U.S. Geological Survey - (Abstract) Forecasting the Impact of Storm Waves and SeaLevel Rise on Midway Atoll and Laysan Island within the Papahānaumokuākea Marine National Monument—A Comparison of Pa**ive Versus Dynamic Inund lyrics

Two inundation events in 2011 underscored the potential for elevated water levels to damage infrastructure and affect terrestrial ecosystems on the low-lying Northwestern Hawaiian Islands in the Papahānaumokuākea Marine National Monument. The goal of this study was to compare pa**ive “bathtub” inundation models based on geographic information systems (GIS) to those that include dynamic water levels caused by wave-induced set-up and run-up for two endmember island morphologies: Midway, a cla**ic atoll with islands on the shallow (2–8 m) atoll rim and a deep, central lagoon; and Laysan, which is characterized by a deep (20–30 m) atoll rim and an island at the center of the atoll. Vulnerability to elevated water levels was a**essed using hindcast wind and wave data to drive coupled physics-based numerical wave, current, and waterlevel models for the atolls. The resulting model data were then used to compute run-up elevations using a parametric run-up equation under both present conditions and future sea-levelrise scenarios. In both geomorphologies, wave heights and wavelengths adjacent to the island shorelines increased more than three times and four times, respectively, with increasing values of sea-level rise, as more deep-water wave energy could propagate over the atoll rim and larger wind-driven waves could develop on the atoll. Although these increases in water depth resulted in decreased set-up along the islands' shorelines, the larger wave heights and longer wavelengths due to sea-level rise increased the resulting wave-induced run-up. Run-up values were spatially heterogeneous and dependent on the direction of incident wave direction, bathymetry, and island configuration. Island inundation was modeled to increase substantially when wave-driven effects were included, suggesting that inundation and impacts to infrastructure and terrestrial habitats will occur at lower values of predicted sea-level rise, and thus sooner in the 21st century, than suggested by pa**ive GIS-based “bathtub” inundation models. Lastly, observations and the modeling results suggest that cla**ic atolls with islands on a shallow atoll rim are more susceptible to the combined effects of sea-level rise and wave-driven inundation than atolls characterized by a deep atoll rim.