Water and Ice-related thermo-mechanical processes in the fractures of Steep alpine bedrock Permafrost – WISPER

The methods include numerical modelling, primarily coupling thermal and hydrological processes. New modelling procedures need to be developed to apply these models to high mountain walls. These new models must then be used to parameterise mechanical models. In addition, energy balance models need to be developed to quantify the water that can infiltrate the walls. Finally, in parallel with the numerical models, geophysics, and in particular geoelectrics, are used to acquire independent data to evaluate the models.

Given the limitations of numerical modelling programmes that combine heat and water transfers, unique instrumentation has been developed to monitor and characterise water flows in fractures and to ensure knowledge development on this subject.

The first coupled models of thermal and hydrological processes have been produced and published in the Journal of Geophysical Research: Earth Surface. The results show the ability to couple these processes using numerical models, and the thermal and mechanical disturbance of water flows in permafrost terrain.

However, their systematisation came up against a numerical bug that was only resolved at the end of the contract of the post-doc working on this issue.

A geoelectrical monitoring system was installed at the Aiguille du Midi and measured data at variable time intervals depending on the season and various hazards (rock falls, frost, damage to equipment, lightning). Analysis of the data acquired between 2019 and 2023 shows the ability of this method to account for changes in permafrost from sub-seasonal to multi-year time scales, in line with climate change. The method has also shown the effect of water circulation in highly fractured sectors. The study is currently being published in the journal The Cryosphere.

An energy balance and hydrological balance model was adapted to Alpine rock faces and demonstrated the variability of the hydrological balance as a function of altitude and exposure to solar radiation. The work was published in the journal ‘Earth Surface Dynamics’ and the model was applied to a case study of a rock avalanche showing the role of water infiltration in triggering this rock avalanche (study published in Earth Surface Landforms and Processes). A simplified version of the model was also applied to 209 rock avalanches to see the statistical distribution of wall temperature before the avalanches, revealing the role of exceptionally high temperatures and the role of permafrost degradation for events with a detachment depth > 4-6 m. The study was published in the journal Permafrost and Periglacial Processes.

Direct measurements of water flows in Alpine rock faces have been set up and provide data that is unique in the world, revealing the link between meteorological signals and the triggering of flows, the presence of meltwater from permafrost ice in the water collected, the role of active layer development and fracture connectivity. An article is being prepared to present these results.

This project has led to a number of projects that are now in the pipeline:

- the funding of 2 actions in the action plan for risks of glacial and periglacial origin (PAPROG) which will directly re-use the models developed or tested in WISPER. Thanks to the energy balance model, new maps of alpine permafrost in the French Alps can now be proposed, and although the difficulties encountered in applying heat and water transfer models have only made it possible to remove and resolve bugs in the WIPSER project, the planned systematisation of their use in the PAPROG project is now envisaged.

- a CIFRE thesis (currently being prepared) to develop permafrost diagnostic methods for operational purposes based on the models developed or tested in WISPER

- an ANR project as part of the PEPR IRiMA programme (letter of intent), which will create a unique database on permafrost and rock falls based on the models developed or tested in WISPER, in order to develop advanced mathematical analyses for decision support.

- The development of energy balance and hydrological models also opens up the possibility of studying alpine morphogenesis, in particular the development of scree slopes and the formation of rock glaciers by analysing freeze-thaw cycles and water availability.

4 communcations in international congresses, 2 publications into international journals with peer-review (1 being published, accepted in september 2021).

WISPER intends to link thermal, hydric and mechanical processes related to water circulation and icing in the fractures of permanently frozen (permafrost) rock walls to address, (i) permafrost degradation processes and (ii) the increasing rock fall hazards affecting mountain areas. Permafrost degradation is attributed to two main processes: (i) the slow heat conduction from the surface through the saturated rock media, and (ii) the water infiltration and circulation in the farctures acting as a shortcut between the surface and the subsurface. Frozen bedrock failures are attributed to two main processes: (i) the ductile-brittle rupture of warming ice-filled fractures, and (ii) the hydraulic-induced strains. While thermal and mechanical dynamics of saturated and homogeneous (no fractures) frozen rock walls have been investigated since about a decade, hydric processes remain the main barrier to lift to quantitatively interpret and predict permafrost degradation and failure mechanisms. These processes have been so far neglected by rock wall permafrost investigations because of (i) their complexity which involves non-linear thermal and mechanical patterns and (ii) the lack of available data to parameterize and evaluate numerical models.
WISPER will tackle this main scientific gap by implementing three work packages (WP) which involve development of innovative numerical modelling procedures and geophysical soundings. It will gather skills, tools and data from four institutes spread between France (EDYTEM Lab. and ISTerre Lab. from CNRS and Université Savoie Mont Blanc), Germany (Technical University of Munich) and Norway (University of Oslo). In the 1st WP (WP1), the unique dataset already collected in the Mont Blanc massif (notably high resolution digital elevation models, > 10 years of rock falls inventory, bedrock temperature, climate variables and fracture kinematics time series) will be used to develop hydro-thermal models, and provide relevant data to parameterize mechanical models in the 3rd WP (WP3). In parallel, cutting-edge geophysical measurements and monitoring will be performed on pilot-sites to image the fractures content and saturation (WP2) and to gain an independent dataset to parameterize and evaluate the numerical models intended in WP1 and WP3. The combination of the three WPs will allow a better theoretical understanding of the thermal dynamics and mechanical behavior of frozen rock walls.
About 260 k€ are demanded to cover non-permanent staff costs (Postdocs and Interns), numerical and geophysical equipment, travelling fees for results dissemination and field work, and publication costs. This project is based on already setup national and international collaborations, is aligned with the coordinator’s background, and will allow reinforcing her position as a leading scientist in high mountain permafrost researches. This project will deliver new numerical models, sensitivity analyses and ground properties images relevant for the research community focusing on steep slopes morphodynamics and cryospheric processes on the short to medium term. Furthermore, on the medium to long term, the delivered results will be a solid basis to draw research directions towards operational solutions to anticipate and mitigate the risks associated to deglaciating environments, which is of primary interest for land planners, policy makers and mountain professionals (e.g. mountain guides). The results will be disseminated via high-ranking international scientific journal, national and international scientific conferences, university courses, public media, and training of mountain professionals.