‘There are eight low level outlets at Hugh Keenleyside Dam. The outlets are used to pass Columbia river treaty water when the reservoir is relatively low,’ said BC Hydro’s maintenance and operations service manager, Gordon Boyd.

‘In 1998, an underwater inspection of low level outlet No 1 indicated extensive concrete erosion in and around the invert of the downstream maintenance gate. Although the outlet has not been used for a number of years, the erosion appears to be increasing in size.’

In June 2002, BC Hydro utilised divers in a depth of 18m to lower a tremmie pipe to the bottom of the low level outlet. Once installed, concrete was pumped through this pipe to fill erosion pockets at the downstream end of the outlet to strengthen the damaged portion of the low level outlet.

The next phase of the work includes the lowering of maintenance gates at both the upstream and downstream ends of the outlet and dewatering the low level outlet in order to perform a detailed structural inspection of this water passageway. However, as a precautionary measure, pressure relief holes are to be drilled into the floor of the passageway to ensure there will not be any uplift pressures on the concrete floor when the passageway is dewatered.

From 22-26 April 2003, diving and diamond drilling contractors will be working to drill eight, 8cm diameter by 2m deep holes into the floor of the low level outlet. The drilling work will be performed from a barge floating in the dam’s tailrace.

‘To maximise safety of the diving crew and minimise environmental impacts, the work will be undertaken when there are no flows being discharged from Hugh Keenleyside dam,’ said project manager David Epp. ‘All discharges to the Columbia river below the dam will be provided through the generating units at Arrow Lakes generating station.’

Upon completion of relief hole drilling, the low level outlet will be dewatered in order to undertake a detailed structural inspection and determine what other remedial works, if any, are necessary.