The Erosional and Depositional Potential of Holocene Tibetan Megafloods Through the Yarlung Tsangpo Gorge, Eastern Himalaya: Insights From 2D Hydraulic Simulations

Abstract

Many outstanding questions in geomorphology concern the relative role that high-magnitude, low-frequency events play in shaping landscapes on Earth and other solar system bodies (e.g.,

The Lasting Legacy of Megaflood Boulder Deposition in Mountain Rivers

“…The upstream extent of the model domain is restricted to the YSR downstream of the moraine dam (Figure 1a)—the same location as the megaflood dam in Morey et al. (2022). We assume a constant rock uplift rate of 0.001 m/yr, similar to typical eastern Himalayan exhumation rates that characterize much of the flood pathway (e.g., Lang et al., 2016), and a concavity of 0.8 is calculated for the equilibrium channel profile and is used as the reference concavity for later calculations (Fig.…”

“…The initial condition is a steady-state river profile generated to roughly match the length and fluvial relief of the YSR between the moraine dam at Namche Barwa (Figure 1) and where the YSR exits the mountains to join the Brahmaputra River (Figure 1a). The upstream extent of the model domain is restricted to the YSR downstream of the moraine dam (Figure 1a)-the same location as the megaflood dam in Morey et al (2022). We assume a constant rock uplift rate of 0.001 m/yr, similar to typical eastern Himalayan exhumation rates that characterize much of the flood pathway (e.g., Lang et al, 2016), and a concavity of 0.8 is calculated for the equilibrium channel profile and is used as the reference concavity for later calculations (Fig.…”

Outburst floods strongly influence valley evolution in the Tsangpo Gorge, Eastern Himalaya

“…1b) lled and were breached repeatedly 20,21,24,26,27 . These very large lakes (80-800 km 3 ) with long durations of blockage (10 3 -10 4 years) may have released mega oods with discharges some ten times greater (> 10 6 m 3 /s) than the 2000 Yigong ood [21][22][23][24]26 . Some 260 previous moraine dams have been documented in the eastern part of the Yarlung River 20 .…”

“…(4) The in ow discharge to the lake is set to 0. The impact of this background ow on the ood depth can be ignored 22 because the discharge of the outburst ood was at least 100 times larger. The HEC-RAS simulations yield hydraulic parameters that include ow depth, velocity, shear stress and discharge for each cross-section.…”

“…Previous studies in the river system have documented late Pleistocene and Holocene dam breaches that released super oods and even mega oods with a peak discharge up to 10 6 m 3 /s (Fig. 1b), based on sedimentary and geomorphic archives [21][22][23][24][25][26][27] . With immense stream power, these outburst oods will have contributed to the long-term evolution of the gorge 19,20 along with high-frequency meteorologic oods.…”