stormwater design for redevelopment in urban...
TRANSCRIPT
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
Bill StackCenter for Watershed
Protection8390 Main Street
Ellicott City, MD 21043
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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Bay P
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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