Laserfiche WebLink
<br />In The Slough hydraulic model, the starting water-surface elevations were based on flood <br />routing studies through John Law Reservoir, Roughness coefficients (Manning's "n" values) <br />for this study were determined by field inspection, survey data, photographs, and engineering <br />judgment. Assigned values ranged from 0,035 to 0.043 in the channel and from 0.070 to <br />0,090 in the overbank areas, Higher roughness coefficients were assigned to areas where <br />structures or heavy trees exist. Expansion and contraction coefficients ranged from 0.1 to <br />0.3 and 0.6 to 0,8, respectively, Lower expansion and contraction rates were assigned to <br />open areas and higher coefficients were assigned to culverts, Near some culverts, <br />encroachments were set to eliminate the ineffective areas on the downstream and upstream <br />cross sections of the culvert to account for the proper expansion and contraction near lhe <br />culverts. <br /> <br />Locations of selected cross sections used in the hydraulic analyses are shown on lhe Flood <br />Profiles (Exhibit I), For stream segments for which a floodway was computed (Section 4.2), <br />selected cross-section locations are also shown on the Flood Insurance Rate Map (Exhibit 2), <br /> <br />'The hydraulic analyses for this study were based on unobstructed flow. The flood elevations <br />shown on the profiles are thus considered valid only if hydraulic structures remain <br />unobstructed, operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD), <br />Elevation reference marks and their descriptions are shown on lhe maps, <br /> <br />4.0 FLOODPI.AIN MANAGEMENT APPI..ICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound floodplain management programs, <br />Therefore, each Flood Insurance Study provides lOO-year flood elevations and delineations of lhe 100- <br />and 500-year floodplain boundaries and lOO-year floodway to assist communities in developing <br />floodplain management measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the I-percent annual chance <br />(IOO-year) flood has been adopted by FEMA as the base flood for floodplain management <br />purposes. The O,2-percent annual chance (500-year) flood is employed to indicate additional <br />areas of flood risk in the community. For each stream studied by detailed methods, lhe 100- <br />and 500-year floodplain boundaries have been delineated using the flood elevations <br />determined at each cross section. Between cross sections, the boundaries were interpolated <br />using topographic maps at a scale of 1:24,000, with a contour interval of 10 feet (Reference <br />8), <br /> <br />The 100- and 500-year floodplain boundaries are shown on the Flood Insurance Rate Map <br />(Exhibit 2), On this map, the lOO-year floodplain boundary corresponds to the boundary of <br />the areas of special flood hazards (Zones A and AE), and the 500-year floodplain boundary <br />corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and <br />5OD-year floodplain boundaries are close together, only the lOO-year floodplain boundary has <br />been shown. Small areas within the floodplain boundaries may lie above the flood elevations <br />but cannot be shown due to limitations of the map scale and/or lack of detailed topographic <br />data, <br /> <br />5 <br />