Report says Okanagan Lake Dam not at risk

Recent geotechnical report concludes Penticton dam could survive a major earthquake

Despite some superficial cracks

Despite some superficial cracks

Its only job is to control a body of water with an estimated volume of 24.6 cubic kilometres, so failure of the Okanagan Lake Dam in Penticton would probably look like something out of a splashy disaster movie.

Fortunately, the dam is expected to survive a major earthquake, according to a geotechnical report prepared in March for the B.C. government, and obtained by the Western News through a freedom of information request.

“In a major earthquake, the dam would likely not fail, but it would likely require significant repairs and structural review. Replacement is a likely option,” the report reads.



‘Very high hazard consequence’

Prepared by Associated Engineering in March 2012, the Okanagan Lake Dam Structural Review and Assessment used site surveys, visual inspections and core samples to assess the integrity of the dam and make recommendations on its upkeep.

The report, signed by engineers Rod MacLean and Dale Harrison, was commissioned by the Ministry of Forests, Lands and Natural Resources, and confirmed the dam is compliant with modern safety regulations and that no seismic upgrades are required.

It does note, however, that the dam is classified by the province as “very high hazard consequence,” and the most serious anticipated results of a breach include: potential loss of life along the Okanagan River Channel in Penticton; inundation of the flood plain between Skaha Lake and Osoyoos Lake; potential flooding of Okanagan Falls, Oliver and Osoyoos; and temporary loss of ecosystems.



Unsettling possibilities

Work on the dam began in 1950 and finished in 1958 at a cost of $4.9 million, according to a history of the structure included with the engineering report.  The build was a joint effort of the provincial and federal governments, and conceived as a response to local concerns about flooding downstream of Okanagan Lake.

Five gates incorporated into the structure’s 36-metre length control outflow from Okanagan Lake. The steel-reinforced dam rests atop two concrete slabs, with a total thickness of about three metres, which are tied into 500 timber pilings that extend 10 metres into the earth below.

While engineers have noted superficial cracks in the concrete, due mainly to the freeze-thaw cycle, it’s the ground underneath the dam that is most vulnerable to movement.

By code, the dam must be able to withstand a 1-in-5,000-year ground motion. A seismic event of that magnitude is expected to cause the soil under the dam to liquefy, or settle, and drop 10 to 20 centimetres.

If the settlement occurs directly beneath the concrete base slab, it could allow water to pass underneath the dam. If the settlement occurs below the pilings, it could cause the dam itself to crack, affecting the operation of the control gates.

“This would be a concern in that we may have problems managing the flows and lake levels as efficiently as we would like,” Shaun Reimer, a ministry engineer who’s part of the team that looks after the Okanagan Lake Regulation System, said via email.

“In an emergency, we could remove a gate or gates and use wooden stop logs to create the necessary gate openings.”



1-in-5,000-year ground motion

The 1-in-5,000-year ground motion mentioned in the report has a gravitational force of 0.22.

Such force would result from a magnitude 7 earthquake originating 20 kilometres from the point of measurement, according to  Garry Rogers, an earthquake scientist with the Geological Survey of Canada.

By comparison, the February 2011 quake that killed 182 people in Christchurch, New Zealand, was a magnitude 6.3, centred 10 kilometres away and five kilometres deep.

But the Okanagan is a rather quiet region for seismologists.



Finding faults

While geological maps of B.C. are scarred with fault lines, Rogers said, “We have not found an active fault that would be a concern in the design of a dam or any other structure.”

Old fault lines, he added, are usually stitched together by minerals and are therefore stronger than the surrounding rock.

“So there’s no evidence that because there’s a fault on a map, that one area is any more hazardous than another area.”

Rogers is unaware of any major quakes in the Okanagan for at least 10,000 years.

“Rarely a year goes by that we don’t have one or two tiny earthquakes in the region,” he allowed, but “it’s not a high-activity region.”



Long-term decisions ‘yet to be made’

Tiny or not, the dam is inspected after any noticeable earthquakes. The last was Nov. 18, 2011, when a magnitude 4.6 shaker centred just south of Osoyoos rattled homes as far away as Penticton.

Reimer, the ministry engineer, conducted a dam inspection later that day and did not note any damage in his report, which was also obtained by the Western News.

He said the dam today “is in good condition for its age,” and receives maintenance that includes filling and monitoring cracks, and concrete sealing. Such work will allow the dam to “function adequately for another 25 years or more,” according to the engineering report.

However, the assessment does recommend a major rehabilitation or replacement of the dam in 20 years, at an estimated cost of $10 million. Reimer said such long-term decisions on the dam “have yet to be made.”