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 Chesapeake Stormwater Network 117 Ingleside Avenue Baltimore, MD 21228 watershedguy@hotmail.com www.chesapeakestormwater.net Bill Stack Center for Watershed Protection 8390 Main Street Ellicott City, MD 21043 bps@cwp.org 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?

• 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

• ur 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 ESD Landscaping 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

• n the site
• Fees are used for retrofit
• r restoration projects in

the same watershed

Photos: Chesapeake Bay Program

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-Way Highway Design Street Lighting Traffic Engineering ESC Wastewater Engineering Stormwater Engineering Traffic 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

• n nutrient load

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

Setting up a stormwater

• ffset 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 removed Per 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