Erosion Constraints and Processes on Arctic Coasts

Shoreline exposure on many parts of the Arctic coast is limited by seasonal or multiyear sea ice. The duration and extent of open water determine the proportionate time exposure and may limit wave energy during that time. In the extreme, as on some islands of the northwestern Canadian Arctic Archipelago, year-round ice cover leads to near-complete elimination of wave activity. Sediment redistribution is severely limited and mostly effected by ice push on the resulting low-energy shorelines.

The cold climate is obviously critical to the erodi-bility of Arctic coasts, where freezing temperatures produce permafrost and ground ice on land in addition to ice cover on the ocean. In places such as the Beaufort Sea and Siberia, permafrost (defined as a condition of perennial ground temperature <0°C), with or without ice bonding or excess ground ice, extends beyond the coast under the seabed, with important implications for nearshore profile adjustment. In this case, erosion is not only a function of nearshore wave, current, and sediment dynamics, but may be limited by ice bonding at shallow depth below the nearshore seabed. Efforts to predict shoreline erosion in such cases have required the development of combined thermomechanical models of nearshore dynamics incorporating both wave-sediment interaction and thaw processes.

The most severe coastal erosion problems affecting communities, infrastructure, or cultural heritage resources in the Arctic occur where structures are built on ice-rich terrain adjacent to eroding shores. Exposure of excess ground ice in the shore zone or on backshore slopes leads to the formation of many of the most distinctive erosional features on Arctic coasts, generally through resettlement as volume is lost on outflow of meltwater.

Ice-wedge polygons (patterned ground formed when contraction and expansion of permafrost creates ice-filled cracks that meet in a geometric pattern to enclose a low or high central area) can facilitate large block failure through undercutting and thaw along ice wedges exposed in cliffs at the coast. In other situations, the development of high pore water pressure in the thin active layer (seasonal thaw horizon) near the ground surface can induce detachment slides or flows of the active layer, sometimes on very low-angle back-shore slopes. Ice bonding in otherwise unlithified sediments also enables the development of thermal niches (undercut notches) beneath cantilevered cliffs. Such niches may cut several meters in from the cliff face before fracturing occurs to produce collapse.

Perhaps the most spectacular erosion process associated with the presence of excess ground ice is retrogressive-thaw flow failure. This occurs where disturbance of an ice-rich slope exposes massive ground ice at the surface, leading to the development of a receding cliff face, usually some distance above sea level. The retrogressive failure may evolve into a growing amphitheater as the icy headwall expands and melts headward, losing volume through ice melt. Sediment liberated with meltwater from the receding cliff is transported downslope in persisting or recurring mudflows, often extending across a narrow beach into the nearshore. Retrogressive-thaw failures may exhibit cyclic patterns of expansion, stabilization, and later reactivation. In places, adjacent retrogressive failures may amalgamate as they expand to form long shore-parallel ice-rich cliffs, which can remain active for many years.

Direct and indirect effects of sea ice in the shore zone also play important roles in high-latitude coastal erosion. Indirect effects include enhanced seabed scour associated with ice wallowing in nearshore waves, as well as snowmelt drainage through holes in fast ice off river mouths, forming eddy or "strudel" scour pits on the seabed. The most obvious direct ice impact involves scour and push when a landward component of wind stress results in ice pressure against the coast and cross-shore or alongshore grounded ice motion. Depending on the shoreline morphology and internal stresses within the moving ice, this may result in ice rideup penetrating as much as 100 m or more onshore, or in ice buckling to form pileup ridges at the coast. Nearshore scour may be significant in such events. In places such as along the northwest margin of the Canadian Arctic Archipelago, persistent ice pressure forms prominent ice-pushed ridges up to 4 m or more in height. Ice rideup or pileup over coastal cliffs as high as 9 m is known from the Alaskan Beaufort Sea coast, where occasional deaths have been reported when ice buried human habitations. There is also evidence to suggest that ice scour on the shoreface may mobilize large volumes of sediment, playing a role in profile downcutting, thereby enhancing erosion at the shoreline.

Ice rafting of sediment is another important process contributing to coastal erosion in high latitudes. This may occur by adfreezing of seabed sediments onto the base of grounded ice, which subsequently floats away, a process that contributes to the common occurrence of boulder-strewn tidal flats in some Arctic settings. More significantly in some places, sediment can be entrained by frazil ice and anchor ice, formed during freezing storms with turbulent supercooling in the water column. It has been reported that sediment incorporated into mobile slush ice and ultimately into the winter ice canopy by this means may amount to more than 15 times the annual river input of sediment on parts of the Alaska coast and may ultimately be transported tens or hundreds of kilometers from the point of entrainment.

Given the range of distinctive and effective erosion processes on Arctic shores, it is not surprising that shoreline recession rates in some areas of seasonal open water on the Siberian, Alaskan, and western Canadian Arctic coasts (in some cases rates are as high as 20 m or more per year) are among the highest in the world, particularly if adjusted for the length of the erosion season. With projections of global warming at accelerated rates in high latitudes, some presently icebound shores may experience rapid readjustment if newly exposed to open water and waves. Other areas may see an extended open-water season (potentially exposing the coast to more storms) and increased open-water fetch (enabling the formation of larger and more damaging waves). With accelerated sea-level rise, higher ground temperatures (leading to more rapid thaw), and reduced sea ice duration and extent (leading to higher wave energy at the coast), there is reason to believe that rates of coastal erosion in some parts of the Arctic may increase significantly in the coming decades.

D.L. Forbes

See also Impacts of Climate Change; Permafrost Further Reading

Aré, F.E., "Thermal abrasion of sea coasts." Polar Geography and Geology, 12 (1988): 1-157 Forbes, D.L. & R.B. Taylor, "Ice in the shore zone and the geo-morphology of cold coasts." Progress in Physical Geography, 18 (1994): 59-89 Kobayashi, N., J.C. Vidrine, R.B. Nairn & S.M. Solomon, "Erosion of frozen cliffs due to storm surge on Beaufort Sea coast." Journal of Coastal Research, 15 (1999): 332-344 Rachold, V., M.N. Grigoryev, F.E. Are,S. Solomon, E. Reimnitz, H. Kassens & A. Antonov, "Coastal erosion vs. riverine sediment discharge in the Arctic shelf seas." International Journal of Earth Sciences (Geologisches Rundschau), 89 (2000): 450-460 Reimnitz, E. & P.W. Barnes, "Sea-ice influence on Arctic coastal retreat." Proceedings Coastal Sediments 87, New Orleans, New York: American Society of Civil Engineers, 1987, pp. 1578-1591 Reimnitz, E. & E.W. Kempema, "Field observations of slush ice generated during freeze-up in Arctic coastal waters." Marine Geology, 77 (1987): 219-231 Trenhaile, A.S.,"Coasts in Cold Environments." In Coastal Dynamics and Landforms, Chapter 12, Oxford: Clarendon Press, 1997, pp. 290-309

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