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Southwestern Portland Cement <br />SHB AGRA Project E92-7075 <br />i~i Haul Road Bridge Floodplain Amlysis <br />' _ B' Page 4 <br />for which any increase in flow causes a directly proportional increase in surface water <br />elevation. In a normal stream channel however, an increase in flow results in both a widening <br />of the channel as well as an increase in flow depth to provide the required channel area. <br />5.0 CONCLUSION <br />In summary, the proposed 85 ton haul trucks preclude crossing the St. Vrain Creek ordinary <br />high water with a single span bridge. As a result of the bridge being located immediately <br />downstream of a split channel flow section, a redistribution of flow between the main channel <br />of St. Vrain Creek and a poorly defined southern channel is predicted by the HEC-2 modeling. <br />Flow in the main channel of St. Vrain Creek is reduced by approximately 373 cfs, resulting in <br />an overall decrease in water surface profile elevation from the North 51st Street bridge <br />downstream a distance of about 2000 feet. Diversion of the additional 373 cfs to the south <br />channel increases the computed water surface of the 100 year floodplain in the south channel a <br />maximum of about 0.22 feet from that predicted in the original floodplain delineation study. The <br />split flow channels rejoin near North 75th Street as is the current condition. No change in Flow <br />wndition is predicted below North 75th Street. <br />Engineering Professionals, Inc., 1987, "Fltwdplain Information Report, North St. Vrain and St. <br />Vrain Creeks, Boulder County, Colorado", consultants report prepared for Boulder County and <br />Colorado Water Conservation Board. <br />Love and Associates, 1992, "Floodplain Re-Analysis and Floodway Delineation, North St. Vrain <br />and St. Vrain Creeks, Boulder County, Colorado", consultants report prepared for Boulder <br />County. <br />COE, 1991, "HEC-2 Water Surface Profiles", computer program developed by the Hydrologic <br />Engineering Center, U.S. Army Corps of Engineers, Davis, CA. 95616-4687 <br />~~ A G R A <br />Earth & Environmen to/Group <br />