Abstract
Abstract. We combine a glacier outburst flood model with a glacier flow model to investigate decadal to centennial variations in outburst floods originating from ice-dammed marginal basins. Marginal basins can form due to the retreat and detachment of tributary glaciers, a process that often results in remnant ice being left behind. The remnant ice, which can act like an ice shelf or break apart into a pack of icebergs, limits a basin's water storage capacity but also exerts pressure on the underlying water and promotes drainage. We find that during glacier retreat there is a strong, nearly linear relationship between flood water volume and peak discharge for individual basins, despite large changes in glacier and remnant ice volumes that are expected to impact flood hydrographs. Consequently, peak discharge increases over time as long as there is remnant ice remaining in a basin, and peak discharge begins to decrease once a basin becomes ice-free. Thus, similar size outburst floods can occur at very different stages of glacier retreat. We also find that the temporal variability in outburst flood magnitude depends on how the floods initiate. Basins that connect to the subglacial hydrological system only after reaching flotation depth yield greater long-term variability in outburst floods than basins that are continuously connected to the subglacial hydrological system (and therefore release floods that initiate before reaching flotation depth). Our results highlight the importance of improving our understanding of both changes in basin geometry and outburst flood initiation mechanisms in order to better assess outburst flood hazards and their impacts on landscape and ecosystem evolution.
Highlights
Glacier outburst floods are sudden releases of water from ice-dammed or morainedammed lakes
In our transient glacier simulations we observed similar trends in flood hydrographs regardless of basin hypsometry and whether the simulations started with the basins filled to flotation depth (Fig. 7) or if the basins were connected to the subglacial hydrological system as they filled (Fig. 8)
The peak discharge will begin to decrease only once the remnant ice is gone. This result is independent of basin geometry and the mechanism of drainage onset and is a consequence of the proportionality between peak discharge and basin storage capacity that occurs for individual basins despite large changes in glacier geometry and remnant ice
Summary
Glacier outburst floods ( referred to as jökulhlaups) are sudden releases of water from ice-dammed or morainedammed lakes. Outburst floods cause semiregular but short-lived perturbations to downstream ecosystems by rapidly changing sediment and nutrient concentrations and proglacial water temperatures (e.g., Neal, 2007; Kjeldsen et al, 2014; Meerhoff et al, 2019). The largest of these floods create major erosional features during glacial periods (e.g., Larsen and Lamb, 2016; Keisling et al, 2020); smaller, more frequent outburst floods are important in driving landscape change (e.g., Russell et al, 2006; Cook et al, 2018; Carrivick and Tweed, 2019). Motivated by observations from Mendenhall Glacier, Alaska, we focus on glacier outburst floods from ice-dammed marginal basins, which form following the thinning, detachment, and retreat of tributary glaciers and often contain remnant ice left behind during deglaciation (e.g., Capps et al, 2010; Kingslake and Ng, 2013a; Kienholz et al, 2020) (Fig. 1)
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