New constrains on infrasound source mechanisms within debris-flows

Debris flows are episodic gravitational currents, consisting of mixtures of water and debris in varying proportions occurring in steep mountain catchments, with volumes commonly exceeding thousands of m³. Given their unpredictability and their capability to transport large boulders, debris flows rank among the most dangerous natural hazards in mountain environments.

The use of infrasound arrays and the combined use of collocated seismic and infrasound sensors have turned out to be efficient systems for reliable detection of debris flows in near real-time, highlighting the strong potential of infrasound for studying and monitoring debris-flows.

Despite these advances, open questions remain about the possibility to infer debris-flow source characteristics and event magnitude from recorded infrasonic signals. This requires theoretical and/or empirical source models describing elastic energy radiation in the atmosphere, in the form of infrasound, and relating it to fluid dynamic processes within a debris flow. Infrasound radiated by debris-flows is thought to be generated by standing waves that develop at the free surface of the flow, but details of the involved dynamic processes are not fully understood.

Here, we present the analysis of infrasonic signals from >20 debris flows and torrential floods recorded with a small aperture array at the Illgraben catchment (Switzerland, Canton Valais) between 2017 and 2021. The comparison between infrasonic signal features (maximum amplitude and peak frequency) and measured flow parameters (front velocity, maximum depth and discharge) showed that the infrasound radiation by debris flows linearly correlates with flow discharge and that the infrasonic peak frequency inversely scales with flow parameters, thus decreasing when flow velocity, depth or discharge increase. In addition, array analysis of infrasonic signals revealed that the infrasound by debris-flows at Illgraben appears to be dominated by clusters of coherent infrasonic detections generated near check dams located along the Illgraben channel.

These pieces of evidence suggest that debris flow infrasound is generated by turbulence-induced waves and oscillations developing at the free-surface of the flow, whose dimensions scale with the magnitude of the flow. As expected from fluid dynamics, these surface oscillations are primarily generated where the flow encounters significant channel irregularities, such as topographic steps, which consequently act as preferential sources of infrasound. To test the validity of our interpretation of infrasound source mechanisms within debris-flows we also compare infrasonic recordings of a water free overfall over a weir with video recordings of the flow, to investigate how infrasound correlates with the dynamic of the surface of the flow.