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<br />001888 <br /> <br />constructed, was 110 feet long and 16.54 feet wide, with an'invert at elevation 5678 [11]. This <br />corresponded to a basin about 8 feet longer, 0.29 feet wider, and 3 feet deeper than Bureau <br />guidelines recommended [12]. However, the basin size was considered to be within nonnal <br />design limits. The outlet works basin design was evaluated for maximum flows up to 4,720 ft3/s <br />at reservoir elevation 6101.6 [13], based on minimum losses in accordance with standard practice. <br /> <br />Since initial construction, several problems resulting from outlet works operation have been <br />identified. The major problems have been concrete abrasion in the stilling basin, cavitation <br />damage on the hollow-jet valves and at selected locations within the upstream pipeline, gas <br />supersaturation of reservoir releases, and flow instability in the pipeline. A brief history of each <br />problem is presented in the following paragraphs. <br /> <br />1. Abrasion damage. - Stilling basin abrasion has been the greatest problem incurred during <br />operation of the outlet works. Stilling basin abrasion damage is a result of rocks and debris being <br />tossed violently around the floor and walls by the highly turbulent flow present in hydraulic jump <br />stilling basins. The main method of controlling abrasion damage is through eliminating local <br />sources of small rock (less than - 12 inches in diameter) and methods of transporting the rock <br />into the basin. Rock sources can be from adjacent spalling slopes, construction or maintenance <br />spoils left on the downstream apron or behind basin walls, and from the downstream riverbed. <br />Methods of transport are by gravity (such as spalling rock or unstable slopes), by people throwing <br />rocks into the basin, and by recirculating flow eddies that move small riverbed or riprap material <br />upstream during operation of the outlet works or adjacent facilities. <br /> <br />Measures are often taken to prevent rocks from entering stilling basins by way of spalling slopes <br />or people. However, flow recirculating near the end of stilling basins is the main method by <br />which rocks are typically transported into basins. Many stilling basin studies have shown, under <br />some flow conditions, strong reverse flow (upstream flow) patterns can fonn along basin inverts. <br />The reverse flow can move rocks from the downstream apron upstream into the basin. This rock <br />typically remains trapped in the stilling basin and is moved around during medium to large <br />releases. <br /> <br />In 1965, severe abrasion damage was found in the Navajo Dam outlet works stilling basin. A <br />synopsis of the operation and observations made during 1964 and 1965 are given below. <br /> <br />Prototype Operating History (1964-1965) <br /> <br />Between May 23 and June 10, 1964, both valves were operated at equal openings up to <br />25 percent. The reservoir rose from elevation 5952 to elevation 5965 (245 feet of head). <br />Some jet penetration probably occurred Operation then began with gate openings <br />between 30 and 40 percent and continued until August 3, 1964. The reservoir dropped <br />from elevation 5965 to elevation 5947 (227 feet of head) in this period. The model <br />studies subsequently showed that this operation resulted in very strong turbulence in the <br />upstream portion of the basin and violent vortices along the downstream faces of the <br /> <br />11 <br />