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<br />SECTION V <br />PREVIOUS FLOOD STUDIES <br /> <br />O'Mara study was the same hydrology used in the previous Flood Insurance Study <br />of 1978. The peak flows in the study area range from 12,500 cfs to 11,700 <br />cfs, <br /> <br />Recent floodplain studies in Littleton for the South Platte River have been <br />performed for the Flood Insurance Agency and Centennial Downs. Centennial <br />Downs is a development on the left bank of the South Platte River, upstream of <br />Belleview Avenue. <br /> <br />The purpose of the Greenhorne and O'Mara study was to develop a revised <br />floodway from that shown on the Flood Insurance map. However, no revised <br />floodplain and floodway map was prepared for the area upstream of Crestline <br />Avenue since this area was not of interest to their client. <br /> <br />Both flood studies were available to Wright Water Engineers for evaluation. <br />The mapping and HEC-II printouts were received. In addition, the engineers <br />who had worked on the previous studies were interviewed. <br /> <br />Flood Insurance Study <br />The Littleton Flood Insurance Study which covered this area was prepared by <br />Gingery and Associates in June, 1978. The 100-year peak flow at Bowles Avenue <br />was 12,500 cfs. This flow was based on an analysis using synthetic <br />hydrographs which reflect the effects of precipitation, ground cover, slope, <br />area and other physical characteristics of the drainage basin. Rainfall data <br />for the synthetic hydrograph analysis were taken from the Denver Regional <br />Council of Governments Drainage Criteria Manual. <br /> <br />Greenhorne and O'Mara used updated cross section information which was <br />obtained from their own 1982 aerial survey. Roughness coefficients were <br />assumed to be 0.03 in the channel and 0.05 in the overbanks. Results of the <br />delineated floodplain and floodway are available. Except for the additional <br />cross sections, the Greenhorne and O'Mara study used the same basic data as <br />the Flood Insurance Study including bridge information. <br /> <br />USACE Channel <br />The U.S. Army Corps of Engineers will channelize the South Platte River <br />through the Bowles Avenue area. Phase 1 of the USACE project extends upstream <br />of Oxford Avenue. Phase 2 is from Oxford Avenue to Crestline Avenue. Phase 3 <br />is scheduled to proceed through the Bowles Avenue area. <br /> <br />The hydraulics were calculated using the Corps of Engineers' HEC-2 model with <br />channe 1 "n" of 0.03 and an overbank "n" of 0.05. The base mappi ng for the <br />Flood Insurance Study was obtained from the U.S. Anny Corps of Engineers. The <br />floodway was developed using an equal loss of conveyance and a one-foot rise. <br />The floodway was delineated to avoid overbank velocities of more than 2 to 3 <br />feet per second. The floodplain and floodway delineation from the Flood <br />Insurance Study is shown on the attachment in the overlay packet. This <br />overlay is to be used with the base mapping in Section VII of this report. <br /> <br />There is a potential for Phase 2 to extend to Oxford, or beyond, in accordance <br />with a request by the City of Littleton, however, this is uncertain at this <br />time. <br /> <br />Phase 2 final design is nearly complete as of June 1984. Initial construction <br />work is scheduled to be let in mid-July, 1984. <br /> <br />A layout of the proposed USACE channel project is shown on Figure 2. <br /> <br />Centennial Downs <br />Greenhorne and O'Mara performed a floodplain and floodway delineation study <br />for Centential Downs development in Littleton. The study reach extends from <br />West Belleview Avenue to Berry Road. Hydrology used for the Greenhorne and <br />