Evolution of debris flow properties and physical interactions in debris-flow mixtures in the Wenchuan earthquake zone

The 2008 Wenchuan earthquake triggered an exceptional number of landslides. The large amount of loose landslide materials on the steep terrains can easily turn into debris flows by heavy rainfall. During the wet seasons from 2008 to 2011, five destructive debris flows occurred repeatedly in Yingxiu, the study area near the epicenter. This paper presents the evolution of debris flow properties and physical interactions in the debris flow mixtures in the past few years. Seven dimensionless numbers are used to describe the physical interactions. The evolution of the physical interactions over time was influenced by changes in debris-flow compositions, which in turn affected the runout characteristics of the debris flows significantly. Grain contact friction was the most important physical interaction for the five debris flows in the study area. With gradual loss of fine particles over time, the viscosity of the debris-flow mixtures became lower and lower, leading to a change from the dominance of viscous shear over solid-fluid interactions and inertial grain collision to the dominance of solid-fluid interactions and inertial grain collision over viscous shear. With inertial grain collision being more and more important, the flow resistance of the debris flows increased. The mobility of the future debris flows may become smaller and smaller.