3D upstream passability of novel river training structures by migratory fish in the river Waal

Longitudinal training dams (LTDs) are novel river training structures that divide a river into a main navigation channel and protected shore channels. High velocities at the inflows of shore channels constructed in the river Waal (The Netherlands) pose a potential bottleneck for migratory fish species swimming upstream. This study assessed the passability of the inflows using flow velocity datasets from governmental monitoring campaigns collected with an ADCP during high river discharges (Q = 3489–5066 m3/s at Lobith monitoring station). The swimming performance of several migratory fish species were estimated from their total lengths (TLs). A new 3D approach to visualize the flow velocity data using Voxler® software is presented. Gasterosteus aculeatus aculeatus was the only fish species with an adult life stage unable to pass the 9 cases tested due to its small size. The juvenile European eel and thicklip grey mullet passed 1 and 0 of the cases, respectively. The most upstream inflow, located in the inner bend of the river, was the most passable. We recommend maximizing the cross-sectional area of the inflow sills in order to reduce the flow velocities experienced by migrating fish during high river discharges. This approach may be useful to assess passability at other locations and training structures.