Researching fish-friendly weir management in the River Meuse

Biodiversity in freshwater systems is under pressure worldwide, and the Netherlands is no exception. As part of the European Union’s Water Framework Directive (WFD), the EU sets requirements for the quality of our freshwater systems. A healthy and balanced fish population is crucial in order to achieve these goals, with migratory fish species in rivers playing a crucial role.

Weirs in rivers form barriers for migratory fish and interrupt the connection between different parts of freshwater systems. Improvement is needed in order to meet WFD objectives.

Weirs and fish migration

The weirs in the River Meuse are vital for water level management and navigation. However, they also form an obstacle for migratory fish. To provide passage for these fish, many weirs have been fitted with fish ladders. A fish ladder is a small stream that flows alongside the weir, in which the large height difference is divided into several smaller steps by means of small weirs or thresholds. This allows the fish to gradually make their way upstream.

Whether fish are actually able to find these ladders strongly depends on the flow conditions near the entrance, which are largely influenced by the configuration of the weir complex.

If the flow near the entrance is too fast or too turbulent, fish are unable to locate it. The visibility of the entrance depends on its position and the discharge (the amount of water flowing through the fish ladder per second), but above all on how the main flow is directed through the weir complex.

Stimulating large-scale fish migration

The project aims to understand how the flow patterns around the various weirs along the Meuse affect the findability of the fish ladders, and how weir operation can be improved to stimulate large-scale fish migration.

The Meuse weirs are approaching the end of their service life, and renovation works will be required to make them fit for the future. The knowledge gained in this project can be used to make these weirs more suitable for fish migration. With this information, we hope that migratory fish will be better able to make use of the ecological value of Dutch rivers and that the biodiversity of river systems will improve.

Research using 3D models

Using detailed 3D models (Computational Fluid Dynamics), Deltares is investigating how different operational settings of the weirs influence the flow near the fish ladders. By simulating these situations, weir management can be optimised to support fish migration. Field measurements are planned for next year to validate the models.

Working together for sustainable rivers

“It’s a challenge to connect the world of hydraulics – which relies heavily on models and numbers – with the world of ecology – where knowledge is often based on observations,” says Nino Zuiderwijk, hydraulic researcher at Deltares. “To make this collaboration work, we need to understand each other and align our approaches. That’s what we’re focusing on right now. This challenge motivates me. Working with people from different backgrounds makes it a fantastic project to be part of.”

This research is part of the joint ambition by both Deltares and Rijkswaterstaat to promote sustainable and ecologically responsible management of Dutch rivers. By combining expertise in hydraulics and ecology, the researchers aim to provide practical guidance for a future in which water management and biodiversity go hand in hand.