Stormwater Design for Redevelopment in Urban Watersheds June 10, - PowerPoint PPT Presentation

Stormwater Design for Redevelopment in Urban Watersheds June 10, 2010 Brought to you by the Chesapeake Bay Trust and the U of MD Mid-Atlantic Water Program

Speaker Info Tom Schueler Bill Stack Chesapeake Stormwater Center for Watershed Network Protection 117 Ingleside Avenue 8390 Main Street Baltimore, MD 21228 Ellicott City, MD 21043 watershedguy@hotmail.com bps@cwp.org www.chesapeakestormwater.net www.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? • G randfathering • 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 A revised version of the Proposed Impervious Surface Area (acres) 3 ESD to the MEP Existing Imperviousness, I pre 62.5% Proposed Imperviousness, I post 75.0% spreadsheet is provided in Resource 2 Development Category Redevelopment Rainfall Depth, P (in) 1.0 Several bugs were fixed Runoff Coefficient, Rv 0.73 including an error in ESD Water Quality Volume, WQv (ac-in) 2.90 Water Quality Volume, WQv (cf) 10,527 sizing for redevelopment Step 4: Calculate Environmental Site Design (ESD) Rainfall Target, P E Throw out your old % Soil Type A 0% version! % Soil Type B 60% % Soil Type C 40% % Soil Type D 0% Updated users guide to Pre-Developed Condition, RCN woods 61 be released next week New Development Soil Type A ESD Rainfall Target, P E (in) 0.00 Soil Type B ESD Rainfall Target, P E (in) 1.32 Soil Type C ESD Rainfall Target, P E (in) 0.80 Soil Type D ESD Rainfall Target, P E (in) 0.00 Site ESD Rainfall Target, P E (in) 1.80 ESD Runoff Depth, Q E (in) 1.31 ESD Runoff Volume, ESDv (cf) 18,949

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 to 65% 66 to 85% 85 to 100% 40% Alternate Alternate Alternate Alternate Surfaces Surfaces Surfaces Surfaces Landscaping Landscaping Landscaping ESD Landscaping ESD ESD 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

City Runoff Can Be More Polluted than Suburban Runoff Stormwater BALTIMORE National Pollutant 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 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 Uni ts