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<br />4.1 <br /> <br />IV_ DET.AILED HYDRAULIC ANALYSIS OF THE CORNET CREEK ALLUVIAL FAN APEX <br />4.1 General <br />The town of Tell uri de provided SLA deta n ed survey data of the Cornet <br />Creek alluvial fan apex. This information consisted of surveyed elevations for <br />most the 25-foot square grid elements in the area of the fan apex. The new data <br />reflects the existence of a historic channel on the east side of the fan apex <br />area and was the 1 i ke 1 y fl ow path for the 1914 fl ood through the town. The <br />present Cornet Creek channel is located on the west side of t.he fan and is <br />confined by a man-made berm. The locat.ion of t.he exist.ing channel on the west <br />side of the fan is apparently t.he result of relocation of the channel aft.er the <br />1914 flood event.. Figures 4.1 and 4.2 show the detai led grid system and <br />topography of the fan apex as defined by the original topographic data and the <br />supplemental survey data, respectively. <br />The surveyed elevations more accurately reflect the potential flow path <br />towards the east side of the alluvial fan if flows were to overtop or remove the <br />berm during a flood event.. The inclusion of elevations reflective of the <br />historic channel on the east side of the fan apex in the hydraulic analysis <br />enhances the potential for overbank flows to continue on a course towards the <br />east side of the fan. <br />The hydrographs for the lOO-year return period event for both water flood <br />and mud flow, as presented in the previous draft study report, were used in this <br />supplementa"j study. These flood hydrographs from the watershed canyon WE!re used <br />as input to the fan apex detailed grid system. The fan apex grid system extends <br />approximately 375 feet downstream from the canyon mouth. As described in the <br />draft study report, the outflow hydrograph from the fan apex model was used as <br />input to the 2-dimensional grid system for the entire Cornet Creek fan. <br />For the detailed fan apex grid system, the IOO-year return period water <br />fl ood and mud fl ow were modeled with and wi thout the berm wh i ch E!X i sts near the <br />canyon mouth. For the without-berm condition, elevations were assigned to the <br />elements representing the berm equal to the elevation of grid elements contiguous <br />to the berm and east of the channel. The new survey data provided both <br />elevations of the berm and the channel bed in the vicinity of the fan apex. <br />In the previous draft study report, it was shown that. the existing channel <br />at the fan apex has insufficient hydraul ic capacity for the lOO'year return <br />period water flood or mud flow events. The channel within the detailed apex grid <br />system was simulated by assigning surveyed elevations of the channel beel to the <br />