Field and numerical assessment of turning pool hydraulics in a vertical slot fishway

Turning pools are common in fishways, they create a more compact design by enabling fishways to fold back on themselves. Despite this, little is known about the hydraulic characteristics of turning pools or how they are influenced by different design elements. This paper presents the results of a study on the hydraulics of turning pools in vertical slot fishways focusing on the Vianney-Legendre vertical slot fishway in Quebec, Canada, which is one of few fishways worldwide to successfully pass sturgeon (i.e., lake sturgeon, Acipenser fulvescens). Field velocity measurements taken in two pools and a computational fluid dynamics (CFD) model study were used to assess the turning pool hydraulics of seven design geometries. Parallel biological studies of sturgeon in the fishway revealed that turning pools were the location with the highest rate of failed passage, apparently associated with large vortices in the centre of the turning pools which serve to delay or inhibit passage. Interestingly, the velocity, and turbulence levels are comparable to results from regular pools in vertical slot fishways. The volumetric energy dissipation rate in turning pools is suitable for fish passage. Based on in silico modelling we revealed that the addition of a baffle wall extending from the inside centre wall of the pool reduced the size of the vortex and provided a resting area for ascending fish. Adding a baffle wall should be considered in turning pools with semi-circular or straight back walls. There is a need for research to evaluate exactly how fish respond to different turning pool designs but in the interim, the approach used here demonstrates the potential for using hydraulic studies to design turning pools in fishways that meet biological criteria and presumably increase passage efficiency.