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.