INVESTIGATION AND ANALYSIS OF "6.21" DEBRIS FLOW IN JIUZHAIGOU COUNTY, SICHUAN PROVINCE

A debris flow hazard triggered by rainstorm occurred in Zhuozhui Gully,Xiajijiehaizi Gully and Zechawa Gully,Jiuzhaigou County,where is the epicenter of the Jiuzhaigou Earthquake on June 20,2019. About 2.79×10⁴m³,3.7×10⁴m³ and 2.67×10⁴m³ solid materials rushed out of the three gullies respectively,caused damage to retaining walls and debris dams,buried part of the scenic road,disrupted traffic temporarily,and had no casualty. This paper identified the cause and mechanism of the debris flow disaster preliminarily,according to the field survey and remote sensing interpretation after the disaster. Cause of disaster is as follows. The earthquake-induced landslide produces a large number of new loose solid material sources and the original gully slope debris(including old debris flow debris) under the action of heavy rainfall to form debris flow. So the debris flows are the results of the earthquake and rainfall. Based on the investigation and analysis of longitudinal profile and typical cross section of gully before and after the debris flow,we find that the mechanisms of debris flow disaster mainly include the follows. (1)The debris flow erodes the old debris flow deposits,at the same time induces the landslide along the bank,so as enhances the debris flow scale and impact force and other dynamic parameters; (2)The channel jam and the landslide slide into the channel and lead to debris flow blockage. Cascade outburst occurs under the action of the subsequent debris flow,and then increase the dynamic parameters such as the peak flow rate and impact force of debris flow. Under the action of the two mechanisms,the parameters such as debris flow size and impact force can exceed the planning and design indexes of the prevention and control project,which leads to the destruction of the prevention and control project and the burying of the road. Therefore,it is suggested that the risk assessment of debris flow and the rationality and reliability of prevention and control engineering planning and design in Jiuzhaigou scenic area under the combined action of earthquake and heavy rainfall should be fully considered to ensure the safety of the scenic area.