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<br />Starting WSELs for Gay Reservoir Channel and City Park Channel were <br />taken from stage-discharge relationships computed for Big Dry Creek <br />for the FIS for Adams County (Reference 7). The starting WSELs for <br />Airport and Brandywine Creeks were computed using the elevations at <br />their confluences with Big Dry Creek. The starting WSEL for North <br />Branch Airport Creek was determined at peak flow at its confluence <br />with Airport Creek. For all other streams, the starting WSELs were <br />based on hand calculations at control sections. <br /> <br />The 100-year shallow flooding for City Park Channel along <br />120th Avenue, West Lake overflow near Grove Way, and Nissen <br />Reservoir Channel near Tennyson Street was evaluated using manual <br />hydraulic computations. Various sheetflow areas are present on many <br />of the streams. The lOa-year floodplain has been delineated to <br />encompass all of these addi tional areas. The complete filling of <br />these sheetflow areas will increase elevations downstream, thereby <br />increasing the 100-year flood hazard. For any channel modification <br />or fill in the sheetflow areas, a detailed review of the improvement <br />should be undertaken to determine its effects downstream. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the North American Vertical Datum <br />of 1988 (NAVD). <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound floodplain <br />management programs. Therefore, each FIS produces maps designed to assist <br />communities in developing floodplain management measures. <br /> <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 <br />the base flood for floodplain management purposes. The <br />O.2-percent-annual-chance (SaG-year) flood is employed to indicate <br />addi tional areas of flood risk in the communi ty. For each stream <br />studied by detailed methods, the 100- and SOG-year floodplain <br />boundaries have been delineated using the flood elevations <br />determined at each cross section. Between cross sections, the <br />boundaries were interpolated using topographic maps at a scale of <br />1:2,400, with a contour interval of 2 feet (Reference 6). <br /> <br />12 <br />