Modelling small ventilated corner baffles for box culvert barrel

Abstract

During the last decades, concerns regarding the ecological impact of standard culverts have led to some design evolution. The installation of baffles along the culvert barrel yields smaller velocities and larger water depths in the barrel, potentially more suitable for upstream fish passage, albeit with a decrease in discharge capacity. Small triangular corner baffles were proposed to facilitate the upstream passage of small-body-mass fish, without compromising the discharge capacity of the culvert at design flow. Although fish benefited from low velocity regions for resting and sheltering, a small fraction of small-body-mass fish were observed to become disoriented by the adverse effect of flow reversal regions in the wake of plain baffles. This study presents the hydrodynamic testing of small ventilated triangular corner baffles for standard box culverts. The baffle ventilation was introduced to reduce the impact of negative wake behind the baffles. Two designs were tested: a baffle with three holes and a brush baffle. Detailed modelling in a near-full-scale culvert barrel showed that the ventilated corner baffles created a smaller negative wake region. A lesser negative velocity magnitude was observed behind the ventilated baffles, in comparison to plain baffles, for the same flow rate, baffle height and spacing. With ventilated corner baffles, the longitudinal distribution of low-velocity zone was more uniform, yielding a better longitudinal connectivity for upstream passage, compared to plain baffles. A comparison between various boundary treatments suggested however that the requirements for continuous, sizeable low positive velocity zone suitable to small-bodied fish might be better fulfilled with an asymmetrically roughened culvert barrel than with triangular baffles, even with ventilation.