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<br />!I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />SECTION IV <br /> <br />INITIAL HYDROLOGY MODELING OF THE WEST OVERFLOW AREA <br />OF SOUTH BOULDER CREEK BELOW HIGHWAY 36 <br /> <br />INTRODUCTION <br /> <br />The Corps' SWMM model was adapted to model the West Valley Overflow by <br />replacing all elements downstream of Node 44, which was estimated to be located <br />above South Boulder Creek's confluence with Viele, and specifically assumed for <br />present modeling purposes to be located at South Boulder Creek and Highway 36. <br />None of the other problems observed upstream were corrected as the purpose of <br />the effort was to conduct an initial reconnaissance of the general behavior of the <br />West Valley Overflow. The Corps' rainfall was used as directed. During this <br />portion of the investigation, comparisons with other hydrology investigations <br />revealed that the Corps' rainfall was different from the other investigations. <br />Therefore, rainfall should be investigated and corrected as part of future phases of <br />work. <br /> <br />INITIAL FLOW ESTIMATES AND METHODOLOGY <br /> <br />Drawing 3 at the end of this report illustrates representative results of the <br />hydrology modeling effort. The model is more complicated than the flow arrows <br />and associated numbers indicate. For example, the flows at and across Baseline <br />are quite complicated, with flows going under the road in a culvert, flows in both <br />lanes traveling east, with the east storage spilling onto the roadway, with flows in <br />excess of the Crossover Ditch ultimately spilling north over a two to three block <br />area. <br /> <br />The Boulder 1993 Aerial Topography (1" = 100' scale, 2 foot contour interval, <br />with supplementary spot elevations) was used as the base, supplemented by <br />measurement of key hydraulic structures and limited hand level surveys to identify <br />relative invert elevations and slopes. <br /> <br />Typically flow splits were determined by evaluating normal depth channel capacity <br />downstream of culverts, and then conducting culvert calculations for both inlet and <br />outlet control. For assumed flows, upstream water surfaces and resultant splits <br />were determined by assuming appropriate weir parameters. At each potential split <br />location, a wide range of values was determined so that spills and normal <br />conveyance pattern flows could be simulated from low to extreme flows. <br /> <br />IV-l <br />