Computational fluid dynamics simulation of passive downstream fish passage over a weir with OpenFOAM®

Many fish species are dependent on migration through river systems. Weirs are an obstacle to downstream migration, which can lead to obstruction as well as injury and mortality. The current level of knowledge about downstream migration over weirs is limited, and the current assessment of this is uncertain. This study uses a computational fluid dynamics (CFD) method for assessing the damage potential of the downstream migration over the weir, evaluating the effectiveness of the current recommendations. Two test cases were simulated using OpenFOAM (R) and an interFoam-based solver, extended with Lagrangian particles to represent passively transported fish. The results of the simulations show that adherence to the recommendation for a necessary tailwater level has no significant impact on the relevant hydraulic stressors, shear stress, pressure changes and collisions in certain situations. A thick nappe, however, leads to a significant reduction in collisions, especially of particles initialized near the surface. The CFD method used in this study proves to be suitable for evaluating downstream fish passage with certain limitations. This means that qualitative comparisons of different situations are possible, but not the determination of the damage potential of a specific situation. Further investigations are necessary to identify factors for safe downstream fish passage.