Forecasting Coastal Geomorphic Evolution at Nelson Lagoon, AK, in Response to a Shifting Arctic Storm Regime

Contemporary warming in the Bering Sea is resulting in a decline in the extent and duration of sea ice, changing the impacts of extratropical cyclones as drivers of coastal change. This regional increase in open water days due to the decline in sea ice is escalating shoreline exposure to extreme cyclone events and magnifying rates of coastal erosion. Barrier beach systems are particularly susceptible to these events because instantaneous disturbances may exceed the resilience of the system, leading to barrier migration or in-place drowning. The implications of which are important to consider, given that there are numerous communities located on spits and barrier islands throughout western Alaska.

This research seeks to capture the local-scale coastal morphological responses to changing ocean conditions at Nelson Lagoon, located on the southern shores of the Bering Sea. Various monitoring methods have been established at Nelson Lagoon, including: repeat cross-shore GPS-collected elevation transects, Structure-from-Motion derived digital surface models, a bathymetric survey, time-lapse photography, and wave and water level time series.

Preliminary results have revealed sections of coastline proximal to the village with ~2.5 m/yr of lateral erosion (1957 to 2018) and up to 3 m of vertical erosion in some places (2014 to 2019). It was also found that ~0.4 km² worth of sediment had been accreted at the tip of the spit, lengthening it by over 1 km (1957 to 2018) at the same time the spit got narrower overall. This appears to be driven by alternating "pockets" of erosion and accretion on the seaward side of the spit, generally moving material parallel to the prevailing longshore current. Hydro-morphodynamic modelling (Delft 3D FM and XBeach) is under development for this site and will be applied to better understand the local wave climate and to forecast nearshore bathymetric and dune face evolution based on storm return interval analyses. The results of this ongoing work will contribute towards understanding how barrier beach systems in Bristol Bay are responding to shifting storm regime in addition to the loss of sea ice as protective buffers. The project will provide useful information for local decision making and mitigation efforts.