Explain the tragedy of Saint-Jean-Viani with science

To understand the Saint-Jean-Vianney mud flow, you must first know that a smaller landslide occurred on April 24, 1971 on farmland. The shattered earth held out for ten days before collapsing.

Geosciences researcher at the Geological Survey of Canada, Didier Perret, explains that all the stars were aligned at Saint-Jean-Viani.

There should be sensitive mud in place. Then there should be dams high enough and have a sufficiently steep slope, as was the case near the Petit-Bras raceway, and then there should be an actuator elementConfirm the world.

In Quebec, a large portion of the inhabited land consists of marine mud, which was deposited after the melting of ice more than 10,000 years ago.

Saint-Jean-Vianney is built on what is called clay sensitive to change. These clays have a very special behavior. This way, on flat ground, it can be very resistant. On the other hand, when these clays are subjected to rework or when they are subjected to shearing, these clays can lose all of their consistency., He explains.

Like a house of cards

Geologist and head of the Department of Public Security’s Coastal Hazards and Earth Movements Team, Jose Desghani, compares these delicate mud to a house of paper.

We can get a lump of clay, stir it like a recipe, or a cake, and then it comes in liquid form.

Terracotta is found under the homes of a large portion of Quebec’s population. It’s Saguenay-Lac-Saint-Jean, but I’d like to tell you it’s all the lowlands of the St. Lawrence River. 89% of the population [du Québec] Which is built into this crust of marine clay or that has the potential to exist, Determines.

However, it has determined that delicate mud is not ubiquitous in this region.

Area map

After the 1971 disaster, the first phase of mapping landslide-prone areas was carried out. Work was also resumed after the 1996 flood.

Since 2003, a team from the Ministry of Transport under the supervision of Public Security has clarified maps intended to inform municipalities of restriction areas associated with landslides.

It is now easier to define the sectors in which housing can be built or even make an explicit decision to eliminate some sectors that are considered high risk.

But not all Quebec homes were built after 1971.

Since 2003, $ 60 million has been invested in Quebec to stabilize soils in problem areas.

The Department of Public Security estimates that in doing so, it is protecting an infrastructure worth a total of $ 600 million and nearly 7,000 people.

Today, there are very few visual clues that Saint-Jean Viani even existed.

Photo: Radio Canada

Could disaster happen again?

Advances in science now make it possible to operate more quickly. Clay flux always has a forward mark. At Saint-Jean-Viani, this was the slip on April 24th.

At present, if a similar landslide is detected, municipalities and the Ministry of Public Security can respond.

This is what happened in 2019 in Lorraine, north of Montreal. A citizen noticed a landslide near his home. Then he informed his municipality.

From three in the afternoon, experts were already at the scene. In the evening, people were evacuated

A unique program in Saguenay

In the region, the city of Saguenay has its own preventive slope monitoring program. It is the only municipality in Quebec that has adopted this approach.

Saguenay’s engineering project manager, François Labarre, estimates that one or two interventions are performed annually.

We follow a certain number of dams each year in order to ensure better public safety, says Francois Labarre. The purpose of these inspections is to spot the warning signs of a landslide that ultimately makes it possible to manage an impending disaster rather than an actual one.

François Labarre of Saguenay and José Desghané of the Ministry of Public Security will also attend a conference organized by the University of Quebec at Chicoutimi (UQAC) on May 13-14. Developments in geotechnical sciences since the landslide at Saint-Jean-Viani will be presented.