The development of a remote sensing based technique to predict debris flow triggering conditions in the French Alps
The effects of mass movements, including debris flows, on the inhabitants of mountainous regions can often be catastrophic, causing serious casualties and property damage. These impacts could potentially be reduced with the development of early warning systems. Debris flows are generally initiated by either heavy rainfall or snow melt. In the past, prediction of debris flow events has been limited to the moment of the flow onset and dependant on the accurate description of free flowing water conditions at the time of initiation. This has remained problematic not least because of the high spatial and temporal variabilities of the triggering phenomena, making their accurate measurement by conventional means, such as by raingauges, difficult. Remote sensing data offers an ideal opportunity to provide information on debris flow triggering conditions, including details of the evolution of triggering rainfall conditions before they initiate a debris flow event. In this paper we outline the development of a remote sensing technique to provide early warning of debris flow triggering conditions using infrared data measured from the Meteosat satellite series, for the Bachelard Valley in the French Alps. The relatively simple relationship and short time interval between the onset of heavy rainfall, and the initiation, movement and deposition of a debris flow allows information on the triggering conditions to be considered as early warning of the actual debris flow event itself, in locations of known debris flow hazard. Predictive information of triggering conditions of a particular hazard is of vital importance to the development of an effective early warning system. The technique outlined in this paper was developed using the debris flow initiation model, of Blijenberg et al ., linked to automated raingauges over a four year period from 1991 to 1994. Of the six case studies identified warning times of 1-12 hours were given in five of these. A false alarm test over a month period for the region revealed false alarms on two days, only. This paper shows that high temporal resolution remote sensing data can be used to provide early warning of atmospheric conditions likely to initiate debris flow events. This information is of importance to the development of a debris flow hazard early warning system.