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I I I I I I I I I I I I I I I I I I I IV. FLOOD PLAINS AND FLOOD DAMAGE Urban Drainage and Flood Control projects are presently evaluated by comparing amortized improvement costs with the average annual flood damage costs. The magnitude of flood damages and costs for flood control improvements are directly dependent upon estimates of flood flows and limits of the respective floodplains. Given the flood hydro- graphs for floods in a range of recurrence intervals, dollar amounts for flood damage and costs for implementing preventative measures are estimated based on an analysis of the existing hydraulic conditions. Hydraulic Determinations Analyses of the hydraulic characteristics of DFA 0054 and its tributaries were performed to provide estimates of the water surface elevations of each of the design floods for the existing channel conditions. A total of approximately 6.3 miles of channel was included in the analysis, however only 2.4 miles has been included in the Phase "A" Study. All floodplains are delineated by routing the peak flood flow through the channel and calculating the depth of flow in the channel and adjacent floodplain. The depth of a flood, or water surface elevation, is directly related to the conveyance characteris tics of the channel and floodplains: channel geometry, roughness, longitudinal slope, and presence of obstructions such as bridges, houses and large trees. Wa ter surface profiles and floodplains were computed using the most recent version of the U. S. Army Corps of Engineers HEC-2 computer program, Reference F. Cross- sections were digitized when the topographic maps were compiled and have been augmented by field surveys at culverts. Locations of the cross-sections and a more detailed description of the 100-year floodplain is contained in the Flood Hazard Area Delinea tion Report. Channel roughness coefficients (Manning's "n" values) for these computations were assigned on the basis of field inspection of the floodplain areas. Hydraulic losses through culverts on Direct Flow Area 0054 have been determined with the use of nomographs published by the Bureau of Public Roads and input into HEC-2 program. The structural integrity of existing culverts, street embankments, detention dams and other channel features has not been evaluated in this analysis. Computed water surface profiles and the floodplain delineation for the 100-year, future basin conditions event are shown in the Flood Hazard Area Delineation Report for the main stem and two tributaries at 1"=200' scale. The original mylar FHAD drawings at a scale of 1"=100' are on file with the Urban Drainage and Flood Control District. The 100-year floodplain is also shown in Figure IV-1 at a scale of 1"=1000'. Floodplains for the 2-, 5-, and 10-year events have also been plotted on working drawings and areon file with the office of the Urban Drainage and Flood Control District. Flood Hazard Areas The DF A 0054 floodplain is presently utilized primarily for residential and agri- cultural purposes. While some reaches are capable of conveying all or part of the design floods, little provision has been made in the reaches for passing flood flows and the hy- draulic analysis revealed that the potential flood induced damages and hazards are sig- nificant. With the exception of some culverts beneath city streets, the entire channel in Reaches 1 and 2 is on private property. Flood hazards in these reaches originate from inadequate culverts and channel facilities, and undersized detention ponds. IV-1