Equation 1: SCS Graphical peak discharge equation
qu=unit peak discharge (csm/in or cfs per sq.mile of drainage area per inch of runoff)
Am=drainage area (mi2)
Fp=pond and swamp adjustment factor.
- Find out the initial abstraction for the rainfall amount, P, using the table provided below or using the formula S=[(1000/CN)-10] with Ia=0.2S.
- Then depending on your region of study, use one of the exhibit for either Type I, IA, II or III rainfall distribution for the time of concentration of your drainage area. This will give you Unit Peak Discharge.
- Then use the Equation 1 to compute the peak discharge in cfs.
- Done !!
Table for estimating Ia for given CN
|Curve Number||Ia (in)||Curve Number||Ia (in)|
Following limitations should be considered when using SCS graphical peak discharge method for estimating peak flows:
- Graphical method provides a determination of peak discharge only. This method cannot be used if you need a hydrograph or if the watershed subdivision is required
- The watershed must be hydrologically homogeneous, that is, describable by one CN. Land use, soils, and cover are distributed uniformly throughout the watershed.
- The watershed may have only one main stream or, if more than one, the branches must have nearly equal Tc' s.
- The method cannot perform valley or reservoir routing.
- The Fp factor can be applied only for ponds or swamps that are not in the Tc flow path.
- Accuracy of peak discharge estimated by this method will be reduced if Ia / P values are used that are outside the range given in exhibit 4. The limiting Ia / P values are recommended for use.
- This method should be used only if the weighted CN is greater than 40.
Exhibit 4-I from chapter 4 of TR-55 for Type I rainfall Distribution
Exhibit 4-IA from chapter 4 of TR-55 for Type IA rainfall Distribution
Exhibit 4-II from chapter 4 of TR-55 for Type II rainfall Distribution
Exhibit 4-III from chapter 4 of TR-55 for Type III rainfall Distribution