Hydraulics of a Megaflood in the eastern Himalaya: Feedbacks between Form and Flood

Abstract:

Megafloods (10⁶ m³/s) from glacially impounded lakes in the Quaternary have been proposed as agents of geomorphic change in the high relief Yarlung Tsangpo Gorge of the eastern Himalaya. Recent numerical simulations of a historical outburst flood (10⁵ m³/s) down the Yarlung-Siang River (YSR) in the Gorge region suggest that interactions between high relief topography and flood hydraulics have the potential to deposit features such as boulder bars within the channel that could impact subsequent annual flow hydraulics, but it remains unclear how hydraulics scale up in megafloods. Here, we use 2D numerical modeling of flood hydraulics on 3D topography to examine interactions between flow, sediment transport, and channel form in a reconstructed outburst megaflood through the rugged YSR drainage.

Megaflood simulations show sustained high shear stresses (>10 kPa) immediately downstream of the reconstructed Holocene moraine dam, within the Gorge, and at select locations downstream of the Gorge. Features like ridges, terraces, and planform channel changes, such as 90° turns or tight double meanders, act as roughness features during the flood, influencing the location of high and low shear stresses. Simulated hydraulics are consistent with the observed distribution of outsized boulder bars within the modern channel and with Holocene slackwater flood sand deposits on hillslopes up to 300 m above the modern channel. Our calculations show that subsequent annual flows and smaller historical outburst floods generate insufficient flow depths and speeds to inundate the slackwater sands or move these boulders within the channel. The boulder bars armor the bed and increase roughness, extracting momentum from annual flows and encouraging deposition. Slackwater sands blanket the hillslopes, affecting sediment storage and hillslope processes. In this way, megafloods may alter the channel form and roughness and the distribution of stored sediment, effectively becoming permanent features in the landscape for decades to tens of thousands of years or more, depending on flood recurrence. Our results suggest that the legacy of a megaflood in the region is both erosional and depositional, with feedbacks between the rugged mountain terrain and megaflood hydraulics that change how the system can evolve post-flooding.