Numerical Approaches to Evaluate the Hydraulics of Vertical Slot Fishways: A Comparative Study of 2D and 3D Simulations

Natural flows in rivers have been strongly impacted by anthropogenic development and water use. Many barriers have been placed in stream corridors, affecting two thirds of rivers worldwide. Fishways have been widely used as a solution to restore river connectivity, with Vertical Slot Fishways (VSF) as the most common type. However, desired passage efficiency is difficult to achieve. A numerical study was conducted to analyze the properties of the flow for two different VSF geometries using computational fluid dynamics. OpenFOAM and TELEMAC-2D were used to perform 3D and 2D depth-averaged flow simulations, respectively. Due to high computational requirements, the 3D simulations were limited to two different cell sizes and no wall roughness, whereas nine different cell sizes and four values of the friction coefficient were tested in the 2D simulations. In all cases, the simulations were found to satisfy the validation criteria. For the steepest VSF, 3D simulations better represented turbulent kinetic energy in the jet zones, while 2D simulations tended to overestimate it. Small variations in the roughness coefficients did not significantly affect the simulation results. Higher 2D grid resolutions improved flow velocity predictions in both cases, but predictions of turbulence did not improve for the steepest VSF. Overall, the 2D simulations had a much lower computational time and still represented the hydraulics of both structures well.