Paraglacial adjustment of sediment slopes during and immediately after glacial debuttressing

摘要

Daily time lapse imagery and pixel tracking was used to monitor and track spatial and temporal changes in sediment-mantled hillslopes, during and immediately after glacier retreat in the Fox Glacier/Te Moeka o Tuawe valley, New Zealand. Observations from 2014 to 2018 of the Fox Glacier and surrounding hillslopes show hillslope failure is primarily coincident with, and triggered by, glacier retreat with rainfall accelerating movement of the hillslope. During glacier retreat, failure of the hillslope primarily occurred through sediment sliding, internal deformation of the sediment and occasional surficial debris falls, flows and avalanches delivering approximately 9.2 M m3 of sediment directly to the underside of the glacier over the study period with a maxi-mum daily averaged movement of 0.4 m per day of the main sediment mass. Following debuttressing, hillslope failure became dominated by localised rainfall-induced debris flows which initiated gullying of the sediment mantled slope. Ongoing instability of the slope and associated movement is maintained by toe erosion from the Fox River and melting dead ice, while continued rapid failure is facilitated through localised debris flows. The tracking of temporal and spatial changes of sediment-mantled hillslopes during glacier retreat has shown broad-scale hillslope response to occur quickly within days of rainfall or accelerated glacier retreat, particularly during summer. Debris flows, commonly thought to be a dominant erosion process within paraglacial environments, only occur following complete debuttressing and are supply-limited, only occurring after sediment sliding has occurred. Unlike many other case studies, sediment connectivity immediately following glacier retreat is high due to a lack of storage space and high rainfall inducing mass movements, efficiently delivering hillslope sediments to the proglacial stream channel. Attempts to quantify displaced volumes of sediment from paraglacial systems are likely underestimated due to a) a lack of focus on the early and latter stages of debuttressing b) a reliance on debris flows being a primary transport mechanism, and c) sediment being delivered sub-glacially rather than supra-glacially, further enhancing connectivity. (c) 2020 Elsevier B.V. All rights reserved.