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<br />3.1 <br /> <br />III. BACKGROUND <br />In the previously submitted draft study report (SLA and O'Brien. 1989) the <br />flood-hazard areas for the 10-, 50-, 100.., and 500-year return period water <br />floods and mud flows along Cornet Creek were dEl'neated. Maximum flow depths, <br />maximum velocities and areas of inundation WE!rl, summarized for each of the <br />val'i ous return peri od flood events. The repo'^t also conta i ned a complete <br />description of the Cornet Creek watershed, its geology and soils, an analysis <br />of the basin hydrology, and the results of the hydraulic analysis. <br />In the draft study report, the delineation of flood hazards on the Cornet <br />Creek alluvial fan involved the analysis of twc flood scenarios. The first <br />scenario assumed that the Cornet Creek channel remained in operation throughout <br />the flood event; the second assumed that the channel was nonfunctional <br />(obstructed by debris) allowing flood flows to follow an unconfinE!d path over' <br />the fan. With the channel in operation, flow depths were determined to be less <br />than I foot for water floods. For mud flows, flood depths were determined to <br />exceed 6 feet at various locations on the fan. Without the channel in operation, <br />water flood..flow depths were also found to be less than I foot in depth over the <br />entire fan. Mud flow depths were calculated to range from 2 feet to 7 feet. <br />In the draft study report the potential fl~N direction from the fan a~ex <br />was defined by use of a detailed grid system for the fan apex area. The detailed <br />grid network consisted of 176 elements, with each element representing a 25-foot <br />square area of the fan apex. The grid element'; were assigned el evations to <br />represent the topography of the apex and an existing berm which confines the <br />Cornet Creek channel in the area. The hydraulics of the area were analyzed with <br />and without the influence of the berm. <br />The results presented in the draft study report of the detailed study of <br />the fan apex, demonstrated that the 100-year mud flow generally followed the <br />existing channel course of Cornet Creek towards the west side of the fan based <br />on available topographic maps. Removing the berm from the simulation was found <br />to have no significant effect on the predicted flow path. To enhance the <br />potential for flow to occur on the east side of the fan, elevations of various <br />grid elements were revised. The modifications were determined to have little <br />effect on the predicted flow path. <br />Since the topography of the fan apex could have a significant impact on <br />the direction of flood flows over the fan, que:';tions were raised during the <br />review of the draft study report concerning the adequacy of the topographic maps <br />