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<br />2,1 <br /> <br />II. MUDFLOW MODEL DESCRIPTION <br />2.1 General <br />1'1 oodi ng over unconfi ned surfaces can occur in a vari ety of geomor' <br />phological situations. Flooding over a broad flood plain from overbank flow, <br />hyperconcentrated sediment flows on ephemeral watersheds and over alluvial fans, <br />and water n oods in wi de all uvi a 1 washes ar'e a few of the natural fl oodi ng <br />conditions that can be simulated with the MUDFLOW model. Principally, however, <br />the model was developed to s imul ate mud fl ows on all uvi ,a 1 fans, <br />Flood hazards on all uvi a 1 fans are characteri zed by trans i ent phenomena <br />such as debris laden frontal waves, surging and channel avulsion. Mathematical <br />representation of these phenomena, however, is not technically feasible at the <br />present time. The most practical approach to model ing water and hypercon, <br />centrated sediment flows on alluvial fans is to consider the flows as a <br />continuum, combining the water and sediment components, This satisfies the <br />objective for predicting a general range of flow properties, including velocity, <br />depth, and the areas of inundation. The MUDFLOW two,dimensional routing model <br />for hyperconcentrated sediment flows on alluvial fans is a modified version of <br />a model developed for rout i ng overbank ri ver channel floods. The ori g i na 1 model, <br />called the Diffusive Hydrodynamic Model (DH~I), was designed by T. V. Hromadka <br />II and C. C. Yen, and published by the U,S, Geological Su:rvey (Hromadka and Yen, <br />1987) . <br /> <br />Simulation of water or mud flow over an unconfined alluvial fan requires <br />two,dimensional flow,routing which distributes the flow across the fan laterally <br />and longitudinally. Alluvial fans are often very steep and have complex physical <br />and topographi ca 1 conditions. The d i ffus i ve pressure gradi ent term of the <br />momentum equation is important for model i ng compl ex topographi ca 1 condit ions. <br />It permits simulation of ponding, water flow over adverse slopes, and backwater <br />effects due to channel constrictions which a kinematic wave model cannot, The <br />model also has an improved channel,flood plain interface routine which is an <br />integral part of flood event simulation, <br />Flows over alluvial fans are simulated using a central difference, finite <br />dtfference routing scheme to solve the continuity and momentum equations for <br />uniform grid elements. The potential flow surface is discretized into uniform <br />grid elements. The number of grid elements is limited only by the constraints <br />of the computer system and the practical limitations of desired computational <br />time. The present array size in the program code restricts the number of grid <br /> <br />'" ' <br />