Low-Velocity Zone in Smooth Pipe Culvert with and without Streamwise Rib for Fish Passage

Abstract

Unimpeded waterway connectivity is a requirement for all freshwater fish. While box culverts are considered the most effective design in terms of upstream fish passage, circular culverts are very common. Detailed hydrodynamic measurements were undertaken under controlled conditions in a near-full-scale smooth pipe culvert operating at less-than-design flows. Two configurations were tested: a smooth semicircular channel and a circular channel equipped with a small streamwise rib placed asymmetrically. For all investigated flow conditions, the channel flow was subcritical and corresponded to less-than-design conditions. Detailed measurements showed high velocities through the entire cross-section, with no obvious low-velocity region along the smooth wetted perimeter. The presence of an asymmetrical streamwise rib induced the formation of a small well-defined low-velocity zone (LVZ) in the vicinity of the rib. The flow resistance was slightly larger than that in a rectangular channel, for identical boundary roughness and flow conditions. The streamwise rib had a limited impact on the flow resistance, although large transverse gradient in skin friction shear stress, conducive of secondary currents, were observed. While the low-velocity zone size relevant to the upstream passage of small fish was smaller in the smooth circular channel than in a comparable rectangular channel, the introduction of the asymmetrical streamwise rib might create preferential swimming paths for small-bodied fish and juveniles of larger fish. The present physical results may serve as a validation data set for future computational fluid dynamics (CFD) modeling, to assist with the development of more efficient designs.