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<br />2,2 <br /> <br />elements to 480. Each grid element is assigned an elevation, roughness factor, <br />and area and width reduction factors. In addition, c6ntiguous elements in each <br />of the four possible flow directions must be identified in the input data file, <br />For each grid element the discharge flux across the four boundaries from <br />the four contiguous grid elements is calculated. The net volume change and <br />change in depth for a given time step area is approximated using a linear trial <br />function between the nodal points on the grids. The flow simulation progresses <br />over the grid elements using an explicit numerical technique, The simulation <br />of split flows is possible. <br />The MUDFLOW model preserves continuity for both the fluid and the sediment. <br />To preserve sediment continuity, the sediment volume is determined by treating <br />the water and sediment as a continuum. A sediment budget array is maintained <br />for each element and is tall ied for each timE! step. Sediment routing is not a <br />feature of the model. The model assumes a rigid boundary and scour and <br />aggradation are not simulated. For a given grid element and time step, the model <br />computes both a water and sedi ment volume, and then ca 1 cuI ates a result i ng <br />sediment concentration by volume. In the next time step, the discharge flux <br />across the four element boundari es is ca 1 cuI ated, each havi ng a spec i fi c sediment <br />con cent rat ion. The net water and sed iment fl ux is determi ned and added or <br />subtracted to the sediment budget of the element in question. A new sediment <br />concentration for the element based on the final water and sediment volumes is <br />computed. <br />The model has been applied to flow over an alluvial wash with a grid system <br />consisting of 460 uniform grid elements, of 200 feet on a side, The simulation <br />was performed on a Dell Computer Company personal computer wi th a 80386 <br />microprocessor and had an approximate 4,hour run time,. A grid system with 95 <br />elements using about half of the grids (300,foDt square), required about 15 <br />minutes computer time. <br />MUDFLOW provides an opportunity to use a three-phase approach to the <br />problem of simulating flows in ephemeral watersheds, watl~rcourses, and alluvial <br />fans. In the first phase, excess rainfall can be routed overland in the upstream <br />watershed. In the second phase, water floods" mud flooqs, and mud flows can be <br />simulated temporally and spatially in the watershed channel, For the third <br />phase, the outflow hydrograph from the watershed channel cou'l d be routed two' <br />dimensionally over the alluvial fan" with or without a channel on the fan. <br /> <br />$' , <br />