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<br />2.2 Hydrology. The discharges u,,'ed for thisl study were those <br />in the 1978 Flood Plain Information Study and the Flood Insut'ance <br />currently being finalized. The dischargeEI are shown in Table One!. <br /> <br />used <br />Study <br /> <br />TABLE ONE: <br /> <br />South Platte River Discharges <br />at Sterling, Colorado <br /> <br />Return Interval <br /> <br />Pe!ak Discharge <br /> <br />10-Year <br />50-Year <br />100-Year <br />sOO-Year <br /> <br />11l,000 c.Ls. <br />5!5,OOO c.f~s. <br />84,000 c.f..s. <br />207,000 c.f.s. <br /> <br />3. <br /> <br />HYDRAULIC ANALYSIS. <br /> <br />3.1 <br />computed <br />The 1988 <br />uses the <br />gradually <br /> <br />Existing Conditions. Water surface profiles and floodways were <br />using the U.S. Army Corps of Engineers HEC-2 backwater program. <br />release for IBM compatible microc:omputers was used. The program <br />standard step backwater method and is applicable to steady <br />varied flow in natural channels. <br /> <br />The HEC-2 data used for the Flood Plain Information study was obtained <br />from the Corps of Engineers computer archive and was updated to meet <br />existing conditions. Minor adjustments WElre made to the bridge models and <br />some cross sections were extended. The Un" values 1tlere assigned based on <br />the field inspection of the area, aerial photos acquired from the Logan <br />County ASeS, and engineering judgement. The selectl3d values arIa shown in <br />Table Two. All existing agricultural levees in the overbanks were <br />determined to be failed for the existing "onditions and floodway water <br />surface elevations calculations. Existing conditions water surface profiles <br />are shown on Plate 1. <br /> <br />4 <br />