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<br />001G70 <br /> <br />from the downstream end of the center wall. A few bars were exposed in the dentated sill <br />and should be repaired <br /> <br />'I <br /> <br />The area just downstream of the basin was riprapped with rounded boulders, cobbles, <br />and gravel rather than angular rock. A comparison of the construction photographs with <br />the unwatered basin indicated considerable movement in the riprap with a small slide on <br />the right side near the downstream comer of the concrete basin. It also appeared that <br />small rock and gravel had been removed from the larger boulders. These two sources <br />account for the material that was found in the basin and caused the severe erosion. <br /> <br />Temporary repairs were made to the stilling basin in May of 1965. The original basin <br />outline was restored by bonding new concrete to the dainaged areas with epoxy. <br />Intermittent outlet works operation was resumed on May 24 and continued until June 3 at <br />valve openings between 16 and 75 percent; the reservoir variedfrom elevation 5975 (255 <br />feet) to elevation 5980 (260 feet) during this period. On June 3, the valve openings were <br />increased to 100 percent and the outlet works operated intermittently, usually 100 <br />percent open, through July 10, 1965. The longest period of sustained operation at 100 <br />percent valve opening was 5 days. The reservoir rose to elevation 6002 (282 feet of <br />head) between June 3 and July 10. <br /> <br />On July 3 a diver examination of the main outlet stilling basin revealed extensive erosion <br />in the downstream end of the basin from the end of the center wall to the dentated sill. <br /> <br />Reinforcement bars were bent across the basin and two large rocks were found wedged in <br />the dentates. The divers removed about 10 gallons of material up to 8 inches in <br />diameter. Some scattered debris in the area of the bent up bars could not be removed <br /> <br />Erosion was observed to a maximum depth of 1.2 feet at a point 25 feet downstream from <br />the center wall and 5 feet from the outside wall. An area about 20 feet long and 20 feet <br />wide was eroded from 0.75 foot to 1.2 feet deep in a floor slab thickness of 4 feet. The <br />upper end of the right chute was eroded about 3 inches and reinforcement in a small area <br />was exposed <br /> <br />On July 10, another diver examination of the stilling basin was made. The report <br />described grooves which had developed along the bottom on both sides of the center wall. <br />The grooves were estimated to be 3 inches deep and 2 inches high extending from the <br />upper end of the horizontal floor to the downstream end of the wall. The downstream end <br />of the wall was undercut 2 to 3 feet and some of the drain tile had been removed The <br />contraction joint in the center wall had opened up one-half inch at the top and a vertical <br />crack had started 6 inches upstream of the joint. <br /> <br />Due to the extensive abrasion damage that occurred in the hollow-jet basin, a 1: 12 scale model <br />study ofthe basin and downstream apron was conducted in 1965, As a result of the model study, <br /> <br />13 <br />