Physical modelling of semi-circular channels and low velocity zones - application to pipe culverts and upstream fish passage at less-than-design flows

Abstract

Unimpeded waterway connectivity is a requirement for all freshwater fish. Culverts have until recently been overlooked despite having a major impact on fish populations. While box culverts are considered a most effective design in terms of upstream fish passage, circular culverts are the most common. In the present study, detailed hydrodynamic measurements were undertaken under controlled conditions in a near-full-scale smooth pipe culvert (O = 0.50 m) operating at less-than-design flows. Two configurations were tested: a smooth semi-circular channel and a circular channel equipped with a small streamwise rib (30mm20mm) placed asymmetrically. For all investigated flow conditions (0.23 < d/R < 0.77), the channel flow was sub-critical. 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. Overall, the flow resistance of the smooth semi-circular channel was slightly larger than that in a smooth rectangular channel, for identical 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 recorded. For both configurations, the skin friction boundary shear stress was less than the total boundary shear stress, with /f ranging from 0.55 to 0.66 mid-channel. In the current study, the low-velocity zone size relevant to the upstream passage of small fish was smaller than in a comparable rectangular channel. The present physical results may however serve as a validation data set for future computational fluid dynamics (CFD) modelling, to assist the future development of more efficient designs in terms of fish passage.