Large Earthquake not only caused enormous fatalities and economic loss, but also made the seismic area more prone to secondary geological hazards. For example, the debris flow event that took place on August 7, 2010 in Zhouqu within the Wenchuan seismic area had more than one thousand people lose their lives. Therefore, the risk assessment of potential debris flows in the Ms 7.1 Yushu Earthquake area is a matter of urgency. However, debris flows in seismic area are often characterized by enormous energy and by a complex behaviour consisting of high velocity and long run-out. It is still challenging to conduct quantitative risk assessment of debris flows by considering their extremely high energy and mobility.

Recently, in a study conducted by Prof. LAN Hengxing, Institute of Geographic Sciences and Natural Resources Research (IGSNRR), Chinese Academy of Sciences, and his research team, a procedure for assessing the risk of debris flows in seismic area has been proposed and applied in the Yushu seismic area. The researches first evaluate the intensity of potential debris flows under different scenarios of rainfall events. A model then is established to assess the displacement of different building structures by both static and dynamic impact from various debris flows. The possible economic loss and fatalities in the Jiegu Township (the economic and inhabiting center of the Yushu County) are quantitatively estimated by taking into account the structure of buildings, the spatial distribution of residents, and the returning periods of debris flows.

The debris risk and associated risk criterion are established for study area. The results suggest that HBC (Hollow Brick Concrete) structure achieves a good balance for the cost-benefit relationship compared with HBW (Hollow Brick Wood) and RC (Reinforced Concrete) structures and thus could be an optimal choice for most of the new residential buildings. The low hazard boundary presents significant risk reduction efficiency in the 100 year returning debris flow event, and the societal risk for the settlement area is unacceptable when the 100 year returning event occurs but reduces to ALARP (as low as reasonably practicable) level as the low hazard area is considered. Therefore, the low hazard area was highly recommended to be taken into account in the reconstruction.

This work has been published in the scientific journal of Natural Hazards and Earth System Sciences (Lan, H. X., Li, L. P., Zhang, Y. S., Gao, X., and Liu, H. J., 2013. Risk assessment of debris flow in Yushu seismic area in China: a perspective for the reconstruction. Nat. Hazards Earth Syst. Sci., 13, 2957-2968, doi:10.5194/nhess-13-2957-2013).