low impact development robb lukes low impact development center june 2008 analysis and design

objectives  design storm approaches  modeling options  design case studies  bmp components  cost analysis  construction and maintenance

Q T Developed Condition, Conventional CN (Higher Peak, More Volume, and Earlier Peak Time) Existing Condition Hydrograpgh Pre/ Post Development Losses

Q T Developed Condition, with Conventional CN and Controls Existing Additional Runoff Volume Developed Existing Peak Runoff Rate Detention Peak Shaving

Q T Developed Condition, with LID- CN no Controls. Existing Reduced Runoff Volume Developed- No Controls Reduced Q p Minimize Change in Curve Number

Q T Developed, LID- CN no controls same Tc as existing condition. Existing More Runoff Volume than the existing condition. Developed, no controls Reduced Q p Maintain Time of Concentration

Q T Provide Retention storage so that the runoff volume will be the same as Predevelopment Retention storage needed to reduce the CN to the existing condition = A 2 + A 3 A3A3 A2A2 A1 Reducing Volume

Q T Provide additional detention storage to reduce peak discharge to be equal to that of the existing condition. Existing Predevelopment Peak Discharge Detention Storage

Q T Comparison of Hydrographs A2A2 A3A3 LID Concepts Conventional Controls Existing Increased Volume w/ Conventional

design storm approaches single event vs. continuous simulation - peak flows / - volume / hydromodification flooding- water quality

hydrologic analysis  NRCS (SCS) methods  curve numbers (0-100)  storm type (I, IA, II, III)  time of concentration  computed process simulations  ex. Hydrologic Simulation Program – Fortran (HSPF)

Models Adaptable Stormwater Models EPA Stormwater Management Model (EPA SWMM) Source Loading and Management Model (SLAMM) Prince George’s County BMP Evaluation Module Western Washington Hydrology Model (WWHM3) / Bay Area Hydrology Model (BAHM) LID Sizing Tools Contra Costa Integrated Management Practice Calculator California Runoff Volume Calculator National LID Manual Technique

EPA Stormwater Management Model (EPA SWMM) DeveloperUS EPA; Oregon State U.; Camp, Dresser and McKee (CDM) Rainfall ModeledSingle Event and Continuous Watershed SizeLarge to Small Watersheds Primary UseFlooding, Quantity, and Quality Land Use & Source Area User defined land uses and source areas Application to LIDCan be adapted to simulate LID controls, models storage and infiltration processes

Source Loading and Management Model (SLAMM) DeveloperDr. Robert Pitt, U of Alabama; John Voorhees RainfallContinuous Watershed SizeLarge to Small Watersheds Land UsesResidential, Commercial, Industrial, Highway, Institutional, and other Urban Source AreasRoofs, Sidewalks, Parking, Landscaped, Streets, Driveways, Alleys, etc. Primary UseRunoff Quantity and Quality Application to LIDInfiltration, Wet Ponds, Porous Pavement, Street Sweeping, Biofiltration, Vegetated Swales, Other Urban Control Device

Prince George’s County BMP Evaluation Model DeveloperTetra Tech Inc.; Prince George’s County RainfallContinuous Watershed SizeSite Level to Small Watersheds Land UsesLow-Medium-High Density Residential, Commercial, Industrial, Forest, and Agriculture Source AreasImpervious or Pervious Primary UseRunoff Quantity and Quality Application to LIDRetention and conveyance options can be adapted to simulate various LID practices

Western Washington Hydrology Model (WWHM3) / *Bay Area Hydrology Model (BAHM) DeveloperWashington State Dept. of Ecology; AQUA TERRA Consultants; and Clear Creek Solutions, Inc. RainfallContinuous Watershed SizeLarge to Small sites in 19 Counties of Western Washington Primary UseRunoff Quantity (Evapotranspiration, Surface Flow, Interflow, Groundwater Flow) Application to LIDPonds, Infiltration Trenches/Basins, Wetlands, Sand Filter, Gravel Trench Beds, Vaults/Tanks, Swales, Green *Model to be released in July 2007

Contra Costa Integrated Management Practice Sizing Calculator DeveloperContra Costa Clean Water Program; Dan Cloak Environmental Consulting Rainfall35 years of continuous Contra Costa rainfall used to sizing factors Watershed SizeSmall Sites in Contra Costa County Primary UseSimplified method for IMP sizing Application to LID Limited sizing options are available for permeable pavement, dry wells, infiltration trenches/basins, vegetated or grassy swales, bioretention, and in- ground planters

National LID Manual Technique DeveloperUS EPA; Prince George’s County RainfallSingle Event Watershed SizeSmall Sites in Contra Costa County Primary UseEstimates a developed sites retention and detention storage requirements Application to LID Applies to any IMP with retention storage: bioretention, infiltration, porous pavement, swales, and planters

Case Studies used to Demonstrate Models Suburban Commercial Site Contra Costa IMP Sizing Calculator SWMM SLAMM Metro West: Dense Urban Site SWMM SLAMM Village at Watt’s Creek: Traditional Neighborhood Development SLAMM Prince George’s Nomograph Oak Creek and Government Center Prince George’s County BMP Evaluation Model

Typical Suburban Commercial Site Existing: Wooded Proposed: 4.0 Acre Commercial Site: 2.25 Acres of Impervious Cover, and 1.75 Acres of Landscaping

Suburban Commercial Site Demonstration Goals This site represents a small office park, retail, or other commercial project common to green field and fringe development. Numerous LID options are available for this type of development, including: swales, bioretention, permeable pavements, cisterns, and flow through planters.

Suburban Commercial Site Modeling Objectives Maintain Pre-Development Peak Flows Reduce, Treat, and Retain Site Pollutants Groundwater Recharge Size Best Management Practices to Meet California Stormwater Requirements

Suburban Commercial Site Contra Costa IMP Sizing Calculator To meet Contra Costa County technical requirements for flow and treatment the following IMP sizes were calculated: Bioretention cell must be sized to 1832 sf w/ underdrain 420 linear ft of vegetated swales to treat and retain permeable /impervious parking lot. IMP design criteria are stated in Appendix C of the Contra Costa County Stormwater C.3 Guidebook

Suburban Commercial Site LID Options Selected Source AreasBest Management Practice Roof (20000 sf) Sidewalk (2700 sf) Bioretention Cell w/ Underdrain -Contra Costa Sizing Tool: 1800 sf -3 ft of media depth -0.5 ft of surface storage depth Parking Lot and Loading Area (70,000 sf) Permeable Pavement sf, located in outer parking spaces -2.5 ft of aggregate depth Grassed Swale -420 linear ft -4 ft bottom width Landscaping Maintain Native Soil Structure Avoid Compaction Deep Soil Aeration

Suburban Commercial Site Modeling Results Rainfall: 1997 Continuous Historical Rainfall for Los Angeles CA. SWMMSLAMM Evapotrans. (acre-ft) Infiltration (acre-ft) Runoff (acre-ft) Pre-Developed Post-Developed Post-Developed w/ LID Reduction in Runoff w/ LID--- 79%75%

Typical Suburban Commercial Site 4 Acre Site C Soils Predevelopment: 4 Acres of Forest Proposed: Commercial Office Park 2.20 Acres of Imp. Cover 1.25 Acres of Landscaping 0.55 Acres of Forest

Suburban Commercial Site Demonstration Goals This site represents a small office park, retail, or other commercial project common to green field and fringe development. Numerous LID options are available for this type of development, including: swales, bioretention, permeable pavements, cisterns, and flow through planters.

Suburban Commercial Site Modeling Objectives Maintain Pre-Development Peak Flows Reduce, Treat, and Retain Site Pollutants Groundwater Recharge Size Best Management Practices to Meet Washington Department of Ecology Standards

Suburban Commercial Site LID Options Selected Source AreasBest Management Practice Roof (0.5 acres) Bioretention Cell w/ Underdrain -2.5 ft of media depth -0.5 ft of surface storage depth Parking Lot and Loading Area (1.6 ac) Permeable Pavement -1.5 ft of aggregate base/subbase Sidewalk (0.1 ac) Landscaping (1.25 ac) Native Landscaping - sidewalk drains to landscaped area - amend soils - avoid compaction - deep soil aeration

Suburban Commercial Site Rainfall Data Site Location: Issaquah, WA Rain Gage: SEATAC Precip. Factor: 1.333

Suburban Commercial Site Pre-Developed Condition

Developed – No Mitigation Half Permeable Pavement Parking Lot Full Permeable Pavement Parking Lot Half Per. Pvt. Lot & Bioretention Cell for Roof Runoff Suburban Commercial Site Half Per. Pvt. Lot & Amended Landscaping Soil

Suburban Commercial Site Peak Flow Results 2 yr-24 hr (cfs) 10 yr-24 hr (cfs) 100 yr-24 hr (cfs) Pre-Developed – Forest Condition Developed – No Mitigation Half Permeable Parking Lot Full Permeable Parking Lot Half Permeable Parking Lot & Landscaping Amended Soils Half Permeable Parking Lot & Bioretention Cell for Roof Runoff

Suburban Commercial Site Peak Runoff Results

Suburban Commercial Site Yearly Peak Runoff Results ( )

WWHM3: Water Balance Comparison

Suburban Commercial Site Modeling Results SWMMSLAMM Evapotrans. (acre-ft) Infiltration (acre-ft) Runoff (acre-ft) Pre-Developed Post-Developed Post-Developed w/ Per. Parking Lot and Bio. Cell Reduction in Runoff w/ LID--- 79%75% Rainfall Data Used: Los Angeles 1997

Metro West Medium to High Density Mixed Use Development Existing: Low Density Residential Proposed: 52 Acre Pedestrian and Transit Oriented Mixed-Use Community: Townhomes, Condominiums, Apartments, Retail, Offices, and Public Spaces Proposed LID: Bioretention, Permeable Pavement, and Green Roofs

Metro West Demonstration Goals A high density development like Metro West may reduce the overall footprint of development, but it is at an extremely high density that will result in high runoff volume and peak rates and concentrated pollutant loads. Modeling will show that strategically placed and integrated best management practices will reduce or eliminate the need for large stormwater infrastructure. Source: Pulte Homes Corporation, Inc.

Metro West Modeling Objectives Maintain Annual Load (Volume, Pollutants) Manage Peak Storm Events (2-, 10-, and 100- yr. 24-hour) Infrastructure Requirements per design manual and physical limitations BMP Sizing based on current regulations

Metro West SWMM Model Results Rainfall Data Used: 1992 Washington Dulles Intl. Rain Gage (total of 41.26”) Runoff (acre-ft) Pre-Developed6.2 Existing24.2 Post-Developed w/ SWM76.4 Post-Developed w/ SWM & LID58.5 Reduction in Runoff w/ LID77%

Metro West SWMM Peak Discharge Results for a 2yr-24hr storm ConditionAreas A & B (cfs) Area C (cfs) Area D (cfs) Without LIDInflow Outflow With LIDInflow Outflow % Reduction in Outflow w/ LID 11%19%43%

Metro West SWMM Peak Discharge Results for a 10yr-24hr storm ConditionAreas A & B (cfs) Area C (cfs) Area D (cfs) Without LIDInflow Outflow With LIDInflow Outflow % Reduction in Outflow w/ LID 22%39%12%

Village at Watt’s Creek LID Options Rain Barrels Bioretention Cells Permeable Driveways/Alleys Street Planters

Village at Watt’s Creek SLAMM Analysis Scenario Catchbasin With Sumps Residential Downspout Disconnection Residential Bioretention Cells Residential Rain Barrels Permeable Pavement for Alleys and Driveways Street Bioretention Planters No BMPs  #1 – All BMPs  #2 – Bio. Cells  #3 – Rain Barrels  #4 – Permeable Pvt.  #5 – Street Planters 

Village at Watt’s Creek SLAMM Runoff Reduction

National LID Manual Techniques Based on NRCS Methods Uses peak storm event Q/V relationships Nomographs that reflect Graphical Peak Discharge Method

8% BMP Determining LID BMP Size

Village at Watt’s Creek National Manual Method Existing CN: 70 Proposed CN: 85 Required Retention Storage Volume =(0.98in)(1ft/12in) (55 ac.) = 4.5 ac.-ft Maintaining Pre-development Runoff Volume

Village at Watt’s Creek National Manual Method Existing CN: 70 Proposed CN: 85 Required Retention Storage Volume =(0.75in)(1ft/12in) (55 ac.) = 3.4 ac.-ft Maintaining Pre-development Peak Runoff Rates

Initial BMP Input Values BMP-A Calculation BMP-B Calculation Evaluation Factor Calculation Routing Sequence Cost Calculation BMP Model Runner Initial Run Results: Evaluation Factor & Cost Values New changeable variable values BMP Optimizer Solution Changeable Variables (C810) (num, min, max, increment) Control Target (C830) Input File Loader Evaluation Factor & Cost Values feed new possible solution to runner based on previous simulation result BMP Model Input File ArcGIS Map Interface BMP Simulation/Optimization DLL Prince George’s County LID Model

HSPF LAND SIMULATION – Unit-Area Output by Landuse – PG BMP Evaluation Model Existing Flow & Pollutant Loads Simulated Flow/Water Quality Improvement Cost/Benefit Assessment of LID design BMP DESIGN – Site Level Design – SITE-LEVEL LAND/BMP ROUTING Simulated Surface Runoff

HSPF Land Use Representation

BMP Physical Processes Possible storage processes include: Evapotranspiration Infiltration Orifice outflow Weir-controlled overflow spillway Underdrain outflow Bottom slope influence Bottom roughness influence General loss or decay of pollutant (Due to settling, plant-uptake, volatilization, etc) Pollutant filtration through soil medium (Represented with underdrain outflow) Depending on the design and type of the BMP, any combination of processes may occur during simulation

Overflow Spillway Bottom Orifice Evapotranspiration Infiltration Outflow : Inflow: Modified Flow & Water Quality From Land Surface Storage BMP Class A: Storage/Detention Underdrain Outflow

BMP Class B: Open Channel Outflow: Inflow: From Land Surface Overflow at Max Design Depth Open Channel Flow Evapotranspiration InfiltrationUnderdrain Outflow Modified Flow & Water Quality Modified Flow & Water Quality

Holtan Infiltration Model veg. parameter void fraction soil porosity soil f c D u (A) background f c D s

Phosphorus Lead Calibrated BMPs!!!

General Water Quality First Order Decay Representation Mass 2 = Mass 1 x e – k t Pollutant Removal is a function of the detention time

Underdrain Water Quality Percent Removal Mass out = Mass in x (1 - PCTREM) Underdrain percent removal is a function of the soil media Mass in = Surface conc * underdrain flow

Government Center Plan

Assessment and Outreach Community meetings Knocking on doors Multimedia outreach Tie-ins to other events Garden clubs Churches Cleanups Neighborhood walkthroughs Identification of hot spots ArcPad mapping

Model Screenshot

Government Center Results

Future Directions More GIS integration with modeling software EPA SWMM EPA Study on SWMM BMP Modeling Improvements Interface w/ SLAMM July 2007, Bay Area Hydrology Model becomes available to the public

Conclusions Continuous hydrologic simulation needed to evaluate stormwater treatment effectiveness. The majority of runoff and stormwater pollution come from storms of 0.5” or less. No runoff model is perfect. Factors to consider when choosing a model: Goals (flow, quantity, quality) User’s Skill Level Project Size and Complexity LID Modeling Capability Available Precipitation Data Cost Optimization New regional models and tools are linking LID integration with regulatory compliance in a simple and easy to use way. Recognize model limitations in results analysis

Construction

Maintenance

Regulatory Implications Continuous hydrologic simulation needed to evaluate stormwater treatment effectiveness. The majority of runoff and stormwater pollution come from storms of 0.5” or less.

Q T Hydrograph Summary Existing Developed, conventional CN, no control. Developed, conventional CN and control. Developed, LID-CN, no control. Developed, LID-CN, same Tc. Developed, LID-CN, same Tc, same CN with retention. Same as, with additional detention to maintain Q Pre-development Peak Runoff Rate

“Initial” Low-Impact Development Hydrologic Analysis and Design Based on NRCS technology, can be applied nationally Analysis components use same methods as NRCS Designed to meet both storm water quality and quantity requirements

Metro West Medium to High Density Mixed Use Development Existing: Low Density Residential Proposed: 52 Acre Pedestrian and Transit Oriented Mixed-Use Community: Townhomes, Condominiums, Apartments, Retail, Offices, and Public Spaces Proposed LID: Bioretention, Permeable Pavement, and Green Roofs

Metro West Demonstration Goals A high density development like Metro West may reduce the overall footprint of development, but it is at an extremely high density that will result in high runoff volume and peak rates and concentrated pollutant loads. Modeling will show that strategically placed and integrated best management practices will reduce or eliminate the need for large stormwater infrastructure. Source: Pulte Homes Corporation, Inc.

Metro West Modeling Objectives Maintain Annual Load (Volume, Pollutants) Manage Peak Storm Events (2-, 10-, and 100- yr. 24-hour) Infrastructure Requirements per design manual and physical limitations BMP Sizing based on current regulations

Metro West: LID Options 9 iterations were modeled in SWMM to find an optimal sizing option for Bioretention Area and Permeable Pavement. Rainfall: Continuous 1997 LA Bioretention (as % of rooftop area) Bioretention Storage Depth (in) Permeable Pavement (as % of paved area) % Reduction in Runoff Volume Option Chosen for SLAMM Analysis

Metro West SWMM and SLAMM Model Results Rainfall: 1997 Continuous Historical Rainfall for Los Angeles CA. SWMMSLAMM Evapotrans. (acre-ft) Infiltration (acre-ft) Runoff (acre-ft) Pre-Developed Existing Post-Developed Post-Developed w/ LID Reduction in Runoff w/ LID--- 78%76%

Choosing SWMM or SLAMM SWMM Goals include: flow routing, peak flow, volume, and pollutant loads Complex site, many source area land types Inputs for BMP performance equations are available SLAMM Goals include: runoff volumes & pollutant loads Site has typical landuses Standard BMPs, including swales and street sweeping are used

Metro West SLAMM Pollutant Load Results Sediment (ton) Phosphorous (lbs) Zinc (lbs) Post-Developed Post-Developed w/ LID % Reduction w/ LID34%20%74%

Village at Watt’s Creek Traditional Neighborhood Development (TND) 55 acre site consiting of mixed-use buildings, townhomes, two-family, single family homes on small lots Other features, alley loaded lots, common green space, narrow and pedestrian friendly streets

Village at Watt’s Creek Demonstration Goals Traditional Neighborhood Developments are an increasingly popular alternative to conventional suburban development. TNDs in combination with LID can mitigate development impacts, protect water resources and provide livable communities. Source: DC Office of Planning, LID Center, and Washington Council of Governments

Village at Watt’s Creek LID Options Rain Barrels Bioretention Cells Permeable Driveways/Alleys Street Planters

Village at Watt’s Creek SLAMM Runoff Reduction

CA Spreadsheet Total Site Area (acres)5.74 Proposed Impervious Area (acres)2.13 Required Area for Non-Structural Treatment (acres)2.02 Required Volume for Structural Treatment (acre-ft)2.09 Credit Options Tree Planting0.30 Disconnect Rooftop Runoff0.46 Disconnect Non-Rooftop runoff0.06 Use of Grass Swales0 Sheet flow to streamway or setback0 Pervious pavers0.60 Green Roof0 Final Area (acres)0.60 Final Volume (acre-ft)0.62

Village at Watt’s Creek SLAMM Runoff Reduction Fully implementing all BMPs will be most effective at reducing runoff and pollutants but also most expensive. Review of flow patterns, BMP capacity, and BMP placement is time consuming but can narrow down the most cost effective combinations.

California Volume Runoff Calculator DeveloperCalifornia State Water Resources Control Board RainfallUses Annual Rainfall Volume and 24 hr – 85th Percentile Storm Event Watershed Size Small Sites Primary UseUses NRCS Method to calculate area required for non-structural and the required volume for structural treatment Application to LID Options limited to disconnected impervious surfaces, tree planting, pervious pavers, green roofs, stream buffer