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<br />III the hydraulic analysis for the current FIS for Rangely, the <br />original section of the existinq levee next to the former <br />filtra1:ion plant was considered only as blocking flow and not as <br />provid:.ng flood protection. When the current FIS was done, the <br />extens:Lon to the levee had not been built, so the hydraulic l~ffects <br />of the extended levee section were not eva.luated. To determine the <br />effect of the extended levee, the HEC-2 hydraulic model from the <br />FIS was modified to account for 1:h,e presence of the leVeE!. The <br />profil,~ of the levee was field-surveyed for the Town of Rangely in <br />Februa:~y 1993. Cross sections 42 a.nd 43 of the HEC-2 model cross <br />the alignment of the levee extension. 'I'hese cross sections were <br />modifi,~d to account for the flow blocked by the levee. 'I'he ground <br />elevat ions on the cross sections 'Here adjusted to those of the <br />levee 1:op. These higher ground elE!Vations were extended back south <br />along the cross sections to higrh ground. These modifications <br />blocked flow through the levee un1:i1 the water surface e,levations <br />were above the top of the levee. <br /> <br />Tile modified HEC-2 model was E~xecuted to compute water surface <br />elevations with the exist:ing levee in place. An increase in the <br />100-ye3.r flood water surface elevations in the vicinity of the <br />levee would be expected as a resmlt of the obstruction to flow <br />caused by the levee. Table 3 giv,~s the computed 100-year flood <br />water surface elevations with the existing levee in place as <br />compared to the elevations before t:he levee was added. <br /> <br />To determine the level of protection provided by thE~ existing <br />levee, the 10-, 50-, 100-, and 500-year flood wa.ter surface <br />elevations computed with the levee in place were compared with the <br />top-of-levee elevations determined by the field survey. To be <br />recognized as providing protection, the levee top elevation must be <br />at or above the computed flood water surface elevation plus <br />freeboard height. The freeboard height is a factor of safety <br />against levee overtopping. For levees protecting developed areas, <br />3 feet of freeboard is required. When a levee lacks the required <br />freeboard, the levee is c(msiderE~d. fai.led for the flood event under <br />consideration. <br /> <br />11 comparison of the computed water surface profiles for the <br />10-, 50-, 100-, and 500-year floods with the surveyed .top-of-levee <br />elevations showed that the 10-year flood event could be passed with <br />adequate freeboard. For the 50-year flood event, the lowest: points <br /> <br />7 <br />