Abstract
Because the majority of sediment originates subglacially, as opposed to proglacially, focus must be given to subglacial sediment discharge. The latter, however is poorly constrained. We present a subglacial sediment transport time-series from two glacierized catchments (Gornergletscher and Aletschgletscher) in the Swiss Alps, based on hourly suspended sediment transport data and bedload transport estimates. This dataset is used to identify interannual and regional variability and to quantify the relationship between sediment transport and water discharge. Analysis of the relationship suggests that the access of water to subglacial sediment exerts substantial control on the quantity of sediment discharged. Historical data from Gornergletscher since the 1970's shows that elevated amounts of sediment were discharged in the 1980's, following the onset of increasing glacier melt. However, by 2016 and 2017, the sediment discharge returns to quantities below those in the 1970's, suggesting that sediment discharge can stabilize to new hydrological regimes over decadal timescales. Erosion rates for the two catchments (0.28\,mm\,a$^{-1}$ to 0.49 \,mm\,a$^{-1}$) are smaller than in other glacierized catchments of the Swiss Alps ($\sim$\,1\,mm\,a$^{-1}$). In some cases they are even less than a third of those reported in earlier decades, highlighting substantial regional and interannual variability in catchment-scale erosion. Empirical models, calibrated with 2016--2017 data and relating water discharge to suspended sediment concentration, fail to capture the increase in sediment discharge over the 1980's. This suggests that processes secondary to runoff, such as changing access to subglacial sediment by meltwater, were responsible for the increase. Because subglacial sediment discharge depends on both water discharge and sediment availability, we argue that physically-based models are required to capture the evolution of subglacial sediment transport as glaciers retreat.
Highlights
The retreat of alpine glaciers changes their hydrologic regime (e.g., Farinotti et al, 2016), but can alter their sediment discharge (e.g., Koppes et al, 2009; Micheletti et al, 2015; Raymond-Pralong et al, 2015; Costa et al, 2018)
In the Gornergletscher catchment, observed, long-term subglacial sediment discharge measurements allow us to examine decadal changes in subglacial sediment transport following glacier retreat. By synthesizing these data we examine processes that can be implemented and considered in physically-based subglacial sediment transport models (e.g., Creyts et al, 2013; Carter et al, 2017)
Empirical models have been previously used to describe subglacial suspended sediment concentration as a function of water discharge (e.g., Müller and Förstner, 1968; Swift et al, 2005). Such models are used to understand the hydraulic drivers and temporal variability of subglacial sediment discharge; notably, we examine the viability of these models to forecast the response of subglacial sediment transport under a variety of hydraulic conditions
Summary
The retreat of alpine glaciers changes their hydrologic regime (e.g., Farinotti et al, 2016), but can alter their sediment discharge (e.g., Koppes et al, 2009; Micheletti et al, 2015; Raymond-Pralong et al, 2015; Costa et al, 2018). Glaciers are known to evacuate large amounts of sediment from their beds (e.g., Østrem, 1975; Collins, 1979; Hallet et al, 1996), which can affect communities downstream of glacierized catchments. This is problematic for hydropower companies that must manage the sediment influx into reservoirs (leading to infill) and water intakes (causing turbine abrasion) (e.g., Anselmetti et al, 2007; Felix et al, 2016). The influence of glacier retreat on sediment discharge in alpine regions remains uncertain
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