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<br />. <br /> <br />Detailed hydraulie analyses were performed by Engineering Professionals, Ine (EPl) for both the <br />Southwest Drainage and the Northern Drainage. Those analyses used the Corps of Engineers <br />baekwater computer model, HEC-2. The hydraulie analysis for the Southwest Drainage eonfirmed <br />that the major problem along the drainage is the railroad erossing. Aeeording to the analysis, the <br />eross-seetional area of the two eulverts would need to be approximately 2.5 times the eurrent eross- <br />seetional area to aeeommodate the 100-year flow. The eurrent eonstriction results in ponding on <br />the south side of the railroad up to a depth of 6 or 7 feet. The terrain in Otis is flat enough that the <br />100-year pond extends from the railroad culverts upstream all the way to Dade Street, at the west <br />end of town. On the Northern Drainage the hydraulie analysis showed flooding from either a 10- <br />year flood or a 100- year flood in the area bounded roughly by Arapahoe to the west, Logan to the <br />east, 5th to the south and the northern eorporate limit to the north <br /> <br />There is a limitation on the aceuraey of the EPI hydraulie analysis for the streteh of the Southwest <br />Drainage upstream of the railroad erossing. Southeast of the railroad culverts is a low point in a <br />ridge, over whieh it appears that floodwaters eould spill during a 1 O-year or a 100-year flood. If the <br />floodwaters did spill over that ridge, they would likely flow east approximately y., to Y, mile before <br />turning north and erossing the railroad at a different point. That spillover would mean the amount <br />of water that actually tried to follow the flowpath that was analyzed by EPI was overestimated. It is <br />likely that a new hydraulie analysis that properly eonsidered the spill to the east would result in <br />ehanges to flood elevations and in the extent of the floodplain. The town has diseussed the <br />possibility of eondueting a revised hydraulie analysis to appropriately eonsider the possible effeet of <br />. the "spillover" on the floodplain of the Southwest Drainage. <br /> <br />The table below summarizes important elevations in Otis' floodplain, including elevations of <br />signifieant ground features and water surfaee elevations, based on the EPl analysis: <br /> <br />Feature <br />Draina e invert immediatel u stream of railroad culverts <br />Railroad culvert invert <br />Ma ed low oint near intersection of S roee and Vine <br />Ma ed low oint near intersection ofS ruce and Washin on <br />Low oint of saddle otential ill to east <br />To of railroad culvert inlet <br />10- ear flood elevation immediate\ u stream of railroad culverts <br />To of railroad embankment <br />100- ear flood elevation immediatel u stream of railroad culverts <br /> <br />Elevation <br />4323 <br />4325 <br />4329.3 <br />4330.2 <br />4331 <br />4331 <br />4333.5 <br />4336 <br />4337 <br /> <br />Several of the elevations in the table above could be expected to ehange as a result of a revised <br />hydraulie analysis. <br /> <br />The CWCB delineated the EPI 100-year floodplain boundaries on a eopy of the FIRM for the south <br />side of Otis. Beeause of the limitation in the hydraulie analysis diseussed above, the floodplain <br />shown on the revised FIRM is eonsidered to be approximate information and subjeet to revision <br />based on a new detailed hydraulie analysis. No flood elevations are shown on the revised FIRM. <br /> <br />. <br /> <br />7 <br />