Excavation of subglacial bedrock channels by seasonal meltwater flow

Abstract

Subglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes – events occurring once per year or less frequently with discharges larger than a few hundred cubic metres per second – are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a one‐dimensional numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed‐ and suspended‐load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D=256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension, which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges. Copyright © 2018 John Wiley & Sons, Ltd.

The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet

“…Persistent subglacial water routing may incise channels into the underlying bedrock (Nye or N-channels; Figure 4; Nye, 1976). On the basis that surface-to-bed connections seem to form in the same locations year-to-year (e.g., Clason et al, 2015) and that channels incised into bedrock have been observed in deglaciated regions (e.g., Walder and Hallet, 1979;Sharp et al, 1989), we think it is likely that N-channels exist in some locations beneath the ice sheet (e.g., Beaud et al, 2018) and may provide preferential flow pathways. Water flow along water-filled Nchannels will melt the overlying ice, possibly forming an N-Rchannel hybrid (i.e., a channel partially melted into the overlying ice and partially eroded into bedrock).…”

“…However, observations of the subglacial environment and constraints on crucial model parameters are sparse, leading to uncertainty in inferences from models: modelled R-channels can form under conditions representative of the upper ablation area if a large (∼1 m 2 ) initial conduit size is used (e.g., Hewitt, 2011;Gulley et al, 2012). Such conditions may occur due to uplift during rapid lake drainage (Das et al, 2008;Pimentel and Flowers, 2010;Andrews et al, 2018) or may be facilitated if persistent surface-to-bed hydrological connections (Catania and Neumann, 2010) enable erosion of preferential flow pathways into the substrate (Gulley et al, 2012;Beaud et al, 2018). In addition, the spatial pattern of surfaceto-bed connections (moulins) strongly influences the spatial organisation of modelled subglacial channels, with a high density of moulins producing widespread and rapid channelisation beneath ice up to 815 m thick (Banwell et al, 2016).…”

Modeling Sediment Transport in Ice‐Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields

“…Subglacial flood flows are often invoked in order to explain the erosive power of water (Kehew et al, 2012;Wright, 1973;van der Vegt et al, 2012), the size of clasts found in eskers, or the large changes in flow regimes inferred from esker sediment (e.g., Brennand, 1994;Burke et al, 2012). Recent numerical modeling studies, however, suggest that pressurized seasonal meltwater flow produces sufficient shear stresses to transport boulder-size clasts and erode the bedrock (Beaud et al, 2016(Beaud et al, , 2018. The formation of eskers has been attributed to either the geology of the substrate (Clark & Walder, 1994), the availability of meltwater (e.g., Storrar et al, 2014), or the availability of sediment (e.g., Burke et al, 2015).…”

Glacial erosion: status and outlook

“…Booth and Hallet, 1993; Kehew and others, 2012; Atkinson and others, 2013; Kearsey and others, 2018). Recent work on occurrence and causes of narrow canyons incised into the floors of glacial troughs (‘inner gorges’) in the Swiss Alps and Scandinavia also argues for vigorous and relatively extensive subglacial fluvial erosion (Dürst-Stucki and others, 2012; Jansen and others, 2014; Beaud and others, 2018). As additional geophysical and borehole data have been collected, buried inner gorges have been identified.…”