Ord 1990-037 - Laserfiche WebLink

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Stream Channel "n" Overbank "n" Purgatory Creek Diversion 1 0.055-0.065 0.060-0.080 Willow Springs Creek 0.030-0.060 0.055-0.085 Stream CC-1 0.040-0.055 0.060-0.065 Blanco River 0.050-0.055 0.065-0.085 Bypass Creek 0.035-0.070 0.070 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 are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in this study are shown on the maps; the descriptions of the marks are presented in Elevation Reference Marks (Exhibit 3). 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS 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 in developing floodplain management measures. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance (lOO-year) flood has been adopted by FE~~ 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. For the streams studied in detail, the 100- and 500-yea~ 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 1:24,000 with a contour interval of 10 feet (Reference 9). For the flooding sources studied by approximate methods, the boundary of the 10O-year flood was delineated using topographic maps and the previous Flood Insurance Study for the City of San Marcos (References 9 and 10). The 100- and 50a-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the lOO-year floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A and AE), and the 50D-year floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and 500-year floodplain boundaries 11