Stormwater Design for Redevelopment in Urban Watersheds

Transcription

1 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

2 Speaker Info Tom Schueler Chesapeake Stormwater Network 117 Ingleside Avenue Baltimore, MD Bill Stack Center for Watershed Protection 8390 Main Street Ellicott City, MD

3 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

4 The skinny on redevelopment in Maryland A copy of May 2010 emergency regs can be found in Resource 1

5 Redevelopment & Stormwater in Maryland PREVIOUSLY ( ) 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.

6 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

7 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

8 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)

9 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)

10 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)

11 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

12 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)

13 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

14 Step 2: Calculate Site Imperviousness and Water Quality Volume, WQv 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 Site Area, A (acres) 4 Existing Impervious Surface Area (acres) 2.5 Proposed Impervious Surface Area (acres) 3 Existing Imperviousness, I pre 62.5% Proposed Imperviousness, I post 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, P E % Soil Type A 0% % Soil Type B 60% % Soil Type C 40% % Soil Type D 0% Pre-Developed Condition, RCN woods 61 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

15 Many Bay States and Cities are Enhancing Stormwater Requirements at Redevelopment Sites For a comparative review, check out Resource No. 3

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

17 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

18 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?

19 The Degree of Difficulty Redevelopment Intensity (Post Development IC) Less than 40% Alternate Surfaces 40 to 65% 66 to 85% 85 to 100% Alternate Surfaces Alternate Surfaces Alternate Surfaces Landscaping ESD Landscaping ESD Landscaping ESD Landscaping ESD IC Reduction IC Reduction Micro ESD Disconnections Micro ESD

20 Why Redevelopment is So Important Incrementally Reduces Untreated Pollution from Existing Development Green Building and Green Infrastructure Movement Sustainable Cities Combined Sewer Overflow Abatement

21 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

22 Street Dirt Contains Many Harmful Pollutants

23 City Runoff Can Be More Polluted than Suburban Runoff 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)

24 City Runoff Has a Very High load of Trash and Floatables

25 Stormwater Runoff is a Leading Cause of Water Quality Impairment

26 Stormwater Compounding ( 5 to 59% IC treated in 25 years) Source: Philadelphia OW

27 Ten Strategies to Integrate Redevelopment with Stormwater

28 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

29 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

30 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

31 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

32 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

33 6. Decompose Site Into Smaller Drainage Units

34 Source: COE,(2005)

35 Source: COE,(2005)

36 7. Roof to Street Design Approach

37 8. Maximize Forest Canopy and Restore Natural Area Remnants

38 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)

39 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

40 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

41 otos: Chesapeake Bay Program Questions and Answers

42 Sustainable Stormwater Practices for the City Green Roofs * Cisterns and Rain Tanks * Permeable Pavers * Bioretention * Expanded Tree Pits Urban Tree Planting Foundation Planters Green Streets After: Courtyard bioretention Sand Filters

43 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

44 Rain Tanks and Cisterns Redevelopment intensity means more internal demand for nonpotable 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

45 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

46 Foundation Planters

47 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)

48 Impervious Cover Removal

49 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

50 Reforestation and Street Tree Credit

51 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

52 Expanded Tree Pits

53 Source: City of Baltimore, MD

54 Design and Construction Issues

55 Underground Sand Filter and Proprietary Practices

56 Green Street Bioretention:

57 Lessons Learned in Green Streets in Baltimore

58 Green Streets are Popular in Neighborhoods Figure 9: Sidewalk enhancements on Collington between Lombard and Baltimore Streets Figure 10: Bump out at Collington and Lombard Street

59 Tree Box Inlet with Curb Extension and Bioretention

60 During Construction B-15: Tree Box Inlet with Curb Extension and Bioretention Post- Construction Pre- Construction

61 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

62 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

63 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

64 The Price of Stormwater Variation Due to: IC and CDA New vs Redevelopment Design Era Your Agenda

65 The Stormwater Version of the Price is Right Show For the technical assumptions for the following cost projections, please consult resources 7 and 8

66 What is the Price of Treating a Cubic Foot of Stormwater? $ $10.00 $ $25.00

67 What is the Cost to Treat: $191,000 $31,700 One Acre of IC of Urban Redevelopment One acre of IC at Pre-ESD Greenfield Development $46,500 $32,500 One acre of IC at Greenfield Development to ESD One Acre of IC with Storage Retrofits

68 What is the Cost to Treat: $167,120 $35,600 $167,100 One acre IC with Green Streets Stream Restoration in length equivalent to one acre IC, expressed in terms on nutrient load

69 Setting up a stormwater offset 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

70 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

71 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

72 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

73 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

74 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)

75 Questions and Answers

76 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