Hydraulic Design of Fish-friendly Cost-effective Box Culverts: Using Hybrid Modelling for Better Design Guidelines

While of importance to deliver a range of socio-economic services, low-level river crossings may have negative impacts on freshwater river system ecology, including blocking upstream fish passage. In order to restore upstream fish passage in culverts, the work develops physically-based design methods to yield cost effective culvert structures in order to form large, contiguous sections of traversable river. In this study, a hybrid modelling technique was applied, combining physical modelling, depth-averaged numerical modelling and three-dimensional computational fluid dynamics modelling (3D CFD) of a standard box culvert barrel. Detailed CFD validation was undertaken against laboratory studies obtained under carefully controlled flow conditions. This study focuses on the upstream passage of small-bodied native species in Australia, with initial tests undertaken with juvenile silver perch (Bidyanus bidyanus) and Duboulay`s rainbowfish (Melanotaenia duboulayi) in a 12 m long 0.5 m wide culvert barrel channel. Overall, the study showed the capacity of using a CFD model to predict the three-dimensional flow field in a culvert barrel, which could be used to design a fish-friendly culvert. The systematic validation against physical data is uppermost critical to ascertain the performances of a numerical model, and can be sensitive to a range of inflow conditions, boundary parameters, and the grid mesh quality and size.