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<br />---" <br /> <br />., <br /> <br />this study are significantly lower than the discharge-probability <br /> <br />profiles developed for the reach from Chatfield Dam to Hampden Avenue <br /> <br /> <br />which were presented in Supplement No.1, Design Memorandum No. <br /> <br />P.C. 20, Downstream Channel Improvement, Volume II, Appendix 4, <br /> <br />dated December 1976. The reasons for the changes are the result of <br /> <br />the following factors: (1) the earlier study did not consider the <br /> <br /> <br />effects of McClellan Dam which controls runoff from about 9 square <br /> <br />miles of the Dad Clark Gulch basin, and (2) the earlier study did <br /> <br /> <br />not consider the effects of 6.5 square miles of non-contributing <br /> <br />drainage area located on the South Platte left bank just downstream <br /> <br /> <br />from Chatfield Dam. In addition, the difference in methodology <br /> <br /> <br />between the two studies also accounted for some of the difference in <br /> <br /> <br />results. The earlier study used a unit hydrograph approach to develop <br /> <br />tributary inflow while this study used the detailed subarea analysis <br /> <br /> <br />technique which is part of the SWMM methodology. After reviewing <br /> <br />the results from both studies, it was our judgment that the profiles <br /> <br /> <br />developed for this reach, which are shown on plates 33 and 34, <br /> <br />represent the best estimate of probability relationships for the <br /> <br /> <br />South Platte River from Chatfield Dam to the mouth of Sand Creek. <br /> <br /> <br />The existing condition profiles shown on plate 33 for this reach <br /> <br /> <br />reflect urbanized conditions as they existed in the early part of <br /> <br />1979. This study was approved by Missouri River Division by letter <br /> <br />dated 31 July 1979. <br /> <br />Sand Creek to State Line - Profile for l-Inch Runoff. For this <br /> <br />reach of the South Platte a peak discharge profile was developed <br /> <br /> <br />representing 1 inch of runoff occurring uniformly over the entire <br /> <br />basin from Sand Creek to the State line. This profile is shown on <br /> <br /> <br />plates 24 to 32. Although a profile representing 1 inch of runoff <br /> <br /> <br />from a 20,000 square mile area in this region is an unrealistic <br /> <br />condition, it was felt that it provided a reliable guide in <br /> <br /> <br />determining the relative magnitude of the discontinuity that should <br /> <br />36 <br /> <br />. <br />