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<br />. <br /> <br />Page 3 <br /> <br />December 7, 1991 <br /> <br />Other variables affecting the sediment transport capacity of the streamflow were estimated and <br />are shown on Attachment 5, The procedure for estimating Manning's 'n' value is shown on <br />Attachment 6. <br /> <br />Desion ADDlicatlon <br /> <br />Once the HEC-6 model was satisfactorily calibrated for this application, the size of the Diversion <br />Dike could be estimated from the results of the program. <br /> <br />The basic assumption used for sizing the Diversion Dike is that it should convey all sediments <br />carried by the flow on the alluvial fan. This ideal situation is not possible due to the slope <br />constraints along the proposed Dike alignment. The alluvial fan surface is at a 4% slope, <br />whereas the maximum Dike slope is 1%. According to our analysis, there is no channel <br />geometry which has equal sediment carrying capacity to the fan flow for the given slopes. <br />Consequently, a Dike channel was designed to carry as much of the sediment from the fan as <br />possible. For the varying slopes along the proposed Dike alignment, a balance of transport <br />capacity was achieved by adjusting the channel geometry (width) to increase or decrease flow <br />velocities as necessary. <br /> <br />. <br /> <br />Another major assumption had to be made regarding the transition of flow from the fan to the <br />Dike when modeling the system. HEC-6 is not very adept at representing abrupt changes in <br />slope or geometry. Therefore, an artificial transition section was developed for the purpose of <br />modeling the sediment transport at the fan/Dike interface. This assumed transition also <br />allowed for the computation of sediment volume deposited at or near the interface. As a <br />conservative approach, it was assumed that the entire volume of sediment deposited through <br />the transition occurred in the Dike channel. <br /> <br />With the above approach in mind, the first step was to separately run the seven single flows <br />through the model with the alluvial fan reach linked to the first segment of the Dike. The results <br />provided the rate of sediment outflow at the downstream end of the fan and the downstream <br />end of the Dike. Power function equations relating the sediment outflow to the discharge were <br />developed as shown on Attachment 7. This was necessary since the Dawdy/FEMA <br />assumption is based on a channel width which is dependent on discharge. Changes in <br />channel width as a function of discharge can not be modeled in HEC-6. <br /> <br />The power function equations were applied to the 1 DO-year hydrograph. A spreadsheet <br />(file = Mass.CAL) was used to find the fraction of the total sediment load that is carried before <br />the peak discharge. The difference in volume of sediment (inflow-outflow) was calculated for <br />each time step. The accumulated volume up to the peak discharge was then divided by the <br />total accumulated volume for all time steps through the hydrograph, It was assumed that the <br />density of the sediment remained constant. It was found that 28% of the total sediment volume <br />occurs before the peak flow. <br /> <br />. <br /> <br />The previously discussed transition section linking the fan to the Dike was assumed to be a <br />parabolic vertical curve with a length of 1,000 feet. This is a mathematical approximation of the <br />physical process of deposition and channel realignment which would occur at the fan/dike <br />interface during the rising limb of the hydrograph. A HEC-6 run was made with the 100-year <br />hydrograph routed through the fan, transition section, and the first reach of the Dike channel. <br />The total volume of sediment deposited in the transition section was calculated by HEC-6 for <br />the entire hydrograph. <br />