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� • The spillway is 32-foot wide, 600-foot long channel excavated in rock and lined with concrete. A side channel ogee spillway weir was constructed in 1962 in order to increase spillway capacity. The spilIway channel concrete was repaired in 1970 and again in 1972. Currently, the spillway carries a maximum flow of over I5,000 cfs which is approximately one-half of the probable maximum flood (PMF). New flood hydrology regulations have been proposed by the SEO. New hydrologic analysis based an the new proposed regulations is yet to be performed for this site. Spillway capacity will need to be enlarged to 84 percent of the current PMF or 100 percent of a site specific probable m�imum precipitation (PMP) using new SEO software. The dam embankment has undergone several changes. The dam itself was enlarged in 1979 to gage height 105. This raising not only allowed more storage, but allowed the spillway to discharge at a higher capacity. As part of the 1982 repair designed by Wheeler, the dam was raised again to its now current elevation of 9470 feet or gage height 111 in order to increase spillway capacity to its current 15,000 cfs capacity by increasing freeboard. This work included raising the dam six feet, as well as raising the spillway one-foot in elevation from 9449 to 9450. Essentially this raise was accomplished by putting a steeper upstream slope (Figure 4} on the upper 30 feet of the dam of 2:1 (horizontal to vertical), and then pushing the raise downstream and putting a rockfill downstream slope at a 2:1, which was flatter than the original 1.5:1 design. In addition, toe drains were installed as part of this downstream raise. These toe drains discharge between 500 to 1,004 gpm when the reservoir is nearly full. In addition another 100 to 500 gpm seeps out from the downstream toe area when the reservoir is nearly fu21. The seepage is negligible from both areas below reservoir gage height 40. As discussed previously, horizontal drains were installed in the Ieft abutment in 1993. The drains flow over I,SOd gpm with a nearly full reservoir and dry-up when the reservoir is below gage height 50. PROPOSED I�IPROVEMENTS F�R REHABILITATION The dam and appurtenant structures will require rehahilitation if the reservoir operation is changed to include a full reservoir for longer periods. The proposed rehabilitation costs are itemized on Table 1 and total $15.8 million. These costs represent reconnaissance level estimates only. The improvements are described below: � Dam Embankment and Left Abutment: In order to control the seepage, I recommend a PVC liner be placed on the upstream slope of the dam and also as a blanket on the left abutment. This should reduce total seepage from about 5 cfs when the reservoir is full to less than 1 cfs. The PVC liner will need to be placed on a}irepared subgrade and bedding, and then covered with bedding and riprap. A more rigorous drain system should be ptaced in the Ieft abutment if water is to be stored for extensive periods at maximum water level. Several landslides have been controlled on dams by construction of a drainage tunnel or gallery as proposed. ■ Outlet Works: It is important to have a reliable outlet works capable of safely discharging 2,500 cfs (+). I recommend installing the air vent shaft below the gates as � � .._ f -- -4- �: �..