Maintenance checks were carried out at 9:30 a.m. on the head valve of a dam’s headrace canal. When its guard gate closes, the workers heard two brief noises. The operation was stopped. No anomalies were noted on the valve. Seepage appeared when the closing operation was resumed. The operation was stopped again. A crack was detected on the upper quarter of the valve body (2.25 m diameter butterfly valve). The partial emptying of the body of water was initiated immediately, via the dam’s by-pass gallery. An operation to save the fish was undertaken and water quality measurements were performed at the same time. The retention reached the threshold of the intake grids 2 days later.

Upon examining the valve, it was determined that a tree trunk, about 10 cm in diameter, was wedged between the lens (moving part) and the valve body. The increased stress on the valve body during the closing action resulted in cracking. The operator noted that the design of the lens’ drivetrain allowed the rated torque to be exceeded. The valve is controlled by a mechanical transmission system, driven by an electric motor. The motor is protected by a timed current limiter. This standard configuration did not allow the manoeuvre to be stopped in time, despite the lens being blocked by the tree trunk.

On the other hand, the upper horizontal panel of the water intake grid was offset from its normal position. This created a pathway for potential logjams. The operator attributes this offset to a probable impact during an avalanche. Indeed, the lowering of the reservoir made it possible to see that the right bank was strewn with tree trunks. These logjams would have been transported by an exceptional avalanche that occurred in early 2015.

In order to avoid this type of incident, the operator is required to take the following measures:

• check the presence of force limiters on the electric motor controls of penstock valves and their correct dimensioning with regard to the operating forces;
• conduct an underwater inspection during major avalanches to check the condition of the water intake grid and the absence of wood debris.