HISTORIC DAM―INNOVATIVE UPGRADE OF SPILLWAY

摘要

Black rock dam is an embankment dam built between 1904 and 1908 on the Zuni Indian Reservation in western New Mexico (United States of America), on the Zuni river. The dam site is a narrow gorge, 24.3 m deep, created by undermining outflows of water impounded by ancient basalt lava flows across the Zuni river. The intensely jointed basalt, overlain by several feet of unconsolidated surface deposits, forms the upper 9.1 to 12.1 m feet of the abutments. Beds of clayey sand and gravels underlie the basalt. The original spillway was excavated in the left abutment basalt. During the first filling in 1909, reservoir water flowed through fractures in the basalt, eroding the underlying soils, resulting in settlements of up to 2.7 m of the basalt blocks, the partial collapse of the spillway and near failure of the left abutment of the dam. Repairs to the abutment and a new spillway were completed in 1912. Despite attempts to limit flows through the basalt, the left abutment nearly failed from the same cause in 1932 and 1936. More recent inspections by the US Bureau of Reclamation under their Safety Evaluation of Existing Dams (SEED) program classified Black Rock dam as high hazard, and identified significant dam safety deficiencies of excessive seepage, inadequate spillway capacity, and poor condition of the outlet works. The outlet works improvements consist of a new intake structure, grouting a steel liner in the existing conduit, and a new terminal structure. Excessive seepage is controlled by construction of a 22.86 m deep cutoff trench through the upstream face of the dam, extending several hundred feet into each abutment. Spillway capacity is increased by modifying the existing spillway, and constructing a new auxiliary spillway. The estimated probable maximum flood (PMF) inflow peak is 1853.2 m~3/s 65,400 (cfs), and the routed outflow is 1800 m~3/s 63,500 (cfs). The spillway capacity before improvements was about 170 m~3/s 6,000 (cfs). Several spillway alternatives for passing the PMF were evaluated. The selected alternative consists of a movable crest in the existing spillway and constructing an auxiliary spillway on the left abutment. Fusegates were selected due to their low maintenance requirements, and their ability to pass normal flows as a service spillway while tipping at prescribed reservoir elevations to pass large floods without depending on mechanical equipment. In addition, an auxiliary spillway channel was excavated in the left abutment, left of the service spillway. The service spillway passes 907 m~3/s 32,000 (cfs) and the auxiliary spillway passes 892.6 m~3/s 31,500 (cfs), totaling the required outflow of 1800 m~3/s (63,500) (cfs).