Evolution of Arctic continental shelves: modelling morphodynamic feedbacks to climate-driven increases in sea states

Arctic coastal environments, including the Alaskan Beaufort Shelf (ABS), are experiencing rapid declines in sea ice coverage and duration leading to increasingly energetic sea states. Observations and modelling show an acceleration of coastal retreat. Climate-change effects and coastal feedbacks with adjoining continental shelves are much less understood and data sparse. Here, we evaluated the morphologic response of the continental shelf to increasing wave energy and examined how a changing shelf profile modifies cross-shelf propagation of wave energy toward the coast, using a Delft3D sediment transport model. Based on available bathymetric, hydrodynamic, and sediment data, we modeled morphologic responses to wave growth on relatively steep (Flaxman Island, AK) and flat (Harrison Bay, AK) cross-shelf profiles. Simulations lasting 1000 years were conducted for both profiles to evaluate the impacts of the present-day wave climate, as well as a future Arctic wave climate projected under the RCP 8.5 climate-change scenario for the years 2081-2100. We find that morphologic evolution and regulation of future waves is mostly dependent on existing shelf morphology. On the steeper Flaxman Island profile, RCP 8.5 waves drive sediment erosion at 017 m water depth and redeposition at 1731 m water depth. Over 1000 years, this redistribution of sediment from the inner to middle shelf results in an 11.4% reduction in wave heights at the 2 m isobath. This morphologic adjustment represents a regulatory feedback in which shallowing of the middle shelf leads to attenuation of waves reaching the inner shelf. In contrast, Harrison Bay is a wider, shallower, and more gently sloping shelf, which is more effective at attenuating wave energy than the shelf at Flaxman Island and experiences little morphologic change given 1000 years of either present-day or RCP 8.5 waves. Contrasting feedbacks between waves, sediment transport, and shelf morphology on these two shelf profiles shows the importance of considering morphologic regulation of wave energy when projecting future coastal erosion in the Arctic.