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Landslide-dammed lake outburst floods (LLOFs) may pose serious safety threats to nearby residents and their livelihoods, as well as cause major damages to the downstream areas in mountainous regions. This study presents the Diexi ancient landslide-dammed lake (DALL), located along the Upper Minjiang River, in the eastern margins of the Tibetan Plateau. The DALL is known to have an estimated previous maximal lake area of 1.1 × 107 m2 and an impounded volume of 2.9 × 109 m3. At approximately 27 ka BP, the ancient landslide dam failed, and catastrophic LLOFs occurred. It was determined that the peak discharge of the Diexi ancient LLOFs could be reconstructed using regression, parametric, and boulder competence approaches. The reconstructed average maximum peak discharge was approximately 79,500 m3/s, with an estimated uncertainty bound of 70,000 to 90,000 m3/s. This indicated that the Diexi ancient LLOFs were the largest outburst floods to have occurred in the Upper Minjiang River Valley since the Late Pleistocene Period. The differences in the widths and slopes within the former and latter reaches of the dam indicated that the geomorphic influences on the river channel resulting from the DALL and its LLOFs have been present for tens of thousands of years. These findings were of major significance in deepening the understanding of the existence and disappearances of important river knickpoints on a time scale of tens of thousands of years. [ABSTRACT FROM AUTHOR]

The Walai rock avalanche (H/L ratio = 0.32), which was laterally and frontally confined occurred in the high mountainous areas of the eastern Tibetan Plateau. According to the field investigation and optically stimulated dating, the initial rock slope failure was determined to be a wedge failure controlled by bedding and two joint sets at approximately 38 ka ago. Based on the topography reconstruction, the deposited volume of this rock avalanche was estimated at 5.2 × 107m3, which was 1.24 times the mass detached from the source area. We observed and mapped trimlines, longitudinal ridges, arched ridges, and lateral ridges associated with the rock avalanche. We also observed well-developed sedimentary features, including carapace facies, body facies, stratified structures, jigsaw fractures, directional arrangements, shear-damaged rock clasts, and interior folds. Measured grain size distributions indicated that the Walai rock avalanche fragmented within a relatively short travel distance after detachment from the source area, with no progressive reduction of grain sizes during the subsequent movement. Based on the morphological and sedimentary features, the emplacement process of the Walai rock avalanche was determined to be controlled by the local terrain and could be divided into four stages: an initiation stage (detachment), an accelerating stage (fragmentation), a longitudinal compression stage (momentum transfer), and a spreading stage (shearing). • A confined rock avalanche that swerved and blocked the river was analyzed. • The trimlines provided evidence for the emplacement process of the rock avalanche. • A contour reconstruction method was used to estimate the ancient landslide volume. • Weak fragmentation effect was found in the later stage of rock avalanche movement. [ABSTRACT FROM AUTHOR]

Landslide events causing rivers to become dammed are common natural disasters in mountainous area. Based on the sedimentary facies analysis of 44 sections between proximal and distal parts of a dammed lake, this study reveals the sedimentary facies distribution of the Diexi Ancient Dammed Lake located in the upper Minjiang River, on the southeastern margin of the Tibet Plateau. Fluvial gravels and sand were deposited in the proximal part of the dammed lake. Rhythmically bedded silt and horizontally laminated silt to clay were deposited in the distal part of lake due to the deep and steady water conditions. After progradation of the fluvial system, flood sediments were deposited in the middle part of the lake. The flood carried a large amount of sediments into the lake, and the decelerating flow generated successively alternating layers of gravels and sand, followed by alternating layers of silt with low-angle cross-stratification, parallel bedding, and climbing ripples downstream. Gravel layers formed by four fluvial progradations were found based on the distribution of sedimentary facies in 44 sections. After the first fluvial progradation, lacustrine sediments at the highest point emerged from the water surface. The interval between the second and third fluvial progradation is relatively short, and only a terrace was formed, namely terrace VI. After the fourth fluvial progradation, terrace IV was formed. Although gravel layers formed by fluvial progradations corresponding to terraces II, III and V were not found, it can be determined that a terrace level does not necessarily correspond to a period of fluvial progradation. Therefore, the analysis of sedimentary facies upstream of landslide dams can be used as a supplementary method for inferring the evolution of the dammed lake. • Logging the lithofacies of 44 sections from the proximal to distal parts of the Diexi Ancient Dammed Lake • Describing and interpreting distribution of sedimentary facies in the longitudinal profile • Linking sedimentary facies with fluvio-lacustrine terraces to further analyze the evolution of dammed lake. [ABSTRACT FROM AUTHOR]