Landslide bodes ill for dam on the Peace

Joe McWilliams
Lakeside Leader

A landslide in the Peace River valley near Fort St. John last fall wiped out the ‘Old Fort’ road altogether. It led from the modern city down to the original site of Fort St. John – back when it was a fur trading post.

When I was in high school it was a place to go to look at the river, maybe launch a canoe or do a bit of scrambling around the grassy breaks and hope to see a cactus or pick some saskatoons. Nobody lived there but now they do.

‘No more grad parties down there, I guess,’ was my flippant comment when my sister posted (or-reposted) an article about the landslide from an anti-Site C dam source. Opponents of that project predict dire things for a dam in such geologic instability.

They might be right. The two dams upstream from there on the Peace were built on solid rock between rock walls. Even so, the giant W.A.C. Bennett Dam shocked the daylights out of everyone a few years ago when a sinkhole appeared in the paved road across its narrow top. Investigating, engineers found a cavern had developed in the middle of the dam over the years. It was found in time to avert disaster, and led to a warning system being installed downstream. The town of Hudson’s Hope would be wiped off the map if that dam ever gave way.

Earth-filled dams aren’t expected to last forever. One of them – the Teton Dam in Idaho – didn’t even last a week. Upon filling the reservoir for the first time in June of 1976 it sprung a leak and within a few hours broke open, wiping out a couple of towns, killing 11 people and washing away over 10,000 cattle.

Back to the drawing board on that one. It has not been rebuilt. Perhaps lessons were learned.

BC Hydro has been pushing the idea of the Site C dam for decades, and for just as long, people in the area have been pushing back. The pro side appears to have won, and construction proceeds. But the instability of the land around the dam has to be giving the proponents second thoughts.

The Peace cuts a deep valley through there and the land is constantly slumping. It slumps on the Taylor hill; it slumps at Peace River, where remediation on the road and railroad never really ends. In June of 1973, near the confluence of the Peace and Halfway Rivers, a huge slide blocked the entire river for a few hours.

Dams fail and the results are often catastrophic. According to the Wikipedia article on dam failures, there have been about two per year since 2000, killing 635 people and doing untold environmental damage. The worst of all time was in China in 1975, when heavy rainfall caused two dams to collapse, with the resulting loss of 171,000 human beings.

The causes vary widely. Heavy rainfall is the most common one. Shoddy construction is another that recurs as you go down the list. So is underestimating the stress on the structure – or on the valley walls that flank it. In the case of the Teton Dam, water was finding its way through the earth-filled structure, under it and around it, simultaneous with the reservoir filling.

Earthquakes have done dams in. Power failures have caused spillways to fail, leading to water breaching dams. Landslides have been known to force water over dams as well. That would seem a highly probable scenario with Site C. Gravity will have its way.

Whatever BC Hydro builds by Fort St. John, it would be wiped out if the big one upstream lets go.

Or rather it would simply not matter, because the volume of water in Williston Lake is gigantic. The ‘Wacky’ Bennett Dam is a mile across and 183 metres high. The lake is 400 kilometres long (depending on how you measure it)

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