Water Flows in Rock Wall Permafrost: A Numerical Approach Coupling Hydrological and Thermal Processes

Abstract

AbstractRock wall permafrost is highly sensitive to atmospheric warming. Its degradation is regarded as a primary factor accounting for the recent increase in periglacial rock slope failures. Investigations of rock wall permafrost have so far ignored hydrological processes acting in bedrock fractures. We propose a 2‐D coupled thermal and hydrological modeling approach applied to alpine rock wall permafrost. Simulations are carried out on the scale of the mountain flank for four case studies with variable saturation levels and hydrological forcings. Results show that cold water infiltration from the summit leads to a deeper permafrost body than with no water flows. Ice‐filled fractures first delay permafrost thawing due to latent heat consumption but then accelerate it when the ice starts to melt. Ice may thus subsist in fractures surrounded by thawed bedrock while thawing corridors may form in fractures embedded in frozen bedrock. As a result, temperature gradients are made steeper. When connected fractures thaw, bottom‐up permafrost degradation can occur through upward propagation of thawing wedges delineated by these fractures. High hydraulic head values are associated with the formation of perched water tables above or within the impermeable permafrost body, and correspond to hydrostatic pressures that could reach critical values in terms of rock wall stability. The proposed model setting is more appropriate for investigating long‐term permafrost changes, rock wall permafrost dynamics and hydrogeology across entire mountain flanks, but could be applied on a finer spatial scale to investigate short‐term permafrost response to climate signals and rock wall destabilization.