Forecast method of multimode system for debris flow risk assessment in Qingping Town, Sichuan Province, China

The Wenchuan Earthquake of May 12, 2008 triggered large numbers of geo-hazards. The heavy rain on 13 August 2010 triggered debris flows with total volume of more than 6 million cubic meters and the debris flows destroyed 500 houses and infrastructure built after the Wenchuan Earthquake. The study area Qingping Town was located in the northwestern part of the Sichuan Basin of China, which needs the second reconstructions and the critical evaluation of debris flow. This study takes basin as the study unit and defines collapse, landslide and debris flow hazard as a geo-hazard system. A multimode system composed of principal series system and secondary parallel system were established to evaluate the hazard grade of debris flow in 138 drainage basins of Qingping Town. The evaluation result shows that 30.43% of study basins (42 basins) and 24.58% of study area, are in extremely high or high hazard grades, and both percentage of basin quantity and percentage of area in different hazard grades decrease with the increase of hazard grade. According to the geo-hazard data from the interpretation of unmanned plane image with a 0.5-m resolution and field investigation after the Wenchuan Earthquake and 8.13 Big Debris Flow, the ratio of landslides and collapses increased after the Wenchuan Earthquake and the ratios of extremely high or high hazard grades were more than moderate or low hazard grades obviously. 23 geo-hazards after 8.13 Big Debris Flow in Qingping town region all occurred in basins with extremely high or high hazard grades, and 9 debris flows were in basins with extremely high hazard grade. The model of multimode system for critical evaluation could forecast not only the collapse and landslide but also the debris flow precisely when the basin was taken as the study unit.