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Excavations under way at Elsie dam

BC Hydro is proceeding with the excavation of a 21m wide, 7m deep slot on the Elsie dam overflow spillway in Canada. The slot on the 100m wide concrete spillway will keep water levels below 326m in the coming winter, about 5m below full pool level.

The slot will also be used to control reservoir levels during a safety upgrade of the dam scheduled for the summer of 2001. Once the remedial work is complete the temporary slot will be plugged with concrete to re-establish normal reservoir levels. The normal operating level of the reservoir is between 315.47m and 330.71m. The reservoir stores about 77M m3 of water.

Test drilling in October 1999 had indicated that if a major seismic event occurred, when the water levels at Elsie lake were above the 326m level, a thin layer of loose fine silty sand in the main dam could liquefy, and may cause unacceptable deformations — possibly resulting in dam failure. The preferred remediation method for the main dam and saddle dam 1 includes removing material from the top of the dam and then rebuilding it along with the addition of a rock berm on the downstream slope. Modifications are also to be made to the low level outlet, which is used to provide riparian and instream flows.

BC Hydro, a provincially owned utility, owns and operates the Elsie dam, in Vancouver Island, British Columbia. The 19m high structure is part of the Ash river hydro facility built in 1959. The Elsie lake reservoir provides water storage and is held by four saddle dams and a main dam. Water is carried from the reservoir through a 7.4km tunnel and pipeline to the 27MW power house equipped with a Francis type turbine. The plant generates an average of 193GWh per year.