To enable water sampling during upcoming tests of the first-time opening of a hydroelectric dam’s bottom valves, a facility technician is requested to install a 2-inch ball valve in the vicinity of a tap on a circuit bypass pipe connecting the upstream guard gate with the downstream regulating valve. Upon verifying closure of the corner valve located on the bypass between the guard gate and the tap, he forgets to close the guard gate. Both the short-circuited section and the bypass segment (including the tap) downstream of the corner valve continue therefore to be fed by the lake. When pulling the plug on the 2-inch tap, pondage water flows into the bottom valve chamber. The adjacent 5-bar water pressure prevents installation of the plug or a new valve. Closing the guard gate, in order to cut water supply to the leak, last 20 minutes during which time 40 to 50 l/s filled the chamber. The personnel in place has to evacuate the premises once the water had reached shoulder height.

The well, plumb bob room, access galleries to and from both banks and the bottom valve chamber all flooded over a 6-m height. Three divers with a specialist firm clogged the leak by screwing back in a plug and opening the door to the bottom valve chamber leading to the outside, helping lower the water level more quickly. First responders installed several pumps. On 11 October, the bottom valve chamber was once again accessible. Various equipment, including electrical panels, bottom valve control devices and pondage measurement instruments, were destroyed, amounting to an estimated €160,000 loss. The two bottom valves were idled for 6 days.

Following this accident, the dam operator installed a second corner valve on the downstream part of the bypass so as to completely isolate the tap. Dam personnel were duly informed of this modification.