TEMPORAL EVOLUTION OF TYPICAL DEBRIS FLOWS IN WENCHUAN EARTHQUAKE AREAS: CASE STUDY OF ZOUMALING-GULLY DEBRIS FLOWS IN MIANZHU COUNTY

The Zoumaling stream is a typical debris flow gully because of the large numbers of loose geomaterials induced by the Wenchuan Earthquake. Three debris flow events have occurred after the Earthquake, and caused serious property loss. Two typical debris flow events occurred respectively on September 24, 2008 and on August 13, 2010.This paper combines the RS image and field investigation data. It analyzes the formation conditions of the Zoumaling debris flows, and compares the evolution characters of the gully after three rainy seasons after Wenchuan earthquake using RS image data of 2008, 2009 and 2010.The results show that the source initiation of the Zoumaling debris flows with a complex supply process in different rainfall intensity. The debris flow initiation began from the branch gullies, and then evolved into a whole basin debris flow. Secondly, after the three rainy seasons of 2008、2009 and 2010,the source volume and its initiation process is changed. The volume of collapses has increased 1.87%.Landslide has increased 25.35%.Sediment of channels and slopes has reduced 67.47%.Most of these have transformed into debris flow sediments. Thirdly, at different rainfall intensity conditions, the boundary of the Zoumaling deposition area has changed obviously. There is an overlay relation among the deposition fans formed in different rainy seasons. Comparing the deposit extent of the Zoumaling -9.24 and the front edge of the Zoumaling -8.13 deposition fan and using the RS images interpretation, the deposition area of the Zoumaling -8.13 is 8.76×104m2. Finally, this paper analyzes the formation and evolution trend of this debris flow gully. The results show that the Zoumaling is a high frequency debris flow gully. The large amount of loose geomaterials are still distributed in the gully, which is the advantageous factor for large debris flow to occur.