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<br />2.) Corps of Engineers <br />The Corps of Engineers report, entitled "Flood Plain Information", <br />Volume IV, Denver Metropolitan Region, Colorado, October, 1968, esti~ <br />mated floods on Big Dry Creek as follows: <br />Q = 4800 cfs (intermediate region"l flood) <br />Q 18200 cfs (standard project flood) <br />Comparing the intermediate regional flood discharge above with <br />the analysis of lOa-year frequency storm (using thc CUIIP and present <br />land use) indicates a very great difference in runoff. This great <br />difference prompted a study of the Rainfall Depth-Duration-Frequency <br />Maps in the Criteria Manual. A graph was prepared o[ the RainEall <br />Intensity versus Rainfall Duration of data Erom the Criteria Manual <br />and compared to similar data from the 'j"ec!:'.]]ic-cal.J<eport__J!0-,__4.Q of the <br />U.S. Weather BureaJ. The Criteria Manual provides a 36 percent higher <br />rainfall value for the same 6-hour duration, 100~year frcquency storm. <br />A 36% increase in the Corps of Enginecrs discharge would provide a Q <br />(intermediate regional flood) = 6528 c[s. The difference of (10,700 <br />cfs - 6528 cfs) 4172 cfs is attribtltahl,c to the Colorado Urban Hydro- <br />graph Procedure of analysis. The soils infiltrated rate and rainfall <br />pattern recommended by the Citeria illanual to establish the unit hydro- <br />graph mayor may not be a true repfesentation of the characteristics <br />of the basin. Furthermore, the report by the Corps of Engincers docs <br />not state the procedure or coefficients used to uerive the est imated <br />discharge rate. <br />In this report, the rate of nll10ff (6-hour uuration, IOO-ycar <br />frequency storm) for present land l1se versus proposed land use inui cates <br /> <br />- '.\ 7- <br />