Estimating Peak Runoff Rates Using the Rational Method

Hydrograph methods are used to identify functional dependencies of the rational-method runoff coefficient, and nested intensity-duration-frequency (IDF) rainfall distributions are used to evaluate the assumption that a uniform rainfall intensity with duration equal to the time of concentration leads to the maximum peak runoff rate. Key findings of this study are that the runoff coefficient can have a substantial dependence on both the IDF function and the time of concentration of the catchment, the runoff coefficient can take on values greater than one, and the peak runoff rate is underestimated by assuming a constant rainfall intensity of duration equal to the time of concentration.