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<br />. Antecedent moisture conditions can irfluence whether a mud flow event will <br />occur for a given rainfall. Anteced(~nt moisture conditions can effect soil <br />erosion, bank and slope stability, and the magnitud," and timing of runoff. <br /> <br />. The availability of adequate sourc," material will contl'ol the magnitude <br />and characteristics of mud flow or mud flood events. The availability of <br />source material will be highly inflnenced by changes in the vegetative <br />cover or land use in watershed areas. Further, the time elapsed since the <br />last major mud flow event will effect the quantity of sediment stored in <br />the channel and watershed which is ava"ilable to produce a mud flow. <br /> <br />For these reasons, the frequency of a mud flow hydrograph was conservatively <br />assumed equal to the probability of exceedance for the rainfall. This assumption <br />is reasonable considering that the probability of exceedance of mud flows can <br />be no greater than that for the rainfall. The delineated flood hazards on the <br />Cornet Creek alluvial fan should encompass both clear-water flooding and mud <br />fl ow events. <br /> <br />Similar to the development of clear-water flood hydrographs, hydrographs for <br />hyperconcentrated sediment flows were generated by model ing runoff from the <br />watershed. The developed outflow hydrographs constitute the input for hydraulic <br />routing analysis across the alluvial fan through the town. Hydrographs were <br />developed for 10-, 25-, 50-, 100-, and 500-YE~ar return period rainfall storms. <br />The excess ra i nfa 11 inputs to the MUDFLO~! model were developed by HEC-) <br />simulations. The resulting runoff was routed over"land and into the main <br />watershed channel by MUDFLOW. To s i mul atE~ the ent ra i nment of sed i ment by the <br />runoff due to bank failures, rill erosion or other processes, the overland water <br />flows were bulked by an assigned sE~diment concentration when it entered waterShed <br />grid elements containing a channel segment. <br /> <br />The runoff was assumed to be bulked with sediment according to a spatially and <br />temporally variable sediment concentration matrix. A detailed discussion of <br />development of the sediment concentration matrix is presented in Reference 2. <br /> <br />A summary of the resulting peak flow estimatE~s for both clear-water floods and <br />mud flows are presented on Table 1. <br /> <br />3.2 Hydraulic Studies <br /> <br />The unconfined topography and historic record of viscous mud nows on the Cornet <br />Creek alluvial fan presents numerous possibilities for flooding in the town of <br />Telluride. The delineation of potential flood hazards requires that a range of <br />flood conditions be assessed. To accomplish this, a 2-d'imensional hyper- <br />concentrated flow-routing model (MUDFLOW) was used to simulate flood hydraulics <br />for various flooding scenarios and a range of water floods and mud flows. <br />Specifically, two general flood scenarios were considered: one which considered <br />the hydraulic capacity of the existing Cornet Creek channel, and another which <br />assumed the channel to be blocked at the fan apex. <br /> <br />In the first flood scenario, th," Cornet Creek channE~l is assumed to remain <br />operational throughout the simulated flood event. Overbank flows occur as a <br />function of the, capacity of the channel. Considering that the blockage of any <br />one of the numerous bridges along the creek by sediment and/or debris is probable <br />during a significant mud flow event, the hydraulic capacity of bridges was not <br />