PORE-WATER PRESSURE MONITORING IN ICE-RICH PERMAFROST
Mountain permafrost and rock glaciers
Permafrost refers to ground material, such as rock or loose rock, that has a permanent temperature of 0°C or below. In summer, the area near to the surface, the so-called active layer, thaws. Permafrost covers roughly four per cent of Switzerland’s land area and is most predominantly found at elevations over around 2,500 m above sea level. In mountainous regions, there are two different main types of permafrost:
- Ice-poor permafrost in rock faces, where ice is only found in the cracks, crevices, fissures and pores of the frozen rock
- Ice-rich permafrost at the foot of steep slopes, where the deposits of mass movements (avalanches and rock falls) accumulate and over many years form a substrate that is oversaturated with ice and that contains more ice than rock material.
In this study we focus on rock glaciers (Fig. 1a). They are characteristic features of ice-rich mountain permafrost. They are tongue-shaped landforms consisting of ice and rock layers that move downhill at a pace of a few centimetres to several metres per year. They transport loose rock material downwards like conveyor belts and their steep frontal lobes are potential starting zones for rock falls and debris flows.
Figure 1: a) Drone image of the Schafberg Ursina rock glacier, Pontresina, Switzerland (photo: A. Bast). The tongue-shaped, structure with its coarse, blocky surface, its steep frontal lobes with strongly sloping edges, and pronounced ridges and hollows is easily visible. The white arrow indicates the position of the borehole with the piezometers. b) Borehole drilling on the rock glacier in August 2020 (photo: N. Bühler). c) KELLER Pressure PAA-36XiW piezometer with textile protection (photo: M. Phillips).
Rock glacier movements and water
A widespread acceleration of rock glaciers is being recorded in the Alps, increasing the likelihood of mass movements such as debris flows from their frontal lobes in steep terrain. The acceleration can be traced back to climate change and the associated warming of the permafrost, which is accompanied by an increase in the water contents of ice-rich permafrost.
The ice and water contents of rock glaciers have previously been modelled to investigate the future availability of water. To investigate rock glacier movements, their hydrology is examined, and aerial photographs, in-situ GNSS data (global navigation satellite system), meteorological data and snow cover timing are analyzed. Additionally, the water outflow from rock glaciers has been quantified and their potential water storage capacity assessed.
Up until 2020 there were no direct measurements of water in permafrost. Yet it is exactly this direct information about rock glacier hydrology, about changes to the ice and water contents and about the formation of unfrozen zones, so-called taliks, that is required before we can gain a better understanding of rock glaciers and their movements. Depending on the soil properties, salinity and pressure, a considerable proportion of unfrozen water can exist well below 0°C, as was shown, for example, by the cross-hole georadar measurements taken by Musil (2006) in the Muragl rock glacier, Oberengadin, Switzerland.
New measurement method for recording rock glacier hydrology
Many rock glaciers are near to the melting point of ice, and borehole temperature data alone cannot differentiate between ice and water as both can coexist at temperatures near 0°C. This means that relative changes in the ice/water contents must be monitored using other methods such as geophysical methods and/or piezometer measurements. This is important, as water contents in permafrost are partly responsible for how quickly ice-rich permafrost moves.
In summer 2020, we drilled three boreholes in the rock glacier Schafberg Ursina, north of Pontresina, Oberengadin in the Eastern Swiss Alps (Fig. 1 a and b). One of the boreholes was equipped with ten KELLER Pressure PAA-36iW piezometers at depths between 2 and 8.5 m (Fig. 1c). In the other two boreholes, multi-core cables were installed to enable cross-borehole electrical resistivity tomography (ERT). Based on the ERT data, resistivity models for the ground can be created, providing information about relative changes in water and ice content and supplementing the piezometer data.
The piezometer data indicate the development of the effective pressure as measured at the sensor diaphragm (measured relative to a vacuum; pressure range 60-230 kPa, accuracy ±11.5 kPa). The sensors were combined with ten PT 1000 temperature sensors (accuracy ±0.1°C). Before installation, the sensors were coated with Vaseline and wrapped in a thin material (face masks) for protection. The sensors are connected to two ARC-1 boxes with 4G data loggers, which also contain a barometer. Data is collected hourly and transmitted to a cloud-based data platform daily via a mobile phone network.
Encouraging results
Pore-water pressure sensors have never been used before in an ice-rich rock glacier. The initial results are encouraging. There are clear indications of the presence of water in the ice-rich rock glacier, which is confirmed by the stratigraphy of the boreholes registered during drilling in August 2020.
Figure 2: a) Average daily piezometric pressure between 2.0 and 8.5 m depth (January 2021 to June 2023). The thin black lines represent the isobars at 1.0, 1.5 and 2.0 bar. The blue and red lines and their accompanying values represent the isotherms of 0°C, -1°C and -2°C (data: WSL Institute for Snow and Avalanche Research SLF, modified in accordance with Bast et al. 2024). b) Borehole stratigraphy in August 2020 and position of the KELLER Pressure piezometer in the borehole (blue dots; GOF: ground surface).
Between 2021 and 2023, the water contents decreased due to low amounts of rainfall and the cooling of the ground as a result of two snow-poor winters (Fig. 2a and b). The lower temperatures and dry conditions meant that the rock glacier was able to cool down and freeze efficiently, which also significantly slowed the speed of the rock glacier. At certain depths, water contents rise during snowmelt or after intense rainfall, which points to a lateral flow of water in the rock glacier. The piezometer data agree with the ERT measurements.
Experiments in the cold lab
As piezometers have not previously been used in mountain permafrost boreholes, the data should be interpreted with caution. If ground temperature drops below 0°C, ice formation might strongly affect the pressure measured in the sensor’s housing and thereby not fully represent the dominant pressure condition at a given depth. Harris and Davies (1998) faced similar problems with their laboratory experiments. Other experiments are currently being carried out under controlled conditions in the SLF’s cold laboratory to determine the behaviour of KELLER Pressure PAA-36XiW sensors at different ground ice and water contents. However, the piezometers in the Schafberg ice-rich rock glacier are delivering meaningful data and indicate the presence of air, water and/or ice as well as seasonal pressure variations in wet layers. All the data presented here highlight the heterogeneous and seasonally variable nature of the substrate, as was revealed by the contrasting borehole stratigraphies.
Sensors in challenging environments
The biggest technical challenges involved in installing the sensors were:
- Borehole walls collapsing between the extraction of the drill head and insertion of the sensors, requiring the use of a stabilising PVC tube in the upper 4 m of the ground.
- Establishing effective contact between the sensors and the borehole walls.
The material used to fill the borehole is different to the original rock glacier substrate. It is not known whether the filling material later settled, whether the sensors were effectively enclosed during filling or whether there were any hollow spaces. Rock glaciers are dynamic landforms. Processes like subsidence, creep or changes to the ice/water contents can represent a challenge for the long-term preservation of sensors in tubeless holes.
Conclusion
Initial analyses show daily but also seasonal to long-term changes to the water contents in rock glaciers.
- This information will contribute towards closing the gap in terms of directly quantifying the water content of rock glaciers and achieving a better understanding of the causes of rock glacier movements.
Piezometer data provides valuable information on local substrate characteristics of rock glaciers. That data contributes to our understanding of the factors that determine the kinematic acceleration of rock glaciers and the future water availability in these landforms.
Sources
Bast, A., Kenner, R., and Phillips, M.: Short-term cooling, drying and deceleration of an ice-rich rock glacier, EGUsphere, 2024, 1-26, 10.5194/egusphere-2024-269, 2024.
Harris, C., and Davis, M.C.R.: Pressures recorded during laboratory freezing and thawing of a natural silt-rich soil, Proceedings 7th International Conference on Permafrost, Yellowknife, Canada, Collection Nordicana 55, 433-439, 1276, 1998.
Musil, M., Maurer, H., Hollinger, K., and Green, A. G.: Internal structure of an alpine rock glacier based on crosshole georadar traveltimes and amplitudes, Geophys Prospect, 54, 273-285, DOI 10.1111/j.1365-2478.2006.00534.x, 2006.
Phillips, M., Buchli, C., Weber, S., Boaga, J., Pavoni, M., and Bast, A.: Brief communication: Combining borehole temperature, borehole piezometer and cross-borehole electrical resistivity tomography measurements to investigate seasonal changes in ice-rich mountain permafrost, The Cryosphere, 17, 753-760, 10.5194/tc-17-753-2023, 2023.
For further references and sources, we refer to the two publications underlying the article by Phillips et al. (2023) and Bast et al. (2024).
