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 $\mathrm{3,950}{\mathrm{m}}^3/\mathrm{s}$ 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%.

Gradation has an important influence on the mechanical properties of earth and stone materials after compaction, and a reasonable gradation is the key to ensure compaction quality. In this paper, a set of earth and stone materials gradation detection system based on digital image processing was proposed, which was suitable for detection in the construction sites. The system obtained images which were taken from multiple directions of materials transport vehicle, used the thresholding method to segment the images, detected particle contours through edge detection algorithm of Canny to realize the image recognition of particle size, and drew the gradation curve of earth and rock materials finally. It was verified by selecting limestone aggregates of different gradations, and the results showed high accuracy. The system can realize on‐site detection of the earthwork gradation rapidly and accurately at a dam construction site.