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1 <br /> SECTION III <br /> ' PAST HYDROLOGY MODELING REVIEW <br /> ' INTRODUCTION <br /> The principal objective of this Phase II effort was to prepare new modeling for the <br /> 1 South Boulder Creek spill at Highway 36 and the West Valley Overflow <br /> downstream to the Wellman Ditch. <br /> The best way would have begun with the identification of a watershed subbasin <br /> delineation map with a corresponding model schematic. Also, Phase 1 questioned <br /> ' many of the parameters used in the Corps' SWMM model, noted that there were <br /> gauge data upstream of Eldorado Springs which presumably had been used for <br /> calibration purposes, and concluded that model verification studies probably had <br /> been conducted and should be reviewed. <br /> A formal letter requesting such information was sent to the Omaha Corps of <br /> Engineers Office. A reply letter provided further study data such as rainfall; basin <br /> of infiltration, percent impervious; and model sensitivity testing for the Boulder <br /> Creek watershed. Unfortunately, no watershed map for the South Boulder Creek <br /> u watershed area below Gross Reservoir was received. Therefore, major and some <br /> �I minor subbasins were delineated sufficient to correspond to the model data <br /> provided by the Corps. <br /> BASIN AND SUBBASIN DELINEATION AND COMPARISON <br /> Detailed subbasins were delineated on the 1993 City topography and on the latest <br /> versions of the USGS quadrangle maps. Major nodes were identified by reviewing <br /> general area magnitudes of basins and subbasins, identifying subbasins that had <br /> unique and larger magnitude areas such as Martin Gulch below Gross Reservoir, <br /> South Draw below Scar Top Mountain which joins South Boulder Creek above the <br /> gaging station; and Shadow Canyon, Bull Gulch, and Dowdy Draw below Eldorado <br /> Springs. Figure 111-1 delineates many of these features. For basins above Highway <br /> 93 directly tributary to South Boulder Creek the model schematic and model <br /> 'I subbasin areas corresponded, with a slight overage in area by the Corps for a few <br /> u subbasins in and just above Eldorado Springs (380 acres). <br /> Unfortunately, the subbasin basin areas and total area downstream of Highway 93 <br /> did not check. As best as we can decipher, changes to some basins were made to <br /> the model, as indicted such basins being out of order in the input data under a <br /> "change card" that modified other hydrologic characteristics. The model appears to <br /> ' III - 1 <br />