A comprehensive analysis of Holocene extraordinary flood events in the Langxian gorge of the Yarlung Tsangpo River valley.

Increasing extreme temperature, precipitation and rapid meltwater events have added stress to the Himalaya's hydrological sensitivity and major flood risks, however, current extreme hydrological dataset and their genesis are insufficient to assess future flood discharge extremes in High Asian' rivers. Here, Holocene extreme floods in the Yarlung Tsangpo River valley were reconstructed by using physic-chemical analysis, optically stimulated luminescence dating and palaeohydraulic techniques. Palaeoflood slackwater deposits (SWDs) were identified by means of palaeohydrological criteria and comparison with SWDs from large flood that occurred in 2018. Palaeoflood SWDs consist of well-sorted silt and sand with a consistent geochemical composition, implying a similar sedimentary source. Such results suggest that these SWDs were transported in suspension over long distances during flood events. The chronological analysis indicates that there are three palaeoflood events, dated to 5.7, 3.9 and 2.9-1.2 ka, during the mid-late Holocene. Palaeoflood peak discharges in the bedrock reach and meandering channel were estimated to be 27,600-35,000 m/s using one-dimensional and two-dimensional hydrodynamic modelling. The simulation results clearly show the potential palaeoflood depositional range in the lower-velocity and eddy backwater environments between Jiacha and Langxian gorge. The palaeoflood magnitudes redefine the regional largest flood discharge, and fit well with global maximum flood curves. And mid-late Holocene extreme flood magnitudes were generally 2.5-3.5 times larger than the current maximum gauged flood, but lower than the Jiedexiu glacial lake outburst floods. Comprehensive analysis highlights the three extraordinary floods were possibly induced by monsoon rains and glacial meltwater. Site-specific palaeoflood information advances our knowledge of rare and extraordinary floods in the highest and largest river in the southern Tibetan Plateau.