Setting the standards for fish passes

The EU Water Framework Directive aims to improve Europe’s river basins by setting environmental goals. Fish populations are key ecological indicators of water health, but weirs used for flow gauging often block their movement. Fish passes help fish navigate these barriers without stress or injury, but they can be costly and affect flow measurement accuracy.

We have been investigating the impact of fishpasses at flow measurement structures for over 20 years. Most recently, we’ve explored solutions like low-cost baffles that make fish passes more affordable and easier to install, even on existing weirs. Our research has informed British Standards for fish passes, including BS ISO 26906:2009 Hydrometry - Fishpasses at Flow Measurement Structures, and the new BS ISO 19234:2024 Hydrometry - Low cost baffles to aid fish passage on triangular profile gauging weirs. 

Testing these structures in our Volumetric Flow Flume (25m long, 2.4m wide and 0.9m deep) was a fundamental part of the process, allowing us to obtain extremely accurate data on the flow discharge curve of weirs that are fitted with fish passes. We’ve also used computational fluid dynamics (CFD) to complement laboratory investigations. CFD allowed for extending data ranges to higher water levels, providing confidence that baffles at an adequate distance from the weir crest won’t affect the gauging curve of the weir. This method also reduces study time and costs.

Weirs are important infrastructure, and this work is crucial for flood risk management and the sustainability of communities it affects, as well as for the well-being of migratory fish. HR Wallingford’s expertise and innovative approaches are paving the way for more effective and economical fish passes.