Abstract
Abstract. Topographic development via paraglacial slope failure (PSF) represents a complex interplay between geological structure, climate, and glacial denudation. Southeastern Tibet has experienced amongst the highest rates of ice mass loss in High Mountain Asia in recent decades, but few studies have focused on the implications of this mass loss on the stability of paraglacial slopes. We used repeat satellite- and unpiloted aerial vehicle (UAV)-derived imagery between 1990 and 2020 as the basis for mapping PSFs from slopes adjacent to Hailuogou Glacier (HLG), a 5 km long monsoon temperate valley glacier in the Mt. Gongga region. We observed recent lowering of the glacier tongue surface at rates of up to 0.88 m a−1 in the period 2000 to 2016, whilst overall paraglacial bare ground area (PBGA) on glacier-adjacent slopes increased from 0.31 ± 0.27 km2 in 1990 to 1.38 ± 0.06 km2 in 2020. Decadal PBGA expansion rates were ∼ 0.01 km2 a−1, 0.02 km2 a−1, and 0.08 km2 in the periods 1990–2000, 2000–2011, and 2011–2020 respectively, indicating an increasing rate of expansion of PBGA. Three types of PSFs, including rockfalls, sediment-mantled slope slides, and headward gully erosion, were mapped, with a total area of 0.75 ± 0.03 km2 in 2020. South-facing valley slopes (true left of the glacier) exhibited more destabilization (56 % of the total PSF area) than north-facing (true right) valley slopes (44 % of the total PSF area). Deformation of sediment-mantled moraine slopes (mean 1.65–2.63 ± 0.04 cm d−1) and an increase in erosion activity in ice-marginal tributary valleys caused by a drop in local base level (gully headward erosion rates are 0.76–3.39 cm d−1) have occurred in tandem with recent glacier downwasting. We also observe deformation of glacier ice, possibly driven by destabilization of lateral moraine, as has been reported in other deglaciating mountain glacier catchments. The formation, evolution, and future trajectory of PSFs at HLG (as well as other monsoon-dominated deglaciating mountain areas) are related to glacial history, including recent rapid downwasting leading to the exposure of steep, unstable bedrock and moraine slopes, and climatic conditions that promote slope instability, such as very high seasonal precipitation and seasonal temperature fluctuations that are conducive to freeze–thaw and ice segregation processes.
Highlights
Climatic-warming-induced permafrost degradation and glacier shrinkage have altered the thermal, hydraulic, and mechanical properties of their adjacent terrains, where the destabilizing slopes increase the risk of hazards in high mountain regions (Krautblatter and Leith, 2015)
The glacier terminus has retreated ∼ 510 m (17 m a−1), its surface has downwasted by −0.88 m a−1, with thinning observed over 83.50 % of the ablation zone study area, and the glacier velocity has slowed from a mean of 0.32 m d−1 to 0.11 m d−1 (2014 to 2018)
The paraglacial slope failure (PSF) and even the whole paraglacial bare ground area provide an ample source of sediment for the glacial environment
Summary
Climatic-warming-induced permafrost degradation and glacier shrinkage have altered the thermal, hydraulic, and mechanical properties of their adjacent terrains, where the destabilizing slopes increase the risk of hazards in high mountain regions (Krautblatter and Leith, 2015). The four predominant modes of slope response in deglaciating catchments are (1) large-scale catastrophic rock slides and rock avalanches (Kirkbride and Deline, 2018; Fischer et al, 2010); (2) ice-contact slope movements (McColl and Davies, 2013); (3) periodic small-scale rock topples or rockfalls (Cook et al, 2013); and (4) deep-seated gravitational creep (Deline et al, 2015b; Ballantyne et al, 2014). These responses provide a useful framework for examining the processes and geomorphological consequences of slope adjustment during or following deglaciation. As one major cold-region geomorphic process following glacial and permafrost-related slope instability, PSFs encompass failure of steep rock walls and lateral moraine slopes following glacier downwasting (Church and Ryder, 1972; Fickert and Grüninger, 2018) and are widely distributed in deglaciating or deglaciated landscapes
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