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<br />001872 <br /> <br />6. The maximum allowable outlet works discharge was reduced to 3,200 cfs. Discharges <br />above this amount caused the stilling action to extend beyond the end of the basin with <br />resulting large waves in the downstream channel. The 3,200-cfs discharge, in <br />combination with operation of the auxiliary outlet works and the 30-inch-diameter bypass <br />valve, will provide the required outlet capacity. <br /> <br />7. Balanced operation of both valves is necessary for satisfactory stilling basin flow <br />conditions because of the absence of a center dividing wall If an emergency requires <br />operation of one valve alone, strong surging and rapid upstream flow in the <br />nonoperating side can be expected <br /> <br />8. The Navajo outlet works stilling basin problem was an individual case and the <br />modifications developed herein apply only to that problem. The hollow-jet valve stilling <br />basin is a relatively short basin whose high efficiency as an energy dissipator depends on <br />the turbulent eddie~ within the basin. If loose material reaches these eddies, severe <br />erosive damage to the basin will occur. These studies confirmed that the standard <br />hol/ow-jet valve stilling basin will operate satisfactorily if adequate precautions are <br />taken to keep the basin free of foreign material. Also, evidence now exists that the center <br />wall must be designed to withstand dynamic loading. <br /> <br />During the model study, the recommended basin design was tested with flows up to 4,720 {fls. <br />The basin performance was described as follows: <br /> <br />Although not an expected operating condition, the maximum discharge capacity of 4,720 <br />cfs was observed with the maximum reservoir elevation of 61 01.6 and tailwater <br />elevations 5715.5 and 5722.0. At the lower tailwater, turbulence was distributed <br />throughout the basin and surface turbulence extended well into the downstream channel. <br />At the higher tailwater elevation, conditions in the stilling basin were more unstable and <br />strong longitudinal surges carried downstream. A pronounced surface boil occurred <br />above the downstream end of the basin. However, wave action in the downStream <br />channel was- weaker than with the lower tai/water. <br /> <br />Drawings 711-D-461 and 711-D-462 show the modifications made to the basin in 1966 as a result <br />of the model study recommendations. The basin was modified to more closely resemble a type <br />ill hydraulic jump stilling basin. The upstream basin slope was changed to 2-1/2: 1 with a steel <br />liner, a I-foot-thick concrete overlay was placed on the basin floor, the downstream apron was <br />off~et 3 feet below the eridsill (at station 24+06.2) to create a rock trap, an IS-inch-thick concrete <br />overlay was placed over existing riprap on the apron for a distance of about 137 feet downstream <br />of the basin endsill, and some regrading of the apron and downstream river channel was <br />performed. The basin modifications were performed to reduce the occurrence and strength of <br />reverse flow currents in the basin. The concrete overlay on the apron was designed to reduce the <br />availability of rocks in the vicinity of the basin. As a further precaution, the maximum flow <br />release from the main outlet works was restricted to 3,200 ft3/S. The restricted flow rate was <br /> <br />15 <br />