The Rational Method for Estimation of Design Surface Runoff Rate for Storm Water Control

The Rational Method is a commonly used procedure for estimating the peak surface runoff rate from an urban watershed for use in design of storm water management facilities, such as storm sewers, storm water inlets, and storm water detention ponds. The Rational equation is q = CiA, where q is the peak surface runoff rate in cfs, C is the runoff coefficient, i is the design rainfall intensity in in/hr, and A is the area of the watershed in acres. The time of concentration, design storm return period, and an IDF relationship for the region are needed to find the design rainfall intensity. The Rational Method works best for small urban watersheds of area less than 100 acres without surface storage such as ponds or swamps.