Causes,Characteristics and Mitigation of 7.21 Catastrophic Debris Flow Hazard at the Longda Watershed of Songpan County,China

摘要

Debris flow is a popular natural hazard in the mountain areas of Western China due to favorable natural conditions,and also exacerbated by mountainous exploitation projects.This paper concentrated on the causes,characteristics and mitigation of a catastrophic debris flow hazard at Longda Watershed in Songpan County,Sichuan Province,on 21 July 2011,which was expanded by mining.This debris flow deposited in the front of the No.1 dam of the tailing sediment reservoir,moved into the drainage channel for flood and then rushed into tailing sediment reservoir and made the No.2 dam breached.The outburst debris flow blocked Fu River and produced dammed lake,whose outburst flood further delivered heavy metal into the lower of Fu River,polluting the drink water source of over 1 million population.The debris flow was featured with a density of 1.87～2.15 t/m3 and the clay content of less than 1.63％.The peak velocity and flux of the debris flow at Longda Gully was over 10.0～10.9 m/s and 429.0～446.0 m3/s,respectively,which equaled to the flux of the probability of 1％.The peak flux of 7.21 debris flow was about 700 m3/s in main channel.The debris flow deposits approached 330,000m3,which deposited in the drainage tunnel (120,000～130,000 m3),the front of No.1 dam (100,000 m3) and the mouth of the watershed (100,000～110,000 m3).It was proved that debris flow would block Fu River when the peak flux and magnitude of debris flow was more than 462 m3/s and 7,423 m3.The rainstorm,with an intensity of 21.2 mm/0.5h,triggered the abundant accumulated loose material,which was composed of landslides,slope deposits,mining wastes,tailing sediments,and formed this debris flow.The losses were mainly originated from the drainage tunnel for flash flood,which was not capable for debris flow drainage,and the irrational location of tailing sediment reservoir,which resulted in chain hazards including outburst debris flows,dammed lake and outburst flood,aggravating the hazards and loss.The mitigation measures for the debris flows induced by mining were presented: (1) The disastrous debris flow watershed should be identified and prohibited from being taken as the site of mining plant; (2) The mining factory was suggested to be constructed at the safe areas; (3) Scoria plots,concentrator factory and tailing sediment reservoir are constructed in low risk areas where the feasible protection measures can be easily made to prevent debris flows; (4) the appropriate system and plan including monitoring,remote monitoring and early-warning and emergency plan is established; (5) The stability of waste dump and tailing reservoir are monitored continuously and controlled to prevent mines debris flows.