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<br />m.s.l., respectively. Dam raise alternatives considered for this study consist <br /> <br />of a reinforced concrete parapet. wall located near the upstream part of the crest <br /> <br />and earthfill raise procedures. Although both dam raise procedures'were costed, <br /> <br />only parapet dam costs are shown on subsequent tables except for the dam raise <br /> <br />alternative with no other project modification. For this alternative, costs for <br /> <br />both dam raise procedures are given. <br /> <br />HYDROLOGIC ANALYSIS OF ALTERNATIVES <br /> <br />These alternatives alone, in combination with each other, or in combination <br /> <br />with raising the crest of the dam were sized in a trial and error procedure using <br /> <br />the Omaha District's Multiple Routing Program. In addition to the full PMF <br /> <br /> <br />routings, 25 percent, 50 percent, and 75 percent of the PMF were routed through <br /> <br />the dam (or dams in the case of Castlewood alternatives) to evaluate the residual <br /> <br />flows. All alternatives were sized using the "fully-developed conditions" PMF. <br /> <br />The results of these routings are shown in table 13. <br /> <br />Many of the alternatives would increase discharges for flood events less <br /> <br /> <br />than the threshold flood, and although all of the alternatives reduce the <br /> <br />discharges on Cherry Creek from a full PMF dam failure, many have very <br /> <br />significant residual discharges. The alternatives which involve enlarging the <br /> <br /> <br />existing spillway would increase the discharges along Toll Gate and Sand Creeks <br /> <br />for all flood events greater than approximately 15 percent of the PMF (with the <br /> <br /> <br />exception of the alternatives which enlarge the existing spillway in combination <br /> <br />with constructing Castlewood Dam.) Only one alternative, which consists of <br /> <br />constructing Castlewood Dam, raising the Cherry Creek dam crest to elevation 5660 <br /> <br /> <br />feet m:s .1. and blocking the existing spillway, would eliminate virtually all <br /> <br />40 <br />