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<br />00lti82 <br /> <br />inspections of all hydraulic features, mechanical repairs, operational testing, and possible <br />structural modifications. <br /> <br />I' <br /> <br />The areas most susceptible to cavitation damage include the 30-inch and 110-inch wye branch <br />locations, the downstream bifurcation, and the two hollow-jet valves for the main outlet works; <br />and the two 4- by 4-foot outlet gates and the downstream tunnel for the auxiliary outlet works. <br />Additional potential cavitation areas include the 20-inch manhole locations inside the 110-inch- <br />diameter pipe, and the 90 degree elbow in the main outlet works intake structure. The vertical tie <br />rods within the 110-inch wye branch and the downstream bifurcation may be subject to both <br />cavitation and vibration damage. These areas should be closely inspected before and after <br />operational testing by unwatering the downstream tunnels. Inspection of the 90 degree elbow <br />would require installation of the circular bulkhead at significant cost, and is not required at this <br />time. Any cavitation damage found should be repaired. <br /> <br />The hollow-jet valve stilling basin is a relatively short basin whose high efficiency as an energy <br />dissipator depends on the creation of turbulent eddies within the basin. Ifloose material reaches <br />these eddies, severe abrasion damage to the stilling basin will occur. Hydraulic model studies <br />conducted in 1965 confirmed that the stilling basin will operate satisfactorily if adequate <br />precautions are taken to keep the basin free offoreign material. Discharges above 3,200 ff/s <br />were found to cause the stilling action to extend well beyond the end of the basin, however, with <br />resulting large waves in the downstream channel. At higher tailwater levels, conditions within the <br />basin became more unstable, with strong longitudinal surges and surface boils; however, wave <br />action in the downstream channel became weaker [13]. These studies modeled the stilling basin <br />and outlet channel configuration without the subsequent power plant and tailrace modifications. <br /> <br />, <br /> <br />U <br /> <br />An underwater inspection was performed for the hollow-jet valve stilling basin in October 1997 <br />[23]. The stilling basin was found to be in very good condition, with no damage reported for any <br />of the structure's surfaces, including the chute blocks. Approximately 2 yd3 of rounded gravels <br />and cobbles, ranging in size from 1 to 12 inches, was reported to extend about 6 or 8 feet from <br />the downstream toe of the outlet chute across the full width of the structure. Approximately 1.5 <br />yd3 of construction debris (chain, pipe, and nails), gravel, and cobbles was reported near the right <br />end of the rock trap beyond the 6: 1 slope. Additional inspections of the stilling basin both before <br />and after operational testing at higher discharges are recommended, using a remotely-operated <br />submersible vehicle (ROV). The TSC can furnish a ROV suitable for this purpose, and perform <br />the necessary basin inspections. These inspections would be used to document the position and <br />approximate quantity of material in the basin, rock trap, and concrete apron, and document the <br />existence of peripheral riprap lying below the maximum tailwater level. An approximate <br />gradation and quantity estimate of rock material less than about 1 foot in diameter in the vicinity <br />of the basin before and after releases would be detennined. It is suggested that the rock trap be <br />cleaned before any operational testing. Existing materials within the basin at the upstream end <br />may be left in place to help determine the flushing capability of the basin at higher discharges. <br /> <br />The 72-inch hollow-jet valves, 72-inch ring-follower gates, and 6- by 13-foot fixed-wheel gate of <br />the main outlet works have experienced flows in the 3,200 ft?!s range for short durations in the <br /> <br />25 <br />