Abstract:

[Objectives] Traditional culvert designs often result in excessively high flow velocities within the channel, impeding the upstream movement of weak-swimming fish species. Installing small triangular baffles inside culverts has the potential to provide upstream passage for small fish while maintaining discharge capacity. This study aims to clarify the hydraulic effects of triangular baffles by arranging multiple small baffles along one side of a flume to simulate internal culvert structures and verify the hydraulic effects through flume experiments. [Methods] Three-dimensional velocity data were collected using an Acoustic Doppler Velocimeter (ADV) to analyze the distribution patterns of turbulent kinetic energy and Reynolds stress. The quadrant analysis method was employed to quantitatively assess the impact of the baffle system on flow velocity distribution, turbulence characteristics, and momentum transport modes. [Results] The results showed that the triangular baffles created stable low velocity zones (LVZs) along the sidewall, with longitudinal velocities ranging from -4 to 15 cm/s, and velocities at the outer edge of the baffles around 25 cm/s, below the critical swimming speed of small fish such as Rhinogobius giurinus. In the mainstream zone, the lateral profiles of longitudinal velocity were nearly identical, ranging from 25 to 30 cm/s, indicating that the small triangular baffles had minimal impact on mainstream flow and thus preserved discharge capacity, achieving synergistic optimization of hydraulic efficiency and ecological function. The proportion of the low velocity zone area remained relatively consistent along the flow path, accounting for 14.80%-18.07% of the total cross-sectional area, demonstrating the feasibility of using triangular baffles to stably expand LVZs. The baffles significantly enhanced turbulence intensity in the region near the baffle-side sidewall, generating clockwise vortices and positive horizontal Reynolds stress that play an important role in maintaining swimming stability. Although the turbulent kinetic energy and Reynolds stress in downstream LVZs were higher than those in high-speed regions without baffles, they remained below the threshold of fish swimming preferences. This moderate turbulence enhancement not only provided energy for swimming but also avoided excessive turbulence that could impair the sense of direction or balance. Momentum exchange was dominated by jetting (Q₂) and sweeping (Q₄) events, whose dominance increased with higher threshold parameter H₀ (with a contribution rate of about 60% at H₀=4). The transient vortices formed had planes parallel to the fish’s spine and body axis, reducing energy loss during upstream movement and improving swimming efficiency through vortex energy transfer. This provided a more favorable flow environment for weak-swimming fish species. [Conclusions] This study identifies the distribution patterns of mean flow and turbulence characteristics and introduces quadrant analysis into the study of culvert turbulence-fish behavior interactions. It reveals the promoting effect of small baffle structures in fish upstream migration and addresses the lack of detailed flow field and turbulence structure analyses in previous research. The findings offer a feasible hydraulic optimization paradigm and model reference for the design of eco-friendly culverts.

Key words: culverts, flume experiment, baffles, low velocity zones, momentum exchange