Forecasting the impact of storm waves and sea-level rise on Midway Atoll and Laysan Island within the Papahānaumokuākea Marine National Monument—a comparison of passive versus dynamic inundation models

Abstract

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 passive "bathtub" inundation models based on geographic information systems (GIS) to those that include dynamic water levels caused by wave-induced setup and run-up for two endmember island morphologies: Midway, a classic 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 assessed 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 setup 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 passive GIS-based "bathtub" inundation models. Lastly, observations and the modeling results suggest that classic 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.

Many Atolls May be Uninhabitable Within Decades Due to Climate Change

“…As shown previously 17 , it is apparent that the passive “bathtub” modeling predicts less inundation than the dynamic flood modeling for these low-lying atoll islands. These differences between the passive and dynamic modeling are especially true for higher sea-level-rise scenarios ( Fig.…”

“…Waves on most atoll reef crests and reef flats are water depth-limited, which suggests as water depth increases with sea-level rise, larger waves 17 will propagate onto or develop via wind forcing on the reef flat or in the lagoon ( Fig. 2 ).…”

“…Mean wave-driven run-up adjacent to the low-lying atoll islands, which is a function of wave height, wavelength, and the slope of the beachface 19 , will similarly increase with sea-level rise. Although there is large heterogeneity in the resulting wave-driven run-up around the islands 17 due to spatial variations in waves around the islands ( Fig. 2 ) and beachface slope, there is a general trend of higher wave-driven run-up on the sides of the islands closer to the reef crest.…”

“…Studies to-date that describe sea-level rise threats to low-lying atoll islands 8 14 15 generally have used passive models to simulate inundation of the islands; these “bathtub” models do not incorporate the interaction between sea-level rise and waves 16 . This study builds on preliminary efforts 17 to explore the combined effect of sea-level rise inundation and storm-induced wave-driven flooding on atoll islands within the Northwestern Hawaiian Islands that are the home to many threatened and endangered endemic species 15 and have high-resolution topographic and bathymetric data, yet this approach is applicable to most populated atolls around the world.…”

Long-distance translocations to create a second millerbird population and reduce extinction risk

“…Laysan, and the other NWHI, are vulnerable to increased frequency and severity of catastrophic weather events, storm-surge, sea level rise, and increased shoreline erosion associated with global climate change (Mimura et al, 2007;Reynolds et al, 2012). These events, in combination with large ocean swells, will result in an increase in wash-over events on atolls and low islands in the NWHI, such as those that have occurred in recent years during severe storms (USFWS, 2011 unpublished data;Storlazzi et al, 2013). Creating a population of millerbirds on a third island, ideally one with greater elevation above sea-level than Laysan, would provide additional insurance for the species' future (Morin and Conant, 2007).…”