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001G73 chosen based on observations from the model study. As described briefly in the model study report (presented above), at 3,200 ft3/s the high velocity jet lifted off the bottom of the floor at about the location of the floor blocks, producing a surface boil largely contained within the basin walls.. Increasing the flow to 4,720 ft3/s progressively moved the surface boil downstream onto the concrete apron. This was deemed undesirable due to the rough surface turbulence that was produced downstream. Surface pressures were measured in the model for both 3,200 ft3/s and 4,720 rels along the 2- 112: 1 upstream slope, on the basin walls, and on the apron concrete overlay downstream of the rock trap. The surface pressures measured for both flow conditions were considered acceptable. The next major change at the facility that affected tailwater flow patterns was the construction of the power plant and downstream tailrace channel. No field data have been found that documents tailwater flow patterns for the current downstream configuration. 2. Cavitation damage. - I A 1979 inspection of the outlet works pipe found several small areas of cavitation damage. The damaged areas were mentioned ina travel report [5] as follows: The wye branch tie rod had smaIl eroded patches near the 'top and bottom. Patches of cavitation erosion were noted downstream at the miter joints in the bends upstream of the holIow-jet valves. The hollow-jet valves showed no progression of damage observed during an inspection pn May 1, 1979. 3. Gas supersaturation. - The concentrations of dissolved gases, primarily nitrogen and oxygen, found in the outlet channels below various hydraulic structures have long been of some environmental concern. Excessive supersaturation of dissolved gases can cause gas bubble disease in fish, which can be fatal. The Environmental Protection Agency (EP A) has recognized this threat to fish and has set water quality standards for dissolved gas levels at 110 percent of saturation [14]. Measurements of gas supersaturation levels resulting from operation of the main outlet works up to 2,000 ft3/s have shown levels of about 115 percent [15]. Gas supersaturation data for higher flow rates are not currently available; however, the 1967 model study data show a flow of2,000 fl?ls already penetrates to the basin floor [13]. Therefore, flows greater than 2,000 fi3/s are not expected to significantly increase gas supersaturation levels above 115 percent., 4. Pipeline flow instability. - On May 23, 1979, during an inspection of the right abutment intake area, a loud thumping noise accompanied by vibration and surging in the air vent to the 72-inch-diameter pipes was reported. 16