Graphical Peak Discharge MethodThe EquationIn the Graphical Peak Discharge Method, runoff is calculated using the following formula:qp = qu Am Q FpWhere:qp = peak discharge (ft3/s, also written as cfs)qu = unit peak discharge (cfs/mi.2/in., also written as csm/in) Am = drainage area (mi.2) Q = runoff (in.)Fp = pond and swamp adjustment factor (no units)ProcedureThe steps used to calculate runoff using the Graphical Peak Discharge Method are as follows:Step 1 - Measure the drainage area (Am).To translate the drainage area from acres to square miles, use the following equation:Drainage area (acres) 0.00156 = Drainage area (mi.2)You will also need to know the drainage area in square feet in order to calculate total runoff volume. To translate drainage area from acres to square feet, use the following equation:Drainage area (acres) 43,560 = Drainage area (ft.2)Step 2 - Calculate a curve number (CN) for the drainage area.In order to calculate the CN, you will need to know the soil type(s) and hydrologic soil group of each soil type. You also need to know the land use and vegetative cover. Then you can determine the CN using Table 5.Curve numbers for heterogeneous watersheds may be determined by dividing the watershed into homogeneous sub-areas and performing a weighted average as in the rational method. Step 3 - Determine runoff depth and volume for the design storm.First, the rainfall depth (P) is found using Diagram 5. Then the rainfall de ...
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