THE GEOMORPHIC LEGACY OF HOLOCENE MEGAFLOODS IN THE EASTERN HIMALAYA: NUMERICAL SIMULATIONS OF OUTBURST FLOOD HYDRAULICS DOWN THE YARLUNG-PO-SIANG RIVER

The eastern Himalaya holds evidence for hundreds of temporary glacial and landslide dam impoundments throughout the Quaternary. What happened during the resulting catastrophic outburst floods? Did these geologically instantaneous events change how rivers function in the region? Holocene glacial moraine dams at the Namche Barwa massif are thought to have impounded large lakes, which drained in megafloods hypothesized to have substantially eroded the Yarlung-Po-Siang River valley. However, constraints on megaflood hydraulics are limited to peak discharge estimates based on extrapolated empirical relations for modern dam failures. Here, we numerically simulate the hydraulics of 80 km³ and 832 km³ paleo-lake megafloods over 3D topography using the depth-averaged shallow water equations (GeoClaw open-source software). For context, we first simulate a smaller (~10⁵ m³/s) recent outburst flood caused by failure of a landslide-impounded 2 km³ lake on the Yigong River; the 2000 Yigong flood followed the same pathway as the ancient megafloods through the Po-Siang River valleys, causing extensive landsliding and slackwater sand deposition. Simulations show that outburst flood hydraulics through rugged topography differ significantly from nonflood flows, in magnitude and in the spatial patterns of flow depth, speed, direction, and shear stress. Directly downstream of the breach and valley constrictions, the Yigong flood simulations produce sustained high shear stresses (>5 kPa) capable of plucking meter‐scale blocks, and megaflood simulations produce shear stresses 20-30 kPa greater. Shear stress patterns suggest outburst floods deposited boulder bars observed on cutbanks of meander bends, which we propose armor the bed, increase channel roughness, and alter incision patterns during nonflood flows. Megafloods inundate the hillslopes >300 m above the modern channel, likely stripping vegetation and causing extensive slope failures during and after the event. Megafloods also backflow ~50 km up major tributaries like the Siyom River, potentially causing slackwater deposition and creating a large floodplain. Our results show that these outburst floods instantaneously perturb river hydraulics, erosion, and deposition, and likely exert lasting control on river and hillslopes processes between floods.