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<br />IV-5 <br /> <br />lOa-year return frequency at the designated "design" points, as shown <br />on Sheet 1 , of Volume 2. Figure IV"2 shows the incremental Increase- <br />peak rate of runoff along the channel. Figure No. IV-3 shows the hy- <br />drographs at specific design poInts for the 100"year flood under future <br />development. <br /> <br />BasIn I was not used in the hydrological analyses of flood peaks <br />because the major flow below the U.P,R.R. is controlled by the routing <br />of water through the rai I road culvert, and the incremental increase of <br />flood peak flow due to the 0.7 square mile Basin I is minimal. <br /> <br />Special Considerations <br /> <br />This study assumed that neither Standley Lake nor Great Western <br />Reservoir would pass any significant flood waters during the IOO-year <br />frequency flood. <br /> <br />The lOa-year inflow hydrographs for each of the reservoirs (Basins <br />A and B) were routed through assumin9 a full re.servolr, using an In- <br />varIable discharge-storage relationship. (Puh Method). it was found <br />that the lOa-year flood routed throu9h Stand I ey Lake drops from above <br />9,200 cfs to less than 400 cfs. The lOa-year flood routed through <br />Great Western also indicated a peak discharge of Jess than 100 cfs. <br />The chances of Standley Lake being spi Ilway-fllll whlen the lOa-year rain- <br />fall comes are much smaller than one in one hundred, so that it was as- <br />sumed that Standley Lake could store the entire 100"year storm when it <br />fa 11 s. <br /> <br />For the IOO-year design hydrology for the channel s downstream of <br />the Standley Lake Reservoir, it was assumed that Standley Lake was <br />making a release equal to the mean release dur Ing the summer, which is <br />250 cfs. <br />