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<br />001(;73 <br /> <br />chosen based on observations from the model study. As described briefly in the model study <br />report (presented above), at 3,200 ftl/s the high velocity jet lifted off the bottom of the floor at <br />about the location of the floor blocks, producing a surface boil largely contained within the basin <br />walls. . Increasing the flow to 4,720 fills progressively moved the surface boil downstream onto <br />the concrete apron. This was deemed undesirable due to the rough surface turbulence that was <br />produced downstream. <br /> <br />Surface pressures were measured in the model for both 3,200 ftl/s and 4,720 fills along the 2- <br />1/2: 1 upstream slope, on the basin walls, and on the apron concrete overlay downstream ()fthe <br />rock trap. The surface pressures measured for both flow conditions were considered acceptable. <br /> <br />The next major change at the facility that affected tailwater flow patterns was the construction of <br />the power plant and downstream tailrace channel. No field data have been found that documents <br />tailwater flow patterns for the current downstream configuration. <br /> <br />2. Cavitation damage. - <br /> <br />! <br />I <br />:1 <br />J <br /> <br />A 1979 inspection of the outlet works pipe found several small areas of cavitation damage. The <br />damaged areas were mentioned ina travel report [5] as follows: <br /> <br />The -wye branch tie rod had small eroded patches near the'top and bottom. Patches of <br />c,avitation erosion were noted downstream at the miter joints in the bends upstream of the <br />hollow-jet valves. The ho//ow-jet valves showed no progression of damage observed <br />during an inspection pn May 1, 1979. <br /> <br />3. Gas supersaturation. - <br /> <br />The concentrations of dissolved gases, primarily nitrogen and oxygen, found in the outlet channels <br />below various hydraulic structures have long been of some environmental concern. Excessive <br />supersaturation of dissolved gases can cause gas bubble disease in fish, which can be fatal. The <br />Environmental Protection Agency (EP A) has recognized this threat to fish and has set water <br />quality standards for dissolved gas levels at 110 percent of saturation [14]. <br /> <br />Measurements of gas supersaturation levels resulting from operation of the main outlet works up <br />to 2,000 fills have shown levels of about 115 percent [15]. Gas supersaturation data for higher <br />flow rates are not currently available; however, the 1967 model study data show a flow of 2,000 <br />fills already penetrates to the basin floor [13]. Therefore, flows greater than 2,000 ftl/s are not <br />expected to significantly increase gas supersaturation levels above 115 percent. <br /> <br />4. Pipeline flow instability. - <br /> <br />On May 23, 1979, during an inspection of the right abutment intake area, a loud thumping noise <br />accompanied by vibration and surging in the air vent to the 72-inch-diameter pipes was reported. <br /> <br />16 <br />