Changes in Arctic Coastal Erosion Mechanism and Implications for Coastal Resilience
The Arctic coast is subject to several interdependent hazards which are exacerbated by the rapid warming of the environment. These include the thawing of coastal permafrost and subsidence, increased wave action, increased storm surge and coastal flooding, salinity intrusion, and coastal erosion. Arctic coastal processes are distinct due to the predominance of both thermal and mechanical processes. Coastal sediments are locked in place by permafrost or seasonal ice and thawing of that permafrost/ice is a prerequisite for later mechanical removal.
Two coastal erosion mechanisms are predominant along the Alaska Arctic coast: thermal denudation and thermal abrasion. Thermal denudation (also called bluff face thaw/slump) is a relatively slow two-step erosion mechanism driven by atmospheric heat transfer. In the first step, solar radiation and other heat fluxes thaw the face of the bluff leading to slumping and deposition on the beach face. In the second step, aggressive waves and currents transport beach and bluff sediment offshore during storm surge events. The majority of the coastal bluffs in Arctic Alaska are suffering from thermal denudation and erosion rates are on the order of 1-2 m/year.
Thermal abrasion (also called niche erosion / block collapse) is a relatively rapid erosion mechanism with erosion rates up to 20 m/year. In contrast to thermal denudation, thermal abrasion is driven by oceanic heat transfer. Thermal abrasion is predominant in locations lacking significant quantities of coarse sediments (sand and gravel), and lacking a high elevation beach. Thermal abrasion proceeds via a 4-step erosion mechanism. First, a storm surge event raises the elevation of the coastal waters enabling direct contact between the sea and the base of the bluff. Second, waves and currents thermally and mechanically cut a niche at the base of the bluff. Third, the undermined bluff suffers block collapse due to an overturning failure. Fourth, the collapse block is thermally and mechanically eroded.
There is evidence that some locations (e.g., Barter Island, Alaska) have recently experienced erosion mechanism “flipping” from the slower thermal denudation to the more rapid thermal abrasion. Preliminary calculations suggest that the flipping may have resulted from the warming of the environment. Between 1980 and 2020, the open water period increased from 40 days to 140 days, and the coast has been subject to an increasingly hazardous wave and storm surge climate. Barter Island locations that experienced thermal denudation and slow erosion rates prior to 2007 abruptly started experiencing rapid erosion rates (of order 10 m/year). We hypothesize that the flipping of the erosion mechanism resulted from a decadal lowering of the elevation of the beach face.
The erosion flipping is likely to have significant impacts on coastal resilience. Certainly, coastal communities and infrastructure will be more vulnerable with a more rapidly erosion coastline. Further, mitigating erosion from thermal abrasion driven by oceanic heat transfer may be more challenging than mitigating erosion from denudation.
