New evidence of high-magnitude Holocene floods in the Purlung Tsangpo River, southeastern Tibetan Plateau

Catastrophic flood events are the most devastating natural hazard in the world, and direct human observation or instrumental/systematic measurement cannot capture the full range of their frequency-magnitudes. It is essential to reconstruct large-magnitude, rare floods that are recorded in landforms and sediments. Recently, palaeohydrology studies associated with Holocene palaeofloods in Himalayan rivers have been gaining attention. Exceptional Holocene outburst floods have been invoked as a highly efficient erosion agent contributing to the rapid incision of the Tsangpo Gorge. Despite its significance, palaeoflood records upstream of Tsangpo Gorge have rarely been explored. This study presents a series of flood deposits consisting of laminated silt and sand or repeated cross-bedding sets, which points to high-magnitude flood. Inverse hydraulic modelling of the step-backwater method based on HEC-RAS software was applied to acquire the water surface profile under varying peak discharge scenarios. The discharges of palaeofloods were estimated by relating elevations of flood features to water surface profiles calculated for the reach. The result provides a minimum estimate of the peak discharge of approximately 3.70 x 10(4) m(3)/s with a maximal depth of up to 47 m. The overestimation of the peak discharge due to post-event channel incision is up to 3000 m(3)/s. Seven charcoal C-14 dates give a narrow age range between 2.2 and 2.6 ka cal BP. This flood occurred only a few hundred years after a period of climate transition at similar to 3 ka BP, which was prone to hydrological extremes. Since the reconstructed palaeodischarge is significantly beyond the scope of either rainfall floods or glacial lake outburst floods, it can be inferred that the breaching of an upstream landslide dam is the most reasonable origin.