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<br />with a contour interval of 40 feet (Reference 42). Flood boundaries supplied by the <br />USACE were transferred directly from the submitted Flood Hazard Information study, <br />because these maps are at smaller scales and thus have more detail and definition <br />(Reference 5). <br /> <br />Approximate loo-year floodplain boundaries in some portions of the original study area <br />were taken directly from USGS Floodprone Quadrangle Maps (Reference 43). <br /> <br />Countywide Restudy <br /> <br />In the countywide restudy of the Yampa River, the Elk River, Waltan Creek, and <br />Burgess Creek as defined in Section 2.1, 2-foot contour interval digital topography was <br />obtained from the City of Steamboat Springs for all areas within its city limits and for <br />some limited surrounding areas. The topography was developed from 1994 aerial <br />photography from 5,000 feet above ground level (AGL), resulting in a scale of 1:1,200 <br />(Reference 22). <br /> <br />The floodplain boundaries for the Yampa River Bypass and split flow reaches at SH 131 <br />were based on topography submitted by Taylor (Reference 44). <br /> <br />Five-foot contour interval topography was obtained from D&D Surveying just south of <br />the city limits of Steamboat Springs, in the vicinity of Lake Catamount. This <br />topography was based on aerial photography from October 1994, flown at 5,000 feet <br />AGL, resulting in a scale of 1:1,200 (Reference 23). <br /> <br />The 100- and 500-year floodplain boundaries are shown on the FIRM. On this map, <br />the tOO-year floodplain boundary corresponds to the boundary of the areas of special <br />flood hazards (Zones A and AE), and the 500-year floodplain boundary corresponds to <br />the boundary of areas of moderate flood hazards. In cases where the l()()- and 500-year <br />floodplain boundaries are close together, only the loo-year floodplain boundary has <br />been shown. Small areas within the floodplain boundaries may lie above the flood <br />elevations but cannot be shown because of limitations of the map scale and/or lack of <br />detailed topographic data. <br /> <br />For the streams studied by approximate methods, only the loo-year floodplain <br />boundaries are shown on the FIRM. <br /> <br />4.2 FloCKlvvays <br /> <br />Encroachment on floodplains, such as structures and fill, reduces flood-carrying <br />capacity, increases flood heights and velocities, and increases flood hazards in areas <br />beyond the encroachment itself. One aspect of floodplain management involves <br />balancing the economic gain from floodplain development against the resulting increase <br />in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local <br />communities in this aspect of floodplain management. Under this concept, the area of <br />the loo-year floodplain is divided into a floodway and a floodway fringe. The <br />floodway is the channel of a stream, plus any adjacent floodplain areas, that must be <br />kept free of encroachment so that the loo-year flood can be carried without substantial <br />increases in flood heights. Minimum Federal standards limit such increases to 1 foot, <br />provided that hazardous velocities are not produced. The floodways in this study are <br />presented to local agencies as minimum standards that can be adopted directly or used <br />'as a basis for additional floodway studies. <br /> <br />37 <br />