Hydrodynamics Matters: Unravelling the Gas Transfer Law for the Energy Dissipation Process in a High-energy Stream

Accurately quantifying air-water gas transfer is pivotal for understanding carbon cycles and assessing aquatic hypoxia prevalence. In the energy dissipation process of high-energy streams, gas transfer is extremely high, but the estimation becomes challenging due to the instantaneous variability in flow properties. Experiments in a laboratory open channel flume for a typical energy dissipation process (hydraulic jump) have been undertaken. The transfer efficiency E for the hydraulic jump lay between 0.037 to 0.162. These values are 4 to 7 times larger than those reported in previous studies with comparable layouts but different scales, highlighting the substantial impact of scale effects in bubble dynamics on gas transfer. Localized gas transfer velocities k600 exhibited a range from 340 to 985 m/day, falling within the order of 100 for estuaries and lowland rivers and comparable to rapids in a large whitewater river. Paired experiments were conducted to explicitly resolve the hydrodynamics of the free surface and bubbles. Subsequently, a mechanistic model was developed by establishing a relationship between gas transfer and hydrodynamics. The model physically clarified the gas transfer contribution from free-surface and bubble-mediated components and elucidated the reasons for gas transfer heterogeneity for different flows. The results provide insights into gas transfer estimation on a large scale.