IN PICTURES – Savoie: the Lac Blanc dam at an altitude of 2,400 m will be destroyed after leaks detected

It's not every day that we witness the destruction of a century-old dam. This is the decision that the State took after discovery of leaks in the Lac Blanc dam in Maurienne in the Sybelles massif in the commune of Saint-Sorlin-d'Arves. Emergency work had to be done before the snow arrived in the mountains.

A breach in the dike to drain the water

It all started with the discovery this summer of leaks at the bottom of this artificial lake. Gilbert Derrier, member of the water commission at the Cœur de Maurienne Arvan community of communes, says: “as part of a maintenance operation, we had to change the drain valve of Lac Blanc. To do this, it was necessary to lower the level of the lake and by lowering the level, we realized that there were what we call fontis. These are subsidences on the upstream face of the dam. These fontis are indicative of water infiltration.

We must therefore react quickly. Because if the dam were to give way, it's half a million m3 (500,000m3) which could threaten the areas below. The State therefore proposes to dig a breach in the dike to drain the water. “The Administration went to the site and noted that this type of event was likely to weaken the dam. And for safety reasons, to prevent the dam from breaking and a wave from breaking, the only solution which was studied and which was adopted to keep the level of the lake low was to create a breach with a channel to prevent water from rising on the dam embankment and causing the dam to collapse.”

Avoid at all costs the lake filling up

Nicolas Revol, environmental mission manager at the Coeur de Maurienne Arvan community of communes, explains that given the deadline, there were unfortunately no other solutions: “We are at an altitude of 2400 meters. And inevitably, the snow will arrive shortly. We were forced to create this breach“.

The excavators have been hard at work since the beginning of October. Luc Saillet follows the construction site for the Hydrostadium design office: “It doesn't happen every day to have to breach a dam. It's quite atypical. The particular thing is that we are at altitude, so the snow can arrive quickly, especially at that time. Here, we are a little lucky, we will be able to enjoy beautiful days at the beginning of November, so the calendar relaxes a little. It's very instructive to see that. Especially since it is a dam that dates back 100 years ago. So with the lowering of the lake, we also see the vestiges of what was done at the time“.

We are losing a water reserve of around 500,000 m3

Lac Blanc is emptying and Gilbert Duriez no longer recognizes this site at the foot of Pic de l'Etendard, very popular both in summer and in winter for skiing. It has lost its beauty and above all, it is now necessary resolve to lose a water supply. “By erasing the dike of the lake, we lose a volume of around 500,000 cubic meters, which is considerable. And in a period where we are talking about global warming, the scarcity of water resources, it is very damaging to lose this resource”regrets Gilbert Duriez.

Until now, Lac Blanc served as a “reservoir” for Lac Bramant, located around thirty meters below. This feeds in drinking water and artificial snow the surrounding communes of Sybelles, such as Saint-Sorlin-d'Arves, la Toussuire, or even Fontcouverte, or 40,000 tourist beds.

The breach dug in the dike of the Lac Blanc dam should soon be completed. The first half of November if the weather holds, the technicians predict.