Multi-scale variations of hydro-mechanical conditions at the base of the surge-type glacier Kongsvegen, Svalbard
Authors: Bouchayer Coline , Nanni Ugo , Lefeuvre Pierre-Marie , Hulth John , Steffensen Schmidt Louise , Kohler Jack , Renard François , Schuler Thomas V. 
Published: April 2023

Abstract. Fast glacier flow and dynamic instabilities, such as surges, are primarily caused by changes at the ice-bed interface, where basal slip and sediment deformation drive basal glacier motion. Determining subglacial conditions and their responses to hydraulic forcing (e.g. rainfall, surface melt) remains challenging due to the difficulty of accessing the glacier bed. In this study, we monitor the interplay between surface runoff and hydro-mechanical conditions at the base of the Arctic surge-type glacier Kongsvegen, in Svalbard, over two contrasting melt seasons. Kongsvegen last surged in 1948, after which it entered a prolonged quiescent phase. Around 2014, flow speeds began to increase, sign of an imminent new fast-flow event. In 2021 we instrumented a borehole to assess subglacial conditions at the local scale and deployed seismometers to monitor the subglacial conditions at the kilometer scale. We measure both subglacial water pressure within the borehole with a water pressure sensor and till rheology with a ploughmeter inserted into the sediments at the bottom of the borehole. We use channel-flow-induced tremors recorded by a seismometer to characterize hydraulic conditions over a kilometre scale at the base of the glacier. The records cover the period from spring 2021 until summer 2022. To characterize the variations in the subglacial conditions caused by changes in surface runoff, we investigate the phase relationship (i.e. how two variables evolve in time) of the following hydro-mechanical condition proxies: water pressure, hydraulic gradient, hydraulic radius, and sediment ploughing forces. We analyse these proxies versus modelled runoff analyzed over seasonal, multi-day and diurnal time-scales. We compare our results with existing theories in terms of subglacial drainage system evolution and sediment shear strength to describe various aspects of subglacial conditions. We find apparent ambiguities in the interpretation of different variables recorded by individual sensors, thus demonstrating the importance of using multi-sensor records in a multi-scale analysis. This study highlights the different adaption of the subglacial drainage system during short, low melt intensity season in 2021, against long, high intensity melt season in 2022. In the short and low intensity melt season, we find that the subglacial drainage system evolves at equilibrium with runoff, increasing its capacity as the melt season progresses. In contrast, during the long and high intensity melt season 2022, we find that the subglacial drainage system evolves transiently to respond to the abrupt and high intensity input of precipitation and melt water conveyed to the bed. In this configuration, the subglacial channels evolution is not rapid enough to adapt immediately to the forcing conditions. The drainage capacity of the main active channels is exceeded, promoting the water to leak in poorly connected areas of the bed, increasing the water pressure, resulting in speed-up events. Another robust outcome of our analysis is, that, on a seasonal scale, till shear strength variations are mainly anti-correlated with water pressure variations (consistent with a Coulomb-plastic behavior), whereas on shorter time scales especially during speed-up events, the two variables correlate, describing a viscous rheology. To our knowledge, such contrasted behaviors of the sediment rheology and subglacial flow at the base of a glacier have not been reported before.