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<br />Greeley and Loveland Irrigation Company - Boyd Lake Spillway <br />November 19-21, 2003 <br /> <br />Agenda Item ad <br /> <br />Water Riahts <br /> <br />GLlC owns two diversion points on the Big Thompson River, the GLlC Canal diversion and the Big <br />Barnes Ditch diversion, with a total water right to divert of 360 cfs. In addition, they have a 49,000 <br />acre-foot storage right for Boyd Lake, and a 12,700 acre-foot storage right for Lake Loveland. On <br />the average GLlC delivers 37,812 acre-feet per year to its shareholders (1644 x 23 AF/share.) <br /> <br />Proiect Description <br /> <br />Three spillway alternatives for conveying the flood resulting from the 75% PMP storm event were <br />evaluated, all located in the south embankment near the east side of the reservoir. Currently, the <br />dam crest of Boyd Lake is at elevation 4964.0, however the GLlC only has the rights to inundate <br />lands surrounding the reservoir to 4962.0. Normal water level is 4959.65, therefore the spillway <br />crest would be set at this elevation for all alternatives, with maximum pool elevation at 4962.0. <br /> <br />1. Labyrinth Weir/Fuse Plug Combination <br />2. Labyrinth Weir ($3,160,000) <br />3. Weir Wall with Overtopping Protection <br /> <br />Alternative No.1, Labyrinth Weir/Fuse Plug Combination, involves a labyrinth weir and fuse <br />plug spillway combination. The labyrinth weir would be the service spillway capable of discharging <br />the 1 OO-year storm flow and the fuse plug would serve as the emergency spillway for the remaining <br />discharge. As the water overtopped the fuse plug it would cause the fuse plug to breach to the <br />invert elevation where it would encounter a concrete slab that would serve as a broad crested weir. <br />In order for a fuse plug to be a viable option, the invert would have to be set below the normal water <br />surface elevation within Boyd Lake due to the low head condition. As a result, approximately 4,800 <br />acre-feet of usable storage and,therefore, revenue would be lost in the event that the fuse plug <br />breached. Additionally the flow resulting from this 4,800 acre-feet would increase the outflow peak <br />of the spillway. For these reasons, this alternative was not considered further. <br /> <br />Alternative No.2, Labyrinth Weir, consists of a labyrinth weir with 44 cycles. Each cycle has a <br />width of 25 feet resulting in a total spillway width of 1,100 feet. The total weir length is <br />approximately 5,720 feet. Grouted riprap would be placed between the spillway and the existing <br />irrigation canal and the irrigation canal would be lined with concrete to prevent undercutting of the <br />spillway. The canal overbank on the downstream side of the spillway would be cut to allow the <br />spillway to operate properly and efficiently. <br /> <br />Alternative No.3, Weir Wall v,tith Overtopping Protection, involves placing a concrete wall within <br />the existing embankment at the normal water surface elevation and then protecting the downstream <br />slope of the embankment with riprap to provide overtopping protection. This alternative is not viable <br />because the weir length would be in excess of 4,000 feet and, which is longer than the south <br />embankment. Also, Greeley's water treatment plant is located directly at the toe of the dam and <br />would be adversely impacted by this alternative, so this option did not receive further consideration. <br /> <br />Selected Alternative No.2, Labyrinth Weirwas selected based on technical considerations, <br />construction cost, site physical constraints, and institutional requirements. Given the site <br />constraints it is the only type of structure that would be feasible. Construction would consist of: <br /> <br />1. Constructing a labyrinth spillway with a width of about 1,100 feet and an effective <br />weir length of 5,720 feet. <br /> <br />Page 3 of 7 <br />