Emergency Response and Back Analysis of the Failures of Earthquake Triggered Cascade Landslide Dams on the Mianyuan River, China

This study has reviewed the emergency action and drainage process of the Xiaogangjian, Xiaogangjian-lower, and Yibadao cascade landslide dams that were formed during the 2008 Wenchuan earthquake in China's southwest Sichuan Province. In order to minimize the hazards caused by the dam breach flood, mitigation works were carried out. Using underwater explosion and mechanical excavation, the dam heights of Xiaogangjian and Yibadao were lowered by 8 and 5 m, respectively. On June 12, the Xiaogangjian Dam was overtopped, followed by the failures of the two downstream dams, resulting in a flood with a peak discharge of 3,950 m3/s measured at the downstream Hanwang gauge station. This paper describes the geographic and geotechnical features, mainly obtained from the field survey and laboratory testing. The back analysis on the breach flood is based on the authors' work on Tangjiashan and Yigong barrier lakes. The results of this study show that the measured breach flood hydrograph can be reproduced using the currently available analytical methods if the input parameters related to the soil erosion and geotechnical properties are well determined based on experience and laboratory testing. The results of the analysis also show that the mitigation works of lowering the Xiaogangjian and Yibadao dam crest elevations effectively reduced the peak flow by nearly 20%.