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<br />following empirical relationships c:an be selected unless otherwise <br />specified: <br /> <br />f3,Cv <br />1) = a, e <br /> <br />(11) <br /> <br />and <br /> <br />T = a2 e f32Cv <br /> <br />(12) <br /> <br />where a, and f3, are empirical coefficients defined by laboratory <br />experiment (see Table 3, O'Brien and Julien, 1988). ThE! viscosity <br />and yield stress are shown to bE! functions of the volumetric <br />concentration Cv of the fine sedimemts. <br /> <br />The model preserves continu,ity for both the water and <br />sediment. For every grid element and time step, the change in the <br />water and sediment volumes and the corresponding change in sediment <br />concentration are computed. The viscous and yield stresses, which <br />are a function of the sediment cc:mcentration, control the flow <br />hydraulics as the sediment is track:ed through the grid system. At <br />the end of the simulation, the mode:lL reports on the amount of water <br />and sediment removed from the s1::udy area and the amount and <br />location of the water and sediment remaining on the fan or in the <br />channel. <br /> <br />5.4 Model Limitations and Assumptions <br /> <br />The resolution of the FLO-2D model is dependent on the size <br />and number of grid elements use,d in a flow simulation. A <br />sufficiently detailed grid systl!m is necessary to properly <br />discretize the inflow hydrographs and boundary conditions. <br />Defining a grid system with small grid elements increases the <br />accuracy of the simulation, but can lengthen the computer run time <br />or possibly inhibit the algorithm stabil i ty. A balance must be <br />struck between the number of grid elements and an acceptable <br />computational time. <br /> <br />Sediment exchange between the:, flow and bed (i. e. scour and <br />deposition) is not simulated by the model because of the <br />complexities involved with sediment transport in viscous fluids. <br />FLO-2D is a rigid boundary model. 'l'he routing of sediment and <br />water are coupled in the mass conservation equations. It is assumed <br />that the average flow hydraulics are not appreciably affected by <br />scour and deposition that might occur in an individual grid <br />element. On the steep slopes of alluvial fans, this is a <br />reasonable assumption. <br /> <br />Two other limitations of the FLO-2D model are: 1) small hills <br />and depressions are ignored because the topography of a grid <br />element is represented by a point E~levation; and 2) the diffusive <br />wave approximation of the momen1::um equation does not permit <br />simulation of special flows such as unsteady phenomena (surging). <br /> <br />22 <br />