RATIONAL-METHOD EQUATION AND HEC TD-15

The rational-method equation for estimating peak flow rates for storm- water runoff is derived from the balanced-design storm unit hydrograph approach presented in the U.S. Army Corps of Engineers HEC Training D ocument 15. The new form of the rational-method equation is Q(p) = (al pha I - phi)A, instead of the well known Q(p) = (I - phi)A; or Q(p) = alpha CIA, instead of the well known Q(p) = CIA, depending on the resp ective loss function used in the unit hydrograph effective rainfall mo del. The preceding fixed constant alpha is found to depend on the type of unit hydrograph used (i.e., S-Graph) and the log-log slope of the rainfall depth-duration curve, and is easily determined by equating to a known unit hydrograph design storm model peak flow rate result. Thi s new development provides a significant foundation for the use of the well-known rational-method equation in small catchments where rainfal l depth-area effects are negligible.