291 / 2024-02-29 09:08:24

Study of the paleo dammed lake outburst flood in Tiger Leaping Gorge, Yangtze River

Jinsha river,dammed paleo-lake,late pleistocene,dam-breaching flood,hydraulics simulation

The Yangtze River course through China's densely populated and economically vibrant heartland with the development of China's hydropower energy sources, the number of high dams on the upstream Yangtze River has increased rapidly, intensifying the risk of dam break floods. Landslide dam outburst floods, renowned for their catastrophic impact and pivotal role in shaping river landscapes, pose challenges as we have difficulty directly observing their magnitude. Examining large-scale ancient landslide dam outburst floods offers invaluable insights into the aftermath of mega-floods breaching dams. The upper reaches of the Yangtze River are located in the Qinghai-Tibetan Plateau and its east margin, and the terrain is mostly deep canyons with frequent geological and tectonic activities, which have formed several landslide dammed lakes in history, among which the Tiger Leaping Gorge dammed-lake is the largest and the impact of the outburst flood is the most far-reaching. Commencing from the ancient landslide blockage in Tiger Leaping Gorge, our investigation involved determining the age, scale, and peak flow discharge of the dammed lake. This comprehensive analysis encompassed remote sensing interpretation, field studies, deposits dating, and hydraulic simulation. We simulated and analyzed the evolution of the flood in the whole downstream course. In summary, our study findings are as follows: 1) a large rockslide occurred on the left bank of the mountain in Tiger Leaping Gorge approximately 30,000 years ago, leading to the formation of a 370m high landslide dam. This rockslide dammed the river, causing upstream water accumulation and inundating an area of approximately 360km². 2) When the dam overtopped, resulting in the breach of the top 30% of the dam, the reconstructed peak discharge of the dam break flood was 5.5×10⁵ m³/s, which were derived through a combination of hydraulic modeling, field observations, and sedimentological analysis. 3) In the hydraulic simulation, careful consideration was given to sedimentation dynamics downstream, which significantly contributed to reducing uncertainties associated with paleoflood estimation. By incorporating sedimentation processes into the hydraulic model, we could more accurately simulate the propagation of floodwaters and better understand the paleohydrological dynamics.