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<br />Big Thompson River, scale 1:2,400, reduced to 1:6,000 with a <br />contour interval of 2 feet (Reference 17) <br /> <br />Coal Creek, scale 1:4,800, with a contour interval of 2 and 4 <br />feet (Reference 19) <br /> <br />In cases where the 100- and SOO-year flood boundaries are close <br />together, only the lOa-year flood boundary has been shown. <br /> <br />The approximate flood boundaries for the South Platte River and the <br />Cache La Poudre River were taken from the 100-year flood boundaries <br />developed by the COE (References 4, S, and 21). <br /> <br />Approximate flood boundaries <br />delineated using topographic <br />as follows for each stream: <br /> <br />on streams in the study area were <br />maps at a scale and contour interval <br /> <br />South Platte River (from <br />Evans and downstream of <br />County), scale 1:24,000, <br />a contour interval of 10 <br /> <br />Adams County boundary to the City of <br />the Cache La Poudre River to Morgan <br />enlarged to a scale of 1:12,000, with <br />feet (References 10 and 23) <br /> <br />Cache La Poudre River, scale 1: 12,000 enlarged to a scale of <br />1:7,200, with a contour interval of 4 feet (Reference 4 and <br />21) <br /> <br />Crow Creek, scale 1:24,000, with a contour interval of 10 feet <br />(Reference 24) <br /> <br />Idaho Creek, scale 1:24,000, enlarged to a scale of 1:2,400, <br />with a contour interval of 2 feet (Reference 2S) <br /> <br />Approximate flood boundaries in some portions of the study area <br />were taken from FEMA's Flood Hazard Boundary Map (Reference 20). <br /> <br />Flood boundaries for the 100- and SOO-year floods are shown on the <br />Flood Boundary and Floodway Map. <br /> <br />Small areas within the flood boundaries may lie above the flood <br />elevations, and therefore, not be subject to flooding; because of <br />limitations of the map scale, such areas are not shown. <br /> <br />" <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities, <br />and increases flood hazards in areas beyond the encroachment <br />itself. One aspect of floodplain management involves balancing the <br />economic gain from floodplain development against the resulting <br />increase in flood hazard. For purposes of the NFIP, a floodway is <br />used as a tool to assist local communities in this aspect of <br />floodplain management. Under this concept, the area of the 100- <br />year flood is divided into a floodway and a floodway fringe. The <br />floodway is the channel of a stream, plus any adjacent floodplain <br /> <br />16 <br />