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the relationship between 100-year event yield and t~le average annual yield were identified in the literature but none could be transferred directly to the Range Wash basin. As a result, it was necessary to undertake an iterative process to determine the event specific sediment yields from the 1 DO-year event. The process began with the estimation of the equilibrium sediment transport down the alluvial fan surface for various recurrence inte,rvals. These values were then integrated to establish an average annual sediment yield based on the fan's ability to deliver sediment. This yield estimate was compared to the PSIA elltimale of average ,mnual sediment yield. When both estimates matched, the individual event yield was assumed to be correct and was used for future analyses and ultimately for dike sizing. The following paral1raph describes this process in greater detai/. The HEC-6 computer program was used to simulate sediment transport thmugh the entire fan/dike system. Yang's formulation of sediment transport equations presented in HEC-6 was used to compute transport capacity. An article published in the Journal of Hydraulic Engineering (Comparisons of Selected Bed-Material Load Formulas, Yang and Wan; ASCE 117(8)973-989) compares several sediment transport Elquations for various sediment concentrations, slopes and Froude numbers. For conditions similar to those encountered along the dike (steep slopes and Froude numbers nea.r one), Yang's formulation was dlltermined to provide the best results when compared to measured results. Yang's methodology is commonly accepted, as demonstratEld by its incllJsion in HEC-6, and appears to be the most appropriate formulation for application on Ran~le Wash (See "Yang paper' Section in Design Notebook.) The process of establishing the event specific yield required several key assumptions which had to be tested. A critical assumption was the determination of material size distributions for both the inflowing sediment load generated in the basin above the apex and the fan surface materials. Eight surface samples were taken on the alluvial fan to '9stimate the particle sizes to be used as the bed material distribution in the transport model. This particle size distribution cOIJld not be used as inflow to the system since no fine materials were found on the aI/uvial fan surface and yet the upstream watershed providllS an abundclnt source of fine particles which are washed through the system. To estimate the fine particle portion of the total sediment load, samples from the val/ey floor further downstream in the Las Vegas Wash system were used. (See the "Particle Size Distribution" Section.) Critical assumptions were evaluated during this calibration and integration process to detl3rmine their sensitivity and reasonableness. These variables included the roughness coefficient, the inflow sediment volume estimate, the temperature 01 the inflow, and the particle size distribution of the inflowing materials. The results of the sensitivity analysis indicate that the estimate of average annual yield as a function of Elach !lpecific yield was relatively insensitive to minor changes in variables with the exception of the percentsge of fine particles in the inflow. The final percentage of fine particles, those less than 0.064 millimeters, was 40% and compared favorably with particle size distribution of materials farther downstream in the Las VElgas Valley. The description of channel cross section geometry in HEC-6 requires a fixed channel bank. The channel bed may move vertically (scour and deposition), but nIl lateral changes in cross- section are possible. The Dawdy/FEMA assumption (Flood FreqUE3ncy Estimates on Alluvial Fans, ASCE Journal of the Hydraulics Division, 10~;(11), 1407-1413) for alluvial fan flooding was used in this study. This assumption establishes a channel geometry on an alluvial fan which is a function of discharge (See "Fan Geometry" Section). Because HEC-6 does not allow for iateral changes in the channel cross-section, it was necessary to run the fan model with a constant cross section. Therefore, a series of constant discharges and fixed cross sections were modeled to establish alluvial fan flow characteristics and E\quilibrium sediment transport. For the 100-year peak example see the Design Noteboo~ "Fan Equilibrium" Section. The model