Efficacy of bedrock erosion by subglacial water flow

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.

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

“…Beaud et al () developed a model of bedrock erosion by subglacial water flow using a fluvial abrasion law (Lamb, Dietrich, & Sklar, ) in a supply‐limited system. They showed that water flow‐induced abrasion rates are negligibly small at the glacier scale, compared to expected ice flow‐induced erosion rates.…”

Paleofluvial and subglacial channel networks beneath Humboldt Glacier, Greenland

“…Subglacial drainage systems modulate ice flow (Iken and Bindschadler, 1986) and the pattern of glacial erosion (Beaud et al, 2016). The seasonal evolution and distribution of channels cut into ice (Röthlisberger channels) and their influence on ice flow (e.g., Cowton et al, 2013) is well studied.…”

The geometry and complexity of spatial patterns of terrestrial channel networks: Distinctive fingerprints of erosional regimes

“…Predicted values of the aspect ratio decrease with increasing θ t . However, typical values of the Shields parameter in the subglacial channel environment are as yet unknown [ Beaud et al , ], although the high hydraulic pressures [>5 MPa, Walder and Fowler , ] suggest larger values than is typical for overland flows, leading to a decrease of R in comparison with fluvial systems. There is a good agreement of the model with the data, for all values of f m , at higher θ t values than those found typically in fluvial systems.…”

“…Obtaining predictions for the minimum channel width and tributary junction angle is also challenging. In particular, deriving a successful model for the minimum channel width can prove problematic, given that the models of subglacial erosion and landscape evolution are mostly underdeveloped [ Beaud et al , ] and require a constraint on the overlying ice thickness. Although the model for the aspect ratio in equation is consistent with the data, the proportionality between R and Δ h t highlights the sensitive dependence on the lateral variations of ice thickness.…”

“…To our knowledge, there is no comprehensive study on the erosion by subglacial channels that develops an appropriate landscape evolution equation [ Beaud et al , ] or gives predictions for the minimum channel width. Understanding the processes that control the junction angles requires a model for the topography of the overlying ice sheet and the pressure gradient distribution.…”