Changes in runout distances of debris flows over time in the Wenchuan earthquake zone

A large number of debris flows occurred in the Wenchuan earthquake zone after the 12 May 2008 earthquake. The risks posed by these debris flows were rather high. An appropriate model is required to predict the possible runout distance and impacted area. This paper describes a study on the runout characteristics of the debris flows that occurred in the Wenchuan earthquake zone over the past four years. A total of 120 debris flows are analyzed. Separate multivariate regression models are established for the runout distances of hill-slope debris flows and channelized debris flows. The control variables include type of debris flow, debris flow volume, and elevation difference. Comparison of the debris flows occurring before and after the earthquake shows that the runout distance increased after the earthquake due to sufficient material supply and increased mobility of the source materials. In addition, the runout distances of annual debris flow events in 2008, 2010 and 2011 are analyzed and compared. There is a tendency that the runout distance decreases over time due to the decreasing source material volume and possible changes of debris flow type. Comparison between the debris flows in the earthquake zone and the debris flows in Swiss Alps, Canada, Austria, and Japan shows that the former have a smaller mobility.