Efficacy of bedrock erosion by subglacial water flow

F Beaud,G E Flowers,J G Venditti

doi:10.5194/esurf-4-125-2016

Abstract

Abstract. Bedrock erosion by sediment-bearing subglacial water remains little-studied; however, the process is thought to contribute to bedrock erosion rates in glaciated landscapes and is implicated in the excavation of tunnel valleys and the incision of inner gorges. We adapt physics-based models of fluvial abrasion to the subglacial environment, assembling the first model designed to quantify bedrock erosion caused by transient subglacial water flow. The subglacial drainage model consists of a one-dimensional network of cavities dynamically coupled to one or several Röthlisberger channels (R-channels). The bedrock erosion model is based on the tools and cover effect, whereby particles entrained by the flow impact exposed bedrock. We explore the dependency of glacial meltwater erosion on the structure and magnitude of water input to the system, the ice geometry, and the sediment supply. We find that erosion is not a function of water discharge alone, but also depends on channel size, water pressure, and sediment supply, as in fluvial systems. Modelled glacial meltwater erosion rates are 1 to 2 orders of magnitude lower than the expected rates of total glacial erosion required to produce the sediment supply rates we impose, suggesting that glacial meltwater erosion is negligible at the basin scale. Nevertheless, due to the extreme localization of glacial meltwater erosion (at the base of R-channels), this process can carve bedrock (Nye) channels. In fact, our simulations suggest that the incision of bedrock channels several centimetres deep and a few metres wide can occur in a single year. Modelled incision rates indicate that subglacial water flow can gradually carve a tunnel valley and enhance the relief or even initiate the carving of an inner gorge.

Highlights

Textbook descriptions of glacial erosion detail mechanisms of abrasion and quarrying, but mention erosion by subglacial meltwater as a potential, unquantified, additional incision mechanism (e.g. Bennett and Glasser, 2009; Anderson and Anderson, 2010)
Most studies of glacial erosion that use measurements of proglacial sediment yield rely on the hypothesis that subglacial meltwater flow is the most important process removing sediment from the glacier bed (e.g. Gurnell et al, 1996; Koppes and Hallet, 2002, 2006; Orwin and Smart, 2004; Riihimaki et al, 2005)
We need to account for these changes when displaying the results, and introduce three quantities: the total transport capacity Qtc = qtcWch (m3 s−1), the total erosion computed with the Total load erosion model (TLEM) Etot = etotWch (m2 s−1), www.earth-surf-dynam.net/4/125/2016/

Summary

Introduction

Textbook descriptions of glacial erosion detail mechanisms of abrasion and quarrying, but mention erosion by subglacial meltwater as a potential, unquantified, additional incision mechanism (e.g. Bennett and Glasser, 2009; Anderson and Anderson, 2010). Subglacial meltwater loaded with sediment has been inferred to carve metre-scale channels in bedrock Humphrey and Raymond, 1994; Gurnell et al, 1996; Hallet et al, 1996; Willis et al, 1996; Alley et al, 1997; Koppes and Hallet, 2002, 2006; Riihimaki et al, 2005; Koppes and Montgomery, 2009), there has been little work quantifying subglacial sediment transport (Creyts et al, 2013), and no work on bedrock erosion by subglacial meltwater. Most studies of glacial erosion that use measurements of proglacial sediment yield rely on the hypothesis that subglacial meltwater flow is the most important process removing sediment from the glacier bed Most studies of glacial erosion that use measurements of proglacial sediment yield rely on the hypothesis that subglacial meltwater flow is the most important process removing sediment from the glacier bed (e.g. Gurnell et al, 1996; Koppes and Hallet, 2002, 2006; Orwin and Smart, 2004; Riihimaki et al, 2005).

Results

Discussion

Conclusion