The water intake (DN 800 mm) on a drinking water production and irrigation dam was blocked and crushed over a length of a few metres. This anomaly was detected following the annual startup of the mobile arm in the current. The sampling capacity on the structure is reduced during this time. Its draining capacity remains unchanged, without affecting the dam’s safety conditions. The population’s water supply is also not affected owing to the reorganisation of the water supply.

An internal inspection of the water intake pipe, using an underwater robot, identified the start of the blockage consisting of concrete residues. Subsequently, an underwater inspection by divers revealed that the pipe had suffered external crushing. Following this discovery, an opening was made in the pipe to remove some of the residues which had been identified as the pipe’s lining material. The maintenance opening was closed up using a shell casing made for this purpose. These operations were accompanied by acoustic measurements and 3D scans of the area concerned which confirmed the anomalies and provided additional information.

The expert assessment, conducted by the operator, allowed several causes to be considered. While the destruction of the pipe by water hammering appears to be unlikely, the most credible scenarios, given the structure’s advanced age, are related to its depressurisation:

• pumping on a blocked pipe: the pipe blockage may have been the result of an equipment failure (failure to indicate the position of the valves and pumping started while the valves were closed), or formation of a plug (gradual degradation of the interior coatings or accumulation of sediments),
• by opening the base drain before opening the dam’s bottom drain: insufficient procedures or deterioration of the pipe due to fatigue (previous repetitive water hammering, especially during the operational phase of the former pumping station).

As a corrective measure, the operator plans to replace the control system within six months. As the expert assessment indicated that the pipe’s critical crushing pressure is low in relation to the physical quantities that it is possible to reach, the operator plans to apply higher margins to the new pipe. While this design change alone should prevent the problem from reoccurring, the expert assessment also recommends that brittle cement-mortar coatings be banned in order to avoid potential problems with the interior coatings, and to secure the valve operating and pump start-up procedures.