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<br />I <br />I <br />1 <br />I <br />1 <br />I <br />1 <br />I <br />1 <br />1 <br />I <br />I <br />1 <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />above the outlet works invert. The normal flow might, in fact, be <br />substantially less than those occurring during June. The average flow for the <br />entire 13-year period of record is 1 cfs, which is far below the capacity of <br />the outlet works, <br /> <br />No changes in outl et works i nl et el evati ons appear to be necessary. The new <br />area-capacity curve shows there to be no storage below the outlet elevation of <br />9276 (zero gage height). Thus, it is expected that, as sedimentation of the <br />reservoi r conti nues, the capacity of the conservati on pool wi 11 decrease. <br />However, as noted previously, the estimated lOO-year sediment yield is 9 acre- <br />feet, Thi s represents only 4 percent of the conservati on pool. Consi deri ng <br />that the reservoir is likely to be drawn down each year and the trap <br />efficiency will be relatively low, reduction of the conservation pool due to <br />sedimentation is not considered to be a significant concern. <br /> <br />3.4 Dam Breach Analysis <br /> <br />A dam breach analysis was performed in order to determine the peak discharge <br />magnitude resulting from failure of the earth embankment. <br /> <br />Assuming there would be no failure of the dam, routing of the PMF through Twin <br />Lakes Reservoi r for the exi sti ng spi 11 way and embankment conditi ons resulted <br />in overtopping the dam by 1.1 feet (maximum water surface elevation 9301.1L <br />The peak discharge was 5660 cfs. <br /> <br />Studies of dam failures indicate there is a minimal depth and duration of <br />overtopping which occurs before actual cutting through the embankment and <br />opening of a breach channel occurs. For the dam breach analysis of Twin Lakes <br />Reservoir, a depth of 0.5 feet of overtopping was used to initiate the breach <br />formation. During the breach formation, the erosion channel will cut <br />vertically to the existing channel or ground surface elevation before <br />spreading laterally across the embankment. Other dam break simulations <br />performed have shown that the peak di scharge usually occurs when the breach <br />openi ng has just cut to the bottom of the embankment. Si de slopes of the <br />breach openi ng depend on the materi al s of the embankment, but cOlllDonly are <br /> <br />-61- <br />