As a continuation of its improvements to WAC Bennett dam in Canada, BC Hydro has begun work on the dam’s drainage system. A filter is to be built at the downstream base of the dam, enabling it to take full advantage of the existing large internal drainage capacity.

According to BC Hydro, the work which is being carried out is described as being a prudent and cost-effective measure to enhance the safety of the dam under the most extreme conditions.

Work on the filter will be done in two phases. The first involves excavating a trench in the canyon section and backfilling with suitably graded rock. This was to be completed as IWP&DC went to press.

The second phase will be continued in 1999 in the remaining sections of the dam. The total cost of the construction work is anticipated to amount to C$4M.

In addition to building the filter, BC Hydro will conduct annual geophysical tests in the areas of the dam disturbed by sinkholes found in 1996 and repaired last year.

Geophysical tests confirmed the success of the sinkhole repairs in 1997 and BC Hydro plans to conduct these tests on an annual basis as part of their ongoing surveillance programme.

In addition to the work at WAC Bennett dam, the Canadian utility is studying Seven Mile and Hugh Keenleyside dams as part of its ongoing dam safety programme. The work will ensure that the dams, built in 1968 and 1980 respectively, meet current-day dam safety standards — even though they fulfilled the criteria for standards applicable at the time of construction.

BC Hydro states that advancements in knowledge and understanding of earthquake activity in the region have led to larger maximum design earthquake loadings being prescribed for the dams. The utility is conducting studies to determine whether improvements are required to strengthen the dams to meet such higher standards.

In 1982, BC Hydro introduced a formal dam safety programme and developed processes to review and document the performance of its dams. The programme includes a full range of dam safety issues from routine surveillance to more comprehensive reviews and upgrades where necessary. To date, the utility has spent more than C$160M on improvements at its dams throughout British Colombia in order to meet current safety standards.