Infrasonic and Seismic Analysis of Debris‐Flow Events at Illgraben (Switzerland): Relating Signal Features to Flow Parameters and to the Seismo‐Acoustic Source Mechanism

Abstract

Debris flows are episodic gravitational currents of high density (∼2,000 kg/m 3 (Wang et al., 2018)), consisting of highly concentrated mixtures of water, mud and solid debris particles in varying proportions (Coussot & Meunier, 1996). Their flow behavior has been described as intermediate between floods and landslides (e.g., Iverson & Vallance, 2001). Within debris flows, the solid fraction typically ranges between 50% and 90% (Coussot & Meunier, 1996), with solid particle sizes varying from clay to meter-sized boulders (Pérez, 2001).Occurring on steep slopes of loose debris, debris flows are often initiated by water supply (Badoux et al., 2009), typically via intense rainfalls, natural or artificial dams collapses or rapid snowmelt (Dowling & Santi, 2014;

Low-Frequency Ground Vibrations Generated by Debris Flows Detected by a Lab-Fabricated Seismometer

“…To develop a robust and effective event warning system for debris flows, easily detectable signals associated with debris flows must be identified. In the past two decades, various ground vibrations sensors, including hydrophones [ 7 , 8 ], geophones [ 9 , 10 , 11 , 12 , 13 , 14 ], seismometers [ 15 , 16 , 17 , 18 , 19 , 20 ], and fiber-optic sensors [ 21 ], have been used to detect and study the seismic ground motions produced by debris flows. The frequencies of such ground vibrations detected using conventional geophones are 10–100 Hz; those at the surge front and flow tail are mainly 10–30 and 60–80 Hz, respectively [ 11 , 22 ].…”

“…For example, mean debris flow velocities can be determined by calculating the time lags of the seismic signals detected at various seismic sensors [ 9 , 11 , 16 , 17 , 23 ]. Moreover, the amplitude of ground vibrations is related to the flow velocity, discharge, and grain size of debris flows [ 19 , 20 , 23 , 24 , 25 ].…”

“…Many studies have reported that debris flows generate not only seismic ground motions but also infrasonic sounds at 4–15 Hz [ 12 , 13 , 14 , 19 , 20 , 31 , 32 ]. Huang et al [ 33 ] demonstrated that a rock falling on a channel bed produced both ground vibrations and airborne sounds, and the spatial decay rate of airborne sounds was significantly smaller than that of ground vibrations.…”

“…Recently, both infrasonic and seismic sensors have been deployed concurrently to increase the reliability of debris flows detection [ 12 , 13 , 14 , 19 , 20 ]. Kogelnig et al [ 12 ] deployed both geophone and infrasound sensors to monitor debris flows at the Lattenbach torrent (Tyrol, Austria) and the Illgraben torrent (Valais Canton, Switzerland).…”

Bi‐Linear Laws Govern the Impacts of Debris Flows, Debris Avalanches, and Rock Avalanches on Flexible Barrier

“…On the other hand, the barrier ring net size (i.e., ring diameter equal to 0.07 m) is determined to retain large particles ($${d}_{\mathrm{p}}$$ = 0.08 m) in a flow while allowing small particles ($${d}_{\mathrm{p}}$$ = 0.04 or 0.06 m) to pass through, which recovers the primary function of a real‐world flexible barrier. In addition to solid volume concentrations and flow dynamics, the flow‐barrier interactions can also be affected by the complex natures of geophysical flows, including varying flow depths (Faug et al., 2012; Iverson et al., 2016), broad particle size distributions (Cabrera & Estrada, 2021), large boulders (Belli et al., 2022; Piton et al., 2022), phase separation of the solid and fluid components (Leonardi et al., 2015; Pudasaini & Fischer, 2020), and erosion (Berger et al., 2011; Pudasaini & Krautblatter, 2021). Meanwhile, the barrier geometry and configuration (e.g., barrier width and height, number of horizontal supporting cables, and the ring‐to‐largest particle size ratio) may also affect the results presented in the following sections.…”