Causes and Triggers

Debris flows are caused by rapid mobilization of loose debris and sediment by water in steep mountain environments. They occur in a wide range of geological and climatological settings and are mostly triggered by intense rainfall events. Non-rainfall related triggers include volcanic eruptions, earthquakes and glacial-, landslide-, or beaver lake outburst floods, among others. The mechanisms for the formation of debris flows range from channel runoff that leads to erosion of sediment in or along a flow path to slope failures that transform to fluidized landslides or feed the channel for further transfer. The difficulty in reliably predicting (hydro-)meteorological trigger conditions for debris flows is associated with the difficulty in rainfall monitoring, the variations in geomorphological and hydrological susceptibility within a watershed and the inherent complexity of possible initiation mechanisms. For improved hazard analysis and forecasting, as well as assessment of climate change impact, a better mechanistic understanding of initiation processes and the interplay of trigger conditions is needed.