Mission: Stormwater Management Technology Pilot Projects, Monitoring, Modeling, Manuals, Training, Education The Low Impact Development Center, Inc. Balancing Growth and Environmental Integrity LID Modeling

Why are We Modeling? Determine Effectiveness Predict/Project (Pre-Post) Calibration “It’s Cheaper than Doing anything” NC State needs more PhD’s Regulatory Requirements Resource Protection

Hard Stuff Peak Flow Water Quality Energy (Light, Thermal, Stream Power) Habitat Optimization Maintenance Optimization Easy Stuff

How well do we maintain the ecological integrity (functions) of aquatic systems (small streams)? ChemicalVariables FlowRegime HabitatStructure EnergySources BioticFactors Nutrients Temperature D.O. pH Turbidity Organics Toxics Disease Reproduction Feeding Predation Competition Sunlight Nutrients Seasonal Cycles Organic Matter 1&2 Production Canopy Siltation Gradient Substrate Current Instream Cover Sinuosity Width/Depth Channel Morphology Soils Stability Riparian Vegetation Velocity Frequency Runoff Evaporation Ground Water Flow Duration Rain Intensity Scale / Spatial / Temporal / Species Ecosystem Integrity

Performance Measures Volume (Seattle Sea Streets) Prescriptive or Presumptive Load vs. Percentage Flow Rate (Discrete or Continuous)

Taxonomy and Classification Design Models Calibration Models Coordinating Needs Critical

Early Models

Simple WQ Stuff L = Load, P=Precipitation

Courtesy Geoanalysis

Today Moving towards Tributary Strategies Loadings and Limits (303d) Site Design Models don’t link to Watershed Models Rapid Assessments that may have significant data or science gaps Costs and Predictability unknown

Civil to Environmental Q = CIA

Bioretention

RECHARGE HYDROLOGIC CYCLE: P+R+E+T

Optimization A bioretention pond costs $2,000 to construct for a ½ site. So it costs $4,000 per acre. First, it’s a cell not a pond!! Region of Feasibility Gazillions of Acceptable Solutions

Conventional Pipe and Pond Centralized Control “Efficiency”

LID Uniform Distribution of Micro Controls

McCuen 2002 III II I Runoff Hydrograph is Sensitive to Bioretention Placement in II only 7 3 I II III

Hoffman and Crawford, 2000

Existing Problems and Future High Risks Hoffman and Crawford, 2000

Remove Large Impervious Areas Oversize Pipes Comparison of Conventional and LID Strategies

An estimate of imperviousness can be derived directly from the satellite image for developed areas. (Water bodies from the USGS topographic maps are overlaid for orientation, and areas identified as undeveloped in the National Land Cover dataset are left white.)

Soil moisture maps can be generated using the vegetation and surface temperature data with a surface climate model. The gray-scale image is dark for surfaces with a dried out top layer and bright or white for surfaces that are wet. This information can be used to locate areas with very moist surface layers near identified wetlands that can be easily converted to wetlands themselves. woody wetlands classified by the National Land Cover Dataset (NLCD ) forested, non-tidal wetlands classified by the Army Corps of Engineers

Defining LID Technology Major Components 1. Conservation (Watershed and Site Level ) 2. Minimization (Site Level) 3. Strategic Timing (Watershed and Site Level) 4. Integrated Management Practices (Site Level) Retain / Detain / Filter / Recharge / Use 5. Pollution Prevention Traditional Approaches

“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

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, LID-CN 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 A1A1 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

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

Disconnecting Impervious Areas to Reduce CN CN c = CN p + (P imp /100) (98-CN p ) (1-0.5 R) Where: CN c = Composite Curve Number CN p = Pervious Curve Number P imp = Percent Impervious R = Ratio of Disconnected to Total Imperviousness

Comparison of Conventional and LID Site Conditions

Comparison of Conventional and L I D Curve Numbers (CN) for 1- Acre Residential Lots Conventional CN 20 % Impervious 80 % Grass Low Impact Development CN 15 % Impervious 25 % Woods 60 % Grass Curve Number is reduced by using LID Land Uses.

8% BMP Determining LID BMP Size Basic Idea: If you can maintain Tc, storage capacity can be based on curve number difference only.

LID Manual H and H Process

Comparison of Conventional and L I D Curve Numbers (CN) for 1- Acre Residential Lots Conventional CN 20 % Impervious 80 % Grass Low Impact Development CN 15 % Impervious 25 % Woods 60 % Grass Curve Number is reduced by using LID Land Uses.

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

Source Node Gross Pollutant Trap Buffer Strip Vegetated Swale Infiltration Dry & Wet Detention Pond Wetlands

BMP Evaluation Computer Module Prince George’s County, Maryland

Target Pollutants Suspended Solids Nutrients –Nitrogen (nitrate, ammonia, organic nitrogen) –Phosphorus Metals (copper, lead, zinc) Oil & Grease

HSPF LAND SIMULATION – Unit-Area Output by Landuse – BMP Evaluation Method 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 N.B.: Good design may need to go beyond period of record.

Phosphorus Lead Calibrated BMPs!!!

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

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

Soil moisture maps can be generated using the vegetation and surface temperature data with a surface climate model. The gray-scale image is dark for surfaces with a dried out top layer and bright or white for surfaces that are wet. This information can be used to locate areas with very moist surface layers near identified wetlands that can be easily converted to wetlands themselves. woody wetlands classified by the National Land Cover Dataset (NLCD ) forested, non-tidal wetlands classified by the Army Corps of Engineers

Ground Truthing Image

Moving From Calibrated “Sophisticated” Watershed Models to Design Tools

Milwaukee Metropolitan Sewer District (MMSD) LID Model Ideas from LID awareness conference/seminar Local Builder liked it and is using it School programs on planting trees and reducing asphalt Keeper of the mega model liked idea

Model Highlights Need retrofit technologies for highly impervious areas Need retrofit approach for build out of conventional and centralized areas Need simple analysis tools for engineers

Highlights of approach Volume based to control low peak rate (x cfs/acre) Uses NRCS Methods Outputs to NRCS Methods Includes “Typical LID CNs Limited Input Graphical Output Minimal Training Transparent (No Big Black Box)

IMP’s CN’s Event and Area Uncontrolled

Instant Graphing of Results (including detention/retention)

Readhyd Card for TR-20

Limitations Accounting on a watershed scale Link to WQ model and WQ calcs ROUTING!!! No groundwater recharge End of pipe answer Size (15 meg) no ing here

Who else is doing this type of Modeling Stuff New Jersey (includes groundwater recharge component, YEAH!!!) North Carolina Delaware (DURMM) Virginia COE (Checklist) Puget Sound (HSPF) WinSlamm Proprietary Engineering Vendors (PCSWMM for Pavers Unilock™)

Where to go from here? Multimedia and calibrated models Clearinghouses and Databases on effectiveness (ASCE) More rigorous calibrated models Optimization!!! Better Costing