Assessment of the Performance of a Protection Bell for a Micro Power Station on the Rhône

VNF protected the operating devices of the Ormes and Drace locks from flooding through a bell designed to trap an air bubble despite the rise of the water level. The operating equipment includes a micro hydraulic power station and an electrical box. A rise of the water in the bell would therefore be prejudicial to the operation of the entire lock. However, during a recent flood (April 2016), the protection system showed some weaknesses that VNF wishes to resolve. Indeed, a flood level reaching nearly the lower part of the electrical box was observed, i.e. a much higher than previously announced in the calculations already made. A 3D model of the bell, without singularity in its close vicinity, allowed reproducing in a satisfactory way the consequences of the event the flood of April 2016. It has confirmed that the static study, considering a body of water at rest and a rise in water only by air compression effect, was insufficient with regard to the technical implementation of the solution. Indeed, the installation of support crossties perpendicular to the flow, added to the vertical part of the tray, generates a dynamic effect of water rising along this surface. The flow coming from upstream passes through the protection rake, rushes under the front of the bell and causes the water line to swell in the bell. This dynamic effect is independent of the outside water level, and depends only on the flow velocity as the structure approaches. An easy-to-implement solution was found to avoid this phenomena.