Intermittent flow regime of water/gas emissions as sinkhole trigger in alluvial plains: a study case at S. Vittorino plain (Rieti, Italy)

[object Object] Abstract A case of natural intermittent flow regime (slug/churn) of a CO2-enriched spring water is discussed as an example of trigger process for the collapse of sinkholes. The hydrodynamic regime of the spring, situated in the area of S. Vittorino alluvial plain (Rieti, Italy), was investigated by monitoring acoustic emission, carrying out geochemical analyses and installing a sedimentation trap during a long-lasting phase of slug flow regime followed by a churn flow phase. The slug and churn flows can erode and transport upward fine and medium size particles of sand, progressively removing them from some underground shallow sandy levels where the groundwater mixes with gas of deep crustal origin to form the intermittent flow that can be observed at the surface. The slug flow regime was characterized by a stable slugging frequency whose value is in agreement with the estimated superficial velocities of Taylor bubbles and liquid slugs. The transition from the slug to the churn flow regime, the latter characterized by an increase of gas flux and by more severe erosion, was also observed during our study. In a span of 15 years, the transport of sand upward could produce a collapse volume similar to those of the sinkholes that recently originated in the same area. The study of such multiphase flows prolonged over time could help to monitor and prevent the sinkhole formation process.