Risk near hydroelectric structures – Determination of critical velocity thresholds for river users (swimmers and floating crafts)

Whether we talk about safety reasons, energy production or regulation, water resources management is one of EDF’s (Électricité De France, French hydropower company) main concerns.

The range of water-based activities is steadily increasing : paddleboarding, canoeing, float-tube fishing, these floating crafts are now widely available at low cost and are becoming popular on rivers. EDF’s hydroelectric facility operators are regularly faced with the intrusion of swimmers and watercraft near the structures. This occurs, for example, beyond the restricted zone upstream of hydroelectric installations. This zone, usually marked by a line of buoys, is intended to prevent drowning accidents that could result from the start-up of a turbine or any other system capable of creating a suction current. Similar risks exist downstream of the structures, where currents and depths attract swimmers and floating crafts.

Given the risky behavior of river users, EDF’s challenge is to secure the vicinity of its installations by assessing the danger and proposing appropriate countermeasures. In this context, EDF aims at determining current thresholds beyond which the swimming ability and maneuvering capacity of floating crafts (paddleboards, canoes, float-tubes) for different user profiles (children, adults, athletes) are no longer sufficient to escape the danger of being sucked in or swept away. This study therefore only concerns areas where users cannot stand in the river. Such work will enable the company to implement the necessary measures to secure zones considered hazardous near hydroelectric structures.

To carry out this work, tests in collaboration with members of the SDIS 81 Water Rescue team were conducted at the Millau whitewater stadium to determine the current speeds beyond which swimmers and light watercraft can no longer escape danger. These trials, carried out under controlled and safe conditions, involved scenarios of swimming and maneuvering floating crafts in currents ranging from weak to strong at the Millau water sports facility. The objective was to assess the swimming and mobility capacities of swimmers and non-motorized watercraft (paddleboards, canoes, float-tubes) across different current speed ranges.

All the tools routinely used by EDF hydrometric teams to measure flow velocities were deployed (ADCP, current meter, SVR radar, LSPIV) to better characterize the different current speeds tested.

The results obtained made it possible to identify the threshold values sought, based on current speed and the presence or not of turbulence. Finally, a theoretical approach based on Newton’s second law helped corroborate the empirical results obtained during the tests.