The operator of a dam was performing qualification testing of its new PLC (Programmable logic controller) while the flow rate of the river Rhine was fluctuating at around the full flow capacity of its channelled portion (Grand Canal d’Alsace). If the canal’s full flow capacity is exceeded, the PLC is programmed to divert a portion of the water to the ‘Alter Rhein’ (the Old Rhine) by opening the dam’s floodgates. The thresholds for opening these floodgates were reached twice that afternoon. At 11:45 a.m., the Old Rhine’s flow rate suddenly increased by 600 m³/s, causing the water level downstream of the dam to rise by 1.75 m. The floodgates closed at 12:40 p.m. At 3:23 p.m., a second sudden 470 m³/s increase in the flow rate caused the downstream water to rise by 1.46 m, trapping three individuals on a small island 300 m downstream of the dam. The individuals had just arrived on the small island although access to it was prohibited. One of them was able to reach the German shore and give the alert. German and French helicopter rescue teams and divers searched for the two missing individuals; they were later found safe and sound on the French shore at 5:00 p.m. The operator of the dam put back into service the old PLC, with which the fluctuations in the flow rates of releases are significantly lower.

The operator’s analysis highlighted organisational failures in defining the new PLC’s control parameters, i.e., the risk analysis performed prior to the old PLC’s replacement focussed on controlling the water level upstream of the dam but failed to properly look at controlling the flow rate diverted to the Old Rhine.