FLOOD08209 - Laserfiche WebLink

Home Browse Search

111-7 the water surface elevation on April 16, 1983 and the actual stream thalweg, the thalweg was surveyed in the field or adjusted by assuming a 1 1/2 foot depth of flow in critical areas. Critical areas were considered to be those locations where the computations showed that overbank flooding would occur and in those locations where the channel bottom profile obtained from the April 16, 1983 mapping varied significantly from the channel inverts used by the Army Corps of Engineers in their floodplain analysis in 1977. All hydraulic analyses were done on the basis of existing channel condi- tions. Estimates of channel and overbank roughness factors were made by field inspection and use of photographs. Typical roughness, or Mannings "nY, used in this study are: for the channel, .03 to .035, for the overbank .04 to .07. It was assumed that none of the bridges will be blocked by deb- ris. Large bridges which were expected to have no effect on flood profiles were not modeled in this study. Most small foot bridges constructed as part of the South Platte River trail system were not modeled because they are designed to break away in times of floods. An exception was the Bronco Footbridge at 1-25 which creates backwater in a critical area of the river. face profile for the 100-year flood based on the assumption that all flows would be contained in the South Platte channel. Using a weir flow calcula- tion, estimates were made of the flow from the channel into the left over.. bank area. An HEC-2 analysis was used to establish rating curves for the cross sections within the overbank area. Particular attention was paid to critical sections at bridges and constrictions in the floodplain which would control flood elevations. The 100-year flow in this reach is about 22,000 cfs and it was determined that the channel capacity was about 17,000 cfs. Therefore, when the flow in the overbank area reached 5,000 cfs moving in a downstream direction from 1-70, no additional flow was assumed to occur in the overbank area. Two major controls in this reach, the York Street drop structure and the Burlington diversion dam, were surveyed to establish crest elevations. The thalweg profile used from York Street to Interstate 70 was based on survey information and estimated depth of water on April 16, 1983. Split Flow Calculations Significant split flows occur between York Street and 1-70 and in the cen- tral Platte Valley area. Split flow occurs where water flows in two dis- tinct and hydraul ically separate channels. In the reach from York Street to 1-70 the profile of the left bank elevations was compared to the water sur- Split flow also occurs between Cherry Creek and approximately 13th Street. This reach was extensively studied by SellardS and Grigg Engineers as part of a study to determine the feasibility of developing the central Platte Valley area. The Sellards and Grigg study employed field surveyed cross sections from Cherry Creek to Interstate 25. These surveyed cross sections were incorporated into the Wright Water HEC-2 model. The Wright Water Engi- neers model produced results that were consistent with those obtained by Sellards and Grigg. In no place was there a difference of more than 1 foot in the 100-year water surface profiles between the two studies, and in gen- eral, the difference was less than .5 feet. The differences which did occur can be attributed to the differences in level of detail of analysis, and differences in mapping. After reviewing these results, it was determined that the Sellards and Grigg profiles should be used in the reach from Interstate 25 to Cherry Creek, and the split flow calculations from Sellards and Grigg from Interstate 25 to 13th Street would be incorporated into the Wright Water study. There are locations between Sand Creek and Oxford Avenue where the flood- plain of the river is lower than the 100-year flood elevation, but the 100- year flood is contained within the river banks. In these areas extensive overbank flooding would occur if the river bankS were overtopped. In some places the freeboard between the 100-year water surface elevation and the top of bank elevation is less than 1 foot and in other places it is several feet. This situation exists in several areas from the 3rd Avenue dam to Sante Fe on the left bank, and in several locations between Sante Fe and Evans Avenue. FEMA regulations as to levees and dikes are not expected to apply in these instances, however, a more detailed analysis will be made in Phase B of this Master Plan. Dams The following dams and drop structures were surveyed to establish crest elevations: the York Street drop structure, the Burlington dam, the 3rd