In the country of the 1/6 dams –
In Tseuzier, the mountain is scrutinized with constant vigilance
Faced with the landslides that multiply in Valais, how do we ensure the safety of large hydroelectric works? Example on the Hauts d’Ayent.
Put into service in 1957, the Tseuzier dam is based on the solid shoulders formed by the shores of the Latée gorges. They were the ones who moved in 1978, cracking the work.
Florian Cella / Tamedia
- Victim of a compaction of the rocky massif, the Tseuzier dam broke in 1978.
- Almost fifty years later, the work was the subject of attentive surveillance.
- This vigilance applies to all the country’s dams, whose movements are scrutinized with constant attention.
- At the time of climate change, there is no shortage of challenges for the hydroelectric sector in Switzerland.
The landslides that occurred in Blatten, In the top Val de Bagnes, or more recently, almost the great dixence Demonstrate unequivocally: the mountain, on surveillance, move.
The hiker who takes advantage of the wild landscape offered The Tseuzier dam And numerous surrounding routes – around the lake, along the Bisse de Sion or even in the COLD COLD WATERS – does not necessarily notice it. The V-shaped scar, clearly visible on the 156 m high wall, recalls, however, that the work located on the Hauts d’Ayent and Icogne itself paid the price for these hazards.
In October 1978, polls took place, for the digging of the Rawyl tunnel, which had to ensure a motorway link between Bern and Valais. This work causes a compassion of the massif: the banks of the Laté gorges, on which the Damage-Volate relies, are tightening 6 cm and the rocky base which supports it is 12 cm, damaging the structure.
By guiding us in the bowels of the dam put into service in 1957, Samuel Lamon stopped in front of the instruments which allowed the operators of the time to note the problem. Three pendulums are suspended in the galleries, from the top of the wall to its base, like a lead wires. “They are used to measure the movements of the dam,” explains the engineer responsible for production and maintenance within the operating company Right. Contrary to popular belief, a dam is built in concrete without metal reinforcement. The work moves from 10 to 11 mm depending on the lake filling level. If the statements of these records leave this standard, an alert is automatically launched. ”
The network of clogged cracks is still visible on the facing.
Florian Cella / Tamedia
Monitoring and understanding
Since 1978, the instruments have evolved: “They allow for example to transmit real -time data to the driving center which pilot the arrangements, while manual statements were necessary at the time”, illustrates Samuel Lamon. But the principle of surveillance remains the same: “We seek to understand all the physical phenomena which occur, apparently as in the depths of the foundations of the dam.”
A vast network of measurement points has been disseminated in the structure itself, as well as throughout the Verse Basin, within a radius of 10 km. “Every five years, we are carrying out a geological surveys campaign over ten days,” continues the engineer.
Samuel Lamon presents the different measures devices that make it possible to ensure the stability of the structure.
Florian Cella / Tamedia
Each year, the stones fallen from the shores of the gorges are also identified. Manometers permanently monitor the water pressure in the rock. In the galleries, water flows, which come from infiltration in concrete and rock, are harvested and measured. If they come out of standards, operators immediately seek to determine the reason.
In the Maison des Barragistes, an exhibition details the course of the 1978 event and the data collected since. A graph shows that the water flows observed have been in recent decades. “The situation is normalized, but the geological phenomenon which has caused the compaction is still active and is monitored carefully,” said Samuel Lamon.
The work located in the municipalities of Ayent and Icogne is not the only one to be the subject of such vigilance. “The Federal Energy Office fixes a very strict framework and the protocols to be respected in terms of surveillance and alert.”
Diving in the bowels of Tseuzier, on the top of the Valais communes of Ayent and Icogne.
Florian Cella / Tamedia
The fear of a giant tidal wave that would drown the old-country in the event of a rupture remains very present in the minds of the Valaisans. Is it founded? “Human beings have a certain ability to imagine the worst,” reacts Samuel Lamon. But that helps us to develop scenarios to prevent disasters. Among those that the Confederation asks us to study figures that of a frank rupture of the dam: a bit as if it was disappearing suddenly. This is not a plausible hypothesis, but it is taken into account in the development of security protocols. ”
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Climate change and new scenarios
The fact remains that faced with climate change, these scenarios are called to evolve. “For example, engineers must today integrate the fact that the temperature of visible facilities in the dam increases, which has an impact on concrete behavior.”
The multiplication of rainy violent episodes is also not without consequences. “Thunderstorms in July, August, September, we know. We always keep a safety volume, which allows them to be absorbed in the lake. On the other hand, we see more and more thunderstorms in winter, when the lake is at its highest level. ” Here again, a safety margin is kept and, in the event of an extreme flood, a system makes it possible to evacuate the overflow. “We can turbine, even drain the restraint by the bottom valve, in an emergency.”
The gorges of the binding being very cashed, the wave caused would have time to smooth before reaching the plain, reassures Samuel Lamon, which evokes surveillance: “In all cases, a flood is not sudden: there are warning signs. Before draining for a reason where another, we must warn the population. In reality, dams are rather a security against floods, because they allow you to play a buffer role. ”
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