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I I I I , I I I I I I I , I a I I I I SECTION IV FLOOD PLAIN ANALYSIS Once the design hydrographs were calculated for the various frequencies, it was possible to evaluate the channel relative to the flood plain for each storm frequency and the area of inundation resulting from each flood. The flood plain eval- uation was completed with the use of the Corps of Engineers' HEC-2, "Water Surface Profiles" computer program. This pro- gram was developed by the Corps of Engineers at the Hydrologic Engineering Center in Davis, California, and has been accepted as one of the most thorough and comprehensive computer pro- grams for calculating water surface profiles in natural and improved channels. This program has also been used in other studies of this type in the Denver Metropolitan area for the Urban Drainage and Flood Control District. To evaluate Slaughterhouse Gulch, 42 channel cross sections were taken from the 1" = 50' scale mapping available for the project. The South Tributary was evaluated with 7 cross sections taken from the mapping. The hydraulic flood flow required was assumed to be subcritical, except for the reach from Datura Street to Powers Park on the main stream and from Powers Avenue to Datura Street on the South Tributary, where supercritical flows will occur. The Mannings' roughness factors, used in the computation of the water surface profile, varied from 0.02 to 0.05 in the channel and 0.04 to 0.07 in the overbank areas. Flood profiles were run on both the streams for the four storm frequencies. Concurrent wi th the channel 'evaluation, structures along the channel were also analyzed. Street- channel crossing structure capacities were computed along with rating curves for each crossing by means of the Water Surface Profiles computer program. In addition, three structures in the study area were evaluated longhand due to their complexity. The structures evaluated by hand were the conduit under Prince Street and Santa Fe Drive, the storm sewer from Powers Park to Windermere Street and the storm sewer on the South Tribu- tary. The conduit under Santa Fe Drive and Prince Street was in- stalled in segments. The sizes range from a 48-inch concrete pipe at the inlet to 66-inch concrete pipe which then empties into a la' x 10' box culvert. The conduit system was anal- yzed assuming inlet control at the 48-inch pipe. The maximum capacity before overtopping was calculated to be approximately 130 cfs. The conduit was found to be interlaced with other utilities which would reduce the capacity of the structure much more. ,IV -1