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II. MUDFLOW ANALYSIS This Section discusses and analyses the potential and magnitude of mud floods and mudflows that may develop due to rainfall events, snowmelt, or rain on snow events, Several different types of models were used to perform these analyses, 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. HEC-l can estimate the quantity of runoff due to melting snow and due to rain on snow, HEC-l does not estimate the quantity of sediment that will be incorporated into this runoff or its effect on the flow. A. RAINFALL ONLY 1. Introduction Mudflows are very viscous, hyperconcentrated sediment flows, whose fluid properties change dramatically as they flow down alluvial fans or steep channels, The behavior of the mudflow is a function of the fluid matrix properties (Le. 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, Smaller rain events . (Le, IO-year or 25-year storm event) are more likely to cause mudflows, Usually, the peak concentration of sediment during a mudflow event is about 45%, and the average sediment concentration is between 20% and 35%, Since mud floods contain ahigher proportion of water than mudflows, mud floods are less viscous and are always turbulent. Mud floods will be produced by larger flood events such as the 100- year flood, 2, FLO-2D a, Introduction FLO-2D is a two-dimensional, fmite 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 9