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4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS - -- - -~----------,.- A prime purpose of the National Flood Insurance ~rogram is to encourage state and local governments to adopt sound flood plain management pro- grams. Each Flood Insurance Study, therefore, includes a flood boundary map designed to assist communities in developing sound flood plain manage- ment measures. 4,1 Flood Boundaries In order to provide a national standard without regional discrimi- nation, the IOO-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain manage- ment. The SOD-year flood is employed to indicate additional areas of flood risk. For streams studied by detailed methods, the boundaries of the 100-year and SOD-year floods were delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated using topographic maps with a scale of 1:4800 feet, with a contour interval of S feet or less (Refer- ence 10). For streams studied by approximate methods, the boundary of the 100- year flood was established according to the professional judgment of engineers familiar with the region, taking into account flood eleva- tions from historical flood data, minor hydrologic and hydraulic analyses, correlations with similar stream basins studied in detail, and field observations. Small areas within the flood boundaries may lie above the flood elevations and, therefore, not be subject to flooding; owing to limitations of the map scale, such areas are not shown. In cases where the 100- and SOD-year flood boundaries are close together, only the 100-year flood boundary has been shown. In areas where the floodway and 100-year flood are close together, only the floodway has been shown. Some areas in Steamboat Springs are subject to sheetflowi that is, broad, shallow, overland flooding generally less than 3 feet deep and characterized by unpredictable flow paths. The water-surface eleva- tions of flooding in these areas are essentially independent of those along adjacent streamways and are affected principally by obstructions in the flooded areas. 4.2 F100dways Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increasing flood hazards in areas beyond the encroachment itself. One aspect of flood plain management involves balancing the economic gain from flood plain development against the resulting increase in flonn 24