When the flow velocity over a river structure exceeds the swimming capacity of fish, it behaves as a velocity barrier. Depending on the hydrodynamic circumstances of the structure as well as the fish’s swimming ability and motivation, the barrier can be permanent, partial, or intermittent. This is the case of flat-V gauging weirs, a common type of velocity barrier in Spanish rivers and in other European rivers. Flat-V weirs are broadly used as they provide precise information about river discharge for water resource management under different hydraulic scenarios, especially during low flow conditions. However, depending on their size, local river morphology, and the river flow scenario, they can produce excessive velocities and thus, reduce or hinder fish upstream movements. Due to their variable geometry, velocity barriers exhibit a non-uniform flow velocity field, which means that flow velocity varies along the barrier. Therefore, any predictive swimming model to assess the barrier effect on fish must consider the spatial variation to achieve a valuable forecast. This work aims to estimate fish passage over Flat-V weirs by linking their 3D hydraulic performance with the swimming capacity of fish. For this, a predictive model is developed using as target species the Iberian barbel (Luciobarbus bocagei), combining research on their swimming ability with 3D models of the structure. Results of the model show the river conditions and weir dimensions that permit the ascent of this species through the sloped wall of the weir. This information has direct implications for the design and assessment of velocity barriers as well as for the retrofitting of velocity barriers, making them compatible with the fish migration.
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