More realistic assessments of future exposure to high-tide flooding in American Samoa

Abstract

American Samoa is experiencing the fastest rates of relative sea-level rise due post-seismic subsidence resulting from the 2009 earthquake. American Samoa is already undergoing severe flooding impacts and requires a planning tool to strategically adapt to future flooding conditions. Here, we propose a more realistic assessment of future exposure to high-tide flooding (HTF) compared to the already available sea-level rise viewers by including site-specific land subsidence projections (Han et al., 2019), several high-tide flooding frequencies, and an updated Digital Elevation Model (DEM). While land subsidence projections reduce the timing uncertainties of sea-level rise exposure, the DEM and HTF frequencies enhance the overall spatial extent of the exposed area. Choosing a less frequent HTF frequency, e.g., 50-days per year, provides a better representation of the useable area throughout the century. On the other hand, the majority of the more realistic subsidence impacts is felt within the next 50 years because of the trend of the projections. The American Samoa relative sea-level rise viewer publicly available on the PacIOOS website provides a risk/tolerance assessment of HTF exposure with a selection of sea-level rise scenarios, and HTF frequencies throughout the century. However, still water level, e.g., sea-level rise is not the only contributor to flooding. Total water level including still water level and the effects of waves can induce more damages on the coastline. The co-variability of wave power and sea-level was evaluated here, during the seasonal cycle and the dominant mode of Pacific tropical variability, i.e., El Niño Southern Oscillations (ENSO). Our analysis suggests that waves and sea-level can be treated independently in American Samoa.