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' FIELD WORK <br /> We measured many of the culvert and bridge entrances including the Highway 36 <br /> and 93 bridges over South Boulder Creek, the Highway 36 barrier, the Lower <br /> Flatiron's levee, the Viele culverts, five culverts along the Anderson Ditch, the Dry <br /> Creek spill structure at the Anderson Ditch, the Baseline and Foothills culverts, two <br /> Crossover Channel culverts, and three locations along Dry Creek Ditch No. 2 in the <br /> vicinity of 55th Street, and selected features of the Wellman. <br /> ' COMPARISONS WITH MWE MODELING OF AREAS TRIBUTARY TO 55TH AND <br /> WELLMAN DITCH <br /> As part of planning efforts for improvements to 55th Street, McLaughlin Water <br /> Engineers (MWE) prepared a CUHP/SWMM hydrology model of the local area. <br /> MWE notes that results were within 20 percent of prior WRC SWMM modeling. <br /> Earlier WRC modeling had no simulation of the flow splits which existed. MWE <br /> simulated the spills at Foothills and Baseline, noting that the flow would leave the <br /> basin. Also they simulated a minor split at 55th and Baseline, which we have not <br /> explored. MWE noted a flow of about 800 cfs arriving at the storages at Foothills <br /> and Baseline, 230 cfs leaving the basin there, with 1320 cfs arriving at the <br /> Wellman at 55th. They recommend a design flow of 1820 cfs for the 55th <br /> crossing of the Wellman Ditch, some 500 cfs greater than their model to account <br /> ' for trans basin water that could come into the Wellman at Bear Canyon Creek. <br /> ' While our initial modeling had similar main element flows, subbasins were noted to <br /> discharge at significantly lower unit discharges (cfs/acre). Numerous parameters <br /> were checked and refined, but the major cause was a significantly different rainfall <br /> ' being used. We used a rainfall similar to MWE's in one version of the model in <br /> which we disconnected the South Boulder spill to test the response of the local <br /> drainage areas. Unit discharge values were then found comparable and similar. <br /> ' LIMITATIONS <br /> In addition to leaving the Corps' model above Node 44 as described above, the <br /> following assumptions were used to facilitate the modeling effort. <br /> Because of the fact that the model is an initial investigation, the runoff from the <br /> individual basins was simplified. Rather than using separate catchments for streets, <br /> ' direct flow from lots, and indirect flow from lots, a single catchment representing <br /> all of catchments as an average was used for each subbasin. Imperviousness, <br /> overland slope, and catchment width attempted to represent these averages. The <br /> ' model does not account for backwater effects, associated storage, or floodplain <br /> storage. <br /> ' IV - 2 <br /> t <br />