FLOOD08209 - Laserfiche WebLink

Home Browse Search

XI-4 Hydraulic Roughness. Hydraulic computations for flow in the South Platte River channel should utilize the following Manning Un" roughness coefficients which are based on USACE or WWE calibrations. f) A factor of 1.65 will be used for total loss calculations in the HEC-2 model. This is the equivalent of ( : 0=8 in the orifice flow equation. Reach Location Manning n 1 Chatfield to Columbine 2 Columbine to Oxford .030 3 Oxford to Bates .030 4 Bates to Colfax .030 5 Colfax to Cherry Creek .030 6 Cherry Creek to 23rd .035 23rd to Franklin .030 7 Franklin to Sand Creek .035 8 Sand Creek to Baseline .035 g) A coefficient of 2.6 will be used for weir calculations. h) The bridge opening will be based on the projected area normal to the flow direction. i) No debris blockage of bridges across the South Platte River is assumed because of Tri-Lakes. Floodplain Determinations. Water surface profiles will be calculated using the Corps of Engineers HEC-2 computer model. Profiles will be determined using the following criteria: Damage Estimates. Damage estimates are to be made for the 10-year, 50-year and 100-year storms assuming ultimate flow conditions and existing channel and floodplain configurations. These damage estimates will provide the baseline against which to evaluate the benefits associated with alternative flood improvement plans. In evaluating damages, the following general criteria will be observed: a) 10-year, 50-year and 100-year peak discharges will be used to deter- mine water surface profiles. a) Damages are to be evaluated by reach. b) The floodway in Denver will be determined assuming equal encroach- ment into the existing floodplain which results in a 1 foot rise in the energy grade line elevations. b) Residential and commercial damages are to be based on actual pro- perty values and curves developed by the Flood Insurance Administra- tion and the Corps of Engineers. c) Friction between cross sections will be calculated by averaging the friction slopes at each section. c) Agricultural damages are to be based on information obtained from Soil Conservation Service. d) A contraction coefficient of .1 and expansion coefficient of .3 will be used unless special factors warrant modification, such as defined in the HEC-2 Users Manual. d) Other damage estimates will be based on the total reconstruction cost. e) The HEC-2 special bridge routine will be used for all bridges. Bridge Freeboard. All proposed bridges crossing the South Platte River should maintain a minimum two foot freeboard between the lowest elevation of the bridge, or attached utility pipes, and the 100-year flood water surface elevation.