CHARACTERISTICS OF DEBRIS FLOWS IN BEICHUAN EPICENTER OF THE WENCHUAN EARTHQUAKE TRIGGERED BY RAINSTORM ON SEPTEMBER 24, 2008

The intense rainfall on 24th September 2008 initiated widespread debris flows in Beichuan County, the epicenter of the Wenchuan Earthquake. These debris flows greatly impacted the community of the Beichuan County and caused 42 fatalities. The debris flows also damaged many roads and other infrastructures in the mountainous areas. Field reconnaisance and interpretations of aerial photographs were used to identify debris flow occurrences. Precipitation data were then utilized to analyze critical rainfall conditions leading to the debris flow initiation. This study examines the process of debris-flow initiation and transport in the study areas and presents field observations on the roles of rainfall, lithology, and faults. Particular emphasis is given to the roles of intense precipitation and source areas. The character and spatial distribution of lithology, faults and topography appear to control the initiation processes of debris flows in the mountainous area. The study results show that after the Wenchuan Earthquake, critical accumulated precipitation and hourly rainfall intensity necessary to initiate debris flow reduced to as low as 14.8%～22.1% and 25.4 %～31.6% respectively comparing with those before the earthquake. Through emergency field investigation, 72 debris-flows triggered by the rainfall were identified. Almost all debris flow gullies in the drainage basins have contained many seismic landslides, which supply debris flow source areas with huge loose material. Two types of failure mechanisms initiated the debris flows: the first initiation involved the mobilization of shallow landslides and the second process, which was explained with term of the fire-hose effect initiated debris flows. The overland flow of water resulting from intense rainfall was concentrated in steep channels and scoured debris from gully-bed loose deposits. The study also indicated that the region affected by the Wenchuan Earthquake become more vulnerable to debris flows in this period. Therefore it is important to assess debris flow risk and take hazard monitoring and early warning in the earthquake areas. The control work is also necessary for debris flow hazard mitigation.