Infragravity wave (IGW) transformation was quantified from field measurements on two shore platforms on New Zealand's east coast, making this the first study to describe the presence, characteristics and behaviour of IGWs on rock platform coasts. Data was collected using a cross-shore array of pressure transducers during a 22 hour experiment on Oraka shore platform and a 36 hour experiment at Rothesay Bay shore platform. A low pass Fourier filter was used to remove gravity wave frequency oscillations, allowing separate analysis of IGWs and the full wave spectrum. Offshore IGW heights were measured to be 7cm (Oraka) and 9cm (Rothesay Bay), which were 21% (Oraka) and 7.5% (Rothesay Bay) the height of incident wave height. At the cliff toe, significant IGW height averaged 15cm at Oraka and 13cm at Rothesay Bay. This increase in IGW height over the platform during both experiments is attributed to shoaling of 40 to 55% over the last 50-60m before the cliff toe, respectively. Shoaling across the platform was quantified as the change in IGW height from the platform edge to cliff toe, resulting in a maximum increase of 1Á88 and 2Á63 on Rothesay Bay and Oraka platforms. IGW height at the cliff toe showed a strong correlation with incident wave height. The proportional increase in IGW height shows a strong correlation to water level on each platform. The rate of shoaling of long period waves on the shallow, horizontal platforms increased at higher water levels resulting in a super elevation in water level at the cliff toe during high tide. Greater IGW shoaling was also observed on the wider (Oraka) shore platform. Results from this study show the first measurements of IGWs on shore platforms and identify long wave motion a significant process in a morphodynamic understanding of rock coast.

Increased flooding due to sea level rise (SLR) is expected to render reef islands, defined as sandy or gravel islands on top of coral reef platforms, uninhabitable within decades. Such projections generally assume that reef islands are geologically inert landforms unable to adjust morphologically. We present numerical modeling results that show reef islands composed of gravel material are morphodynamically resilient landforms that evolve under SLR by accreting to maintain positive freeboard while retreating lagoonward. Such island adjustment is driven by wave overtopping processes transferring sediment from the beachface to the island surface. Our results indicate that such natural adaptation of reef islands may provide an alternative future trajectory that can potentially support near-term habitability on some islands, albeit with additional management challenges. Full characterization of SLR vulnerability at a given reef island should combine morphodynamic models with assessments of climate-related impacts on freshwater supplies, carbonate sediment supply, and future wave regimes.

We present new detail on how future sea‐level rise (SLR) will modify nonlinear wave transformation processes, shoreline wave energy, and wave driven flooding on atoll islands. Frequent and destructive wave inundation is a primary climate‐change hazard that may render atoll islands uninhabitable in the near future. However, limited research has examined the physical vulnerability of atoll islands to future SLR and sparse information are available to implement process‐based coastal management on coral reef environments. We utilize a field‐verified numerical model capable of resolving all nonlinear wave transformation processes to simulate how future SLR will modify wave dissipation and overtopping on Funafuti Atoll, Tuvalu, accounting for static and accretionary reef adjustment morphologies. Results show that future SLR coupled with a static reef morphology will not only increase shoreline wave energy and overtopping but will fundamentally alter the spectral composition of shoreline energy by decreasing the contemporary influence of low‐frequency infragravity waves. “Business‐as‐usual” emissions (RCP 8.5) will result in annual wave overtopping on Funafuti Atoll by 2030, with overtopping at high tide under mean wave conditions occurring from 2090. Comparatively, vertical reef accretion in response to SLR will prevent any significant increase in shoreline wave energy and mitigate wave driven flooding volume by 72%. Our results provide the first quantitative assessment of how effective future reef accretion can be at mitigating SLR‐associated flooding on atoll islands and endorse active reef conservation and restoration for future coastal protection.

Wave-driven flooding is a serious hazard on coral reef-fringed coastlines that will be exacerbated by global sea-level rise. Despite the global awareness of atoll island vulnerability, little is known about the physical processes that control wave induced flooding on reef environments. To resolve the primary controls on wave-driven flooding at present and future sea levels, we present a globally applicable method for calculating wave overtopping thresholds on reef coastlines. A unique dataset of 60,000 fully nonlinear wave transformation simulations representing a wide range of wave energy, morphology and sea levels conditions was analysed to develop a tool for exploring the future trajectory of atoll island vulnerability to sea-level rise. The proposed reef-island overtopping threshold (RIOT) provides a widely applicable first-order assessment of reef-coast vulnerability to wave hazards with sea-level. Future overtopping thresholds identified for different atoll islands reveal marked spatial variability and highlight distinct morphological characteristics that enhance coastal resilience.

No abstract