FLOOD06537 - Laserfiche WebLink

Home Browse Search

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 SOO-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries of the 100- and SOO-year floods have been delineated using the flood elevations determined at each cross section; beLween cross sections, the boundaries were interpolated using topographic maps at scale of 1:24,000, with a contour interval of 10 feet (Reference 10), enlarged to a scale of 1:7200 and super- imposed on aerial photographs (Reference S). Boundaries for the AD zone near the upstream limit of study were delineated using field investigations and engineering judgment. In cases where the 100- and SOD-year flood boundaries are close together, only the lOa-year flood boundary has been shown. Flood boundaries are indicated on the Flood Boundary and Floodway Map (Exhibit 3) and the Flood Insurance Rate Map. On these maps, the 100-year flood boundary corresponds to the boundary of the areas of special flood hazards (Zones AO and A2); and the SOG-year flood boundary corresponds to the boundary of the areas of moderate flood hazards (Zone B). 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. 4.2 Floodways 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 flood hazard. For purposes of the National Flood Insurance Program, the concept of a floodway is used as a tool to assist local communities in this aspect of flood plain management. Under this concept, the area of the IOO-year flood is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment in order that the 100-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Insurance Adminis- tration limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodway for this study was computed on the basis of equal conveyance reduction from each side of the flood plain. The 6