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SECTION II I 100-Year Flood The Urban Drainage and Flood Control District's Hydrology for the South Platte River (May 1983) was utilized for the Writer Corporation's current floodplain and floodway analysis. The scrne hydrology has been used in the District's Major Drainageway Pl anning Study for the South Platte River through the Denver Metro area. It represents future basin development conditions. F. I. S. USACE UD&FCD Year of ,S tudy 1978 1979 1983 Present Condit ion 12,500 6,000 Future Condition HYDROLOGY 12,000 8,000 An adjustment was made to the District 100-year flood discharge to allow for a 500 cfs release from'Chatfield Reservoir coinciding with the rainstorm induced flood occurring downstream of the dam. This resulted in a 100-year flood dis- charge at Bowles Avenue of 8,500 cfs. To simulate the rainfall-runoff process, the UDFCD's hydrology used the Storm- water Management Model (SWMM) computer program. The SWMM model was originally developed by the Environmental P~eotection Agency and later modified by the U.S. Army Corps of Engineers and'the Urban Drainage and Flood Control Dis- trict. In determining the 100-year flood, the South Platte River basin from Chatfield Dam to Sand Creek (see Figure 1) was divided into 452 sub-basins. It is intended in the South Platte River Master Plan that, in the event of the design 100-year flood occurrence, Chatfield Reservoir will have it's outlet gates closed (or near closed) for flood control purposes, rather than adding to the downstream flood conditions. For purposes of this WWE 1984 Study, an allowance of a 500 cfs release was made. The storm used to develop the discharge profile for the South Platte River was derived from the Urban Storm Drainage Criteria Manual. Storm distributions were developed based on areal reductions. A 100-year flow of 12,000 cfs was also used in the analysis for comparison with 1978 FIA Floodpl ain Study. The 12,000 cfs was the 1978 FIA 100-year flood at Bowles Avenue based on existing basin conditions. The estimated 100- year flow based on the U.S. Army Corps of Engineer hydrology study in 1979, assuming future basin development conditions, is also 8,500 cfs. Apparently, 3,500 cfs was added to the USACE discharge for Chatfield release to derive 12,000 cfs which then compares with the Federal insurance Study value. The corresponding estimated USACE 100-year existing condition flood is 6,000 cfs, according to the CWCB. For flood routing, the South Platte River was modeled using the cross-section- al information provided by Corps of Engineers HEC-2 runs. A typical section was selected for each reach of the river. Rainfall-runoff data for calibra- tion was obtained from gage records provided by the Denver Regional Council of Governments. The District hydrology for the South Platte River Master Drainage Plan was re- viewed by the Colorado Water Conservation Board Flood Control and Floodplain Management Section in October 1983 (see attached letter). The 100-year flood discharge at the Bowles Avenue bridge is 8,000 cfs. A comparison of this 100- year flow with several previous hydrologic study results is listed below.