Experiences of rapid landslides monitoring and warning at catchment scale in the Pyrenees. The example of the Rebaixader test site.

Monitoring of rapid landslides, as rockfalls and debris flows, improves the understandings of these processes and also provides fundamental information for an efficient early warning and alarm system (EWAS). The Rebaixader site is a typical high mountain catchment, where sort of torrential phenomena occur with a sub-annual frequency. To investigate the triggering conditions and the post-failure behaviour of the rapid landslides, a monitoring system consisting of five stations was set up progressively since July 2009. Two stations monitor landslide propagation characteristics by using eigth geophones, an ultrasonic device and a video camera and infrared spot lights. Until March 2013, six debris flows, eleven debris floods and four rockfalls (two of them were large boulder falls, 55 and 18 m3) were recorded at the site. The analysis of ground vibration signal allowed defining preliminary thresholds for distinguishing between debris flows, debris floods and rockfalls. The identification of processes was checked by the analysis of the video images and the post-event field evidences. Field surveys, ground vibration records and the movies recorded at the video camera provided unique information on the rapid landslides occurrence and dynamics, and gave an excellent opportunity to show the importance of monitored data for calibrating of simulation models. The monitoring results show that the data gathered by the geophones provides valuable information for the design of alarm systems for the event detection and distinction between processes.