Formation and characteristics of post-earthquake debris flow: a case study from Wenjia gully in Mianzhu, Sichuan, SW China

During the three flood seasons following the Wenchuan earthquake in 2008, two catastrophic groups of debris flow events occurred in the earthquake-affected area: the 2008-9-24 debris flow events, which had a serious impact on rebuilding; and the 2010-8-13/14 debris flow events, which destroyed much of the progress made in rebuilding. The Wenjia gully is a typical post-earthquake debris flow gully and at least five debris flows have occurred there. As far as the 2010-8-13 debris flow is concerned, the deposits of the Wenjia gully debris flow reached a volume of 3.1 x 10(6) m(3) in volume and hundreds of newly built houses were buried. This study took the Wenjia gully debris flow as an example and discussed the formation and characteristics of post-earthquake debris flow on the basis of field investigations and a remote sensing interpretation. The conclusions drawn from the investigation and analysis were as follows: (1) Post-earthquake debris flows were a joint result of both the earthquake and heavy rainfall. (2) Gully incision and loose material provision are key processes in the initiation and occurrence of debris flows and a cycle can be presented as the following process: runoff-erosion-collapse-engulfment-debris flow-further erosion-further collapse-further engulfment-debris flow enlargement. (3) The amount of rainfall that triggered debris flows from the Wenjia gully was significantly less than the average daily rainfall, while the intraday rainfall threshold decreased by at least 23.3%. (4) The occurrence mechanism of Wenjia gully debris flow was an erosion type and there was a positive relationship between debris flow magnitude and rainfall, which fitted an exponential model. (5) There were five representative characteristics of Wenjia gully debris flow: the long duration of the occurring process; the long distance of deposition chain conversion during the process of damage; magnification in the scale of debris flow; and the high frequency of debris flow events.