Shoreline Thermokarst

Thermokarst activity along the shorelines of rivers, lakes and seas involves thermal erosion and thermokarst subsidence. Thermal erosion at shorelines that dissect ice-rich unconsolidated sediments causes undercutting and rapid bank retreat. Undercutting by waves and currents excavates a horizontal cleft (thermo-erosional niche) that may extend 10 m or more laterally into the bank, at about water level (Fig. 13.4). Above the niche, the undermined permafrost episodically collapses in large blocks, often along ice wedges. Such erosion is common in the very ice-rich permafrost fringing the Arctic Ocean, particularly around the Laptev Sea, where mean retreat rates due to thermal erosion of the ice complex are 2-6 m per year (Are 1983).

Retreat rates show high spatial and interannual variability. For example, on the Lena River, northern Siberia, retreat exceptionally reaches 19-24 m per year, or even 40 m per year (Are 1983). On the Colville River, northern Alaska, the long-term retreat rates rarely exceed 3 m per year, although block collapse can generate an almost instantaneous retreat as much as 12 m, protecting the bank from further retreat for periods of up to a few years (Walker et al. 1987). Numerical analysis and experimental simulation of fluvial thermal erosion suggest that exceptionally high retreat rates reflect a combination of high water temperatures and river discharge, in association with some particular channel geometry (Costard et al. 2003). Along coasts exposing ice-rich permafrost, exceptionally

Fig. 13.4 Thermo-erosional niche developed in massive ice beneath the floor of a retrogressive thaw slump along the Beaufort Sea coast at North Head, Richards Island, Tuktoyaktuk Coastlands, Canada. Spade for scale

high retreat rates result from storm events. For example, a maximum rate of 19 m per year estimated during a stormy year contrasts with a long-term rate of 1.9 m per year for the same coastal segment of the Beaufort Sea coast, NWT (Dallimore et al. 1996).

Thermokarst subsidence occurs along coastal margins where excess ice in subsea permafrost thaws beneath the seabed. For example, where the warm waters of the Mackenzie River enter the Beaufort Sea, sea-bottom temperatures of ~2°C exist year-round in water depths shallower than ~10 m and deeper than the zone where sea ice freezes to the seabed (Rachold et al. 2000). Thus, ice-bonded permafrost can degrade continuously in a narrow coastal band. Rates of seabed subsidence of 5-7 mm per year are estimated along parts of the Canadian Beaufort Sea Shelf.

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