Quantitative risk assessment of two successive landslide dams in 2018 in the Jinsha River, China

Two large successive co-site landslide dams blocked the Jinsha River in the Sichuan Province of China in 2018. A quantitative risk analysis was carried out to quantify potential human and economic losses resulting from the failure of these dams, especially the interaction between the co-site landslide dams, and to further investigate the influence of digging a diversion channel on risk mitigation to better evacuate upstream waters. Flood routing for three scenarios (i.e. after the first dam formation, when the second landslide mass was added, and with a diversion channel after failures) were simulated using HEC-RAS. The human and economic losses were evaluated using a human risk assessment model together with empirical formulations. The results show that the risk of breaching floods had increased significantly after the second co-site landslide dam formation on the pre-existing loose deposits of the first dam. This amplification effect of outburst floods was so great that the peak outflow resulting from the breaching of the second landslide dams was more important, leading to greater economic losses than those resulting from the breaching of the first dam. However, the expected loss of life caused by the breach of the two landslide dams appeared small because of the sufficient time lag provided by the long distance between the residential area and the dam site. The simulations also outline the importance of the diversion channel in decreasing the peak outflow rate and hence downstream risks. A parametric analysis shows that a deep channel with a moderate longitudinal gradient can significantly decrease the peak outflow discharge at the dam site. Flood intensity and risks to downstream towns did not change because of the relatively small attenuation rate of the peak outflow rate. However, a shorter height of the residual dam can be obtained by optimizing the diversion channel, and the amplification effects of such co-site dam can be significantly reduced in the futur