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<br />. <br /> <br />Detailed hydraulic analyses were performed by Engineering Professionals, Inc (EPI) for both the <br />Southwest Drainage and the Northern Drainage. Those analyses used the Corps of Engineers <br />backwater computer model, HEC-2. The hydraulic analysis for the Southwest Drainage confirmed <br />that the major problem along the drainage is the railroad crossing. According to the analysis, the <br />cross-sectional area of the two culverts would need to be approximately 2.5 times the current cross- <br />sectional area to accommodate the 100-year flow. The current constriction 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 hydraulic 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 corporate limit to the north <br /> <br />There is a limitation on the accuracy of the EPI hydraulic analysis for the stretch of the Southwest <br />Drainage upstream of the railroad crossing. Southeast of the railroad culverts is a low point in a <br />ridge, over which it appears that floodwaters could spill during a I 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 crossing 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 hydraulic analysis that properly considered the spill to the east would result in <br />changes to flood elevations and in the extent of the floodplain. The town has discussed the <br />possibility of conducting a revised hydraulic analysis to appropriately consider the possible effect 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 />significant ground features and water surface elevations, based on the EPI 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 change as a result of a revised <br />hydraulic analysis. <br /> <br />The CWCB delineated the EPI 100-year floodplain boundaries on a copy of the FIRM for the south <br />side of Otis. Because of the limitation in the hydraulic analysis discussed above, the floodplain <br />shown on the revised FIRM is considered to be approximate information and subject to revision <br />based on a new detailed hydraulic analysis. No flood elevations are shown on the revised FIRM. <br /> <br />. <br /> <br />7 <br />