REMOTE SENSING IMAGES BASED OBSERVATIONAL ANALYSIS ON CHARACTERS OF DEBRIS FLOW SOURCE AREAS IN BEICHUAN COUNTY OF WENCHUAN EARTHQUAKE EPICENTER REGION

Wenchuan Earthquake produced abundant debris left behind on the hillside slopes. The debris would later serve as the source materials for rainfallinduced debris flow hazard. On 24th September 2008,a total of 72 debris flows were triggered by heavy rainfall in the central part of the epicenter region affected by the Wenchuan Earthquake. The Beichuan County in the central epicenter region is chosen for analyzing the change of debris flow source area after the earthquake and also following a rainstorm. Significant differences after the earthquake are noticed in producing a large amount of landslides for debris flow occurrence. Interpretations of aerial photographs and SPOT images as well as field investigations are used to identify landslide occurrences in the debris flow watershed for exploring the initial conditions triggering debris flows. Comparison of the aerial photographs taken on 18th May 2008(after the 512 Wenchuan earthquake) with the SPOT 5 images taken on 14th October 2008(after the rainstorm event on 24th September 2008)indicates that the landslide area in eight debris flow watersheds increased from 1537104 m2 to 1912104 m2 during that period of time. Measurement of the SPOT images reveals that the loose sediment areas on the channel beds had further expanded to 97104 m2 after the rainstorm. The present results show the Wenchuan Earthquake affected area is particularly susceptible to debris flow occurrence after heavy rainfall events．