Civil engineering

Will Québec’s dams fail?

The hundreds of dams scattered throughout Québec do not threaten local populations. “But there is no such thing as zero risk,” maintains Claude Marche, a professor in the École Polytechnique de Montréal, affiliated with University of Montréal.

Marche, who has followed all the major building projects closely since the 1970s, and who helped to analyze the effects of the Saguenay floods in 1996, believes we should not forget that between 1 and 3 of the 30,000 big dams around the world break every year, producing their share of disasters. And only a careful, detailed study of the reasons for the ruptures can tell us about the threats the dams represent for public safety.

At the request of Hydro-Québec, Mr. Marche set up a team of twenty individuals, three quarters of whom are university students and professors, to calculate the repercussions of a breach in a major dam. A “full-scale” test was even carried out in Norway. A breach was deliberately made in a dam so that the complex mechanisms could be filmed and better understood. “It was all over in 30 minutes,” says the engineer, showing striking photos of the experimental breach. When the water breaks over the top of a dam, the break expands as the flow increases. The effect is very fast, especially when the dam is made of earth and rocks. The surge can wipe out whole towns.

One of the best known disasters was in Johnstown, USA in 1889, where 2,209 people were killed. More recently, in 1959, 420 people perished when the Malpassat dam in France ruptured. In the 20th century, more than 200 incidents have resulted in 8,000 deaths and considerable material damage. To date, Québec has been spared this sort of large-scale drama.

“The risk of rupture is greater for earthwork and rockfill construction than for concrete structures according to recent statistics, which also show that the probability of a dam rupture is on the order of 1% over a service life of 100 years,” we read in the research report submitted to the Natural Sciences and Engineering Research Council of Canada last October. Signed by three engineers, this report sought to “provide a reliable estimate of the scope of the consequences in potential loss of life and direct damage from a rupture.”

Mr. Marche does not want to be seen as a prophet of doom, even though his work involves imagining the impact of an accident. The authorities have changed their attitude considerably in terms of what they’re interested in. “In two decades, there has been a total reversal. They have shifted from utmost discretion about this subject to complete transparency.” The expert remembers the early years of his work, when he had to carefully conceal all his data in locked folders in order to prevent leaks (no pun intended). Today, dam owners are anxious to show the public that everything possible is being done to ensure their safety.