HYDRAULICS OF A HOLOCENE TIBETAN MEGAFLOOD THROUGH THE YARLUNG TSANGPO GORGE, EASTERN HIMALAYA

Quaternary megafloods (≥10⁶ m³/s) sourced from valley blocking glaciers on the Tibetan Plateau have been implicated in the geomorphic evolution of the Yarlung-Siang River (YSR). However, past estimates of megaflood erosion in the region have relied on simple estimates of peak discharge and erosional potential, making it difficult to fully understand how megafloods shape this rugged, mountainous landscape. Here, we use 2D numerical simulations of a megaflood sourced from a reconstructed 81 km³ lake on the Tibetan Plateau to analyze flood hydraulics over 3D topography and to examine the erosional and depositional legacy of these large floods. Simulations reveal an intense >60 hour flood with a peak discharge of 2.6x10⁶ m³/s, twice as high as predicted by previous work The flood inundates hillslopes 250 m above the modern channel and reaches flow speeds up to 70 m/s. Complex patterns of shear stress and flood power result from dynamic interactions between inundated topography and flood hydraulics. Most major tributaries experience between 8 and 60 km of backflow inundation. Simulated megaflood power and energy expenditure is at least an order of magnitude higher than that of the annual monsoon. Ridges adjacent to the modern channel that are not inundated during the annual monsoon have no control on the stream power of annual flows; however, when inundated during a megaflood, ridges directly influence patterns of depth, speed, stress, and flood power. The erosional patterns of a megaflood will therefore not mimic those of the annual monsoon. Megafloods are capable of moving and depositing sediment from large ≥4 m boulders to sand and gravel along the flood pathway. Large boulders deposited during a megaflood are likely too large to be moved by smaller magnitude flows. The deposition of these large boulders within the modern channel during a megaflood could increase roughness, armor the channel bed, and impede bedrock erosion. Fine-grained deposition on the hillslopes could inhibit hillslope erosional processes, especially along inundated tributaries. Megafloods have the potential to substantially alter the landscape both via erosion and deposition. Their legacy is complex as they leave behind landscape scars and sediments that have the potential to influence modern annual erosional patterns along the YSR.