At around 8 p.m. during a thunderstorm, the flood spillway on the left bank of a dam accidentally opened 13 cm. The on-call technician received an alarm. The operator on site noted that the downstream flow rate of the dam had been 9 m³/s for 1 hour. The 3 m³/s spillway release warning, provided for in the operating instructions, was not respected. The technician closed the flood spillway gate. The incident had no impact on third parties.

Over the summer, a landslide above the reservoir was closely monitored. In late July, roughly 400,000 cubic metres of material fell into the reservoir, which did not affect the dam. However, in order to quickly regulate water level in the reservoir, the operator had decided to remove the lockouts on the flood spillway gates. In doing so, the gates could be operated more quickly. During normal operation, the spillway gates are locked out. Removing their lockout can only be done when they are in standby mode, with the permanent physical presence of the operator. This organisation was implemented this summer.

Investigations revealed that the opening of the spillway gate was caused by lightning which had struck the gate’s remote control cable, resulting in an opening command to be issued and then the shutdown of the operation due to the power supply being switched off. A control cabinet is located in the gate room near the gates. However, the operator had gotten into the habit of operating the spillway gates remotely from the dam’s control room. The remote control cable had been accidentally cut three years earlier and was reconnected to a junction box, but the earthing was not guaranteed.

The operator took the following immediate action:

• earthing of the remote control cable;
• lock-out of the flood spillway when an agent is not on site;
• update of internal instructions stipulating that the flood spillway gates must be controlled locally by default.

The hydraulic structures oversight body demanded a detailed formal analysis of this incident. One of the feared events, central to the dam’s hazard analysis, was avoided by only one of the five planned defensive barriers. The oversight body reminded the operator of the importance of conducting risk analysis for all phases of operation that are different from the usual methods, and of establishing measures to maintain the facility’s safety level.