FLOOD07631 - Laserfiche WebLink

Home Browse Search

SECTION VI I I FINDINGS AND CONCLUSIONS reasonable when its HEC-2 model computations are checked against the WWE 1984 Study. While the Centennial Downs 1983 Study used erroneous bridge data for the Crestline Avenue Bridge, the effect of this data was not pronounced upstream as with the 1978 Flood Insurance Study. The reader is referred to Figure 3 for comparison of computed water surface profiles. The regulated floodplain and floodway of the South Platte River, in the vici- nity of Bowles Avenue, is not applicable for use in floodplain administration under 1984 conditions. Based upon the WWE 1984 Study and comparison with previous flood studies and checking with field surveys and inspections, it is the opinion of the consultants that: The lack of applicability of the 1978 Flood Insurance Study to the Bowles Ave- nue areas is due to: 1. Hydraulic profiles of the 1978 Flood Insurance Study are in error due to bridge survey data. The bridges were field checked by Wright Water Engi- neers, Delta Surveys for the District, and by TriConsultants of Denver. The WWE Crestline Bridge data has been confirmed. o The WWE 1984 Study and delineation of the floodplain and floodway is reasonable and dependable. It should be adopted for regulatory purposes on the South Platte River near Bowles Avenue in Littleton, Colorado. We recommend that the WWE 1984 Study be adopted commencing at a point 2,000 feet downstream of Bowles Avenue and extending upstream to a point 2,000 feet upstreif1l of Bowl es Avenue. 2. The channel bottom has changed since the 1978 study. The WWE channel data is based upon 1983 mapping, field inspection, and a field surveyed water surface performed in May, 1984. The various HEC-2 computer model outputs are available for analysis along with field survey data, profiles, cross sections, and aerial photos. 3. The 1 percent (100-year) flood discharge of 12,000 cfs used in the 1978 Flood Insurance Study was to represent existing conditions of basin devel- opment. It is not suitable for this purpose. The District 100-year flood discharge is 8,000 cfs generated from land downstream of Chatfield Reser- voir. The USACE 100-year discharge for the future development is 8,500 cfs. A release of 500 cfs from Chatfield Reservoir during the 100-year flood occurrence is a reasonable addition to the 8,000 cfs computed 100-year flood. A release of 4,000 cfs is not a reasonable assumption as pointed out by the Colorado Water Conservation Board and it is not in accordance with master planning of the Urban Drainage and Flood Control Di strict. The Centennial Downs 1983 Study performed in the same reach of the South Platte River, i.e. upstream of Belleview Avenue, has been found to be