Debris flow

Debris flows constitute a significant hazard in the Alps and other mountainous regions.  They occur typically in steep torrent channels in catchments with steep slopes and abundant sedimentary debris.  A debris flow is a dense and rapid mass movement of water and sediment containing a wide range of grain sizes from clay up to boulders.  In debris flows, roughly equal amounts (by volume) of sediment and water are well mixed and travel at approximately the same speed in a wave-like form down a torrent channel.  In comparison, flash flood flows also may have a steep wave-like front, but the boulders generally travel slower than the front. In comparison with rapid landslides, debris flows tend to be well mixed and the sedimentary structure of a landslide mass (if the flow initiated as a landslide) are destroyed.  Debris flows tend to erode and entrain loose sedimentary debris along a channel bed and deposit sediment in the form of lateral levees at the flow margins.  

In Switzerland debris flows typically occur during unusually intense storm periods that last more than a day or two (e.g. 1987, 2000, 2003, 2005 and 2011).  Debris flow events cause damage mainly on populated alluvial fan areas.  Additionally, debris flow deposition may greatly reduce the water-carrying capacity of the channel, leading to flood damage caused by subsequent flood waves.

To help reduce the risk and design mitigation measures and concepts, a good understanding of the process is necessary.  Debris flow research at the WSL is directed at improving our understanding of the process in a way that is directly relevant for practical applications.  Recent projects and papers span the range from observations of the debris flow process at full-scale automated monitoring stations, to the investigation of mitigation measures such as ring-net barriers to the evaluation of existing debris flow runout models to the development of the next generation of process models.