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tit tit e mudflow is a function of the fluid matrix properties (i.e. density, viscosity, and yield stress), channel geometry, slope, and roughness. Viscosity is in turn a function of the type of sediment (clay or silt), the sediment concentration, and the water temperature. Mudflows have high sediment concentrations and high yield stresses, which may produce laminar flow. Usually, the peak concentration of sediment during a mudflow event is about 45% to 55%, and the average sediment concentration is between 20% and 35%. The objective of this study was to develop a methodology and model that will enable the City of Aspen to estimate the location and magnitude of a possible mudflow event through the City. This model can then be used to develop mitigation measures to eliminate or reduce the effect of a possible mudflow event, to estimate the effect that future development at the base of the Mountain may have, and to establish regulations or guidelines for future development. The computer program, FLO-2D, provides the City of Aspen with an effective tool to achieve these goals. METHODOLOGY FLO-2D Description FLO-2D estimates the amount of runoff that will occur during a rain event and the depth of flow, water, and sediment that will occur due to this rain. FLO-2D is a two-dimensional, finite difference flood routing model, which uses a kinematic wave or diffusive wave equation to estimate overland flow. In addition to modeling water-only flow, the program will also model hyperconcentrated sediment flow. Hyperconcentrated sediment flow is runoff that contains a very high concentration of sediment. FLO-2D requires a representation of the topography of the study area. This is accomplished by establishing a network of nodes and assigning x-y coordinates and elevations to each node. The nodes must be placed in a rectangular grid with equal spacing between nodes. A typical grid node spacing is 200 feet to 500 feet. Decreasing