UC Santa Cruz

The native Inupiaq community of Shaktoolik, in northwestern Alaska, is located on a low-lying barrier spit in Norton Sound. The inhabited portion of the spit is 7.1 m above MLLW at its highest and only ~200 m across. The community is vulnerable to marine flooding on both the open ocean and lagoon sides of the spit during large storms. Storm events in this region typically occur during the fall and winter months, often when the coastline is protected from flooding and erosion by shorefast ice. High latitudes are experiencing the greatest increases in temperature due to global warming and the reduced duration and extent of sea ice is affecting Alaskan coastal communities. Continued reduction of sea ice, which typically protects the coastline from exposure during large storm events in late fall and winter, may result in the need to relocate many Native Alaskan coastal settlements.

The goal of this study was to quantify changes in storm surge and wave runup during ice-free months for the mid- to late-21st century. A combination of field data, including beach profiles, offshore bathymetry, debris line position, and sediment grain size, as well as modeled meteorological data were used to address the research goals. An analytical approach was developed to quantify storm surge in Alaskan coastal communities with historical meteorological data from the North American Regional Reanalysis. This analytical model was used to calculate projected storm surge flooding levels in Shaktoolik for the mid- and late-21st century for both a moderate and high greenhouse gas emissions scenario (RCP 4.5 and RCP 8.5 respectively), with meteorological output from the MIROC5 global climate model. Additionally, projected wave runup heights during storm events were determined numerically using WAVEWATCH III to calculate wave height and period from the projected MIROC5 meteorological data and SBEACH to model the maximum runup heights along the Shaktoolik shoreline.

Total storm water levels (storm surge plus wave runup height) were calculated for 236 projected storm events in the Bering Sea and used to find the return periods of flooding for each emissions scenario. When compared to modeled historic storm surge, the results show that the moderate emissions scenario is similar to the historical. The 100-yr surge level is +6.3 m above MLLW for the moderate emissions scenario and +6.1 m historically. The high emissions scenario produces lower storm frequency than the moderate emissions scenario and the return periods of flooding for storm surge are lower than the historical values, the 100-yr surge level is +5.0 m. When wave setup and runup are included in the total flood levels the 100-yr flood level is +10.2 m above MLLW for the moderate emissions scenario and +8.7 m for the high emissions scenario, compared to +10.4 m historically. All three of these flooding levels overtop the highest ground elevation in the community.