U.S. Geological Survey - (Introduction) 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 I lyrics

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.