Calculation of Total Event Flows.

A generally accepted method used to compute runoff from small urban catchments is found in Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs, published by the Metropolitan Washington Council of Governments. This method is commonly known as the Schueler Method. This method uses the following equation:

R = (0.05 + (0.009 x I)) x P/12


I = imperviousness of the drainage area (percentage of pavement and roof areas within total area contributing runoff to the outfall.) Example: Input 85% as 85.

P = precipitation in inches. Input in decimal form. Example: Input 1" as 1.25.

R = runoff, in cubic feet per square foot of drainage area.

Thus the total flow volume for the outfall of concern can be calculated:

Outfall: SDO-_______

Vtotal = R x (Area in square feet draining to Outfall).

Vtotal = ((0.05 + (0.009 x I)) x P/12) x (Area in square feet draining to Outfall), or

Vtotal = _____________ cubic feet

The monitoring report form supplied by the NCDWQ requests that the total flow volume be reported in units of million gallons (MG). Multiply Vtotal by (7.48 x 10-6) to change from cubic feet units to MG units.

Vtotal = _____________ MG




  1. One acre equals 43,560 square feet
  2. (7.48 x 10-6) equals 7.48 divided by 1,000,000

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