Identifying spatial patterns of storm driven flooding and erosion at Nelson Lagoon, Alaska

Abstract

This project quantifies localized potential for shoreline change and flooding at Nelson Lagoon, a small fishing community located on the Bering Sea coast of the Alaska Peninsula. The overall goal of this project is to generate societally relevant and locally applicable map and data products through synergistic relationships with federal, state, private, tribal, and public partners. This project intends to substantiate anecdotal observations by local residents, with the ultimate goal of informing erosion and flooding mitigation efforts moving forward. Long-term trends of shoreline change were measured using multi-temporal orthorectified aerial imagery between 1983 and 2019, while annual changes in shoreline morphology were measured via cross-shore elevation profiles using a survey grade Real-time-Kinematic Global Navigational Satellite System (RTK-GNSS). Shoreline positions were extrapolated using linear regression techniques. A digital surface model (DSM) of the community was derived using Structure-from-Motion (SfM) with >2,400 aerial images collected with an Unmanned Aerial Vehicle (UAV) and used to assess flooding vulnerability after being geodetically referenced and related to a local tidal datum computed by this project. New and existing topographic and bathymetric datasets were compiled and refined into a 6,000 km² topobathymetric "seamless elevation" model of the Nelson Lagoon area, over which storm-tide induced currents were simulated using Delft3D FM Suite HMWQ. Remote sensing records indicate that the Nelson Lagoon spit elongated by more than 800 m and narrowed with an average Net Shoreline Movement (NSM) of -16.9 m between 1983 and 2019 (distal end not included). Though, NSM values show high variability ([sigma] = 21.9 m) and the lagoon and seaward sides of the spit are exhibiting very different erosional regimes. On both sides of the spit, episodes of rapid erosion mainly occurred during high storm-tide events that coincided with significant wave action. For this reason, the long-term erosion rates ultimately reflect the combined erosional impact of just a few storm events. Sand dunes in the supratidal zone on both sides of the spit are eroding at the vegetation line while the dune face retreats landward. By assessing erosion and flooding vulnerabilities for the Nelson Lagoon community, this study adds to an ever-growing database of such assessments statewide; which, ultimately, advance our understanding of regional coastal change in a shifting environment.