Ground vibrations produced by rock motions and debris flows

Abstract

[1] In this study we compare ground vibrations generated by various rocks and debris flows at the same channel to explore the origin of ground vibrations caused by debris flows. In field experiments we identified the ground vibrations caused by motions of individual rocks. Ground vibrations were detected by geophones and analyzed in the time-frequency domains. We found that ground vibrations caused by individual rocks were 10-150 Hz and that larger stones generate ground vibrations with lower peak frequencies. A sphere-packing model of granular rocks was used to estimate the propagation speed of ground vibrations. This was found to be underestimated when compared with our experimental data. The signal decay rate was obtained under the assumption that ground vibrations propagate as exponentially attenuating cylindrical waves. Ground vibrations produced by a debris flow at Ai-Yu-Zi Creek, Nan-Tou, Taiwan, on 2 July 2004 revealed that the debris flow forefront produced vibrations lower than 50 Hz and ranged between 50 and 100 Hz after the front had passed. When the main front was closest to the sensor, the frequency spectrum covered a wide range, from 10 to 250 Hz. The mean velocity of the debris flow front was 13.3 m/s. Frequencies of ground vibrations caused by individual rocks are within the frequency range of ground vibrations captured during the debris flow, confirming that one of the main sources of ground vibration caused by debris flows is the interaction of rocks or boulders with the channel bed.

Analysis of the ground vibration generated by debris flows and other torrential processes at the Rebaixader monitoring site (Central Pyrenees, Spain)

“…The efficiency of the geophones to monitor the occurrence of torrential processes has been widely proved, and so it is their convenience for warning purposes (Suwa and Okuda, 1985;Arattano and Moia, 1999;LaHusen, 2005b;Bessason et al, 2007;Huang et al, 2007;Badoux et al, 2009). However, there is a great variety of data recording systems, highly conditioned by the technical details of each monitoring station and many site-specific factors that affect the ground vibration measured.…”

“…Some analyses of geophone signals induced by debris flows have been published during the last decades (Arattano and Moia, 1999;Berti et al, 2000;Hürlimann et al, 2003;Huang et al, 2007;Chou et al, 2010;Arattano et al, 2012). All these studies have substantially increased our knowledge on the dynamic behaviour of debris flows and the ground vibration they induce.…”

“…Thus, on one hand debris flows can be distinguished from other torrential processes and, on the other hand, the different phases of a debris flow can be detected (e.g. Huang et al, 2007;Navratil, 2013).…”

“…Other authors found some general patterns in the frequency domain. For instance, LaHusen (1996) described the typical peak frequency range of the debris flows between 30 and 80 Hz, or Huang et al (2007) suggested this range from 50 to 100 Hz.…”

Correction to “Experimental study of bedrock erosion by granular flows”

“…The mass removal by granular flow erosion into bedrock is minor compared to loose debris entrainment, but could be the dominant process cutting valleys into steep landscapes [e.g., Stock and Dietrich, 2003]. During debris flow passage, video cameras have recorded huge boulders sliding on and colliding with the bed [Swartz and McArdell, 2005], force plates have documented large normal and shear stress values [McArdell et al, 2007], and ultrasonic gauges have measured significant ground vibrations [e.g., Arattano and Moia, 1999;Arattano and Franzi, 2004;Itakura et al, 2005, and references therein; Huang et al 2007], all observations suggesting potentially significant collision-driven erosion. Concrete check dams in debris-flow-dominated channels exhibit grooves, knicks, and missing blocks (see Figure S1 in the auxiliary material).…”