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Roughness coefficients (Manning's "n") were estimated by field inspec- tion at each cross section and checked for reasonableness against a recognized standard text (Reference 4). Roughness values for the main channel ranged from 0.03S to 0.040; roughness values for the flood plain ranged from 0.060 to 0.065. Water-surface profiles of floods of the selected recurrence intervals were developed using the u.s. Army Corps of Engineers HEC-2 computer step-backwater model (Reference 5). Starting elevations were taken from the upstream elevations of the La Plata County Flood Insurance Study (Reference 6). Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). The flood profiles shown in this report are based on the effects of flow through unobstructed hydraulic structures and are, thus, consi- dered valid only if these structures remain unobstructed and do not faiL All elevations are referenced to the National Geodetic vertical Datum of 1929 (NGVD). Elevation reference marks used in the study are shown on the maps. 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS 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 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 discrimina- tion, the 100-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain manage- ment measures. The SOD-year flood is employed to indicate additional areas of flood risk in the community. For the stream studied in detail, the boundaries of the 100- and 500-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated using topographic maps enlarged to a scale of 1:4800, with a contour interval of 20 feet (Reference 7). Flood boundaries are indicated on the Flood Insurance Rate Map (Exhibit 2). On this map, the 100-year flood boundary corresponds to the boundary of the areas of special flood hazards (Zone A3); and the SOQ-year flood boundary corresponds to the boundary of the areas of moderate flood hazards (Zone B). 6