Turbulence and Secondary Motion in Rectangular Channels: Implications for Upstream Fish Passage at Road Crossings

Abstract: A culvert is a covered channel designed to pass water through an embankment. The adverse ecological impacts of road crossings on upstream fish passage have driven the development of new design guidelines with a focus on small-bodied native fish species and juveniles of larger fish. Recent studies recommended the usage of low velocity zones (LVZ) in the culvert for upstream fish passage. To date, there is limited knowledge on the hydrodynamics in the LVZ, particularly the turbulence characteristics, despite its relevance to fish kinematics. New research was carried out to study the turbulence and secondary motion in the low velocity zone in a rectangular culvert. Detailed turbulence measurements were undertaken in a near full-scale box culvert barrel. Velocity and Reynolds stress measurements showed some strong secondary motion of Prandtl's second kind. The turbulent characteristics, including burst events and secondary currents, were carefully detailed, presenting a novel insight into the turbulence of fish's preferential swimming zone. Keywords: Culvert, Turbulence, Open Channel Flows, Fish Passage, Environmental Fluid Mechanics, Hydrodynamics