Abstract:

Landslide-dammed lakes and outburst floods have been widely developed along the Upper Minjiang River valley, due to the complex geological environmental conditions in this region. Hazard evolution reconstruction has major significance for regional disaster prevention and mitigation. This study is dedicated to the Diexi ancient landslide-dammed lake (DALL) triggered by a strong paleoseismic in the Upper Minjiang River in Western Sichuan Province, at the southeastern margin of the Tibetan Plateau. We focus on the fluid dynamics of downstream megaflood characteristics. The estimated former lake size was 1.1×10⁷ m², and the impounded volume was 2.9×10⁹ m³. The maximum peak discharge and paleohydraulics of the landslide-dammed lake outburst flood (LLOF) were reconstructed with empirical equations and one-dimensional (1D) HEC-RAS hydraulic numerical model. The results reveal that the maximum peak flow of the Diexi ancient LLOF by HEC-RAS was 73,060 m³/s, largely similar to the results by the empirical methods (74,500-76,800 m³/s, avg. 76,000 m³/s), with an error of < 5%. The corresponding maximum flood depth and flow velocity are 70.1 m and 16.78 m/s, respectively. The inundated area during the modeled reach of Minjiang River was approximately 6.08 km². The uncertainty bound of the maximum peak discharge was determined to be 69,000-81,000 m³/s. The maximum peak discharge of the Diexi ancient LLOF was more than one hundred times the average annual flow of Minjiang River (approximately 700 m³/s), indicating that it was an abnormally large outburst flood in Diexi Region once in a hundreds-of-thousand years in history, and also one of the world’s largest LLOFs. The direct consequence of the high-energy outburst flood was the formation of band- or terrace-shaped outburst deposit bars and boulder deposit terraces on both sides of the gorge and river-bed in the downstream. These findings are consistent with previous hydrological and sedimentary studies on the high-energy outburst floods, demonstrating the high reliability of our results. In addition, this study also shows that the 1D HEC-RAS model can be used to reconstruct the hydraulics of an ancient LLOF in deep-confined gorge environments. This study is of great significance to the paleoenvironment reconstruction and geomorphic evolution in the upper reaches of Minjiang River.

Key words: ancient landslide-dammed lake, outburst flood, peak discharge, HEC-RAS, uncertainty assessment, Upper Minjiang River