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.. Mr. TravisSItlitI1.. . .' .. . . S.an LpisValley ]rrigatipn District RioGrarideDamPropo~ Apri17~ 2003 J>age2 . To c3ptw"eadditionalstoragewillrequire that the spillway crest be raised. We plan to study two wa,ysfor this to be accomplished One approach would utilize an 1nflatable rubber dam section. The secondappro~hwould involve construction of a concrete ogee on top of the existing spillway crest, or as arephlcement for it.. This is a less promising alternative than a rubber dam. Asyour~aIl the exiSting spillway capacitY doesnotallow safe discharge of the. histOric estimate oftl1eProbableMaximum Flood. The State of Colorado is still engaged in a study of meteorological cOnditions that ()ccur at high elevations and their stUdy affect large flood estimates, The resu1tsoftheState's investigations will ultimately influence whether any further enlargettlelltofthespillwaychamiel mightbe warranted at Rio Grande Dam. For the time being we suggest no efforts be spent to develop the modifications and costs that mightbe needed to expand the capacity of the spillway. . OncetheState'sstl1dy is complete the spillway channel can be appropriately sized. hi my opinion the sOlution would involve widening, and possibly deepening the existing .chanIlel. .. Material removed from the channel would potentially be used on the dam face orinconnectlon witi1 raising the crest, provided the channei enlargement was . accoinplishedbefore, or in connection with, raising the dam. INFORMATION .NEEDED FROM DISTRICT We will usetheinfonnation in the Chen Report for geotechnical parameters to perform several stability analyses. Eventua1ly some additional drilling and sampling will be needed in the detailed design phase,. if the project proceeds. We need photos, a drawing(s), and possibly some smvey information from the spillway "bathtub" area to do a layout and cost estimate for the modifications at the entrance to the spillway. We have some topo infonnation of the upstream face of the dam near the crest and the Chen Report said the slope was 4 horizontal to I vertical but I believe that was before the crest was raised. . It would be helpful if we had some recent, more detailed information on the slope of the top 20 feet or so of the upstream face. Also, we need to have some topo along the alignment a dike might take from the left abutment of the "dam," across the "corral" to a connection to high ground somewhere along the entrance road. We have some information on the eleVation of the ground across the corral area. Along this alignment a conventional earthfill dike would probably be the most economical solution. I understand you do not want to consider possible improvements to the existing outlet works system that might be implemented as part of the reservoir enlargement project. That will enable us to concentrate on how best to raise the dam and modify the spillway entrance conditions. I will, however, note several possible improvements for your information and future consideration, and we will perform our evaluations recognizing these outlet works modifications may some day be implemented. The two improvements are: 1. Construction of a vertical air vent shaft to introduce air into the gate chamber just downstream of the gates. This would greatly reduce, or perhaps eliminate, the violent surging that occurs under high discharges when the tunnel begins to flow full. 2. Construction of a bypass tunnel section around the existing gates and inStallation of a large flow control valve such as a sleeve valve or ball valve or jet flow gate. These