Following storms that occurred the previous night, the prefecture notified the department in charge of inspecting hydraulic structures that the LIESBACH, a stream, had overflowed its left and right embankments and flooded a commercial area (see ARIA 43884). An inspection was conducted on 21 June. Signs of mud along the highway indicated that the stream had flooded over a bridge. The dyke downstream of the bridge prevented the water from flowing back into the stream. With nowhere else to go, it spilled into the adjacent fields. It therefore appears that the bridge was not sized for floods that could cause the flow of water to be blocked by eroded material from the foot of the upstream embankment slope. Downstream of the bridge, several zones of erosion and slippage of the embankment visible on the left bank could cause the structure to fail during a future flood. Signs of overflow are visible along the crown of the dyke on both the right and left banks. The water stopped just 5 cm below the crown on the left bank and opposite the landing strips of an airport. An excavator was used to enlarge the Liesbach’s bed, with the excavated material simply being placed on the dykes without any compacting. As a result, they will not strengthen it and may be carried away the next time a flood occurs. The rise in the water level during the flood was compounded by the dense vegetation along the downstream portion of the stream’s bed. Two dams of accumulated debris were found during the inspection. The bed of the LIESBACH will have to be extensively maintained to reduce overflow.

The inspection authorities concluded that the structures were no longer able to protect endangered areas and could create a substantial hazard if they ever failed. A special overhaul of these structures, already suggested during a visit back in 2011, is not possible for the moment because there is no managing entity.