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<br />Starting water-surface elevations for the Gay Reservoir Channel and <br />City Park Channel were taken from stage-di8char~e relation.hips <br />computed for Big Dry Creek for the Flood insurance Study for Adltms <br />County (Reference 7). The starting water-surface elevations for <br />Airport and Brandywine Creeks were computed using the elevations at <br />their confluences with Big Dry Creek. The starting water-surface <br />elevation for North Branch Airport Creek was determined at peak <br />flow at its confluence with Airport Creek. For all othet" streams, <br />the stat"ting watet"-surface elevations wet"e based on hand <br />calculations at contt"ol sections. <br /> <br />The 100-yeat" shallow flooding fot" City Puk Channel along 120th <br />Avenue, West Lake ovet"flow near Grove Way, and Nissen Reservoit" <br />Channel neat" Tennyson Street was evaluated using manual hydraulic <br />computations. Various sheetflow areas are pt"esent on many of the <br />stt"eams. The 100-yeat" floodplain has been delineated to encompass <br />all of these additional at"eas. The complete filling of these <br />sheetflow areas will increase elevations downstream, thereby <br />increasing the 100-year flood hazat"d. For any channel modification <br />or fill in the sheetflow areas, a detailed review of the <br />impt"ovement should be undet"taken to determine the effects <br />downstream. <br /> <br />The hydraulic analyses for this study wet"e based on unobstt"ucted <br />flow. The flood elevations shown on the profiles at"e thus con- <br />sidered valid only if hydraulic stt"uctut"es remain unobstt"ucted, <br />operate propet"ly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vet"tical <br />Datum of 1929 (NGVD). Elevation reference marks used in this study <br />are shown on the maps. <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encout"ages State and local governments to adopt sound <br />floodplain management pt"ograms. Thet"efore, each Flood Insurance Study <br />produces maps designed to assist communities in developing floodplain <br />management measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without t"egional disct"imination, the <br />1 percent annual chance (lOO-year) flood has been adopted by FEMA <br />as the base flood fot" floodplain management purposes. <br />The 0.2 percent annual chance (SOO-yeat") flood is employed to <br />indicate additional at"eas of flood risk in the community. For each <br />stream studied by detailed methods, the 100- and SOD-year <br />floodplain boundat"ies have been delineated using the flood <br />elevations determined at each cross section. Between cross <br />sections, the boundaries were interpolated using topographic maps <br />at a scale of 1:2,400, with a contour interval of 2 feet <br />(Reference 6). <br /> <br />13 <br />