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3.2 Hydraulic Analyses Analyses of the hydraulic chara9teristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the se1~cted recurrence intervals. For all streams studied by detailed methods, the water-surface elevations for floods of the setected recurrence intervals were computed using the U.S. Army Corps of Engineers HEC-2 step- backwater computer program (Ref~rence 5). Cross sections used in the backwater analyses for all streams were obtained by aerial photogrammetry (Reference 6). The below-water sections of all cross sections were obtained by field measurement. All bridges, dams, and cu1vertsjwere field surveyed to obtain elevation data and structural g~ometry. , Locations of selected cross sec~ions used in the hydraulic analyses are shown on the F100djProfi1es (Exhibit 1). For stream segments for which a f100dway was computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and F100dway Map (Exhibit 2). . Roughness factors (Manning's "n\') used in the hydraulic computations for the detailed sfudy streams were chosen by engineering judgment and based $n field observations. Roughness values for the main channels oflthe streams studied ranged from 0.02 to 0.065; overbank roughness values ranged from 0.02 to 0,10. Starting water-surface e1evatiorts for the Gay Reservoir Channel and City Park Channel were'taken from stage-discharge relationships computed for Big Dry Creek for the Flood Insurance Study for Adams County (Reference 7). The starting water-surface elevations for Airport and Branaywine Creeks were computed using the elevations at their conf1ue~ces with Big Dry Creek. The starting water-surface elevatio~ for North Branch Airport Creek was determined at peak flow at }ts confluence with Airport Creek. For all other streams, the star~ing water-surface elevations were based on hand calculations at control sections. The lOa-year shallow flooding for City Park Channel along 120th Avenue, West Lake overflow neariGrove Way, and Nissen Reservoir Channel near Tennyson Street wa~ evaluated using manual hydraulic computations. Various sheetf10~ areas are present on many of the streams. The lOa-year f100dp1a~n has been delineated to encompass all of these additional areas. I The complete filling of these sheetf10w areas will increase e~evations downstream, thereby increasing the lOa-year flood h~zard. For any channel modification or fill in the she~tf10w areas, a detailed review of the improvement should be undertaken to determine the effects downstream, 12 )