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<br />The starting water-surface elevation and discharge for the USBR study (Reference 35) <br />where the overflow channel converges with the main channel are not the same as those <br />used in the Simons, Li & Associates, Inc., study. This is due to the very significant <br />reduction in discharges from losses to Cooper Slough. A rating curve was drawn from <br />Simons, Li & Associates, Inc., data, and elevations corresponding to tnc discharges <br />resulting from the reductions were u.sed. <br /> <br />For Dry Creek, the starting water-surface elevations were taken from the existing USACE <br />profiles of the Cache La Poudre River at the confluence. The HEC-2 computer program <br />was used to compute the water-surface profiles (Reference 36). Roughness coefficients <br />ranged from 0.03 to 0.05 in the channel and from 0.015 to 0.100 in the overbanks. <br />Cross sections along Dry Creek were taken from topographic maps with a 7-foot contour <br />interval. <br /> <br />The approximate flood boundaries fot Boxelder Creek were taken from an Augu,t 198 J <br />Simons, Li & Associates, Inc., Master Drainageway Planning Study for lkselder Creek <br />and Cooper Slough (Reference 2). <br /> <br />The hydraulic analyses for this study were based on unobstructed flow. The Good <br />elevations shown on the profiles are thus considered valid only jf hydraulic suuctures <br />remain unobstructed, operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGYD). <br /> <br />4,0 FLOODPLAIN MANAGEMENT APPLICA TlC1."l'S. <br /> <br />The NFIP encourages State and local govenmlcnts to adopt sound floodplain management <br />programs. Therefore. each Flood Insurance Study produces maps designed to assist communities <br />in developing floodplain management measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the I-percent armual <br />chance (100-year) flood has been adopted by FEMA as the base flood for r100dplain <br />management purposes. The 0.2-percent annual chance (500-year) flood is employed to <br />indicate additional areas of flood risk in the community. For each strean: studied by <br />detailed methods, the 100- and 500-year tloodplain boundaries have been delineated using <br />the flood elevations determined at each cross section. For Dry Creek, between cross <br />sections the boundaries were interpolated using topographic maps at a scale of 1:4,800, <br />with 2-foot contour intervals (Reference 37). The floodplain boundaries for Cooper <br />Slough were delineated using topographic work maps at a scale of I: 1 ,200, Wilh 2-foot <br />contour intervals (Reference 5). Flood boundaries for Spring Creek were Illtcrpolated <br />using topographic work maps ranging in scale from 1:480 to 1: 1 ,200, with contour <br />intervals ranging between 1 and 2 feet (References 38 and 39). <br /> <br />For the Cache La Poudre River. the flood boundaries for the loo-year ana 500-year <br />floods were delineated using the flood elevatIons determined at each cross ~ection. The <br />flood boundaries were plotted on L2AOO-sca!e orthophoto maps (Reference 40). There <br />were several low-lying areas protected from flooding from t.he illJ;n channel. by gravel <br /> <br />18 <br />