C154141 Feasibility Study - Laserfiche WebLink

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II. Conveyance Systems As presented in the Clay Report, water supply for the Grand Valley Lake would be conveyed from a gravity diversion on the main stem of the Gunnison River, downstream of the confluence with the North Fork of the Gunnison. Both gravity-fed and pumped conveyance systems are presented below. Based on the reservoir yield analysis presented in Section IV, two capacities; the 300 cfs capacity used in the Clay Report and a 115 cfs capacity were considered for the two alternative reservoir sizes presented in the next section. Three conveyance alignments were considered for both capacities, resulting in six conveyance system configurations. Field maps prepared for site reconnaissance are included in Appendix A. A. Gravity Flow Alignments Two gravity-flow conveyance alignments were considered in this study. Conveyance A approximates the alignment in the Clay Report and include siphons, tunnels and flumes. Conveyance B eliminates the tunnels, siphons and flumes to determine whether there would be a significant reduction in overall cost by using only open canal conveyance. The purpose of the tunnels, siphons and flumes is to reduce the length of the conveyance system by going through ridges and directly across streams and valleys. Even though open canal construction would be significantly longer, the unit cost per foot of construction is considerably less. Both gravity flow alignments divert water from the Gunnison River just east of Orchard City to provide sufficient head to convey flows from the diversion to the proposed reservoir. A diversion structure in the Gunnison River will be required for both of the gravity flow conveyance systems. The concept (location and height) presented in the Clay Report was used in this study. The streambed elevation at the proposed diversion site is about 5062 and the height of the diversion dam would be approximately 28 feet with the crest of the dam at 5090. The maximum water surface elevation in the canal would be about 5085 feet. Geologic considerations for the diversion dam are discussed in section II.C. The location of the diversion for both gravity-flow alignments is shown on Figure 1 on page 12. Conveyance A, estimated to be approximately 447, 000 feet (85 mi) long, is significantly longer than the comparative alignment in the Clay Report of 275,900 ft(53 mi). One potential explanation for this is that the mapping used to establish the alignment in the Clay Report was at a greater contour interval than used in this study (i.e., 20 feet from U.S. Geological Survey digital topographic mapping). The other possibility is that the alignment reported in the Clay Report envisioned substantial reaches of open canal in cuts and fills up to several tens of feet high to cut across rather than follow the contours along the rugged topography the alignment traverses. In this study, the open canal reaches generally followed the contours to minimize the amount of higher cuts and fills (both to reduce unit earthwork cost and minimize potential instability of the canal cut and fill slopes). Conveyance A and the associated stationing is shown in Figure 2 on page 16. 14 AECOM 2/22/2010