Characteristics and formation mechanism of a catastrophic rainfall–induced rock avalanche–mud flow in Sichuan, China, 2010

A catastrophic rock avalanche–mud flow was triggered by the heavy rainfall in Sichuan, China, on July 27, 2010. A mass of strongly weathered basalts with a volume of ∼480,000 m3 was initiated from a valley side slope and then moved downstream along the valley, entraining a large amount of unconsolidated substrate and bilateral materials and colluviums. The entrainment increased the volume of slide to ∼1.0 million m3 and may also enhance the mobility of the landslide. Approximately 30 min after the first failure, the deposits of the rock avalanche in the steepest part of the valley started to creep slowly down as a mud flow. It reached a small town at the foot of the slope after several hours, causing the damage of 92 houses and the urgent evacuation of 1,500 people. The field investigation, mapping, grain size test, and aerial photo interpretation were applied to analyze the dynamic process and the formation mechanism of the landslide. The strongly weathered and fragmented basalts as well as the most vulnerable combination of joint sets were revealed to be the most contributive factors. The antecedent torrential rainfall is the direct trigger, which affected the slope stability in three aspects: induced debris flow that scoured the toe of the sliding surface of rock avalanche; caused the increase of the slope unit weight, and penetrated into the steep joints reducing the strength of the materials.