Abstract
Atoll islands’ alongshore sediment transport gradients depend on how island and reef morphology affect incident wave energy. 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). Schematic atoll models with varying morphologies were used to evaluate the relative control of individual morphological parameters on alongshore transport gradients. Incident wave transformations were simulated using a physics-based numerical model and alongshore erosion and accretion was calculated using empirical formulae. The magnitude of the transport gradients increased with SLR: initial erosion or accretion patterns intensified. Modeled morphologic parameters that significantly influenced alongshore transport were the atoll diameter, reef flat width, reef flat depth, and island width. 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. 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. 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. 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.
Highlights
Atolls are shallow, ring-shaped, coral platforms upon which small, low-elevation carbonate islands are often perched
Throughout, the magnitude of the transport patterns increased with sea-level rise (SLR); shorelines that were initially erosive or accretive with lower sea level generally displayed greater erosion or accretion magnitudes with SLR
Because atoll islands are generally dynamic features (Kench and Brander, 2006; Rankey, 2011) and these alongshore wave-energy gradients will increase with SLR, future island alongshore transport patterns will change considerably
Summary
Atolls are shallow, ring-shaped, coral platforms upon which small, low-elevation carbonate islands are often perched. 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) 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. There has been limited exploration modeling of the erosional and accretional effect of changing wave conditions along atoll islands (Shope et al, 2017), it is currently unclear how alongshore erosion and/or accretion along different atoll islands’ shorelines will respond to future SLR. Shope et al (2017) and other studies have provided rationale as to how observed or projected erosion patterns may be influenced by atoll morphology, but there has not been a rigorous evaluation of how much influence morphologic variability affects alongshore processes on atolls
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