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<br />PART 111- RATIONAL METHOD <br /> <br />1. RATIONAL METHOD. The so-called rational method is a popular, easy-to-use <br />technique for estimating peak flow in any small drainage basin having mixed land use. It <br />generally should not be used in basins larger than one square mile. The p~,ak flow can <br />be calculated by the following equation: <br /> <br />(;1 = CIA <br /> <br />(111-1) <br /> <br />where: <br /> <br />Q= <br />c= <br />I = <br />A= <br /> <br />peak flow (cfs) <br />runoff coefficient <br />rainfall intensity (in./hr.) <br />drainage area (acres) <br /> <br />The coefficient is the proportion (Of rainfall that contributes to runoff. Table I is an <br />example, from the literature, of the relationship between this coefficient and land use. In <br />basins having a significant non-homogeneity of land use, an average coefficient can easily <br />be determined by muitiplying the percElIltage of each land use in the basin by its <br />appropriate coefficient from Table 7-8. <br /> <br />The rainfall intensity is specifically defined for an event or the frequency of interest <br />and for a duration equal to or greater than the time of concentration of the watershed. <br />Time of concentration (To) is defined as the time for runoff to travel from the most distant <br />point of the watershed to the watsrshed outlet. To influences the shape and peak of the <br />runoff hydrograph and is a parameter used in may simplified techniques. Numerous <br />methods exist in the Iterative for estimating To' The SCS has developed a method that <br />takes a physically based approacl1 to calculating To ' which can be found in Chapter 2 of <br />the SCS TR-55 (June 1986) publication. <br /> <br />Use of the rational method for large drainage areas should be discouraged <br />because of the greater complexity of land use and drainage pattern, and the unlikelihood <br />of having uniform rainfall intensity for a duration equal to the time of concentration. The <br />method assumes the peak flow occurs from uniform rainfall intensity over the entire area <br />once every portion of the basin is contributing to runoff at the outlet. <br /> <br />TABLE 7-8 <br />TYPICAL C COEFFICIENTS (FOIR 5 TO 10 YR FREQUENCY DESIGN) <br /> <br />DESCRIPTION <br /> <br />BUSINESS <br /> <br />Downtown Areas <br /> <br />I 0.70 - 0.95 <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-.3~1 <br /> <br />fRIJFT <br />