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tit mudflow event at or through the development, and it can increase the water surface elevation in properties adjacent to the development. INTRODUCTION The City of Aspen is located in Township 10 South, Range 85 West of the 6th Principal Meridian, in Pitkin County. The City lies at the base of the north side of Aspen Mountain adjacent to the Roaring Fork River. Runoff from the north side of the mountain flows through the City on its way to the Roaring Fork River. The River then empties into the Colorado River. The selected study area is bounded on the north by the Roaring Fork River, on the west by the drainage boundary of Castle Creek, and on the east and south by the drainage basins on Aspen Mountain tributary to the City. The 2.5 square mile study area for this tit project includes the Spar, Pioneer, Vallejo, and Copper Gulches to the south. The study area also includes that portion of the City of Aspen that is bounded by the Roaring Fork River to the north and Castle Creek to the west. Mudflow events occur when runoff from a precipitation event "fluidizes" the typically non- cohesive sediments on an alluvial fan and causes this mud to flow down the mountain slope. Compared to "water-only" floods, these mudflows have a slower velocity but a greater depth. Aspen Mountain has had a history of mudflow events. There is geologic evidence to suggest that mudflows down the face of the mountain have been an ongoing process. In fact, as recently as May 1996, a mudflow event occurred on the alluvial fan adjacent to the watershed above the City of Aspen. Development continues to occur at the base of the Mountain, slowly proceeding upslope. This development increases the risk of a mudflow event and the magnitude of the damages that may occur due to such an event. e 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