Modeling Steep-Slope Flow across Staggered Emergent Cylinders: Application to Fish Passes

Designing efficient rock-ramp fish passes with flows over a bottom with roughness on the same scale as the water depth requires a precise knowledge of hydrodynamics in order to avoid or limit characteristics unattractive for fish, particularly for small fish. This paper considered the numerical modeling of free-surface flow across a steep-sloped ramp covered with staggered surface emergent cylinders. Considering the importance of complex flow features for fish passage, computational fluid dynamics (CFD) was adopted because it is capable of predicting such features. Because of the longitudinal periodicity of the arrangement of the obstacles, cyclic boundary conditions made this fine simulation possible. Two computational meshes (coarse and fine) and two turbulence models [shear stress transport (SST) k-ω and Smagorinsky large-eddy simulation (LES)] were used. The SST k-ω coarse mesh model gives correct time-averaged values, the main flow unstationarities and is usable for rock-ramp fish pass design, but a fine model using LES turbulence closure can provide detailed flow characteristics in the wakes in order to provide possible rest zones, particularly for smaller fish.