Initiation processes for run-off generated debris flows in the Wenchuan earthquake area of China

The frequency of huge debris flows greatly increased in the epicenter area of the Wenchuan earthquake. Field investigation revealed that runoff during rainstorm played a major role in generating debris flows on the loose deposits, left by coseismic debris avalanches. However, the mechanisms of these runoff-generated debris flows are not well understood due to the complexity of the initiation processes. To better understand the initiation mechanisms, we simulated and monitored the initiation process in laboratory flume test, with the help of a 3D laser scanner. We found that run-off incision caused an accumulation of material down slope. This failed as shallow slides when saturated, transforming the process into debris in a second stage. After this initial phase, the debris flow volume increased rapidly by a chain of subsequent cascading processes starting with collapses of the side walls, damming and breaching, leading to a rapid widening of the erosion channel. In terms of erosion amount, the subsequent mechanisms were much more important than the initial one. The damming and breaching were found to be the main reasons for the huge magnitude of the debris flows in the post-earthquake area. It was also found that the tested material was susceptible to excess pore pressure and liquefaction in undrained triaxial, which may be a reason for the fluidization in the flume tests.