On a dam, the breakage of a keyed joint caused a deformation of the transmission shaft, then unavailability of the brake used to restrain one of the 3 valves used to evacuate flood waters (not in control mode). This valve, having undergone a fall of 1.5m, became blocked in half-open position (under tension on the deformed shaft). The manager released it by sawing the transmission shaft. The valve ended its fall on the foundation raft. This fall caused localised puncturing of the foundation raft by 0.5cm. The shaft being inoperative, the valve was unavailable.

The manager urgently took the following precautionary measures:

• visual and structural diagnosis of civil works and gates and valves;
• topographic surveys to analyse any displacements of the structure;
• specifications for repair of the valve opening mechanism;
• hydraulic modelling to quantify the new bankfull discharge and the consequences of the closing of a valve in the event of flooding: the hydraulic study concluded that the clogging of the watercourse by the valve (blocked in closed position) did not alter the bankfull discharge.

Pending repair of the valve and because, over a period of less than one year, the keyed joints of 2 out of 3 valves had been faulty, the inspection authority recommended that precautionary management measures be applied. The risk was of having only a single valve in a flood situation. Moreover, the impact strength of the foundation raft should be verified as part of the general stability study stipulated by prefectural order. The authority also requested that the keying system for the third valve which had not been replaced should be replaced after the valve giving rise to the event was operational again.