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ig i <br />i~i <br />1.0 INTRODUCTION <br />Southwestern Portland Cement <br />SHB AGRA Project E92-7075 <br />Haul Road Bridge Floodplain Analysis <br />Page 1 <br />This report presents the results of an analysis of the impacts of construction of a proposed bridge <br />across St. Vrain Creek at the location shown on Figure 1. The bridge is to be utilized by 85-ton <br />capacity haul trucks transporting quarried rock from the Dowe Flats quarry site to the <br />Southwestern Cement Plant. <br />The objective of this analysis is to evaluate the impacts of construction of the bridge on the flow <br />capacity and surface water elevation of the St. Vrain floodplain during a 100 year recurrence <br />interval flood event. <br />The proposed bridge is to be a 60 foot wide structure with a total span length of approximately <br />116 feet to cross the ordinary high water level of the St.Vrain. Because of the loading by the <br />85 ton capacity haul trucks, a two span structure will be required with the center pier located <br />within the normal channel of St. Vrain Creek. The location of the haul road crossing of the St. <br />Vrain is restricted by land ownership and the Colorado Department of Transportation <br />requirements for crossing Highway 66. Grades for approach fills to the bridge aze established <br />by the required bridge deck elevation and the grade of the crossings of the Burlington-Northern <br />Railroad and Colorado Highway 66. <br />2.0 EXISTING FLOODPLAIN DEFINITION <br />The floodplain of the St.Vrain Creek was initially evaluated by Engineering Professional, Inc. <br />(1987) for Boulder County and the Colorado Water Conservation Board. This analysis was <br />subsequently revised by Love and Associates (1992) to correct certain errors in the initial <br />evaluation as well as to determine a 100 year floodway which is defined as the portion of the <br />flood channel that conveys the design flow with a maximum rise in energy grade line of 1 foot. <br />In completing the analysis, the HEC-2 Water Surface Profiles program developed through the <br />U.S. Army Corps of Engineers Hydraulic Engineering Center was used. <br />Love and Associates determined that within the reach of the drainage between the Highway 36 <br />bridge and the bridge at North 51st Street, the main channel conveyance was inadequate to carry <br />the full 100 year discharge and a split in flow occurs. The discharge split from the main channel <br />flows in a poorly defined drainage channel south of the main channel to near North 75th street <br />at which point the main and southern channels converge to a single floodplain. The area between <br />the two channels has been defined as an area of shallow (0 to 3 foot) flooding. The proposed <br />haul road bridge is to be located about 520 feet upstream of the North 51st Street bridge, near <br />the downstream end of the channel section where the split in flow to the southern channel occurs <br />as shown on Figure 1. <br />The total 100 year flow in the main channel above the Highway 36 bridge was estimated at about <br />9100 cfs. The Love and Associates analysis estimates about 5000 cfs is conveyed by the main <br />channel below the North 51st Street bridge. The remainder is diverted to the south channel <br />floodplain. <br />AG RA <br />EsrM & Environments/Group <br />