Air–water gas transfer in open channels in the presence of counter-rotating streamwise vortices

The rate at which weakly soluble gases transfer through natural air–water interfaces can be difficult to model because the transfer velocity depends on complex multi-scale dynamics at or near the interfaces. The impact of counter-rotating streamwise vortices, which occur in wind-driven water bodies and open channel flows, on interfacial gas transfer is not well understood. Laboratory studies were conducted in a wide, recirculating, open channel flume to quantify the impact of said vortices on gas transfer velocity. The counter-rotating streamwise vortices were stabilised using fixed longitudinal bed bars. Cases with bed bars were compared to cases without bed bars at three flow velocities (with depth-based Reynolds numbers from $1.7\times 10^4$ to $5.8\times 10^4$ ). Cases with bars on average exhibited 9–15 % faster gas transfer, 42–100 % more surface turbulent kinetic energy, and 20–50 % faster key turbulence time scales, likely due to enhanced shear and vertical transport of subsurface turbulence. Turbulence measurements demonstrate that the presence of the longitudinal bed bars leads to significant lateral heterogeneity in gas transfer.