Morpho-sedimentary and stratigraphic characteristics of the 2000 Yigong River landslide dam outburst flood deposits, eastern Tibetan Plateau
Introduction
High-energy floods occur when a dam is breached by overflow or seepage and its impounded water catastrophically drains. There are two broad classes of alluvial dams: formed by glacial moraines or volcanic debris (Baker and Bunker, 1985; Rudoy and Baker, 1993; Bjorck, 1995; Capra et al., 2002; Jakobsson et al., 2007), and by landslides (Casagli and Ermini, 1999; Ermini and Casagli, 2003; Korup, 2004; Dai et al., 2005; Wu et al., 2009). The scale of the floods is often extraordinarily large, with peak discharges up to 1.0 × 106 m3/s (Baker, 1973; O'Connor and Baker, 1992; O'Connor, 1993; Baker et al., 2002; Rudoy, 2002; Montgomery et al., 2004; Komatsu et al., 2009). Ancient catastrophic outburst floods on Earth (Bretz, 1923; Malde, 1968; Baker, 1973; O'Connor, 1993; Carling, 1996a, Carling, 1996b; Montgomery et al., 2004; Herget, 2005; Lamb et al., 2008a; Baynes et al., 2015) and megafloods on Mars (Baker and Milton, 1974; Baker, 2001; Goudge and Fassett, 2018) have been well studied in recent years. Many studies of the flood geomorphic impact (Baker, 1973; O'Connor, 1993; Maizels, 1997; Russell and Knudsen, 1999; Marren and Schuh, 2009) and sedimentary characteristics (Benito and O'Connor, 2003; Marren and Schuh, 2009; Carling, 2013) demonstrate that they can entrain and transport various materials ranging in size from clay to boulder and that downstream deposition depends on the interaction between local hydraulics and topography. Although there has been significant research on the linkages between the geomorphological, stratigraphic and sedimentary characteristics of outburst flood deposits and their hydrodynamics, including ancient (O'Connor, 1993; Miyamoto et al., 2006; Carling, 2013; Larsen and Lamb, 2016) and modern outburst floods (Cook et al., 2018; Kougkoulos et al., 2018; Turzewski et al., 2019), some issues remain open, e.g. the relationship between peak flow stage and sedimentary surface. Transport and subsequent deposition of sediment grains by free-surface flow does not occur until a flow first exceeds and then falls below a threshold for sediment transport allowing bartop deposition, so there is a likely relationship between the maximum flood level and the uppermost surface of deposits (Litty et al., 2016). Consequently, the elevations of the surface of coarse-grained giant bars deposited by paleofloods have been used as an indicator of maximum flood stage (Carling et al., 2010; Bohorquez and Ancey, 2015; Carling et al., 2020). However, caution needs to be exercised as some studies of modern floods have shown that the height of the flood deposits can be less than the peak stage (Greenbaum et al., 2001). Slackwater deposits (SWDs), representing rapid accumulation of fine sediments deposited from suspension, have also been used to determine paleoflood stage, but not all SWDs are associated with peak discharge (Turzewski et al., 2019).
Outburst floods are common in the margins of the Tibetan Plateau and there is abundant evidence for valley blockage by glacial and landslide dams in the eastern Himalaya (Zhu and Li, 2000; Montgomery et al., 2004; Zeng et al., 2007; Chen et al., 2008; Korup and Montgomery, 2008; Korup et al., 2010; Yuan and Zeng, 2012; Huang et al., 2015; Hu et al., 2018; Turzewski et al., 2019). In term of megaflood research on the Tsangpo River in the eastern Tibetan Plateau, Montgomery et al. (2004) identified at least two Holocene outburst floods from Gega dammed lake, located immediately upstream of the Tsangpo gorge, and estimated peak discharges of up to 5,000,000 m3/s, while Liu et al., 2014, Liu et al., 2015 suggested lower discharges of c. 500,000 m3/s. Lang et al. (2013) also found evidence of slackwater deposition associated with Gega lake megaflooding.
The large-magnitude modern outburst flood in the Yigong River in 2000 has been well documented in terms of landslide dam volume and height, duration of dammed lake impoundment, timing of breach, peak discharge of the outburst flood and its impact on downstream areas (Shang et al., 2003; Evans and Delaney, 2011). Numerical simulations have also been made for the rockslide mechanism and outburst flood hydraulics (Delaney and Evans, 2015; Turzewski et al., 2019). However, the geomorphic and sedimentary record of the 2000 event has been little discussed. This study aims to describe quantitatively the geomorphology and sedimentology of boulder bars and SWDs and their interaction with outburst flood hydraulics, as a contribution to enhancing understanding of sediment transfer and deposition processes under extreme hydrodynamic conditions.
Section snippets
Background
The outburst flood occurred on June 10, 2000, in the Yigong River, a tributary of the Yarlung Tsangpo. Yigong river originates at the southern foot of the Nyenchen Tonglha Mountains and extends 286 km to join the Parlung River (Liu and Lu, 2000) (Fig. 1). With a drainage area of 13,533 km2, the Yigong accounts for 47.3% of the Parlung catchment. The river is located in the eastern Tibetan Plateau, in the transition zone between the high plateau to the west and alpine valleys.
Due to collision
Methods
A combination of field survey, laboratory analysis and remote sensing interpretation was used to map and evaluate geomorphic and sedimentologic characteristics of the 2000 Yigong outburst flood from the breached dam to Tongmai Bridge (c. 17 km). The 1:200,000 geological survey map of Bomi county obtained from the Bureau of Geology and Mineral Resources of Tibet Autonomous Region, was used as the baseline for field survey. Four remote sensing images with a spatial resolution of 30 m, obtained
Channel migration
The main channel has undergone substantial lateral migration, and boulder bars contain numerous meter-scale blocks. The lateral erosion caused extensive translational landslides and left a heavily scoured channel characterized by a steep gradient and several erosion scarps. These are in agreement with previous studies that outburst floods can cause significant lateral erosion in rapidly incising rivers (Hartshorn et al., 2002; Turowski et al., 2008; Barbour et al., 2009; Beer et al., 2017; Cook
Stratigraphic and sedimentological characteristics
The stratigraphy of outburst flood deposits was analyzed at three exposures in the upper (section 1), middle (section 2) and lower parts (section 3) of the Yigong River study reach (see Fig. 5 for location). All sediments within the sections are cohesionless. The upmost section is at the tail of the residual dam, and comprises a poorly sorted, massive accumulation of granular gravels and boulders. The second section comprises coarse parallel-bedded deposits located above the old debris flow
Conclusions
The 2000 Yigong landslide dam outburst flood has a remarkable effect on river channel development due to its huge lake storage volume, high peak discharge, and strong hydrodynamic conditions. The impact of this event on river morphology is investigated with respect to morpho-sedimentary and stratigraphic records. Remote sensing analysis of pre- and post-flood channel planform using ETM+ Landsat 7 images indicates that the new channel migrated downstream relative to the original and increased in
Declaration of competing interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgements
This research was supported by the National Natural Science Foundation of China (Grant No. 91747207), the National Key Research and Development Program of China (Grant No. 2018YFC1505205), and the National Natural Science Foundation of China (Grant No. 41790434). Many thanks to two anonymous reviewers, Dr. Barbara Rumsby, Prof. Paul A. Carling, and Prof. Yong Li for their valuable comments and advices.
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