Session 2B Stormwater Design for Redevelopment Practices

1. Stormwater Strategies for Redevelopment

2. Preferred Redevelopment Practices 3. Green Streets 4. Setting Offset Fees

5. Questions and Answers

Session 2B Agenda

• Maryland and DC Redevelopment Spreadsheet • PDFs of Todays Presentations • Key Stormwater Design Specs • Redevelopment Design Resources • CSN Technical Bulletin No. 5

• Please copy and return !!!

Key Resources on your disk

Why Redevelopment is So Important • Growing as a Share of Total

Development • Poor Runoff Quality from our

Urban Watersheds • Drains to Impaired Waters (with

TMDLs) • Long term strategy to

incrementally treat existing impervious cover and combined sewers

• Links to green building and sustainable cities

Redevelopment expected to increase 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

Stringent Stormwater Standards for Redevelopment an Effective Strategy for TMDL Pollutant Reduction Liabilities

Source: Philadelphia OW

Stormwater in the City vs. the Country

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 LID practices

developed in suburban areas don’t work in our cities • Designers have little or no experience in designing

the practices that do

Most designers have no experience with the new LID practices that work for redevelopment

• Green Roof: 67% • Amended Soils: 65% • Rainwater Harvesting: 50% • Disconnection: 50% • Filter Strip: 47% • Dry Swale: 46% • Permeable Pavement: 45%

Why Redevelopment is So Hard – 2

• 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 objective of land use

efficiency

Technical Bulletin No. 5 Stormwater Design for High Intensity

Redevelopment Projects in the Chesapeake Watershed

• Unique Design Approach Required • Special Performance Standard • Preferred LID Practices in the City • Design Recommendations • Special Focus on Green Streets • Stormwater Costs and Offset Fees • Nutrient Reduction Credits

1. Understand the Urban Watershed Context

• Pollutant of Concern • Combined or Separate Sewers • Age of watershed development • Limited hydraulic capacity of

existing stormwater conveyance and floodplain

• Historical flooding conditions

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 • 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 Technical Bulletin 5

4. Really Reduce Impervious Cover at the Site

• Designers have a strong incentive to change site footprint to reduce IC from existing condition

Reduce Impervious Cover at the Site

• 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

Proposed Design Guidelines for IC Removal

• 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

5. Decompose Site Into Smaller Drainage Units

Units may be as small as a few thousand square feet up to a few acres

Source: COE,(2005)

Source: COE,(2005)

6. “Roof to Street” Design Approach

7. Maximize Forest Canopy and Restore Natural Area Remnants

Proposed Design Guidelines for Reforestation

It can be treated as a “vertical” disconnection

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

Or as an Expanded Tree Pit (2000 to 4000 sf)

8. Careful Urban Infiltration and Recharge

• Most redevelopment sites will

be on urban fill soils • Past development has

destroyed soil structure and porosity

• Urban soil infiltration rates are usually very low

• Not sure what is really under the ground (rubble fill, soil pollutants, etc.)

Brownfield and Hotspots

• If a site is a brown-field or hotspot, shift to “closed” water quality treatment that doesn’t interact with groundwater (don’t worry about runoff reduction)

• Otherwise, conduct on-site geotechnical tests to confirm whether infiltration is feasible and/or desirable

Urban Redevelopment Site Conditions and Infiltration Restrictions Site History or Condition

Risk Infiltration Restriction

Site is Designated as Brownfield

Infiltration increases risk of leaching pollutants

Cap or liner, and ensure no intentional or unintentional infiltration over affected area

Site was Previously Mass Graded and is Classified as Urban Fill Soils

Geotechnical concerns. Prior compaction suggests poor infiltration rates. Unsure of soil quality and risk of leaching

Unless your on-site testing proves otherwise, avoid intentional infiltration and rely on “closed” practices that do not interact w/ groundwater (e.g., sand filters, green roof, and rain tanks)

Undisturbed Soils

Small risk of damage to underground infrastructure and foundations

Infiltration encouraged but confirm infiltration rates and respect setbacks

Site Expected to be a Potential Hotspot

Polluted stormwater contaminates groundwater

Treat at least half of the treatment volume in closed practice prior to infiltration

Site Expected to be Severe Hotspot

Polluted stormwater, spills, leaks and illicit discharges into gw

Avoid intentional or unintentional infiltration, and used closed practices

NJ Soil Investigation Methods • 1 Soil Permeability Test per Facility • Test must be performed at proposed

bottom of infiltration practice. • Permeability tests can include:

– Percolation Test – Pit Bailing Test – Tube Permeameter Test – Basin Flooding Test

Stormwater Practices for Redevelopment

Hi Density Redevelopment Projects Preferred Adequate Restricted Marginal

Impervious Cover Removal Sand Filters Infiltration Ponds &

Wetlands Green Roof and Rain Tanks Bioretention Proprietary

Practices Wet Swales

Permeable Pavers* Soil Restoration Dry Wells

Grass Channels & Filter Strips

Foundation Planters Tree planting

Disconnection credits

Expanded Tree Pits Dry Swales

Green Street Retrofits

Green Roofs • Extensive or Intensive • Structural design

considerations • High installation cost • Increased roof longevity • Additional urban environmental

benefits • Can be major element of

compliance at urban redevelopment sites

• 70% of Bay engineers have never designed one

Feasibility and Applications Structural Capacity of the Roof. Must be designed to support an additional 15 to 30 pounds per square foot (psf) for an extensive green roof. Roof Pitch. Storage volume is maximized on relatively flat roofs (a pitch of 1 to 2%). Green roofs can be installed on steeper rooftops if baffles, grids, or strips are used. Roof Access. Adequate access to the roof must be available to deliver construction materials and perform routine maintenance, either by an interior stairway roof hatch or trap door of minimum dimensions. Roof Type. Certain roof materials, such as exposed treated wood and uncoated galvanized metal, may not be appropriate Setbacks. Green roofs should not be located near rooftop electrical and HVAC systems. A 2-foot wide vegetation-free zone is recommended along the perimeter of the roof, with a 1-foot vegetation-free zone around all roof penetrations, to act as a firebreak.

• Lightweight media mixture, low compost

• Design per ASTM specifications

• Min 4” media plus 2” drainage layer

• DC Retention Credit: 0.25” per inch of media depth

Green Roof Plants

Plant Light Moisture

Requirement Notes

Delosperma cooperii Full Sun Dry Pink flowers; grows rapidly

Delosperma 'Kelaidis' Full Sun Dry Salmon flowers; grows rapidly Delosperma nubigenum '' Full Sun Moist-Dry Yellow flowers; very hardy Sedum album Full Sun Dry White flowers; hardy

Sedum lanceolatum Full Sun Dry Yellow flowers; native to

Sedum oreganum Part Shade Moist Yellow flowers; native to

Sedum stoloniferum Sun Moist Pink flowers; drought tolerant

Sedum telephiodes Sun Dry Blue green foliage; native to region

Sedum ternatum Part Shade-Shade Dry-Moist White flowers; grows in shade

Talinum calycinum Sun Dry Pink flowers; self sows Note: Designers should choose species based on shade tolerance, ability to sow or not, foliage height, and spreading rate. See Snodgrass and Snodgrass (2006) for definitive list of green roof plants, including accent plants.

Maintenance Considerations

Activity Schedule • Water to promote plant growth and survival. • Inspect the green roof and replace any dead or

dying vegetation.

As Needed (Following

Construction) • Inspect the waterproof membrane for leaking

or cracks. • Weeding to remove invasive plants (no digging

or using pointed tools). • Inspect roof drains, scuppers and gutters to

ensure they are not overgrown or have organic matter deposits. Remove any accumulated organic matter or debris.

• Inspect the green roof for dead, dying, or invasive vegetation. Plant replacement vegetation as needed.

Semi-Annually

Rainwater Harvesting

March 9, 2011

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

Applicability/Feasibility • Minimal space or setback requirements. • Filters, pumps, and overflow devices are generally

necessary. • Risk Assessment needed to determine any treatment

requirements. • ESD reduction determined through cistern design

spreadsheet.

VADCR Cistern Design Spreadsheet

Simple inputs to optimize the size of tank based on indoor and outdoor water demand for the building

50%

55%

60%

65%

70%

75%

80%

85%

90%

0 20,000

Run

off R

educ

tion

Cre

dit (

%)

Cistern Storage Associated with Treatment Volume Credit (gallons)

Runoff Reduction Volume Credit Chart

Runoff Reduction Credit Chart

depends on size of tank and dedicated demand.

Rainwater Harvesting Demand

Also: Cooling towers Vehicle washing

Slow Bleed to Storm Drain System

• Constant drain from tank to storm drain at a gallon per minute dewaters a 3000 gallon tank in two days

One tenth the flow rate of a standard ½ inch garden hose

Maintenance Notes • Maintenance requirements vary with use.

Activity Frequency Keep gutters and downspouts free of leaves and other debris

O: Twice a year

Inspect and clean pre-screening devices and first flush diverters

O: Four times a year

Inspect and clean storage tank lids, paying special attention to vents and screens on inflow and outflow spigots. Check mosquito screens and patch holes or gaps immediately

O: Once a year

Inspect condition of overflow pipes, overflow filter path and/or secondary stormwater treatment practices

O: Once a year

Inspect tank for sediment buildup I: Every third year Clear overhanging vegetation and trees over roof surface I: Every third year Check integrity of backflow preventer I: Every third year Inspect structural integrity of tank, pump, pipe and electrical system

I: Every third year

Replace damaged or defective system components I: Every third year Key: O = Owner I = qualified third party inspector

Foundation Planters

Green Streets

Engineered Tree Pits

Source: City of Baltimore, MD

Blue Alleys

Actual – St. Louis Design – Baltimore

Construction Issues

Maintenance Issues

Stormwater Offset Fees

BMP Cost Research

Bioretention Costs

LID Economics • Lots of recent studies (about ten) • Most show LID cost-effective in

greenfields • Savings in pipes, paving, and storm drain

infrastructure • Actual LID practices cost more than

traditional ones (economies of scale) • Prototype effect: costs should drop

some

The Stormwater Version of the Price is Right Show

For the technical assumptions for the following cost projections, please consult Technical Bulletin No. 5

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

Suggested Stormwater Offset Fees

Fee Should be expressed in unit terms such as: Per pound of phosphorus to be removed Per acre of untreated impervious cover To be equitable, the minimum 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

Recommended Accountability Elements in Offset Programs

• 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

Questions and Answers