Numerical simulation of pipe culverts equipped with bristle clusters for fish passage

A three-dimensional numerical study was conducted on standard pipe culverts equipped with flexible bristle clusters to evaluate their effectiveness in improving fish passage, while considering their impact on culvert capacity. The Reynolds-averaged Navier–Stokes equations were coupled with the Darcy–Forchheimer saturated drag model, in which the bristle clusters could be modelled as homogeneous porous blocks, owing to the low Cauchy number of the bristles. This approach allowed the use of mesh sizes 20 times larger than the bristle scale, significantly reducing computational costs. The model was validated with experimental results with less than 4% error for flow depth. Simulations demonstrated that bristles shift flow conditions from supercritical to subcritical regimes, thereby reducing flow velocity and creating low-velocity zones that serve as energy-efficient resting areas for small-bodied fish. Unlike solid elements, bristles do not produce recirculation regions, which can disorient small-bodied fish. Under design flow conditions, bristles resulted in approximately 15% less water rise compared to non-porous solid blocks. The results indicate that flexible bristle clusters represent a promising and cost-effective retrofitting solution for smooth pipe culverts, improving ecological connectivity while preserving hydraulic efficiency.