Subglacial sediment production and snout marginal ice uplift during the late ablation season of a temperate valley glacier

Abstract

Sediment export from glaciated basins involves complex interactions between ice flow, basal erosion and sediment transfer in subglacial and proglacial streams. In particular, we know very little about the processes associated with sediment transfer by subglacial streams. The Haut Glacier d'Arolla (VS, Switzerland) was investigated during the summer melt season of 2015. LiDAR survey revealed positive surface changes in the ablation zone, indicating glacier uplift, at the end of the morning during the period of peak ablation. Instream measures of sediment transport showed that suspended load and bedload responded differently to diurnal flow variability. Suspended load depended on the availability of fine material whereas bedload depended mainly on the competence of the flow. Interpretation of these results allowed development of a conceptual model of subglacial sediment transport dynamics. It is based upon the mechanisms of clogging (deposition) and flushing (transport/erosion) in sub‐glacial channels as forced by diurnal flow variability. Through the melt season, the glacier hydrological response evolves from being buffered by glacier snow cover with a poorly developed subglacial drainage system to being dominated by more rapid ice melt with a more hydraulically efficient subglacial channel system. The resultant changes in the shape of diurnal discharge hydrographs, and notably higher peak flows and lower base flows, causes sediment transport to become discontinuous, with overnight clogging and late morning flushing of subglacial channels. Overnight clogging may be sufficient to reduce subglacial channel size, creating temporarily pressurized flow and lateral transfer of water away from the subglacial channels, leading to the late morning glacier surface uplift. However, without further data, we cannot exclude other hypotheses for the uplift. © 2018 John Wiley & Sons, Ltd.

Ecosystem engineers: Biofilms and the ontogeny of glacier floodplain ecosystems

“…Measurements close to glacier margins (Mao, Comiti, Carrillo, & Penna, 2019) show distinct timescales of variation in suspended sediment concentration (SSC)-discharge relations. At the daily timescale (Clifford, Richards, Brown, & Lane, 1995;Perolo et al, 2019), clockwise hysteresis can result because discharge forces shear stress to rise above the critical value needed for bed sediment transport and subglacial bed-accumulated fines are released. These become exhausted during the day, leading to lower concentrations on the falling limb.…”

“…Bedload transport appears to be controlled by discharge magnitude and variability. Perolo et al (2019) showed how late season bedload…”

“…Additionally, summertime has intense diurnal flow variations (Lane & Nienow, 2019;Nienow et al, 1998;Swift et al, 2005). The increase in peak daily discharge and in the intensity of diurnal discharge variation through the melt season, which also drives high rates of sediment delivery (Perolo et al, 2019), leads to the morphodynamically most active period. Not only may such activity erode the sediments to which biofilms are bound, the high sediment transport rate (Milner & Petts, 1994), and flow shear stresses (Cullis, Stanish, & McKnight, 2014) may scour biofilms from bed sediments.…”

A Numerical Model for Fluvial Transport of Subglacial Sediment

“…The relationship between sediment and water in the subglacial system has been used to interpret glaciological processes (e.g., Perolo et al, 2018) and has given insight into the availability of sediment beneath glaciers (e.g., Riihimaki et al, 2005;Swift et al, 2005;Willis et al, 1996). Therefore, describing the capacity of a model to capture these processes is useful in presenting its ability to capture real-world phenomena (Beaud et al, 2018).…”

“…Including feedbacks between hydraulics and subglacial sediment discharge would not only allow for some ad-hoc conditions to be removed (such as limiting the till height in equation (10b)) but could also provide further insight into the interactions between sediment and glaciers. These include the formation of eskers (e.g., Beaud et al, 2018;Hewitt & Creyts, 2019), subglacial hydrology (e.g., Walder & Fowler, 1994), diurnal glacier uplift (Perolo et al, 2018), the effect of till on glacier sliding (e.g., Truffer et al, 2001), processes in glacial overdeepenings (e.g., Cook & Swift, 2012;Creyts et al, 2013), and creation of sediment shoals in tidewater glaciers (e.g., Brinkerhoff et al, 2017).…”