Engineering Feasibility of Long Draw Reservoir Enlargement

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occurrence cannot be labeled with a return period, hydrologists often consider the approximate frequency of a maximum probable flood as about once in 10,000 years. The natural drainage basin has 8.5 square miles of high mountain land which is about 85 percent forested. Much of the unforested land is above timberline. The basin ranges from an elevation near the dam of 10,065 feet to a maximum of 12,265 feet at Iron Mountain. It was concluded that the maximum probable flood would have a peak inflow of 1,700 cubic feet per second. Added to this would be a potential maximum of 525 cfs flowing into the basin from the western slope via the Grand River Ditch, based on the assumption that the ditch would not breach on the Western Slope during this flood. Based upon maximum probable conditions, the following characteristics 2,225 cfs .?("YCA OP7/, 3,090 Acre Feet 6 4 'r 3,590 Acre Feet $?,0 y Peak Inflow to Reservoir 24 Hour Volume 48 Hour Volume the design flood inflow has The dampening effect of the reservoir storage on the peak of the hydro - graph would reduce the peak to approximately 1,725 cfs. This 1,725 cfs is the rate of flow which the spillway should be capable of passing, Outlet Works The existing outlet works, consisting of two 36" x 48" conduits con- trolled by, two Hardesty No. 191 slide gates, would be modified to suit the new embankment and water pressure conditions, and would be lengthened to the new embankment toe. We have inspected the tubes, and except for minor trans- verse cracking from past settlement, they are in good condition. Some grouting and patching will be necessary after the new embankment weight is in place and most of the new settlement has occurred. The capacity of the outlet works has been reported to be about 275 cfs. HowF computations indicate a capacity nearer 600 to 700 cfs, if the conduit were to be allowed to flow under some pressure in the upstream portion. The existing gates have a design maximum water pressure of about 50 feet. Increasing of the maximum normal water surface to 74 feet of head will require that new gates be installed. It is anticipated that the gates would be Armco Model 100 -30, 36" x 48" in size, with new wall thimbles, new hydraulic opera- tors, and a new hydraulic control system. A new gatekeeper's shack on the dam crest would be installed to house the hydraulic valves, oil tank, and power source and oil pump. A standby hand pump would be included. A concrete headwall addition upon which to anchor the gate thimbles is required, though it is believed that the existing trash rack can be reused. 21