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Landslides and rock avalanches triggered by the 2008 Wenchuan Earthquake produced 257 landslide dams, mainly situated along the eastern boundary of the Qinghai-Tibet Plateau where rivers descend approximately 3,000 m into the Sichuan Basin. The largest of these dams blocked the Tongkou River (a tributary of the Fujiang River) at Tangjiashan. The blockage, consisting of 2.04 × 10 m of landslide debris, impounded a lake with a projected maximum volume of 3.15 × 10 m, potentially inundating 8.92 km of terrain. Its creation during the rainy season and the possibility of an uncontrolled release posed a serious, impending threat to at least 1.3 million people downstream that could add substantially to the total of 69,200 individuals directly killed by the earthquake. Risk assessment of the blockage indicated that it was unlikely to collapse suddenly, and that eventual overtopping could be mitigated by notching the structure in order to create an engineered breach and achieve safe drainage of the lake. In addition to the installation of monitoring and warning instrumentation, for emergency planning we estimated several outburst scenarios equivalent to 20, 25, 33, and 50% of the dam failing suddenly, creating, respectively, 3.35, 3.84, 4.22, and 4.65 km of flooded area, and overbank water depths of 4.6, 5.1, 5.7, and 6.2 m, respectively, in Mianyang, the second largest city in Sichuan Province, 48 km downstream from the blockage. Based on these scenarios, recommendations and plans for excavating a sluiceway, draining the lake, and downstream evacuation were proposed and later were implemented successfully, with the blockage breached by overtopping on June 10, less than a month after dam emplacement. The peak discharge of the release only slightly exceeded the flood of record at Mianyang City. No lives were lost, and significant property damage was avoided. Post-breaching evaluation reveals how future similar mitigation can be improved. Although initial breach erosion was slow, later erosion was judged uncontrollably rapid; increased slope of the engineered channel and adoption of a compound, trapezoid-triangular cross-section can be considered, as can other measures to control the rate of breach incision. Evacuees from Mianyang City spent an unnecessarily long time (12 days) in temporary settlements; more precise risk management planning can reduce this time in the future. [ABSTRACT FROM AUTHOR]

This paper assesses the probability of the main stream in the Dujiangyan-Wenchuan reach of the Minjiang River becoming blocked by debris flows in response to the Wenchuan Earthquake of 12 May 2008, on the basis of analyses of remote sensing data and ground investigations. Background information of debris flow gullies was developed from field investigations and high-resolution remote sensing image interpretation, and was used to estimate the maximum discharge per unit width by means of the revised model of debris flow magnitude according to the situation after the Earthquake. Based on improved discrimination equations of river blocking induced by debris flow, 29 debris flow gullies were evaluated and classified as four probability levels: extreme high, high, moderate and low. The results show that most of the debris flow gullies were at or over moderate hazard level, accounting for 72.4% of the total area. The present results provide a scientific basis for the prevention, elusion and mitigation of debris flow hazards and will contribute to the reconstruction of an earthquake prone area. [ABSTRACT FROM AUTHOR]