Stormwater BMP Interactive Model

The Stormwater BMP Interactive Model is intended to allow users to observe the general stormwater runoff reductions that can be gained by applying stormwater BMPs in urban areas. The model allows the application of various BMPs to percentages of the total surface area within the model view window. This is intended as a demonstration and general planning tool only. The underlying calculations upon which the model results are based are not rigorous technical calculations that would be needed to provide a detailed design and outcome analysis.

Landsat imaging at 30 meter resolution was used for the model map generation. Existing land use conditions are available within the model. For example:

  • NLCD_2001_Impervious_Surface: Provides the impervious surface coverage in the view window
  • NLCD_2001_Canopy: Provides the tree canopy coverage in the view window
  • NLCD_2001_Land_Cover: Provides overall land uses including impervious surface, tree canopy, and agriculture

The model uses several assumptions to calculate its results. Technical Release (TR-55): Urban Hydrology for Small Watersheds procedures are used for estimating runoff volumes. The TR-55 method estimates runoff depth with the following equation:


Ia = 0.2s
s = (1000/CN) - 10

Q = runoff depth
CN = overall curve number for the watershed
Ia = initial abstraction (inch), the losses before runoff begins (surface depressions, interception by leaves, evaporation, infiltration)
P = precipitation (inch)
Q = depth of runoff over entire watershed (inch)
s = potential maximum watershed water retention after runoff begins (inch)

Runoff volume is calculated as the runoff depth times the area:

Runoff volume in gallons = (Q/12) x Area in square feet x (7.48 gallons/1 cubic foot)

For the model calculations, each of the land uses and stormwater BMPs was assigned a Curve Number (CN). The curve numbers are used to estimate the characteristic runoff from a land surface. The curve numbers that were estimated for land uses and BMPs include the following:

Practice/Land Type     CN    
Impervious area 98
Downspout disconnection     80
Filter strips 80
Infiltration practices 40
Pocket wetlands 80
Porous pavement 40
Rain barrels/cisterns 75
Rain gardens 35
Soil amendments 60
Tree box filters 85
Vegetated roofs 75
Vegetated swales 60
Tree cover 70
Urban area 90

The weighted average curve number for the model viewing area is calculated using the ratio of land uses and BMPs and their respective curve numbers with the following equation:

Baseline conditions are calculated using the existing land use data contained within the model. Runoff conditions following the application of stormwater BMPs are calculated using the TR-55 equation and weighted average curve number.

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