Full-height sidewall baffles in box culvert to assist upstream fish passage: physical modelling

River crossings and culverts are important for delivering a range of socio-economic services, including transportation and hydrological control, but have negative impacts on freshwater river ecology, including small-body-mass native fish species and juvenile of larger fish. In relation to upstream fish passage, the excessive barrel velocities have a major negative impact on weak-swimming fish. Recently, full-height sidewall baffles have been proposed to assist upstream passage of small fish across a wide range of discharges, although the impact on the culvert hydraulics is not well understood. Physical modelling was conducted under controlled flow conditions in a 0.5 m wide rectangular channel, equipped with full-height sidewall baffles. Detailed free-surface and velocity measurements were performed at several locations along and across the channel for a broad range of discharges and baffle geometries. The findings demonstrated a drastic reduction in discharge capacity of the box culverts with the installation of full-height sidewall baffles, with increasing adverse impact as the flow rate increases for a given structure. In many cases when the discharge capacity cannot be compromised, alternative designs may be considered to assist upstream fish passage, e.g. small corner baffles, asymmetrical roughness.