Characteristics and Causes of Disastrous Debris Flows on July 4, 2013, in Shimian County, Sichuan, China

Debris flow is an abrupt phenomenon of earth surface movement and typical disaster in mountainous areas with steep terrace, quantity of loose soil and abundant surface runoff. Intense rainfall and rainstorm easily triggered debris flows and generated huge losses. The disastrous debris flows, on July 4, 2013 at the gullies of Hou, Heilinzi and Xiongjia in Shimian County, Sichuan Province, resulted in 18 casualties and endangering Shimian city with a population of 50,000. These debris flows were characterized by low viscosity with only 0.9% - 1.4% clay soil of less than 0.05mm, density of 1.77 - 1.84 t/m³, velocity of 4.4 - 13.5m/s and discharge of 827 - 1248 m³/s, respectively, and also delivered sediment of 16.8 × 10⁴m³, 12.7 - 13.5 × 10⁴ m³, 20.5 × 10⁴m³ out of the outlet, respectively. These three events all generated a hazard chain, which involved in flash flood, channelized debris flow, dammed lake and outburst flood. The threshold conditions of debris flow blocking Nanya River and forming this hazard chain are that the unit width peak discharge and the deposition volume in river channel are more than 37.0 m³/s and 4500 m³, respectively. These debris flows were initiated by intense rainfall with the antecedent rainfall of over 52 mm and triggering rainstorm of over 36 mm/h. And, the property losses and casualties mainly originated from impacting and scouring, burying and blocking, highway destructing and river channel rising. The irrational location of constructions and the destruction of under-standard prevention constructions were responsible for loss worsening. It was strongly recommended for mitigating that hazards reassessment, integrated control, emergency plan and integrated risk management were made at mountainous urban areas, especially in high-hazard areas.