Dam repairs

Re: BC Hydro in hot water over dam repairs (Globe and Mail, Dec. 9)

To help reassure our customers, I am writing to provide a fuller perspective on BC Hydro’s Bennett dam riprap project and the extent of our disclosure of the 2012 Expert Engineering Panel report on the WAC Bennett dam.

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BC Hydro’s highest responsibility is public safety and that's why we maintain a strong dam safety program. It includes 24/7 instrumentation monitoring, weekly inspections, bi-annual engineering reports and regular external reviews of all our dams. Our work, and the panel's review, confirm that there is no immediate concern for the safety of the Bennett dam, while long-term investigations and capital upgrades such as the riprap project continue into the future.

BC Hydro recently filed an application with our regulator, the BC Utilities Commission, for the riprap project. It includes excerpts from a 2012 Expert Engineering Panel report on the Bennett dam. For simplicity, only a portion of the report that was relevant to the project was included with the application. There was no intention of excluding the report from the Commission. The report was provided to the Province's Comptroller of Water Rights in 2013 and has now been provided to the Commission. It’s also available for our customers to read on our website at bchydro.com.

The WAC Bennett dam, completed in 1967, remains one of the largest earthfill dams in North America. Two sinkholes developed in the crest of the dam in 1996 and received immediate attention. Following this incident, the dam has become one of the world's most instrumented and studied dams, and there are a number of upgrades underway. The riprap project is one of these, and we will be replacing much of the zone of large rocks on the upstream face of the dam that protect the dam from wind and wave action.

In 2011, we invited three world experts in embankment dams to conduct their own, fully independent, bottom-up review of the dam design and performance. The work was not focused on the riprap project. It had an emphasis on evaluating the control of seepage and a process known as internal erosion. The intent was to better understand the actual conditions and the behaviour of the dam since its construction, and following 15 years of monitoring and investigations since the sinkhole developed.

In its 2012 report, the independent panel noted that the Bennett dam design will prevent ongoing internal erosion and that there are no situations where erosion, if initiated, would continue. The panel recommended a number of further investigations and work to confirm this conclusion. Since that time, we have been addressing the panel's recommendations. The panel reconvened last month to assess our progress, and reassess the situation. This latest report should be available in early 2016.

Based on the panel’s verbal update, we expect the report to state that the work over the past three years confirms their earlier opinion regarding the safety of the dam. We also expect to receive recommendations for further work, which we will continue to follow. The panel’s follow-up report will be made public after it is received and submitted to the Comptroller of Water Rights.

We believe the timing of the riprap project is appropriate and its pace is as fast as practical in view of the complexity of the project. The project was released in 2011 before the panel was convened, following our dam upgrade priorities. The timing was influenced by the observation of the extent of the ongoing deterioration of the riprap.

When the panel was in session in 2011, we believed that relatively simple repairs to the riprap would be possible using a local rock quarry and anticipated project completion before now. However, early project planning and definition pointed us to a more comprehensive and complex solution that required higher quality rock from a more distant quarry. This solution has required more time and cost, but it provides a permanent fix to the problem. This much larger project involved more extensive environmental and First Nations consultation and a formal Commission application.

—Stephen Rigbey, Director of Dam Safety, BC Hydro

© Copyright 2018 Alaska Highway News

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