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I I I I I I I I I I I I I I I I I I I large to cause overbank flooding in several areas along the creek. This magnitude of discharge \would be expected') in a rapt dly develop- ing urbanized area, such as lower Bear Creek. Starting water surface elevations were obtained assuming a simultaneous 10-year runoff event on the South Platte River. A water surface elevation for this 10-year event was estimated from available information. TABLE I In many areas along the study reach where heavy development has taken place, there is very little area left unobstructed for conveying flood flows. One reach in particular is in the mobile home parks near the mouth of Bear Creek. Conveyance in this area is confined to the channel and a small area on the overbank. Discharge Summary Location Peak Discharge (cfs) 100-Year Bear Creek Dam Above Weaver Creek and Left Bank Tributary South Kipling Street Bear Valley Shopping Center At the Mouth 1,000 3,600 6,650 7,910 8,150 In several locations, flood flows diverge from the main flow path of the stream and become hydraulicaly disconnected from flows in the channel. These flows generally spread over large sheet flow areas on the floodplain mapping (Sheets 3 to 21), and no water surface elevations were determined. The extent of such flooding was estimated using normal depth calculations and judg- ment of the engineer. Hydraulic Analysis A hydraulic analysis was completed along the channel of Bear Creek to determine water surface elevations for the potential 100- year storm event. This hydraulic analysis was completed using the HEC-II Water Surface Profile Computer Program developed by the Army Corps of Engineers (Reference 4). Cross section data for Bear Creek were obtained photogrammet- rically by digitizing sections marked on the aerial photography of the stream and floodplain. These sections were taken from the same photography flown for the topographic mapping used in this study (Reference 1). The digitized sections were also supple- mented with cross sections taken from this mapping as needed. All bridge cross sections were obtained from actual field measurements. Roughness coefficients (Manning's "n") for the channel and overbank areas were estimated by field inspection of the study area. The ranges for these values are as follows: Special Floodplain Analyses Some of the floodplain areas shown on the Flood Hazard Area Delineation Map Sheets were determined by special hydraulic analyses. The floodplains in these areas are described in some detail in the following paragraphs. /'" ,:' Overbank Channel .025 .035 to .09 On Sheets 3 and 4, the shallow flooding, on the south side of the Bear Creek channel is the result of side spill from the main channel. This side spill occurs from the east side of Federal Boulevard downstream to Clay Street. Approximately 2,000 cublic feet per second (cfs) of the 100-year discharge of 8,150 cfs spills into this shallow flow area and attains a water surface elevation below that of the main channel (see cross-section No.6, Sheele 23) These flows remain separated from the main channel downstream to about station 6+00. The average depth of the shallow flooding is 2.0 feet. ,^ U 'J~(' -4-