Elsie Dam Safety Upgrade Project

Construction of the Elsie Dam on Ash River was completed in 1958, and the Ash River Generating Station entered service in 1959. An intake on the south shore of Elsie Lake Reservoir diverts water through a tunnel and pipeline to a 27 MW powerhouse on Great Central Lake. 

As part of its Dam Safety Program, BC Hydro conducts regular reviews of all its dams. Although the condition of Elsie Dam has not changed since it was built, and the dam has operated without incident since it was constructed, a review in 1999 identified a need for an upgrade to enhance dam safety during a major earthquake, based on current earthquake design standards. Development of plans for those improvements is now under way. Elsie Dam, consisting of a main dam and four saddle dams, is located in an area that has experienced large earthquakes in the past.

Eric Johnson, Elsie Dam Project Manager
Phone: (604) 528-3059
E-mail: eric.johnson@bchydro.com
Tom Veary, Maintenance and Operation Services Manager, Vanc. Island
Phone: (250) 286-6616
E-mail: tom.veary@bchydro.com
Stephen Watson, Community Relations, Vancouver Island
Phone: (250) 755-4795
E-mail: steve.watson@bchydro.com

Why Upgrade Elsie Dam?

Analysis of horizontal layers of loose material found in the Main Dam and Saddle Dam 1 during a review in October 1999 suggested the dams would likely deform if the reservoir level were above these layers and a large earthquake occurred — which could lead to failure of the dams.

How Will Work Proceed?
The first stage of the upgrade will be the excavation of a slot, or channel, in the overflow spillway during August to October 2000 to help keep the reservoir level below elevation 326 metres. This will minimize the risk of deformation. The Main Dam and Saddle Dam 1 will be upgraded during the second stage of the project. This second stage work is scheduled for the low inflow period of June through September 2001.

The next stage of the dam improvement work will include complete removal of the loose layer in each of the two dams. They will then be rebuilt. To increase the safety of the dams even further, rock berms will be placed on the downstream face of each dam. The low level outlet, located in Saddle Dam 1, also needs to be upgraded to safely operate at full capacity. This work will be done after the upgrade work on the dams.

The safety upgrade work will take place August through October of 2000 and through the spring, summer and early fall of 2001. During this time, access to the areas under construction (Main Dam, Saddle Dam 1 and the overflow spillway area) will be restricted. Access to other parts of Elsie Lake Reservoir will not be affected, but the water level will be lower than normal.

Dam Safety Program

BC Hydro recognizes the importance of operating and maintaining its dams to ensure public safety. In 1982, BC Hydro introduced a formal Dam Safety Program and developed processes to review and document dam safety status. The program includes a full range of dam safety initiatives — from routine surveillance to periodic comprehensive reviews and upgrades — that are implemented whenever necessary.

You Asked Us….
July 2000

QUESTION: Where did the loose layers come from?

ANSWER: The dams were built by the BC Power Commission in 1957/58, and unfortunately there is very little detailed information available about the construction. A report did, however, note interruptions by strikes and fire closures which delayed completion of the dams until the early fall of 1958. Heavy rains also interrupted construction. It may have been this combination of wet weather, which would make compaction of soils difficult, and the need to quickly finish the dams because of rising water levels, that layers were not compacted as well as they should have been.

QUESTION: Why not make all the remedial improvements now?

ANSWER: We have to wait for a period of sustained dry weather when inflows are low (typically June through September) so we don’t recreate the problems of 1958. Work on the Main Dam will include excavating down about nine metres and then rebuilding the dam, which requires excavating about 18,000 cubic metres ( m³) and replacing it with mostly new material. In addition, about 19,000 m³ of rock will be placed in the down-stream side rock berm. Saddle Dam 1 requires about 8,000 m³ to be excavated and replaced, along with about 9,000 m³ for the downstream rock berm.

QUESTION: Why do you need to excavate a slot in the spillway?

ANSWER: With the dam improvement construction scheduled for the summer of 2001, the spillway slot will keep the reservoir water level below the loose layer over the coming winter, except following an extreme rainstorm. If the loose layer is dry, it can’t deform in the unlikely event of a large earthquake.

QUESTION: What happens if an extreme rainstorm occurs?

ANSWER: If there is an extreme rainstorm (very infrequent and usually short term) and the water level rises above 326 metres. 24-hour on-site surveillance will be in place. This surveillance plan is linked to the Port Alberni emergency services plans and will be in place until the dams have been repaired.

QUESTION: How were the remedial plans chosen?

ANSWER: In considering the repair alternatives, BC Hydro staff looked at potential impact on the environment, reservoir draw-down, risk during construction and estimated costs. The selected short- and long-term alternatives were then reviewed and endorsed by an independent advisory board of engineering experts in June 2000.

QUESTION: Will the Ash River Generating Station continue to operate?

ANSWER: Yes, the generating station will operate until Elsie Lake Reservoir drops below elevation 320 metres, at which time plant output is reduced to ensure there is sufficient water for fish.

QUESTION: What happens to the spillway slot when all repair work is finished?

ANSWER: The spillway slot will be filled with a concrete plug and Elsie Lake Reservoir will return to its normal elevations.

QUESTION: Will the work impact fish?

ANSWER: During the seismic upgrade construction of the Main Dam and Saddle Dam 1, the low level outlet will continue to operate, providing required water for fish downstream in the Ash River. The required amount of water can be provided at any time. The work on the dams will take place above the reservoir level and will not impact lake water quality.

QUESTION: Will this work impact the upcoming Water Use Planning process?

ANSWER: BC Hydro is beginning a Water Use Planning (WUP) process for its Ash River hydroelectric facility as part of an overall review of its licences to store and divert water for electricity generation. Once the planned dam improvements are completed, Elsie Dam and Reservoir will return to normal operation. This work will, therefore, not affect the upcoming WUP process.