On upstream fish passage in standard box culverts: interactions between fish and turbulence

Abstract

ABSTRACTThe challenge to understanding the fluid mechanics of fish swimming is knowing exactly what the water is doing where the fish swims. Recent field and laboratory observations in box culvert barrel showed that fish tend to swim preferentially close to the channel sidewalls, in regions of slow velocity and often high turbulence intensity. An analogy with human swimming is developed herein. Fish minimise their energy expenditure by swimming in inter-connected low-velocity zones (LVZs) and minimising acceleration-deceleration amplitudes. In a box culvert barrel, the mechanical energy expenditure is drastically reduced in sidewall and corner flow regions, characterised by low velocities and secondary current motion. These regions were “sweet spots” used by small bodied fish to minimise their rate of work. Both bed and sidewall roughness must be scaled to the fish dimensions. More generally, the methodology brings rigorous scientific insights into why certain culvert designs, possibly equipped with baffles and apertures, are more efficient in promoting fish passage. One may foresee an evolution of the scientific approach towards using advanced physics-based theory supported by high-quality data sets. The results also raise questions on limitations of current fish swim tunnel tests, and matching swimming performance data to hydrodynamic measurements.