A number of recent studies (Grinsted and others, 2009; Jevrejeva and
others, 2009; Merrifield and others, 2009; Milne and others 2009)
suggest that not only is global sea-level rise (SLR) occurring and
accelerating in response to global climate change, but that rapid,
meterscale SLR due to ice sheet collapse has occurred in the past
(Blanchon and others, 2009). Furthermore, syntheses by Grinsted and
others (2009) and Nicholls and Cazenave (2010) suggest that global
mean sea level in 2100 may exceed the average projection by the
Intergovernmental Panel on Climate Change (2007) of approximately
60 cm above 2000 levels, with some studies suggesting extreme
(although less likely) rises as high as 200 cm in that time frame
(Vermeer and Rahmstorf, 2009; Pfeffer and others, 2008;
Rahmstorf, 2007). Beyond the year 2100, or perhaps sooner according
to some scientists, rapid collapses of the Greenland and West
Antarctic ice sheets could lead to a SLR of many meters
(Overpeck and others, 2006). Although coral reefs can accrete into
the accommodation space provided by sea-level rise over geologic
time scales, published vertical reef-flat accretion rates for
exposed fringing reefs (1–4 mm/yr; see Buddemeier and Smith, 1988;
Montaggioni, 2005) are as much as an order of magnitude smaller
than the rates of SLR projected for the years 2000–2100
(8–16 mm/yr; see Grinsted and others, 2009; Nicholls and Cazenave,
2010). It is therefore likely that projected SLR will outpace
potential new vertical reef-flat accretion, resulting in a net
increase in water depth over exposed reef flats on the order
of 0.5–2.0 m during the 21st century.
Satellite observations from 1993 to 2010 (Leuliette, 2012) show
global SLR occurring at almost double the rate cited in the
Intergovernmental Panel on Climate Change (2007) report, and
above-average rates have been observed in the Northwestern
Hawaiian Islands (fig. 1). Rising sea levels have the potential
to exacerbate the impacts of storms and wave action on coastlines
and coral reefs by reducing wave-energy dissipation, primarily by
reducing wave breaking at the reef crest and increasing the water
depth relative to hydrodynamic roughness over the reef flat
(see, for example, Storlazzi and others, 2011). By reducing
wave-energy dissipation at the reef crest and over the reef flat,
SLR will cause larger waves to directly affect the coastline
and potentially drive coastal erosion. These larger waves at the
shoreline increase the potential for marine inundation that can
extend inland considerable distances. The maximum vertical extent
of wave-driven inundation is primarily a function of the wave
height, wavelength, and coastal slope. Because storm wave heights
and wavelengths vary in time and space, and coral reefs are
spatially heterogeneous, wave- and SLR-induced inundation will
vary spatially and temporally. This variation is particularly
large for Pacific Ocean islands and atolls that are exposed to
waves in excess of 5 m high numerous times each year (U.S. Army
Corps of Engineers, 2011). On low-lying atolls that typify much
of the central and western Pacific Islands, a small rise in sea
level may cause large horizontal migrations of the shoreline,
impacts to terrestrial infrastructure, loss of critical
terrestrial nesting and foraging habitat, or even complete
inundation of the atoll islands.
Although there have been a number of efforts to investigate how
reefs may respond to SLR (for example, Ogston and Field, 2010;
Field and others, 2011; Storlazzi and others, 2011), there has
been little information presented on how infrastructure and
natural resources of atoll islands may be affected by changes
in sea level. Studies to date that describe SLR threats to
atolls (for example, Baker and others, 2006; Krause and others,
2012) have generally used pa**ive inundation models to simulate
flooding of the islands (fig. 2). These pa**ive models, often
referred to as “bathtub” models, do not project the cumulative
effects of SLR and storm-driven waves on the adjacent
terrestrial landforms, infrastructure, and natural resources.
Although pa**ive inundation represents an important element of
SLR, islands are likely to be affected by a broader, more
complex, and interrelated set of processes, including the
following: loss of land due to erosion; island migration,
breaching, and segmentation; wetland drowning, accretion, or
migration; saltwater intrusion; and increased frequency of
storm flooding (Gesch and others, 2009). The unique
characteristics of a particular location affect the relative
importance of each of these processes. More comprehensive
modeling techniques that consider sediment transport,
morphological changes to the island, currents, stratified
and density-driven flows, and salt-water intrusion require
additional data, including substrate an*lyses, grain size,
current measurements, and detailed hydrodynamic roughness
(Deltares, 2012).
This study explores the combined effect of SLR and
storm-induced wave events for Laysan Island and Midway Atoll's
Sand, Spit, and Eastern Islands within the Northwestern
Hawaiian Islands. Wave and water-level model simulations under
the present conditions and four SLR scenarios were used to
map inundation and provide estimates of potential impacts to
infrastructure and natural resources. These dynamic SLR model
estimates that include wavedriven set-up and run-up are
compared to pa**ive SLR estimates to understand the relative
importance of these processes on inundation and impacts to
terrestrial habitats on Laysan Island and Midway Atoll's Sand,
Spit, and Eastern Islands. By providing information on the
range of forcing parameters (for example, SLR scenario,
wave climate) that may threaten habitats, wildlife, and
infrastructure, this study will help managers to prepare
for possible climate-change scenarios and extreme weather events.