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<br />PART III - RATIONAL METHOD <br /> <br />1. RATIONAL METHOD. The so-called rational metllod is a popular, easy-to-use <br />technique for estimating peak flow in any small drc,inage basin having mixed land use. It <br />generally should not be used in basins larger than one square mile. The peak flow Can <br />be calculated by the following equation: <br /> <br />Q,~ CIA <br /> <br />(111-1) <br /> <br />where: <br /> <br />Q~ <br />C= <br />I = <br />A= <br /> <br />peak flow (cfs) <br />runoff coefficiElnt <br />rainfall intensity (in :hr.) <br />drainage area (acrEls) <br /> <br />The coefficient is the proportion of rainfalllhal contributes to runoff. Table I is an <br />example, from the literature, of the relationship between tilis coefficient and land use. In <br />basins having a significant non-homo!,eneity of land use. an average coefficient Can easily <br />be determined by multiplying the percentage 01 each land use in tll'" basin by its <br />appropriate coefficiElnt from Table 7-B. <br /> <br />The rainfall intensity is specifically defined for an event or the frequency of interest <br />and for a duration equal to or greatElr than the tirne of concentration of ::he watershed. <br />Time of concentration (T,) is defined as the time for runoff to travel from tile mosl distant <br />point of the waterslled to the watershed oullet. To influences the shape a nd peak of the <br />runoff hydrograph and is a parameter used in may simplified techniques. Numerous <br />methods exist in the Iterative for estimating T,. Tile SCS has developed a method that <br />takes a physically based approach to calculating To ' whicll Can be found In Chapter 2 of <br />the SCS TR-55 (June 19B6) 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 tile 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 />TABL.E 7-8 <br />TYPICAL C COEFFICIENTS (FOR S TO 10 YR FREQUENCY DESIGN) <br /> <br />DESCRIPTION <br /> <br />BUSINESS <br /> <br />.--.---J------. . <br />0.70.0,"5 <br />------ ------"------ <br /> <br />Downtown Areas <br /> <br />Colorado Flood <br />Hydrology Mallual <br /> <br />7-33 <br /> <br />CPIfT <br />