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SECTION I EXECUTIVE SUMMARY The previous flood studies, the 1978 Federal Insurance al Downs 1983 Study, were analyzed for reliability. approximate checks were made to resolve the questions. Study and the Centenni- Where questions arose, The urban renewal project of the City of Littleton at the South Platte River and Bowles Avenue resulted in questioning the validity of the regulatory floodpl ain and floodway. It was found that the 1978 Federal Insurance Study was not applicable. The Centennial Downs 1983 Study was found to be reasonable. The analysis undertaken utilized accepted hydraulic engineering methods coup- led with field checking and surveys. The WWE 1984 Study was performed using verified basic data and accepted engin- eering procedures. This study included verification runs using 12,000 cfs as a flood discharge under conditions similar to the previous studies. The WWE work also included floodplain and floodway delineations for the 8,500 cfs dis- charge. This included both having the Crestlirie Avenue Bridge in place, and removed. Wright Water Engineers was engaged by Writer Corporation, with the approval of the City of Littleton and the Urban Drainage and Flood Control District to in- vestigate the validity of the regulatory floodplain. The one percent (100-year) flood used in past studies was determined to be in- consistent with current South Pl atte River hydrological studies and master planning findings. Much of the inconsistency was due to a 1978 flood dis- charge for existing basin conditions (12,000 cfs) and the discharge assumed to be released from Chatfield Reservoir (4,000 cfs) concurrent with the 100-year flood caused by runoff downstream of Chatfield Reservoir. A more approrpriate release from Chatfield Reservoir during the downstream 100year flood is 500 cfs. This was used by Wright Water Engineers as being consistent with the flood control nature of Chatfield Reservoir and master planning. Comparisons of the floodplain and floodway delineations of the various studies and the water surface profiles are given in the attachments and in Figure 3. It has been concluded that the regul atory floodpl ain and floodway of 197!J is not applicable to the Bowles Avenue area which is proposed for urban renewal. The WWE 1984 Study is applicable for regulatory purposes. This floodplain and floodway was derived for the 8,500 cfs flood discharge value. Basic data used in the WWE 1984 Study was obtained from the previous studies, field surveys, current (1983) mapping, and field inspections. All basic data is available for inspection. Where critical dimensions of bridges were dif- ferent between studies, additional confirmation was made. The channel rough- ness coeffi cients were confi rmed from a water surface profile surveyed for a discharge of 1700 cfs. This delineation include Crestline Avenue Bridge as being in place, unless, at the time of the review of the WWE 1984 Study, the bridge has been removed. Information available to the consultant is that the Crestline Avenue Bridge may be removed during June 1984. '-..........._"---_. The South Platte River floodplain and floodway will be moot upon completion of the U.S. Army Corps of Engineers flood control channel through the City of Littleton. This will control flooding in this reach of the river.