Initiation mechanisms of granular debris flow

Debris flows are gravity driven two-phase motions intermediate between intensive bed-load transport and landslide, which cause considerable damages in mountain regions. The knowledge of the initiation mechanisms of debris flow is useful for the planning of hazard mitigation measures. One of the main aims of a research project of the Board of the Swiss Federal Institute of Technology (being conducted jointly at VAW and three other institutes) is to investigate experimentally the critical conditions for debris-flow initiation with particular attention to the specific conditions of the European Alpine regions.

One of the most frequent initiation mechanisms of debris flow in the Alps is channel failure, generally triggered by surface water flow. Observations of debris flow initiation processes in nature are extremely rare and only few specific laboratory investigations were carried out up to now to analyse initiation mechanisms. Theoretical considerations, analysis of debris flow initiation zones and flume experiments suggest that debris flow formation by channel failure is characterized by a destabilisation of the torrent bed by surface run-off and a successive, progressive and fast transition from intense bed-load transport to hyperconcentrated flow and finally to mature debris flow. The experimental investigations are now being carried out in a new laboratory flume of 10.7 m length, 1.0 m width and slope adjustable between 0 and 40°. The flume is equipped with eight ultrasonic distance measuring devices, eight water pressure cells and one sediment balance.