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

**Where:**

**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 1¼" 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-_______**

V_{total} = R x (Area in square feet draining to Outfall).

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

V_{total} = _____________ 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 V_{total }by (7.48 x 10^{-6}) to change from cubic feet units to MG units.

V_{total} = _____________ MG

Notes:

- One acre equals 43,560 square feet
- (7.48 x 10
^{-6}) equals 7.48 divided by 1,000,000

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