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\ \ sections were made in the vicinity of bridges to determine the backwater effect of these structures. Supplementary data were obtained from U.S. Geological Survey quadrangle maps (Reference 5). Topographic data were augmented with field observations. Selected c~oss sections are shown on the Flood Boundary and Floodway Maps. Approximately lOO cross sections were used in the study. :) Hydraulic roughness coefficients (Manning's "n" values) were com- puted from stream gage dat:a or determined by field observation. The coefficients used ranged from 0.035 to 0.040 for channels and from 0.060 to 0.070 for overbank areas. Water-surface profiles were developed using a HEC-2 computer step- backwater program (Reference 6). Profiles were determined for the lO-, 50-, lOO-, and 500-year floods to an accuracy of 0.5 foot (Exhibit l). Starting wa1:er-surface elevations were determined by the slope-area method. All elevations are referenced to the National Geodetic vertical Datum of 1929 (NGVD). Elevation reference marks are shown on the maps. The hydraulic analyses reflect unobstructed flow conditions. Flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed. For streams studied by approximate methods, the elevation of the lOa-year flood was established by professional engineering judg- ment, giving consideration to available data and field observations. 4.0 FLOOD PLAIN MANAGEMENT APPLICNrIONS A prime purpose of the National Flood Insurance Program is to encourage state and local governments to adopt sound flood plain management pro- grams. Each Flood Insurance S':udy, therefore, includes a flood boundary map designed to assist communi1:ies in developing sound flood plain management measures. 4.l Flood Boundaries .. In order to provide a national standard without regional discrimi- nation, the lOO-year flood has been adopted by the Federal Insur- ance Administration as the base flood for purposes of flood plain management measures. The 500-year flood is employed to indicate additional areas of flood risk in the study area. For each stream se~ent studied in detail, the boundaries of the lOa-year and the 500-year floods were delineated using the flood elevations deter- mined at each cross section; between cross sections, the boundaries were interpolated using topographic maps at a scale of l:24,000, with contour intervals of 20 or 40 feet (Reference 5). In cases where the lOa-year and the 500-year flood boundaries are close together, only the lOO-year boundary has been shown. ~ . l4