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c . . . December 7,1991 Page 2 Model Calibration Before the HEC-6 model could be used as a design tool for sizing the Diversion Dike, it was necessary to confirm that the computed sediment yield output closely matched the report average annual yield in the Plan. In order to model the flood flow down the alluvial fan, a representative channel width had to be estimated. It was assumed that apex flows reaching the Dike would be confined to a single channel with a top width equal to that given by Dawdy /FEMA regime equation. T 9,500.4 = where: T Top width of flow (feet) Discharge (CFS) = o = This equation also relies on the assumption that the flow forms the channel and the depth of flow is at critical depth. The major unknown variable affecting the sediment carrying capacity of the flood flows was the percentage of wash load materials (silts) in the inflowing sediment. The concentration of these fine particles affects the total sediment carrying capacity. The distribution of inflow particles larger than wash load size was based on the distribution of grain sizes found in the fan. The fraction for the silt range was selected according to the percentage of wash load material that is commonly found in materials in the valley alluvium. This procedure led to a silt and sand distribution of 40% and 60%, respectively. The estimation of an equivalent average annual sediment yield from the HEC-6 model was done using a numerical integration procedure. The HEC-6 model was run for seven single discharges, each with a channel width and sediment inflow estimates as described above, over the alluvial fan to produce sediment outflow rates for each. A generalized power function equation was developed to relate the sediment outflow to the discharge, which was then applied to the 2,5, 10,25,50 and 100-year flood hydrographs. The volume of sediment yield for each of the events was computed by summing sediment flow for each time increment in the hydrograph. Attachment 4 shows this procedure, The average annual yield of 11.7 acre.feet matches the target rate and reflects a slightly lower annual rate than the 16 AF stated in the Plan. The average annual yield estimated using this procedure reflects the integration of storm events in excess of the 2-year storm. Some sediment yield is expected for lesser events. A slight adjustment must also be made to account for the annual series statistical analysis represented by this analysis. The adjustment factor of 1.3 and was computed by integrating the total individual storm volumes for the Flamingo Wash at Las Vegas, NV and Las Vegas Wash at North Las Vegas, NV gages and comparing this to the total annual runoff volume of those gages. Both gages resulted in comparable relationships, therefore the target annual sediment yield of 16 AF was reduced by 1.30 for an actual target of 12.3 AF, The 11.7 AF was determined to be within calibration accuracy (5%) of the target.