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<br />2.22 <br /> <br />as fgll. The model may perform poorly if the time step (I1t) versus the grid size <br />(Ax) ratio becomes too large. The model has a,n inherent procedure to all evi ate <br />this instability by decreasing the time step increment until convergence in the <br />computed results in achieved. In essence, thE! sensitivity of the model to the <br />t'ime step selected is dependent on the wave celerity or slope of the discharge <br />hydrograph I1Q/l1t. If the time step is decreased, the calculation of inflow and <br />outflow di scharges wi 11 be refi ned and the depth vari ati on at the nodal poi nts <br />will be smoother. Increasing the grid spacin!j has a similar effect by creating <br />additional water storage. For large variations in discharge (i .e., large I1Q/l1t), <br />the 1 ag in outflow from the source vol ume can generate unac:ceptabl e computat i ona 1 <br />error. <br /> <br />It. . <br /> <br />Duri ng model verifi cat i on, the parameters of slope, di scharge, and <br />Manni ng' s roughness coeffi ci ent were vari eel over a range of val ues. The <br />parameter found to have the greatest impact on the hydraLll ic characteristics of <br />depth and velocity was the friction factor (Hromadka et al., 1985). Changes in <br />nodal spacin!j were found to have 1 ittle effect on mode" results. Variation of <br />the mud flow parameters of viscosity and yield stress have a dramatic effect on <br />flow depth and velocity for flows with sediment concentrations greater than <br />approximately 20 percent by volume. <br /> <br />2.9 Model Calibration <br />Simulation of a mud flow event will rl~quire SOmE! cal ibration of input <br />parameters. Sediment con cent rat ion, vi scosity and yi e 1 d stress parameters, flood <br />plain and channel roughness, flood plain area and width reduction factors, and <br />laminar flow resistance factor can all be var'ied to achieve a desired result. <br />If a worst'case scenario is developed in the simulation, then the sediment <br />concentration can be varied by employing a uniform concentration value or one <br />that varies temporally. If historical flow data are available, then roughness <br />values can be used for calibration. Ideally, a calibration of the MUDFLOW model <br />to a specific site might follow this prOCedUl"e: <br /> <br />1. If the location produces very viscous mud flows, choose a set of parameters <br />from Table B.l in Appendix B; such as Glenwood Springs sample #4 that would <br />produce high viscosity and yield stress. <br /> <br />2. Use a relatively high concentration of 45 percent by volume to represent <br />the viscous flows. <br />