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<br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the <br />l-percent-annual-chance (loo-year) flood has been adopted by FEMA as the base flood <br />for floodplain management purposes. The O.2-percent-annual-chance (500-year) flood <br />is employed to indicate additional areas of flood risk in the community. For each <br />stream studied by detailed methods, the 100- and 500-year floodplain boundaries have <br />been delineated using the flood elevations determined at each cross section. <br /> <br />Town of Hayden <br /> <br />Flood boundaries for the original areas studied by approximate methods were <br />determined through interviews with residents, historical records, the rational formula <br />method, and maps (References 36 and 37). <br /> <br />Town of Oak: Creek <br /> <br />Flood boundaries for the original study were interpolated using USGS topographic <br />maps at a scale of 1 :24,000, with a contour interval of 20 feet (Reference 38). <br /> <br />For the streams studied by approximate methods, only the loo-year floodplain <br />boundaries were shown. Initially, approximate loo-year floodplain boundaries in some <br />portions of the original study area were taken from the Flood Hazard Boundary Map <br />(FHBM) for the town (Reference 39), but through correspondence with the CWCB, <br />these boundaries were modified to account for changes in topography since publication <br />of the FHBM in December 1975. The topographic map published for the modified <br />boundaries was prepared at a scale of 1:2,400, with contour intervals of2 feet <br />(Reference 40) and was used to aid in establishment of the loo-year approximate <br />floodplain boundaries. <br /> <br />City of Steamboat Springs <br /> <br />For streams studied by detailed methods in the original study, the boundaries of the <br />loo-year and 500-year floods were delineated using the flood elevations determined at <br />each cross section; between cross sections, the boundaries were interpolated using <br />topographic maps with a scale of 1 :4,800 feet, with a contour interval of 5 feet or less <br />(Reference 41). <br /> <br />Some areas in Steamboat Springs are subject to sheetflow, that is, broad, shallow, <br />overland flooding generally less than 3 feet deep and characterized by unpredictable <br />flowpaths. The WSELs of flooding in these areas are essentially independent of those <br />along adjacent streamways and are affected principally by obstructions in the flooded <br />areas. <br /> <br />Routt County <br /> <br />Floodplain boundaries for the original study were plotted at surveyed and synthesized <br />cross sections and interpolated between cross sections based on the contours shown on <br />the 7.5-minute topographic quadrangle maps for Steamboat Springs, Rocky Peak, MudsCreek, Cow, Creek, Blacktail Mountain, and Clark, Colorado, at a scale of 1 :24,000, <br />36 <br />