60 Water / Wastewater
MEASURING MOUNTAIN PERMAFROST USING BOREHOLE PIEZOMETER DATA
© WSL-Institut für Schnee- und Lawinenforschung SLF
Up until now, researchers have not been able to research seasonal changes in ice- rich mountain permafrost, or have only been able to do so in imprecise terms. The Schafberg rock glacier in the eastern Swiss Alps is being monitored to a depth of 12 metres using a new combination of borehole temperature, borehole piezometer and cross-borehole electrical resistivity tomography (ERT) data in order to give researchers a better understanding of accelerating rock glacier kinematics and future water availability.
A
n acceleration of ice-rich rock glaciers is being recorded in the Alps, caused by global warming and increasing water
content. This widespread acceleration increases the likelihood of mass movements, such as debris fl ows.
At the moment, there is little information available about the internal hydrology of rock glaciers, whilst borehole temperature data does not enable a differentiation between ice and water. Many Alpine rock glaciers are close to their melting points. Depending on the soil properties, salinity and pore- water pressure, then, a substantial unfrozen water content can persist well below 0°C. Although pore-water pressure is routinely monitored in other environments, up until this point no piezometer data has been obtained in ice-rich mountain permafrost. Now the WSL Institute for Snow and Avalanche Research SLF can present the fi rst data snapshots resulting from a novel combination of borehole temperature, borehole piezometer and cross-borehole electrical resistivity tomography (ERT) measurements. This new method is designed to investigate changes occurring in ice-rich rock glaciers - particularly changes in the ice-to-water ratio - through continuous measurements.
Two KELLER ARC-1 boxes and 4G data loggers, wich contain a barometer. © WSL-Institut für Schnee- und Lawinenforschung SLF
Location and methods
The measurement site is located on the ice-rich Schafberg rock glacier, 2750 m above sea level and above Pontresina in the Engadine in the eastern Swiss Alps. In August 2020, three additional vertical bores were drilled into the existing temperature sensor bore, allowing the new sensors to be inserted. The boreholes were fi lled with a sand-gravel mixture in order to establish contact between the sensors and the borehole walls, and to minimise air circulation. At ground surface level, the boreholes and instrument boxes are protected by concrete chambers with iron lids.
Borehole B5 is located 10 m north-west of B3 and was equipped with 10 KELLER PAA-36iW piezometers. The piezometer data indicate the development over time of the effective pressure as measured at the sensor diaphragm (measured relative to a vacuum; pressure range 60-230 kPa, accuracy ±1.7 kPa), combined with 10 PT1000 temperature sensors (accuracy ±0.1°C) between 2.0 and 8.5 m in depth. To protect against glaciation, the sensors were coated with Vaseline and wrapped in a thin material (Fig. b). They are connected to two KELLER ARC-1 boxes and 4G data loggers, which contain a barometer. Data is collected hourly and transmitted to a cloud-based data platform daily via a mobile phone network.
First results
The multi-method approach presented here is an innovative combination of techniques for ice-rich rock glaciers and has delivered some encouraging initial results.
Figure (a) Drill and protective PVC pipe used to drill boreholes B3, B4 and B5 on Schafberg rock glacier in August 2020. (b) Piezometer and temperature sensors ready to be installed in B5. (c) Stainless steel ring mounted on the cross-borehole ERT electrode to improve contact with the ground. (d) Cross- borehole ERT logging system for B3 and B4 in a concrete chamber.
IET ANNUAL BUYERS’ GUIDE 2023/24 © WSL-Institut für Schnee- und Lawinenforschung SLF
The temperature data collected from boreholes B1 and B5 are almost identical in summer; however, they differ signifi cantly in winter in the active layer, which indicates local differences in
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