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<br />VII. RUNOFF COMPUTATION <br />The quantity of rainfall which eventually reaches a major dra,inage <br />channel is dependent upon the duration and intensity of the rainfall, <br />upon the quantities of rain evaporated 01" intercepted py vegetat'ion, upon <br />the soil infiltration characteristics (which are affected by the slope of <br />the land) and by the volume of local runoff r'E'tained in depressions. <br />The S.C.S. has made extensive studies and observations on storm runoff <br />from different ,agri cultura 1 1 and useS and different soi 1 groups. The S. C.S <br />runoff computation procedure as modified by the Bureau of Reclamation was <br />used for each subbasin. <br />The S.C.S. runoff computation method is described in the NatiQnal. <br />IlliL~1eering Handbook, Section 4, HYdrology and further descri bed in Procedures <br />fOLJJetermining Peak Flows in Colorado. . <br />Using the S.C.S. soils data, CN-values and minimum infiltration rates <br />were established. CN-values were developed from the hydrologic runoff designa- <br />tion for each soil type (type A through D), combined with land use and ground <br />COVE'!' da ta. <br />Table 5 indicates the parameters used to calculate the runoff Inydrographs <br />for E!ach of the i ndi vidua 1 subbas i ns. The hydrographs vlere ca 1 cul ated based on 5 <br />mim te time increments. Figure 11 shows a sample computer hydrogrilph output. <br />The rational formula (Q = CxIxA) was used to define localized runoff <br />quantities at the 16 problem areas within Montrose for the 5 year )'ecurrence <br />interval storm. A minimum storm duration of 10 minutes was used. Time of <br />concEntra t'ion (in minutes) was determi ned from 1 and use, 1 oca 1 topography, <br />and flow quantity. The corresponding rainfall intensity rates (1) were taken <br /> <br />15 <br />