Although these approaches elucidate large-scale trends, most of the analyses involve a few atoll islands (excepting Webb and Kench, 2010 Rankey, 2011), and/or have only considered the effects of recent SLR. Many studies have explored atoll island response to SLR ( Roy and Connell, 1991 Woodroffe, 2008 Webb and Kench, 2010 Rankey, 2011 Ford, 2012 Yates et al., 2013 Purkis et al., 2016) and extreme wave events ( Hoeke et al., 2013 Smithers and Hoeke, 2014), as well as reef controls on shoreline sediment mobilization and erosion ( Sheppard et al., 2005 Storlazzi et al., 2011 Grady et al., 2013). Incident wave dynamics, atoll morphology, and sediment composition (size, induration, etc.) influence shoreline erosional and accretional patterns therefore, changes in atoll island alongshore erosion and/or accretion with SLR are location dependent and findings for one island may not necessarily apply to another. Global SLR and reef degradation (bleaching, ocean acidification, etc.) will reduce the reef platform’s ability to attenuate wave energy and protect shorelines from wave-driven erosion and flooding hazards ( Sheppard et al., 2005 Storlazzi et al., 2011 Grady et al., 2013 Ferrario et al., 2014 Shope et al., 2017). As global climate warms, sea levels are projected to potentially increase by more than 2.0 m by 2100 due to seawater thermal expansion and glacial/ice cap melt ( Church et al., 2013 Kopp et al., 2014 Slangen et al., 2014) the rate of sea-level rise (SLR) is projected to exceed the rate of coral reef platform vertical accretion ( Montaggioni, 2005), leaving the future stability of these low-lying islands and their associated infrastructure, agriculture, and habitats uncertain. Communities living on atoll islands are vulnerable to large wave events and changing climates ( Storlazzi et al., 2015). These findings make it possible to evaluate the relative risk of alongshore erosion/accretion on atolls due to SLR in a rapid, first-order analysis.Ītolls are shallow, ring-shaped, coral platforms upon which small, low-elevation carbonate islands are often perched. ![]() Oblique island, oriented parallel to the incident deepwater wave direction, shorelines are forecast to build out leeward along the reef rim and toward the lagoon while eroding along regions exposed to direct wave attack. Windward island shorelines are projected to accrete toward the island’s longitudinal ends and lagoon due to SLR, whereas leeward islands erode along lagoon shorelines and extend toward the island ends. Modeled atolls with comparably small diameters, narrow and deep reef flats with narrow islands displayed greater magnitudes of erosion and/or accretion, especially with SLR. ![]() Modeled morphologic parameters that significantly influenced alongshore transport were the atoll diameter, reef flat width, reef flat depth, and island width. ![]() The magnitude of the transport gradients increased with SLR: initial erosion or accretion patterns intensified. Incident wave transformations were simulated using a physics-based numerical model and alongshore erosion and accretion was calculated using empirical formulae. ![]() Schematic atoll models with varying morphologies were used to evaluate the relative control of individual morphological parameters on alongshore transport gradients. It is unclear, though, how potential atoll morphologic configurations influence shoreline erosion and/or accretion patterns, and how these relationships will respond to future sea-level rise (SLR). 2Pacific Coastal and Marine Science Center, US Geological Survey, Santa Cruz, CA, United StatesĪtoll islands’ alongshore sediment transport gradients depend on how island and reef morphology affect incident wave energy.1Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA, United States.
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