INVESTIGATION OF SEVERE DEBRIS FLOW HAZARDS IN WENJIA GULLY OF SICHUAN PROVINCE AFTER THE WENCHUAN EARTHQUAKE

Many debris flow hazards were triggered recently in the epicenter area of the 2008 Wenchuan Earthquake. The severest debris flow is the debris flow in Wenjia Gully on Aug.13, 2010.Wenjia Gully was a simply and ordinary gully and had no debris flow at all before the Wenchuan Earthquake.The formation conditions of debris flows were totally changed by a giant landslide caused by the earthquake.The giant landslide formed a landslidedebris deposition in the gully.The debris deposition had a volume of more than 30million cubic meters.The sediments were easily eroded by flash flood and formed debris flows.In the three raining seasons after the earthquake, a total of five large scale and giant scale debris flow were triggered.The largest and most harmful debris flow occurred on Aug.13, 2010.The total rainfall during the debris flow was 227mm,and the duration of debris flow was 2.5 hours.The volume of debris flow was 3.1 million cubic meters.The debris flow resulted in 7 fatalities, 5 persons listed as missing, 39 persons injured, 479 houses buried,and a direct economic loss of RMB 430 million. About 16%sediments of the landslide-debris deposition flew down out of the gully during the five large scale debris flows.New debris flows will be triggered by rainfall in the gully in the future.It will be a long-term effort for prevention and mitigation of the debris flow hazards in the gully.