Designing free-surface bypasses for fish-friendly inclined bar rack: A CFD parametric study

Hydroelectricity production using run-of-river power stations meets multiple challenges, including the growing demand for renewable energy and the need to preserve local biodiversity and ensure ecological continuity in rivers. Various infrastructures are implemented to address these requirements. For downstream fish migration, fish-friendly inclined bar rack systems with surface bypasses may be installed at water intakes. The design of these systems is predominant to ensure the hydraulic attractiveness of the entrances with effective guidance of fish toward the downstream passage. However, the design of such bypass systems remains rarely studied, particularly regarding flow distribution among multiple entrances. This study presents a parametric investigation of a bypass system with two entrances, using Computational Fluid Dynamics (CFD) simulations based on the Volume of Fluid (VOF) multiphase model. The bar rack with narrow bar spacing is modeled as a porous volume to enable feasible meshing size, representing a relatively innovative approach. The influence of important factors on the flow distribution between the bypasses, the hydraulic attractiveness and the entry flow characteristics is examined. These factors include the width, shape, and orientation of the bypass entrances, the implementation of an additional pinching adjustment system, and the design and separation of the downstream channel. This study contributes to advancing current knowledge by providing recommendations for future water intake infrastructures, particularly for configurations similar to those investigated here. It critically evaluates the effectiveness of previously implemented designs while proposing new solutions. Furthermore, the results highlight the benefits of optimizing entrance orientation and incorporating a pinching device to achieve a more balanced discharge distribution between bypasses.