Laboratory experiments of turbulent density currents and implications for near-surface CO2 rivers dispersion

Highly concentrated geogenic CO₂ emissions are frequently observed in volcanic and tectonic areas. Specific topographic and meteorological conditions can lead to surface accumulation in the form of buoyancy-driven “CO₂ rivers.” While history records catastrophic events, such as the deadly limnic eruption of Lake Nyos in 1986, the dynamics of these CO₂ rivers are not well understood. Current modeling efforts are often limited by a lack of controlled empirical data, hindering the development of robust hazard assessment and mitigation strategies. To address this issue, we simulate CO₂ rivers in scaled analog laboratory experiments by turbulently injecting high-density saline water into a tank of lower-density fresh water over a rough, inclined surface. We vary the volume flow rate, slope angle, and surface roughness between experiments. We characterize the flow dynamics by measuring the front and lateral spreading velocities as a function of time. The acquired experimental datasets are then used to calibrate TWODEE, a depth-averaged, shallow-layer numerical model for buoyancy-driven flows that relies on several empirical parameters to describe entrainment. To test the new range of parameters, we apply the calibrated model to our field data on airborne concentration and surface flux of CO₂ collected at the Ribeira Grande CO₂ degassing zone on São Miguel, Azores, Portugal. The results validate the experimentally calibrated model and demonstrate that our refined set of model parameters significantly improves the modeling of turbulent dense-gas flows, enabling more robust predictions of the behavior of hazardous CO₂ rivers in volcanically and tectonically active regions.