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Table Of Contents
Using the TR-55 Graphical Method to Calculate the Peak Runoff Flow
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You can obtain the rainfall distribution amounts from the local SCS office (now
NRCS) in the form of a .rf file, or you can manually enter the data into the
Rainfall Distribution Editor.
Graphical Peak Discharge Method Formula
The TR-55 Graphical Peak Method determines a peak discharge rate. Use this
method for larger drainage areas and storm water detention calculations. This
TR-55 Method uses a 24-hour rainfall event to calculate runoff.
The Graphical Peak Discharge Method formula is as follows:
qp = qu Am QFp
where:
qp = Peak discharge (cfs)
qu = Unit peak discharge (cm/in)*
Am = Drainage Area (mi2)
Q = Runoff (in)
Fp = Pond and swamp adjustment factor
*cubic feet per second per square mile per inch of runoff [cfs/mi
2
in]
Calculating the Peak Runoff Flow by Using the
TR-55 Graphical Method
The Graphical Peak Discharge Method formula for runoff calculation is used to
calculate peak discharge runoff for a watershed without creating a hydrograph.
The TR-55 Graphical Peak Discharge Method calculator simplifies the process of
calculating peak runoff by doing the calculations for you. You enter the
required data (rainfall distribution type, runoff curve number, time of
concentration, pond and swamp area adjustment factor, and 24-hour rainfall
amount), and the calculator determines the peak flow value.
For more information on this method, see the SCS TR-55 manual, Chapter 4,
Graphical Peak Discharge Method.