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<br />Mr. Frank M. Akers <br />September 23, 1991 <br />Page 2 <br /> <br />. <br /> <br />Downstream of the existing dam crest, fill is being placed in channel to <br />create the boatchutes. Review comments received from members of <br />the advisory committee (City of Englewood, Urban Drainage and <br />Flood Control District, South Suburban Metropolitan Recreation and <br />Parks District, and others) after the preliminary design was completed <br />(October 1989 submittal) resulted in changes to the crest geometry of <br />the downstream fills. <br /> <br />. <br /> <br />The geometric changes included shortening the lengths of the <br />embankment crests and modifying the crest profiles. The crests were <br />horizontal in the preliminary design. For the final design the crests <br />were sloped upward from the boatchute to the channel banks (see <br />attached calculations). The effect of these changes was a slightly <br />higher water surface elevations downstream of the existing dam (station <br />380+00 to station 366+95). The 100-year flood remains within the <br />channel banks and channel right-of-way as shown on the drawings. <br /> <br />. <br /> <br />Even though the final downstream water surface elevation is higher <br />than before, the control is still the existing dam weir crest (station <br />366+95). The water passes through critical depth near the crest. <br /> <br />. <br /> <br />No changes of hydraulic significance were made upstream of the <br />existing dam crest. The channel cross sections upstream of the existing <br />dam crest do not change and the downstream control remains the <br />same, i.e., critical depth at the crest. The water surface profile for the <br />100-year flood upstream of the crest does not change from the Corp <br />of Engineers water surface profile. This situation is true for both the <br />preliminary (1989) and the final (1991) design. <br /> <br />. <br /> <br />The calculations for the 1989 water surface profile upstream of station <br />366+95 were based on different assumptions than the final <br />calculations. The difference (about 1-1/2 feet) in water surface profiles <br />resulted from the assumptions made for cross section locations, cross <br />section configurations, and the flow behavior through the various <br />channel transitions upstream of the existing weir crest (see the water <br />surface profile calculations dated April 1989). The final water surface <br />profile (see calculations dated June 1991) revised these assumptions <br />and the resulting water surface profile more closely matched the COE <br />water surface profile. <br /> <br />Brown and Caldwell <br />Consultants <br /> <br />21.5993/ js <br />