Quantitative assessment on landslide dam risks and mitigation: an in-depth study on the Baige lake breach

The emergency response actions against an impending barrier lake breach flood involve multiple social and engineering approaches, such as evacuating the affected people downstream, lowering the breach opening by excavating a diversion channel, and regulating the downstream hydraulic structures if present. Analytical evaluation of the breach flood under different scenarios of emergency mitigation is crucial for a rational decision when a rescue plan is being made. This study reviews the emergency responses undertaken for the "11.03" Baige barrier lake that occurred in the Jinsha River of China in 2018, which involved the evacuation of 27,500 downstream people in the Shangri-La and Lijiang tourist attraction areas and the engineering measures by excavating a diversion channel and treating the two existing dams. The numerical model for dam breach and flood routing analysis performed by the computer programs DB-IWHR and Hec-Ras allowed quantitative risk assessments for various scenarios of mitigation approaches. The results showed that the excavation of the 15-m-deep diversion channel reduced the peak flood discharge from 39,580 to 31,000 m(3)/s. The demolition of the Suwalong cofferdam avoided a cascade dam failure that would have created a peak flood of 22,841 m(3)/s, resulting in a 2.25-m-high additional inundation on an area of 1.13 x 10(5) m(2). The numerical assessment confirms the substantial reduction in the social and economic impact achieved by pre-emptying the Liyuan Hydropower Station, which avoided a sustained 10-h spillway discharge of 12,231 m(3)/s.