unit fee of $35,000/IC acre

20,000 sf Commercial Redevelopment

90% Impervious Pre-development

5000 sf of IC treated by on-site ESD practices

0.3 untreated acres of IC

0.3 * 35,000 = $10,500

Please make check payable to “chesapeake stormwater network”

Stormwater Smackdown

So at what level should the offset fee be set?

Tom: Set fee to recover the public sector cost to implement storage retrofits in a suburban setting ($32,500 per IC acre)

Bill: Set fee to recover the public sector cost to implement Green Streets or ESD practices in urban setting ($167,100 per IC acre)

Questions and Answers

Session Resources

Redevelopment Reqmts in Other Bay States

Links to good Redevelopment Specs and Design Manuals

Urban Bioretention Guide

Portland Specs for Foundation Planters

Retrofit Appendix with Cost Data (Appendix E)

Technical Documentation for Cost Estimates