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,- RATIONAL METHOD 1. RATIONAL METHOD. The so-called rational method is a popular, easy-to-use technique for estimating peak flow in any small drainage basin having mixed land use. It generally should not be.- used in basins larger than one square mile. The peak flow can be calculated by the following equation: Q = CIA (1 ) where: Q C I A. peak flow (cfs) runoff coefficient rainfall intensity (in./hr.) drainage area (acres) The coefficient is the proportion of rainfall that contributes to runoff. Table I is an example, from the literature, of the relationship between this coefficient and land use. In basins having a significant non-homogeneity of land use, an average coefficient can easily be determined by multiplying the percentage of each land use in the basin by its appropriate coefficient from Table 1. The rainfall intensity is specifically defined for an event or the frequency of interest and for a duration equal to or greater than the time of concentration of the watershed. Time of concentration (T.) is defined as the time for runoff to travel from the most distant point of the watershed to the watershed outlet. To influences the shape and peak of the runoff hydrograph and is a parameter used in may simplified techniques. Numerous methods exist in the Iterative for estimating To' The SCS has developed a method that takes a physically based approach to calculating To ' which can be found in Chapter 2 of the SCS TR-55 (June 1986) publication. Use of the rational method for large drainage areas should be discouraged because of the greater complexity of land use and drainage pattern, and the unlikelihood of having uniform rainfall intensity for a duration equal to the time of concentration. The met.hod assumes the peak flow occurs from uniform rainfall intenSity over the entire area once every portion of the basin is contributing to runoff at the outlet. 7-85