Best Management Practices: Evaluating the Benefits

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Best Management PracticesEvaluating the Benefits
An Interactive Tool for Performing a
Comprehensive Site Assessment
Developed by Tetra Tech, Inc. for the Prince
Georges County Department of Environmental
Resources
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BMP Evaluation Method
Existing Flow Pollutant Loads
HSPF LAND SIMULATION
Unit-Area Output by Landuse
SITE-LEVEL LAND/BMP ROUTING
BMP Module
Simulated
Surface Runoff
BMP DESIGN
Simulated Flow/Water Quality Improvement
Cost/Benefit Assessment of LID design
Site Level Design
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BMP Module

• Input Data
• Surface runoff flow and pollutant loads from
  contributing areas in site

• BMP Simulation
• User designs/selects each BMP and defines the
  flow routing at the site
• Flow and pollutant time series routed through the
  BMP or IMP network
• Physical processes are simulated in the BMP
• Output timeseries and data summary available at
  each land, BMP, or watershed outlet

BMP Module
BMP DESIGN
Site Level Design
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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
  (Underdrain outflow)
• Depending on the design and type of the BMP, any
• combination of processes may occur during
• simulation

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BMP Class A Storage/Detention
Evapotranspiration
Outflow
Inflow
Modified Flow Water Quality
From Land Surface
OverflowSpillway
Storage
Bottom Orifice
Underdrain Outflow
Infiltration
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BMP Class B Open Channel
Outflow
Inflow
From Land Surface
Evapotranspiration
Overflow at Max Design Depth
Open Channel Flow
Modified Flow Water Quality
Underdrain Outflow
Infiltration
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BMP Class A Example
Bioretention
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BMP Class B Example
Grassed Swale
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BMP Underdrain System
Holtan Infiltration Method
Soil and subsurface flow configuration is
available for both Class A and Class B BMPs
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LID Modeling and Evaluation

• BENCHMARK (example)
• Representative pre-development condition is
  runoff from a forested site of the same area

• LID ECONOMICS
• User can weigh costs of BMP implementation or
  retrofit against long-run environmental benefits
  and cost savings
• RATIONALE
• The closer to pre-development conditions, the
  more credits received
• Optimum BMP Design Low cost high environmental
  benefit

Simulated Flow/Water Quality Improvement
Cost/Benefit Assessment of LID design
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For More Information, please contact

• Dr. Leslie Shoemaker or John Riverson
• Tetra Tech, Inc.
• 10306 Eaton Place, Suite 340
• Fairfax, VA 22030
• [phone]