Underground thermo-erosion of ice wedges: numerical simulation of tunnel freeze- back

On Bylot Island in the Eastern Canadian Arctic archipelago, Fortier et al. (2007) observed and characterized the formation and development of tunnels initiated by the process of underground thermo-erosion of ice wedges networks. These tunnels often collapsed during the course of one or two summers and developed into gullies. However, observations of such tunnels in permafrost exposures indicate that they can be preserved in the permafrost record. The objective of this study is to estimate the freeze-back time of tunnels filled with water and slurry in cold and warm permafrost conditions. Ultimately, the goal is to evaluate time the tunnels remain "open"" for groundwater flow. We used numerical thermal modeling to conduct simple simulations of the conductive heat transfer during freeze-back of the tunnels. The thermal analyses were performed using the GeoslopeTM unsteady finite element heat conduction model TEMP/W. We used Bylot Island, Nunavut, Canada (Mean air temperature around -15 C) as a cold permafrost study case and Beaver Creek, Yukon Territory, Canada (Mean annual air temperature around -5.5C) as a warm permafrost study case. The air temperature was converted to ground surface temperature by the n-factor method. Zero heat flux was applied at the vertical and bottom boundaries due to the permafrost which is several tens to hundreds of meters thick. Based on previous studies, we simulated tunnels partly cut in ice-wedges and in the adjacent permafrost. The syngenetic permafrost of the case studies was assumed to be fully saturated with 110% gravimetric water content. The geometry of the tunnels was based on field measurements on Bylot Island. We considered three scenarios for the slurry filling the tunnels: 1) 100% water; 2) fully saturated sand with 30% gravimetric water content; and 3) an air layer at the top of the tunnel with water and saturated sands partly filling the bottom of the tunnel. We used three water/slurry temperatures: 1) 0.5C which simulates the water temperature of early snowmelt run-off, a period of active underground thermo- erosion; 2) 2C corresponding to the water temperature of run-off over a partly frozen active layer, which is typical for early summer undergrouns thermo-erosion ; 3) 5C and 15C corresponding to the water temperature lakes at the end of August on Bylot Island and at Beaver Creek respectively. This scenario simulates underground thermo-erosion due to lake drainage through ice wedges. The volumetric heat capacity of the ground was calculated as the sum of the volumetric heat capacities of the three phases multiplied by their volumetric fractions. The thermal conductivity of the permafrost and slurry was calculated by the geometric mean model. The apparent heat capacity method was applied to deal with latent heat generation. The initial permafrost temperature was -12C for the cold permafrost case and -2.5C for the warm permafrost case. The results suggest that thermo-erosion of ice wedges creates underground water flow paths that remain unfrozen for significant period of time, particularly in warm permafrost conditions. Tunnels entirely filled with water took about 1.5 to 2 times longer to freeze back. The presence of air pockets in the tunnel significantly delayed the freeze back time. In the cold permafrost cases, the tunnels froze back in about 3 to 6 months whereas they required between 8 months to about 3 years to freeze back in the warm permafrost cases.