Experimental Examination of the Rational Method

Rainfall and runoff data collected in Baltimore, Maryland, from 20 gaged urban drainage areas ranging in size up to 150 acres have been used in a study of the Rational Method. The results suggest it may be nearly correct to assume that the frequency of occurrence of the computed design peak runoff is the same as the frequency of occurrence of the rainfall intensity selected by the designer if an appropriate C value has been selected. Presently used values of C are not adequately based on measurements of rainfall and runoff in urban areas, so large errors can occur when the Rational Method is used. In accordance with their usual design procedures, five storm drainage designers used the Rational Method to estimate 5 yr design peak runoff rates for six gaged drainage areas. These values are compared with runoff values from runoff frequency curves for these gaged areas.