The Geomorphic Impact of Outburst Floods: Integrating Observations and Numerical Simulations of the 2000 Yigong Flood, Eastern Himalaya

Abstract

Outburst floods in mountainous landscapes traverse complex topography and interact with the channel and valley walls, producing intense flow hydraulics that drive geomorphic change and impact people and infrastructure. Evidence of modern and ancient outburst floods is scattered around the eastern Himalaya, but hydraulics related to these geomorphic features are uncharacterized, limiting our understanding of the role of large floods in long‐term evolution of the region. Here we combine remote and field observations of the 2000 Yigong River landslide‐dam outburst flood with 2‐D numerical flood simulations using the software GeoClaw. Modeling results agree with field evidence to the extent that we judge the simulated hydraulics to be relevant to flood hazard and geomorphic investigations. Results show that the hydraulics of outburst floods through rugged topography differ from those expected for nonflood flows, in magnitude and in the spatial patterns of flow speed, direction, and shear stress. The flood produced sustained high bed shear stresses capable of plucking meter‐scale blocks immediately downstream of breach, in the steep Tsangpo Gorge, and in isolated locations associated with valley constrictions. Simulated shear stresses suggest that outburst floods deposited numerous kilometer‐scale boulder bars observed along the flood pathway, armoring the bed, increasing channel roughness, and inhibiting incision in locations that would not be predicted for nonflood flows. Our findings highlight the potential for different magnitude flows to promote not only different amounts, but also different patterns of bedrock erosion, with implications for the role of prehistoric megafloods in the topographic evolution of the eastern Himalaya.

Timing of exotic, far-travelled boulder emplacement and paleo-outburst flooding in the central Himalaya

“…Cenderelli and Wohl (2001) and Fort et al (2010) compared seasonal high flow floods (SHFFs) with discharges of recent GLOF and LLOF events and they appeared to be at least one order of magnitude higher than monsoonal precipitation peak discharges in central Himalayan catchments (Kali Gandaki and Mount Everest region). Peak discharges reaching 10 5 m 3 /s have been documented or suggested for a few historical events in the Himalaya mainly associated to LLOF such as the Great Indus flood of 1841 (Mason, 1929;Shroder et al, 1991), the great outburst in April 2000 in the Tibetan Yigong Zangbo River (Shang et al, 2003;Delaney and Evans, 2015;Turzewski et al, 2019), and the large LLOFs at Dadu River and Yalong https://doi.org/10.5194/esurf-2020-17 Preprint. Discussion started: 20 March 2020 c Author(s) 2020.…”

“…Costa and Schuster 1988;Cenderelli, 2000;O'Conner and Beebee, 2009;Korup and Tweed, 2007;Wohl, 2013;Cook et al, 2018). LOFs represent both a significant hazard (Kattelmann, 2003; and an active geomorphic agent of landscape evolution (Wohl, 2013;Cook et al, 2018;Turzewski et al, 2019).…”

Reconstruction of the 1941 GLOF process chain at Lake Palcacocha (Cordillera Blanca, Peru)

“…A total of 882 high-mountain lakes were identified by Emmer et al (2016). Some of these lakes are susceptible to GLOFs (Vilímek et al, 2005;Vilímek, 2013, 2014;ANA, 2014;Iturrizaga, 2014). A total of 28 geomorphologically effective GLOFs originating from moraine-dammed lakes have been documented (Emmer, 2017).…”

Chronology and sediment provenance of extreme floods of Siang River (Tsangpo‐Brahmaputra River valley), northeast Himalaya

“…Lang et al (2013) studying undated flood deposits of the Siang, and using U-Pb zircon ages to determine sediment provenance, suggested that floods facilitate sediment erosion from the Namche Barwa syntaxis zone. Turzewski et al (2019), in investigating sediments laid down by the 2000 flood, suggested that sediment was mainly generated from the Tibetan plateau; and the fast exhuming Namche Barwa syntaxis zone was the secondary source. Our study suggests the occurrence of two types of floods in the Siang valley: (i) episodic lake outburst megafloods that deposit massive sand piles along the channel, for example as observed at Pangin, Geku, Yinkiong, Bombao and Tuting; and (ii) floods caused by a strong southwest monsoon that produced meter-scale sand-silt couplets, for example as observed in the sequence at Ranaghat.…”