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<br />The actual routing algorithm is relatively simple. The change <br />in depth for a given time step is approximated using a linear trial <br />function between the grid nodal points. For each grid element and <br />time step the discharge flux across 1:he four boundaries is <br />calculated using the new depth approximation. The time step <br />increment/decrement is controlled, in part, by diffusive wave <br />stability criteria which restricts the size of the time step based <br />on the choice of grid size (Ponce, et. al., 1978) <br /> <br />The watershed subroutine calculates the overland flow <br />discharge into the upstream waters,hed channels. A separate grid <br />system for the upstream watershed is required for this subroutine. <br />For the simulation of sediment yield from the upstream watershed, <br />a sediment concentration matrix is specified based on the grid <br />element location in the watershe,d and the time interval for <br />sediment loading. To simulate hillslope or channel bank failure, <br />the overland flow from a grid element is bulked as it enters the <br />channel. This purpose of this subroutine is to analyze the effect <br />of varying sediment concentration on the apex flood hydrograph <br />before it spreads over the alluvial fan. <br /> <br />The usefulness of the model is embodied in its versatility to <br />simulate diverse flow problems associated with flow obstructions. <br />Buildings, walls, levees or other obstructions are simulated by <br />specifying area and width reduction factors for each grid element. <br />Area reduction factors are defined by the amount of storage area in <br />each element that is lost to buildings or even natural features <br />such as hills or embankments. Wid.th reduction factors reduce the <br />discharge flux calculated across a grid element boundary. For <br />example, a wall might obstruct flm, across an element boundary by <br />eighty percent. These factors can gr'~atly enhance the simulation <br />accuracy of flow through an urban area. <br /> <br />Flow through bridges or cul vert:s either in the channel or on <br />the floodplain is determined by specifying rating curves for the <br />grid element outflow. Flow through any structure with a known <br />discharge rating curve can be model,~d with this routine. Backwater <br />effects upstream of bridges or culverts as well as blockage of a <br />culvert or overtopping of a bridge can be simulated. <br /> <br />Options have been incorporatl~d in the model to format the <br />output files as either spatially (lr temporally varied. General <br />output parameters include water surface elevation, flow depth, <br />velocity, discharge and sediment concentration. Grid element flow <br />hydraulics may be viewed individually or the elements can be <br />grouped together t.o produce floodplain cross sections. Summary <br />tables listing maximum velocity and flow depths and their times of <br />occurrence appear at the end of th,'~ output file. <br /> <br />FLO-2D data preparation and computer run times vary according <br />to the number and size of the grid e!lements and the dura.tion of the <br />flood being simulated. The computer run t.ime for an 80386-33 mHz <br />machine may be ten times less than that for an 80286 computer. On <br /> <br />18 <br />