Retrogressive Thaw Slumping

Retrogressive thaw slumping is a slope failure characterized by thaw of exposed ground ice and slumping of thawed soil. Slumping usually starts where ice-rich permafrost is exposed by erosion, mass movement, forest fires, construction or mining (Burn and Lewkowicz 1990). Where the exposure reveals massive ice, large ice wedges or dense concentrations of segregated ice, slumping may quickly enlarge it to produce a steep or vertical headwall (1 m to > 15 m high) that overlooks a low-gradient floor covered by slumped soil.

Headwall ablation occurs mainly by radiation and sensible heat transfer, and often leads to rapid slope retreat. Net radiation is dominant in some High Arctic slumps, but sensible heat transfer is more important in warmer permafrost regions (Lewkowicz 1988). Retreat rates depend on atmospheric conditions and ground-ice concentration. Rapid ablation is favoured by clear, warm and windy conditions, when radiative inputs and turbulent transfer of heat to the ice are high, and during rainstorms, which wash thawed soil from the thaw face. Rates of headwall retreat often reach several metres per year, with rates as high as 16 m per year and 23 mm h-1 measured in central Yukon (Burn and Lewkowicz 1990).

Permafrost degradation beneath slump floors occurs by heat conduction or convection. In the boreal forest near Mayo, central Yukon, Burn (2000) measured increases in ground temperature with time, and increased depths to permafrost with distance from a slump headwall. Permafrost degradation between 1949 and 1995 resulted from surface disturbance by slumping, which raised mean annual ground temperature (MAGT) by ~3-4°C at 1 m depth beneath the slump floor. As permafrost degraded — primarily by conductive heat flow in fine-grained soil — the active layer thickened to > 4.8 m. Where permafrost had degraded longest and reached a depth of 7 m or more, a residual thaw (unfrozen) layer developed above the permafrost and beneath the depth of seasonal frost penetration. Where slump-floor sediments are sandy and permeable, as on Summer Island, NWT (Murton 2001), convective heat flow from percolating groundwater probably contributes to degradation.

Thaw slumps eventually stabilize. Stabilization results when all of the excess ice has melted, where slumped soil insulates the headwall, or where the slope gradient above the headwall is less than that of the slump-floor deposits, which therefore bury the excess ice. The duration of thaw slumping varies from a single summer to several decades or more (e.g., Lewkowicz 1987; Burn 2000). After slumps stabilize, permafrost may re-aggrade beneath the slump floor and vegetation re-establish. Near Mayo, re-establishment of a birch/white spruce sere similar to that of the original boreal forest takes ~35-50 years after slumping (Burn and Friele 1989). Cycles of slumping and stability may occur where erosion episodically removes slumped debris.

Soil and organic material fall, slide or flow from ablating headwalls onto the slump floor, where they are often reworked by debris flows or meltwater. On eastern Banks Island, debris-flow morphology, size and activity are largely determined by the liquid limit, permeability and water content of the thawed soil (French

1974). The resulting debris-flow deposits usually comprise a mixture of soil, peat and vegetation (Murton 2001).

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