Near-surface turbulent dissipation at a laboratory-scale confluence: implications on gas transfer

River confluences contribute to the outflux of saturated dissolved gases in the water resulting from high dam discharges. This process is related to gas transfer across the water–air interface, which is primarily controlled by turbulent dissipation near the water surface. However, the near-surface turbulence dissipation is rarely reported in confluence hydrodynamics studies. This study conducted experiments with different discharge ratios to investigate near-surface turbulent motions at a laboratory-scale confluence. The higher dissipation rate H∕U3 m of near-surface turbulence was mainly located inside the interfacial shear layer between the two incoming streams (~ 10–4) and the bank separation zone (10–4–10–3) where high shear was found in the mean flow. By contrast, the dissipation rates were much lower inside the incoming flows and outside the two regions of high shear (~ 10–5). The magnitudes of the dissipation rate inside the shear layer were comparable in experiments where the mixing interface was in the Kelvin–Helmholtz mode or in the wake mode. The dissipation rate was found to increase away from the free surface outside the shear layer, while it was more uniformly distributed over the depth inside the layer possibly due to the presence of strongly-coherent, vertically-orientated vortices. In the far field, the mean shear within the shear layer was largely weakened. Nonetheless, the effects of flow separation persisted and laterally expanded to occupy the entire cross section. The dissipation rate H∕U3 m of the confluent flow was more than 10–4 even at a distance of 10 times the channel width in the post-confluence channel.