Field measurements of the attractivity of bypasses for fishfriendly trashrack

Abstract

Fish-friendly trashracks, combining a low bar-spacing rack and bypasses, are a solution towards the fish downstream migration issue at hydroelectric intakes and rehabilitate the ecological continuity, as aimed by the European Water Framework Directive (2000). To comply to the French Environmental Code, the Hydropower plants (HPPs), located on rivers listed in list 2 according to the article L-214-17, must ensure an adequate sediment transport and free movement of fishes along the river. Each infrastructure has to be managed, and equipped according to the rules set by the administrative body in concertation with the owner or at least the operator. This study concerns inclined trashracks, which have an acute angle starting from the ground, to guide fishes to one of the surface bypass entrances. Well dimensioned bypasses should allow an efficient attraction of the fishes, while limiting the discharge in the bypasses that cause loss for hydraulic operators. In this context, a measurement campaign in a hydropower plant on the Ariège river has been carried out to characterize the velocity profiles at different positions upstream of a 26° angle trashrack with an ADCP and the flow discharge of each of the 3 bypass entrances with flowmeter measurements. The measurements show that the streamwise velocities are quite homogeneous along the rack, without significant acceleration or deceleration, and that the tangential and normal velocities, Vt and Vn, are consistent with their theoretical values. These results tend to validate the angular criteria proposed for inclined rack by Courret et Larinier [6] and studied experimentally by Raynal et al. [4] on a down-scaled model. However, it must be considered that the turbine discharge was low the day of measurement. So, these in situ measurements have to be conducted again, with a turbine discharge closed to its maximum.