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1 <br /> ' SECTION IV <br /> INITIAL HYDROLOGY MODELING OF THE WEST OVERFLOW AREA <br /> OF SOUTH BOULDER CREEK BELOW HIGHWAY 36 <br /> INTRODUCTION <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 /> ' INITIAL FLOW ESTIMATES AND METHODOLOGY <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 /> ' 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 /> 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 /> IV - 1 <br />