Supported by the Swiss National Science Foundation, 2020-2024
PhD students: Matt Jenkin and Davide Mancini
Research Technician: Floreana Miesen
- Introduction
Proglacial margins, exposed following glacier and ice sheet retreat, have been described as one of the most rapidly changing landscapes on Earth. In the Swiss and Austrian Alps alone, c. 930 km2 of proglacial margins have formed since the Little Ice Age (Carrivick et al. 2018). Our knowledge of the geomorphic processes that follow glacier retreat is well-developed (McColl and Draebing, 2019; Porter et al. 2019). Deglaciation implies debuttressing (Cossart et al. 2008) such that previously immobile sediment can be more readily mobilised. It leads to (1) increased rockfalls (Ravanel and Deline, 2011; Heckmann et al. 2016); (2) moraine evolution through erosion (Curry et al. 2006; Lane et al. 2017); (3) dead ice melt-out (Bosson et al. 2015); (4) paraglacial landsliding (Hugenholtz et al. 2008; Cossart et al. 2013); (5) debris flow formation (Haas et al 2012); and (6) rock glacier formation/response (Käab et al., 2007; Micheletti et al. 2015). The paraglacial model (Church and Ryder, 1972) argues for peak geomorphic activity immediately after deglaciation then declining due to sediment exhaustion and negative feedbacks (e.g. vegetation development; Eichel et al.2015).
A crucial element of proglacial margins that can develop following deglaciation is the proglacial forefield (Figure 1). If valley slope is low enough to prevent deep river incision, there is sufficient lateral accommodation space for river migration and there is no natural or artificial overdeepening and proglacial lake formation a morphodynamically active proglacial alluvial plain, or “forefield” may form (figure 1). These latter are of crucial importance as they buffer valley side-slope hazards (rockfalls, debris flows,…; Oppikofer et al., 2008) and they can potentially act as filter for sediment exported from glaciers, and hence on downstream sediment yield from glaciated basins.
Figure 1. The proglacial forefield of the Otemma Glacier South-West Switzerland
Sediment mobilisation processes have been largely described (e.g. Church and Ryder, 1972; Mercier et al., 2008) and quantified (archival imagery, sediment budgets, sediment cascades; e.g. Warburton, 1990; Staines et al., 2015; Lane et al., 2017). The effects of glacier thinning and recession on glacier erosion are increasingly well known: a thinning glacier should deform and so erode more slowly; but the enhanced melt (“meltwater dividend”; Collins, 2008) may encourage glacier acceleration and sliding-enhanced erosion (Koppes et al. 2009). However, the relationship between climate warming and sediment yield in deglaciating basins remains highly uncertain notably (1) how glaciers evacuate sediment through their marginal zone and into the forefield; and (2) the role that forefields play in filtering the flux of evacuated sediment (Heckman and Morche, 2019; Porter et al 2019). These observations lead to the core aim of this proposal: undertake the first, coupled study of the relationship between subglacial sediment export, proglacial forefield morphodynamics and sediment flux to understand the role of proglacial margins in filtering sediment export from retreating temperate Alpine glaciers. The study areas for this project are located in the Swiss Alps (Wallis), and more in particular on the proglacial forefields of the Glacier d’Otemma and the Gornergletscher (Figure 1).
- Subglacial sediment export (PhD candidate to be appointed)
Controls on the rate of sediment export by a glacier to its forefield are poorly known (Koppes and Montgomery, 2009). Export is a function of the ability of a glacier both to erode (abrasion, plucking) and to transport sediment to the glacier margin. It is assumed that erosion at a glacier bed translates directly into transfer of eroded sediment to the glacier outlet and that erosion continues only if subglacial channels are capable of evacuating eroded sediment (Riihimaki et al. 2005). That said, observations suggest that sediment export is variable over a number of timescales: (1) within-daily, bedload transport can be locally capacity limited due to diurnal discharge minima (Perolo et al. 2019); and (2) seasonally, sediment export declines through the melt-season, attributed to the progressive evacuation of winter-eroded sediment (Riihimaki et al. 2005; Gimbert et al. 2016).
Sediment export rates and volumes from retreating glaciers are studied coupling three methods. First, bedload and suspended sediment load are measured directly in front of the snout of the glacier to determine variation in sediment export rates. Second, repeated GPR surveys are undertake to establish the topography, but also the dynamics, of the subglacial channels in the snout margin zone. Third, particle tracking method, injecting pit-tagged sediments into crevasses and moulins, is used to quantify storage duration and transfer delays in space and time.
- Proglacial forefield dynamics following glacier retreat (PhD student Davide Mancini)
Forefield response to deglaciation is a function of the ratio of sediment supply to sediment transport capacity. The dividend of increased runoff during glacier retreat (Collins, 2008) may increase sediment transport capacity dramatically (Lane et al. 2017) but morphodynamic response then depends on whether or not there is concomitant increase in glacial sediment export. With glacier recession, there is evidence (de Winter et al. 2012; Cordier et al. 2017) that supply does not keep up with capacity. This is reflected in incision in front of retreating glacier margins, which provides sediment necessary for downstream aggradation, aided by reductions in downstream bed slope (incision-aggradation, Marren, 2002; Beylich et al. 2009). It is also reflected in terrace formation (Marren and Toomath, 2013). Over seasonal to decadal time scales, formation of proglacial lakes has been shown to reduce gravel supply (Bogen et al. 2015) and promote downstream incision-aggradation (Chew and Ashmore, 2001); but conditions have been observed where supply can keep up with capacity increases as aggradation following glacier retreat is observed throughout the forefield (Curran et al. 2017). Thus, understanding how forefields respond to climate retreat must consider the effects of both the meltwater dividend and glacier sediment export, with reference to large-scale topographic forcing (e.g. lateral accommodation space and valley slope) within the forefield.
Proglacial margin response to glacial retreat is studied coupling sediment budget technique (analysis of continuous suspended sediment and bedload discharges at specific proglacial stream spots) with remote sensing analysis of surficial geomorphological changes (repeated DEMs of differencing starting from UAV imagery; figure 3) through time. In addition, in collaboration with Prof. A. Nicholas (Exeter University), collected data will be used to develop the HSTAR model to simulate how different sediment supply rates impact morphodynamics and sediment flux downstream.
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