Beyond average velocity: modelling velocity distributions in partially filled culverts to support fish passage guidelines

Abstract

ABSTRACTAn important consideration in managing a river basin is how to treat the many stream crossings within the basin. Historically, corrugated metal pipe (CMP) culverts have been commonly selected for stream crossings throughout the world. The associated reduction in cross-sectional flow area causes an increase in water velocity at the culvert, which may become a barrier to fish passage. In Canada, existing guidelines compare the swimming performance of fish that may use the culvert with the average velocity within the culvert at some design flow. It is known that a velocity distribution occurs within culverts, and it is hypothesized that fish may possess the ability to sense and locate preferential swimming paths within low-velocity zones in a culvert. To facilitate the design of culverts in a manner that may better consider fish swimming performance, this paper compares the results of empirical and numerical modelling of the velocity distribution within partially full CMP culverts at uniform depth. An additional simplified model to estimate the percentage of the cross-sectional area with a water velocity less than any reference velocity is presented. Finally, the concept of using two-dimensional velocity distributions in combination with fish preference data for water velocity and depth is presented.