Slowing the Flow: the Hydrodynamics of Benthic Protrusions and their Impact on European Eel (Anguilla anguilla) Kinematics

Eels are often unable to navigate fast flows caused by anthropogenic modifications (e.g. tidal gates, hydrokinetic turbines) due to their limited swimming performance and this is a contributing factor in the dramatic population decline of the European eel (Anguilla anguilla). A novel solution is the use of ‘eel tiles’ to create more favourable conditions allowing eels to navigate upstream. These tiles, composed of two strips of protrusions, one side with smaller diameter protrusions and the other with larger diameter protrusions, were placed on the bed of a laboratory flume. The hydrodynamics of the tiles were recorded using Particle Image Velocimetry (PIV) under conditions of varying bulk velocity and flow depth with diameter based Reynolds number in the range 925 - 7642. A reduction in mean velocity in the shear layer was observed immediately above the protrusion layer, which could aid eel passage during fast flows, and a periodic upwards shedding was also identified composed of a large scale turbulent structure among the small scale shedding from the individual wake protrusions, resulting in a highly turbulent and complex surface flow layer. We then observed the kinematic behaviour of European eels swimming in the flume over the different sections of the tiles and related the kinematic parameters of different swimming modes back to the hydrodynamic conditions of the flow. These pilot experiments show promising results for the widespread application of this technology and investigate the swimming kinematics of eels in turbulent flows, for which a knowledge gap exists