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<br />4.0 FLOOD PLAIN ~~~AGE~ENT APPLICATIOKS <br /> <br />A prime purpose of the ~ational Flood Insurance Program is to <br />encourage state and local governments to adopt so~nd flood <br />plain ~anagement programs. Each Flood Insurance Study, there- <br />fore, includes a flood boundary map designed to assist com- <br />munities in developing flood plain management measures. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a ~ational standard without regional <br />discrimination, the laO-year flood has been adopted by <br />the Federal Ece~gency Management Agency as t~e nase flood <br />for purposes of flood plain management measures. The <br />500-year flood is employed to indicate additional areas <br />of flood ris~ in the community. For each stream studied <br />in detail, the boundaries of the lOa-year and the 500- <br />year floods have been delineated using the flood eleva- <br />tions determined at each cross section; between cross <br />sections, the boundaries have been interpolated using <br />topographic maps (Reference 2). In cases where the 100- <br />year and 500-year boundaries are close together, only <br />the lOa-year boundary has been shown. <br /> <br />The boundaries of the 100- and 500-year floods are shown <br />on the Flood Boundary ~ap. <br /> <br />Small areas within the flood boundaries ~ay lie above <br />the flood elevations and therefore, not be subject to <br />floodi~g; owing to limitations of the ~ap scale, such <br />areas are not shown. <br /> <br />Identification and outline of the sheet r~no:f areas <br />were delineated using the accounts of flooding fron <br />residents and officials of Longmont, photographs of <br />flooding (Figures 6 and 7), and field reconnaissance of <br />the areas to confirm previous observations. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, <br />reduces the flood-carrying capacity and increases flood <br />heights, thus increasing flood hazards in areas beyond <br />the encroachment itself. One aspect of flood plain <br />managenent involves balancing the economic gain from <br />flood plain developnent against the resulting increase <br />in flood hazard. For purposes of the Flood Insurar.ce <br />Program, the concept of a floodway is used as a tool to <br />assist local conmunities in this aspect of flood plain <br />management. Under this concept, the area of the lOa-year <br />flood is divided into a floodway and a floodway fringe. <br /> <br />18 <br />