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� � proposed by HARZA in order to reduce vibrations on the gates at high flow rates. In addition a new modern gate should be installed in a 150-foot Iong bypass tunnel conshvcted around the existing gates. These improvements will provide reliability, redundancy, and additional emergency release capacity. • S iln lwav: New hydrology calculations are reguired in order to obtain the required capacity of the rehabilifated spillway. I have assumed the construction of a more efficient weir, such as a labyrinth, some channel widening and deepening improvements, construction of channel drop structures, and construction of a dam crest four-foot high concrete parapet wall for additional freeboard will be needed. PROPOSED IMPROVEMENTS FOR ENLARGEMENT I believe that a raise of 10 feet is probably feasible. Additional studies of the left abutment landslide and the reservoir landslides are warranted to confirm this conctusion. There is little room to attempt a raise larger than 10 feet. I recommend a downstream raise as shown on Figures 4 and 5. The dam crest would be shifted downstream and new rockfill placed at a 2:1 (horizontaI to vertical) slope as shown on Figure 5. The improvements are similar to rehabilitation requirements discussed above except additional dam fill for the raise, a new redundant outlet tunnel system, and larger and higher spillway improvements. The total costs for raising the reservoir 1 � feet are $35 million and are itemized on Table 2. This would yield 11,000 additional acre-feet at a cost of less than $3,200 per acre- foot. 1'his cost is competitive with most reservoirprojecfs. In order to constnzct a downstream raise efficiently, the river should be diverted around the work area. lfiis would be accomplished by constructing a new outlet tunnel that exits near the spillway (Figure 5). In addition, a new upper level inlet tunnel would be constructed. This would provide water quality and temperature control of reservoir releases. The outlet would contain a nine-foot diameter steel pipe that would allow pressurized release through a jet valve at the downstream end with a capacity of 3,500 cfs. Once the new tunnel was operational to divert the river, the dam raise and the old outlet tunnel extension could be constructed. The old outlet would be used as a redundant emergency system only and provide a total release capacity (combined with the new outlet} of 6,000 cfs_ CONCL USIONS I. Rio Grande Dam is nearly a 100-year old dam and reservoir constructed at a challenging location on the main stem of the Rio Grande R.iver at an elevation of 9450 feet. 2. The dam, spiliway, outlet gates, and left abutment have required multiple episodes of repair and enlargement. 3. Significant improvements to all four components are required to provide a da.m and reservoir that meets modern current safety standards. Seepage througlZ the dam and lefl � ./.. t ~ � ' � 5 � �' � �-..