Rehabilitation of Black Rock Dam "Seepage & Inadequate Spillway"

Black Rock Dam is a 79 ft. high historic embankment built by the Bureau of Indian Affairs in 1908 on the Zuni Indian Reservation, located about 40 miles south of Gallup, New Mexico. The structure consists of an embankment dam, an ungated spillway at the left abutment, and outlet works at the right abutment. Black Rock Dam was classified as high hazard by BUREC under their Safety Evaluation of Existing Dams (SEED) program because of the Pueblo of Zuni three miles downstream. The major problems with the dam are excessive seepage and its inability to pass the large floods without overtopping. The probable maximum flood (PMF), which was chosen as the design flood for dam safety modification, corresponded to an overtopping of the dam by several feet. A failure was very likely to occur under this overtopping condition. Excessive seepage is controlled by construction of a 75 ft deep cutoff trench through the upstream face of the dam, extending several hundred feet horizontally into each abutment. The alternative solutions for the rehabilitation of the spillway, which is the main focus in this study, developed as part of the SEED program by the Pueblo of Zuni (Tribe) consisted in the following: (1) Providing a new spillway on the right abutment of the dam, (2) Overtopping protection (RCC) on the downstream of the dam, (3) Enlargement of the existing left abutment spillway. After evaluation of the alternatives against cost efficiency and suitability, the choice was the construction of a free emergency spillway in addition to the existing service spillway, equipped with labyrinth crested steel fusegates, which required no major work to the embankment as there was no increase in the Maximum Water Level. The innovative approach of ifFusegated Spillwayle not only increased the discharge capacity of the existing by 500%, but also reduced the environmental impact of the rehabilitation. 20 pp., 16 figures.