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Hydraulic analyses for portions of First Creek and Senac Creek were performed using topographic maps at a scale of 1:2,400, wi th a contour interval of 10 feet (Reference 27). Field surveyed cross sections were used and normal-depth calculations were performed in order to obtain top widths at the selected cross sections. A revised hydraulic analysis was performed for a reach of West Toll Gate Creek between Chambers Road and a point approximately 1,600 feet downstream (Reference 28). As a result of the channel improvements along this reach, the 100- year flood is contained within the channel. This improvement was the basis for a Letter of Map Revision issued on July 19, 1988. ' The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations Vertical Datum in this study Insurance Rate are referenced to the National Geodetic of 1929 (NGVD). Elevation reference marks used and their descriptions are shown on the Flood Map. 4.0 FLOODPLAIN MANAGEMENT APPLICATION The NFIP encourages state and local governments to adopt sound floodplain management programs. Therefore, each Flood Insurance Study provides 100-year flood elevations and delineations of the 100- and 500-year floodplain boundaries and 100-year floodway to assist communities in developing floodplain management measures. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the I-percent annual chance (lOO-year) flood has been adopted by FEMA as the base flood for floodplain management purposes, The 0.2-percent annual chance (500-year) flood is employed to indicate additional areas of flood risk in the community. Por each stream studied by detailed methods, the 100- and 500-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2,400, with a contour interval of 2 feet (References 21 and 22). The boundaries for Sand Creek and Westerly Creek were interpolated using topographic maps at a scale of 1:6,000, with contour intervals of 2 feet (References 20, 23, and 24). The flOOdplain boundaries for Cherry Creek Spillway and Unnamed Creek were interpolated by field survey spot 21