FLOOD07631 - Laserfiche WebLink

Home Browse Search

SECT! ON VI WRIGHT WATER ENGINEERS 1984 STUDY The two subsequent studies were similar to the previous two studies (12,000 cfs and 8,500 cfsl, except that here the Crestline Bridge was removed from the model. In addition, for the 8,500 cfs discharge, minor cleanup along the left bank, just upstream of the Bowles Avenue Bridge and cleanup of the banks under the bridge, were included in the model. The floodplain and floodway analysis of the South Platte River in the vicinity of Bowles Avenue in Littleton was commenced in May 1984. The work was under- taken for the Writer Corporation of Denver, Colorado after receiving clearance to perform the assignment from the Urban Drainage and Flood Control District and the City of Littleton. For all modelling, the U.S. Army Corps of Engineers Hydrologic Engineering Center computer model HEC-2 was utilized. The WWE 1984 Study commenced with a review of previous floodplain delineation engineering and after conferring with engineering personnel who had performed the previous work. In the analyses, floodways were determined based on equal loss of conveyance. Starting elevations were obtained by developing a rating curve at the Belle- view Avenue Bridge based on previous studies. All bridge pier coefficients, expansion and contraction coefficients, weir coefficients, and total loss coefficients were based on the recommendations in the HEC-2 Users Manual. The topographic mapping of the District was utilized after expanding the cov- erage to include the full floodplain area. The mapping uses two-foot contours and was originally checked to insure conformance with national mapping stan- dards. A summary of studies performed by Wright Water Engineers is presented below. Study A: 12,000 cfs flood; existing conditions; floodplain and floodway delineation Roughness factors for the channel were determined using field surveys of the profile of a discharge in the river of 1700 cfs. Overbank roughness factors were field estimated. All roughness factors were checked against those used in previous flood studies and found to be in conformance. Study B: 8,500 cfs flood; existing conditions; floodplain and floodway delin- eation Study C: Crestline Bridge removed, flow rate of 12,000 cfs The WWE 1984 Study was first directed to utilizing a flood discharge of 12,000 cfs to check with previous flood studies. This, in effect, was a calibration effort to determine Where similarities and differences existed. The flood- plain ands floodway are shown in an Overlay in the packet. Study D: Crestline Bridge removed with improvements at the Bowles Avenue Bridge, flow rate of 8,500 cfs The second part of the effort was to determine the floodplain and floodway un- der existing conditions using a flood discharge of 8,500 cfs. The results of this analysis are shown on an Overlay in the packet. Table VI-1 displays the floodplain and floodway data for the lOa-year floodplain using an 8,500 cfs di sch arge.