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<br />001561 <br /> <br />equation for maximum losses and the 4- by 4-foot regulating gate fully open, <br /> <br />Q = 101.9 (RWS - 5771) O.S <br /> <br />with Q equal to the discharge in fels, and RWS equal to the reservoir water surface elevation in <br />feet. Reservoir head is measured from the centerline of the regulating gate, or elevation 5771. <br />This equation suggests a maximum discharge of 1,850 fi3/s at the maximum reservoir elevation <br />6101.6, or about 4 percent greater than the discharge curve indicates, and a discharge of 1,800 <br />fi3/s at elevation 6085.0. The Project Data information for Navajo Dam [3] cites an auxiliary <br />outlet works discharge capacity of 1,700 fi3/s at elevation 6085.0. <br /> <br />Assuming a discharge of 1,800 fi3/s at reservoir elevation 6085 (using the discharge equation), a <br />flow velocity of 112 ftls would be required through the 4- by 4-foot gates. Using the detailed <br />hydraulic loss computations included in the Technical Record of Design and Construction, <br />applying an adjustment for the larger (1,800 ft3/s) discharge, and using the assumptions discussed <br />previously for the main outlet works, the cavitation index at the gates for this operation is 0.25, <br />which suggests a strong potential for cavitation damage. Even with the reservoir water surface at <br />the restricted elevation 5920.0, the flow velocity at the gates is about 78 fils and the <br />corresponding cavitation index is 0.38. Cavitation indices at the upstream end of the discharge <br />tunnel, just downstream from the regulating gate, are approximately 0.14 with the reservoir at <br />elevation 6085, and 0.29 with the reservoir at elevation 5920. Aeration was provided for the flow <br />in this area in 1972 to help prevent cavitation damage to the concrete-lined tunnel, and a flip was <br />added at the downstream end in 1977 to help reduce total gas supersaturation effects. <br /> <br />By memorandum dated September 26, 1979 (included as Appendix C), permission was granted to <br />operate the auxiliary outlet works at reservoir levels above elevation 5920.0. Recent operations <br />of the auxiliary outlet works at gate openings up to 3.95 feet, and reservoir levels up to elevation <br />6076, have resulted in some reports of minor cavitation damage to the regulating gate leafand <br />frame, but no reports of cavitation damage-to the concrete surfaces. <br /> <br />Ifflow regulation was moved to the downstream end of the discharge tunnel, some improvement <br />in discharge capacity of the auxiliary outlet works could be realized. Replacement ofthe tandem <br />4- by 4-foot outlet gates with a single 5- by 7-foot guard gate, installation ofa 76-inch-diameter <br />(or equivalent area) steel liner within the downstream tunnel, and provision of a 72-inch jet-flow <br />gate for releases directly into the spillway stilling basin, could increase the auxiliary outlet works <br />discharge capacity by about 12 percent, or by about 220 fi3/s at reservoir elevation 6085 (from <br />1,800 fi3/s to 2,020 fi3/s). The maximum flow velocity at the regulating gate would be reduced to <br />70 ftls, compared to 112 ftls under the existing conditions, and cavitation potential would be <br />significantly reduced. <br /> <br />It should be noted that the intake sill of the auxiliary outlet works, at elevation 5775.0, is 9 feet <br />below the projected 100-year sediment level at the dam, elevation 5784.0. Based on this, the <br />reservoir evacuation study performed in 1985 concluded the auxiliary outlet works may not be <br /> <br />6 <br />