“Vegan” culvert: application of hybrid modelling in modern hydraulic structures

A culvert is a covered road structure constructed to pass flood and drainage. Modern society with a raise in environmental awareness demands modification in culvert design to incorporate a fish-friendly perspective. The migration of fish upriver is important for their breeding and feeding activities, contributing to a stable population and species diversity, and culverts have negative impacts on freshwater river system morphology and ecology, including blockage of upstream fish passage. Herein, a review is presented, highlighting the use of hybrid modelling in developing a fish-friendly culvert design solution. The hybrid modelling involves using both physical and numerical Computational Fluid Dynamics (CFD) modelling to test a range of design alternatives. Among the proposed design alternatives, a widening of the smooth culvert barrel proved to be best suited for the primary purpose of a culvert, i.e. flood passage, as well as fish passage. Field test is still needed to verify the effectiveness in practice. Full-height rectangular baffles and longitudinal beams both provided sizable and continuous low velocity zones for fish to traverse. However, relatively high turbulence can be induced by full-height baffles and the impact on fish behavior is unknown. The installation and maintenance of these two types of intrusive structures can also be difficult, while both reduce the flood capacity of the culvert by 30 to 50%