Application of remote sensing monitoring to the spatiotemporal variation in debris flow activity in the catastrophic Wenchuan seismic area

Debris flows are typical compounded geohazards with long-term activity in earthquake-stricken areas. Numerous debris flows were simultaneously induced by subsequent extreme precipitation and caused serious property losses and casualties. Monitoring the dynamic variation in debris flow activity and its decay process over time is key for debris flow risk management and mitigation of the impact of debris flows. In this study, the spatio-temporal variation in debris flow activity of the Wenchuan seismic area over the past ten years was monitored based on multiperiod satellite data and field monitoring. The hillslope landslide sediment delivery to the channel gradually decreased, and the area with higher sediment transfer potential gradually changed from southern Wenchuan County to the northern area, which caused the area with high susceptibility to debris flows to gradually move towards northern Wenchuan County. Impacted by the decrease in available materials and coarsening of channel materials, the power of rainfall runoff required for driving debris flows gradually increased. This resulting debris flow is more likely to occur in this basin with a larger catchment area, longer channel length, and higher elevation difference. The changes in the debris flow activity index indicated that the debris flow activity evolves with an impulse type, and it may last for decades. The decay processes of the debris flow activity were summed up in three periods, i.e., high-activity period (2008-2015), unstable period (2015-2035), and recession period (after 2035), based on the dynamic evolution of material, hydrodynamics, and the coefficient of debris flow activity. Simultaneously, the active characteristics of debris flows in different periods were revealed, which is of great significance to the risk management of debris flows in the next period.