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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 /> <br />Floodplain Hvdroloqv Modelinq below the Canvon throuoh the Urban Area <br /> <br />While the model here gives reasonable results for the tributaries of the foothills and <br />floodplain areas, the actual main stream routing is deficient because of the inherent <br />limitations of UDSWMM. <br /> <br />Review of the total discharges proceeding from Highway 93 downstream reveals <br />that there is essentially no reduction of total flow until past Baseline. Also from <br />comparison of the hydrograph shapes at various points down the main stream and <br />the West Valley Overflow reveals that the spiky peaks are not being attenuated. <br />Floodplain dynamic storage routing is a well documented phenomena. We believe <br />different modeling techniques should be used such as HEC-1 stream routing based <br />on HEC-2 model storage routing data, a more advanced version of SWMM with <br />Routing capability EXTRAN, or UNET. Most importantly, the peaks of the main <br />stream hydrographs are being spilled to the West Valley urban area. Any <br />attenuation of the peaks in the main stream results in direct reductions to the West <br />Valley. Section V presents comparative hydrographs to illustrate these points. <br /> <br />The 500-year flood event will require further review. The flows may extend <br />beyond the range of the flow splits. For example, flow may overtop into the RTD <br />parking structure or east of Foothills. <br /> <br />Reasons for Chanoe from Previous Model <br />, n'f"lfJ <br />Basica~e total flows are higher than the 1977 Corp's mode~because of <br />increa~d and longer duration rainfall, increased area, and to a lesser degree, <br />because of more appropriate land surface and stream routing characteristics. The <br />flows would have been even higher, but Gross Reservoir was found to have larger <br />storage than that previously used by USACE based on both the Colorado State <br />Engineer's Office and Denver Water, <br /> <br /> <br />GENERAL CONCLUSIONS <br /> <br />While we are satisfied with the results of the modeling effort and feel that the <br />report will meet regulatory requirements, allow reliable and safe delineation of <br />floodplains, hydraulic structure sizing, and a base for initiating the master planning <br />effort; there are several caveats or conditions to be aware of: <br /> <br />1. Gross Reservoir in reality provides significant flood storage below the crest of <br />its spillway, which is to the benefit of the downstream residents so long as <br />its operation is sustained. The model does not depict storage below the <br />spillway crest as directed by the sponsors and per CWCB and FEMA <br />regulations. If a formal agreement was made with Denver Water to sustain <br /> <br />1-14 <br /> <br />