Debris-flow monitoring at the Rebaixader torrent, Central Pyrenees, Spain: results on initiation, volume and dynamic behaviour

The sophisticated monitoring system installed in the Rebaixader catchment incorporates a total of 6 different stations: four stations recording information on the initiation mechanisms (two meteorological stations and two infiltration stations), and two stations focussing on the debris flow detection and the dynamic behaviour of the flows. Between August 2009 and August 2013, seven debris flows and seventeen debris floods were detected. The volumes of the debris flows ranged from 2,100 to 16,200 m3, while debris floods normally included smaller volumes. Debris flows and debris floods showed clear differences in both the recorded data and field observations. The distinction was clearly visible in the ultrasonic measurements of the flow height and also in the ground vibration registered by the most downstream geophone located near the fan apex. The analysis of the rainfall data indicated that most events were provoked by short high-intensity rainstorms that occurred during summer. A preliminary threshold for debris-flow initiation of about 15 mm in 1 h may be adequate for the summer season. However, some events took place in spring, when the melting of snow cover and frozen soil may have played an important role in the triggering mechanisms