On debris flow front evolution along a rorrent

Abstract

Field experiments carried out in a debris flow prone torrent show that ground vibrations produced by debris flows are a function of flow depth and grain size. The passage of the debris flow produces a sharp increase of ground vibration velocity that can be clearly detected by seismometers placed on the banks of the torrent. The ground vibration velocity usually reaches a peak that lasts only a few seconds and is attributable to the passage of the debris flow front in the vicinity of the seismic detector. Further field experiments reveal that such a characteristic peak of ground vibration velocity is not always detectable anywhere along the channel, but can be consistently detected only on the fan. This is probably due to the fact that debris flows sometimes develop a well defined, bouldery front only after they have reached the fan. It is also observed that debris flow volume can significantly increase after the debris flow has reached the lower part of the basin, close to the fan apex. Further field tests are needed to confirm these observations, observations that could affect the design and disposition along the torrent of defensive measures against this type of mass movement.