Towards the end of the day, residents observed an abnormally high flow along the Boulan stream. Upstream, where the lagoon feeds into the stream, they noted a significant leak on the dam’s drain valve. In conjunction with the owner (a private individual) and the gendarmerie, the municipality defined the actions to be undertaken:
- the leak was stopped with a metal plate placed upstream from the valve, and 40 t of soil was placed in front of the plate;
- the installation was covered with a tarpaulin and a layer of pebbles.
However, the flow of water resumed the following day. Local farmers took the initiative to haul in manure and straw to clog the leak. Despite these efforts, the land immediately downstream of the drain outlet was flooded.
On site, the department in charge of inspecting hydraulic structures confirmed that the leak was caused by a failure of the dam’s drain valve. It is a vertical-lift type gate valve operated by a worm gear. The inspectors noted abnormalities on the valve’s operating mechanism: the sprocket wheels driving the worm gear in rotation were no longer meshing together. Without being able to examine the valve directly, they would not be able to determine whether or not the failure was caused by a failure of the valve causing the sprocket wheel to be raised, or by an act of vandalism. They noted that the planks making up the valve had been replaced with new ones five months ago. They also noted that the wooden supports were in good condition and the worm gear was lubricated.
The operator erected cofferdams upstream of the valve to stop the leak and make the necessary repairs. The operator believes that the malfunction was the result of a malicious act: a nut located under the manoeuvring device had been unscrewed. The valve’s lifting system was most likely lifted under the pressure of the water, causing the sprocket wheels to disengage. Moreover, the valve’s lower plank had been torn off and was found downstream. The operator took the following corrective actions:
- the valve and its operating device were completely reconditioned;
- a protective cover was added to prevent unauthorised access.