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<br />cross-,sections were comp leted for this study at the "Reese Lake" area, 'in order <br />to defi ne the new river confi gurat i on after the fa i 1 ure of the Reese Lake Levee. <br /> <br />Levee profiles were surveyed along the existing gabion wall adjacent to the <br />"Riverside" area and also ,~long the bank adjacent to the Pepsi Cola plant and in <br />the "Connected Lakes" areL Survey data was supplemented as required with <br />elevations taken directl:1 from topographic mapp ing provided from the Corps' <br />Study (Reference 3). <br /> <br />The HEC-II model was calibrated based on the estimated disch,~rges and high water <br />elevations from the 1983 flood. Aerial photographs taken on June 29, 1983 were <br />provided by the Colorado ~Iater Conservation Board, and were used as a basis for <br />comparison with the predicted floodplain for that discharge. The model was <br />calibrated based on condi':ions after the failure of the levee at "Reese Lake", <br />with a maximum peak discharge on June 29th of 51,220 cfs downstream of the <br />Gunnison river and 33,500 cfs upstream of the Gunnison River. Peak discharges <br />were obtained through the Colorado Water Conservation Board from gaging records <br />provided by the U.S. Geological Survey. <br /> <br />Manning's roughness coefficient ("n") for the river channel was adjusted in <br />various locations in order to calibrate the model based on estimated water <br />surface elevations for June 29th. Mannings "n" values were varied in the <br />"Reese Lake" area from 0,035 to 0.043, in the "Connected Lakes" region from <br />0.035 to 0.039, and at the Grand Avenue Bridge from 0.028 to 0.035. Mannings <br />roughness coefficients are dependent upon surface roughness, vegetation, channel <br />irregularity, silting and scouring, obstructions, and size (l,nd shape of channel. <br />It is, therefore, not uncommon for estimated "n" values to vary significantly <br />throughout a channe 1 reach. <br /> <br />The calibration was completed by attempting to match computed water surface <br />elevations with high water marks observed by City and County Staff, and by <br />matching floodplains with the flooded areas shown on aerial photography. The <br />information used was more general than specific. Sheet 1 shows the plotted <br />water surface profiles for the 2-, 5-, 10-, 50-" 100-, and 500-year events, in <br />addition to profiles of the left and right river bank elevations. .J.L(:'Q!TIR,9-x:ts,QI') <br />. between the bank profEt?...AIJ.!;Lj'L9-t~L surfa,fegJ.~y'a.ti.o"~~",.s..hQ.!tL!.h!lLl!P,1Q",a~Q~L,~_ <\.1 , <br />10-year flood event, the floodplain is genEOrally confjned tJlr:9..l1g,b,o~!.~I),E;!",~i.u,gy,., I <br />reach:-F or Ta'rger-'f1 000 -event s ,Tevee'~s' i n"var f6us"area s are overtopp ed and <br />---~ <br />flooding becomes widespread. Floodplain maps (Sheets 2 and 3) are provided which <br />show flood plains for the 10-, 100- and 500-year events for existing conditions. <br /> <br />When flood depths exceed levee height floodin() becomes widespread, and increased <br />discharges add generally to flood depth and not to floodplain extent. The 100- <br />year and 500-year events are out of the channel banks throughout most of the <br /> <br />4 <br />