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<br />Starting water-surface elevations for the Gay Reservoir Channel and <br />City Park Channel were taken from stage-di.scharge relationships <br />computed for Big Dry Creek for the 'tood Insurarice Study for Adamo <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 other streams, <br />the starting water-surface elevations were based on hand <br />calculations at control sections. <br /> <br />The lOa-year shallow flooding for City Park Channel along 120th <br />Avenue, West Lake. overflow near Grove Way, and Nissen Reservoir <br />Channel near Tennyson Street was evaluated using manual hydraulic <br />computations. Various sheet flow areas are present on many of the <br />streams. The lOa-year floodplain has been delineated to encompass <br />all of these addi tional areas. The complete filling of these <br />sheet flow areas will increase elevations downstream, thereby <br />increasing the lOa-year flood hazard. For any channel modification <br />or fill in the sheetflow areas, a detailed review of the <br />improvement should be undertaken to determine the effects <br />downstream. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus con- <br />sidered valid only if hydraulic structures remain unobstructed. <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <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 encourages State and local governments to adopt sound <br />floodplain management programs. Therefore, 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 regional discrimination, the <br />1 percent annual chance (lOO-yead flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. <br />The 0.2 percent annual chance (SOO-year) flood is employed to <br />indicate additional areas of flood risk in the community. For each <br />stream studied by detailed methods, the 100- and SOO-year <br />floodplain boundaries 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 /> <br />-' <br />