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April, 2004 <br />6.0 CONCLUSIONS <br />A need for additional site access during gravel mining operations at the Turnpike Pit in <br />Weld County, CO requires that a low water crossing be constructed across Boulder <br />Creek. The location of the proposed crossing of the creek was chosen because this <br />location was used previously for a similar crossing. Approach roads are already in <br />place in this area and the existing channel bend in the area make this a logical location <br />to construct the crossing. The channel bend will provide a diversion for existing flow <br />while the west half of the crossing is constructed. Once the west half of the crossing is <br />completed, the creek flow can be diverted through three, 48-inch HDPE pipe while the <br />east half of the crossing is then completed. <br />The 6 barrel, 48"diameter HDPE pipe crossing is considered to be a temporary <br />structure to accommodate flows of up to 780 cfs. This flow can only be realized if the <br />water surface of the creek were to reach an elevation of 4940. An estimated average <br />annual peak flow of 500 cfs was chosen as the design flowrate for the crossing. This <br />flow is conveyed by the six culvert barrels with essentially zero headwater, (HWlD = 1). <br />Stream flow greater than 780 cfs will overtop the low water crossing causing erosion of <br />the structure and eventual failure. The failure of the crossing structure at water surface <br />elevations greater than 4940 is required so that backwater effects due to this channel <br />constriction do not inundate upstream properties. Setting the roadway surface <br />elevation on the crossing at 4940 will limit backwater effects to the immediate area <br />(owner's property). No flooding of adjacent properties is expected with this crossing <br />design, however, the owner assumes all responsibility for offsite damage of adjacent <br />property as a result of flooding due to construction of this low water crossing. <br />When gravel mining operations at the site have ceased, the owner is required to <br />remove the low water crossing structure from the floodway and restore the disturbed <br />area to meet or exceed historic floodplain conditions as directed by Weld County. <br />Pipe performance under vertical loading depends on the interaction of the pipe <br />structure and the surrounding soil. It is essential that the construction contractor bed <br />the pipe according to manufacturer's recommendations. General notes regarding <br />bedding of flexible pipe are provided on the drawing of low water crossing design <br />details included with this report. <br />of 6 <br />