Vulnerability of a subarctic barrier spit to global warming induced changes in storm surge and wave runup: Shaktoolik, Alaska

The native Inupiaq community of Shaktoolik, in northwestern Alaska, is located on a low-lying barrier spit located on Norton Sound. The inhabited portion of the spit is 7.07 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 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 protecting the coastline from exposure during large storm events may result in the need to relocate many Native Alaskan coastal communities. 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. 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 inundation 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 calculated 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 inundation for each emissions scenario. When compared to historical total storm water levels, the results show that the moderate emissions scenario has a higher storm frequency and greatest change in return periods of inundation for storm surge. The 100-yr surge level is +6.26 m above MLLW for the moderate emissions scenario and +5.26 m historically. The high emissions scenario produces lower storm frequency than the moderate emissions scenario and the return periods of inundation for storm surge are similar to the historical values. When wave setup and runup are included in the total inundation levels for the moderate emissions scenario the results are similar to historical values, and when wave setup and runup are included in the high emissions scenario the total inundation levels are lower than historical values. The 100-yr flood level is +10.20 m above MLLW for the moderate emissions scenario and +8.62 m for the high emissions scenario, compared to +10.36 m historically. All three of these inundation levels overtop the highest ground elevation in the community.