Hydraulic modelling of pipe culverts and low-velocity zones at less-than-design flows

Abstract

The impact of waterway crossings and culverts on fish populations have often been underestimated despite their major impact on fish. Recent research in box culverts has shown that they can be more effective in terms of upstream fish passage, but circular pipe culverts are most common. In the current study, the hydrodynamics of smooth pipe culverts operating at less-than-design flows was characterised, including a quantitative estimate of the low-velocity zone relevant to upstream fish passage in a near-full-scale smooth pipe culvert (D = 0.50 m) operating with d/D < 0.5. The flow was sub-critical for all investigated flow conditions. Visual observations and detailed velocity measurements showed high velocities through the entire cross-section, with no obvious low-velocity region in the smooth circular channel. The flow resistance was slightly larger than that in a rectangular channel, for identical boundary roughness and flow conditions. The increased flow resistance was caused by the secondary motion resulting from the circular channel shape, while the skin friction boundary shear stress was less than the total boundary shear stress. The present physical results may serve as a validation data set for future computational fluid dynamics (CFD) modelling to assist with the development of more efficient designs.