Coral atoll islands, common in tropical and subtropical oceans, consist of low-lying accumulations of carbonate sediment produced by fringing coral reef systems and are of great socio-economic and ecological importance. Previous studies have predicted that many coral atoll islands will become uninhabitable before the end of this century due to sea level rise exacerbating wave-driven flooding. However, the assumption that such islands are morphologically static, and will therefore ‘drown’ as sea levels rise, has been challenged by observations and modelling that show the potential for overwashing and sediment deposition to maintain island freeboard. However, for sustainable habitation, reliable predictions of island adjustment, flooding frequency and the influence of adaptation measures are required. Here, we illustrate the effect of various adaptation measures on the morphological response of an atoll island to future sea level rise using process-based model simulations. We found that the assumption of a static island morphology leads to a significant increase in the predicted frequency of future island flooding compared to morphodynamically active islands, and demonstrate that natural morphological adjustment is a viable mechanism to increase island freeboard. Reef adaptation measures were shown to modify the inshore wave energy, influencing the equilibrium island crest height and therefore the long-term morphological response of the island, while beach restoration mainly delays the island’s response. If embraced and implemented by local communities, allowing for natural island dynamics and implementing well-designed adaptation measures could potentially extend the habitability of atoll islands well beyond current projections.

Timothy Scott

and 7 more

Improved understanding of how our coasts will evolve over a range of time scales (years-decades) is critical for effective and sustainable management of coastal infrastructure. Globally, sea-level rise will result in increased erosion, with more frequent and intense coastal flooding. Understanding of current and future coastal evolution requires robust knowledge of the wave climate. This includes spatial, directional and temporal variability, with recent research highlighting the importance of wave climate directionality on coastal morphological response, for example in UK, Australia and California. However, the variability of the inshore directional wave climate has received little attention, and an improved understanding could drive development of skillful seasonal or decadal forecasts of coastal response. We examine inshore wave climate at 63 locations throughout the United Kingdom and Ireland (1980–2017) and show that 73% are directionally bimodal. We find that winter-averaged expressions of six leading atmospheric indices are strongly correlated with both total and directional winter wave power (peak spectral wave direction) at all studied sites. Coastal classification through hierarchical cluster analysis and stepwise multi-linear regression of directional wave correlations with atmospheric indices defined four spatially coherent regions. We show that combinations of indices have significant skill in predicting directional wave climates (r= 0.45–0.8; p<0.05). We demonstrate for the first time the significant explanatory power of leading winter-averaged atmospheric indices for directional wave climates, and show that leading seasonal forecasts of the NAO skillfully predict wave climate in some regions.

Erin Victoria King

and 6 more

Embayed beaches separated by irregular rocky headlands represent 50% of global shorelines. Quantification of inputs and outflows via headland bypassing is necessary for evaluating long-term coastal change. Bypassing rates are predictable for idealised headland morphologies; however, it remains to test the predictability for realistic morphologies, and to quantify the influence of variable morphology, sediment availability, tides and waves-tide interactions. Here we show that headland bypassing rates can be predicted for wave-dominated conditions, and depend upon headland cross-shore length normalised by surf zone width, headland toe depth and spatial sediment coverage. Numerically modelled bypassing rates are quantified for 29 headlands under variable wave, tide and sediment conditions along 75km of macrotidal, embayed coast. Bypassing is predominantly wave-driven and nearly ubiquitous under energetic waves. Tidal elevations modulate bypassing rates, with greatest impact at lower wave energies. Tidal currents mainly influence bypassing through wave-current interactions, which can dominate bypassing in median wave conditions. Limited sand availability off the headland apex can reduce bypassing by an order of magnitude. Bypassing rates are minimal when cross-shore length > 5 surf zone widths. Headland toe depth is an important secondary control, moderating wave impacts off the headland apex. Parameterisations were tested against modelled bypassing rates, and new terms are proposed to include headland toe depth and sand coverage. Wave-forced bypassing rates are predicted with mean absolute error of a factor 4.4. This work demonstrates wave-dominated headland bypassing is amenable to parameterisation and highlights the extent to which headland bypassing occurs with implications for embayed coasts worldwide.

Gerhard Masselink

and 4 more

Coral reefs are widely recognised for providing a natural breakwater effect that modulates erosion and flooding hazards on low-lying sedimentary reef islands. Increased water depth across reef platforms due sea-level rise (SLR) can compromise this breakwater effect and enhance island exposure to these hazards, but reef accretion in response to SLR may positively contribute to island resilience. Morphodynamic studies suggest that reef islands can adjust to SLR by maintaining freeboard through overwash deposition and island accretion, but the impact of different future reef accretion trajectories on the morphological response of islands remain unknown. Here we show, using a process-based morphodynamic model, that, although reef growth significantly affects wave transformation processes and island morphology, it does not lead to decreased coastal flooding and island inundation. According to the model, reef islands evolve during SLR by attuning their elevation to the maximum wave runup and islands fronted by a growing reef platform attain lower elevations than those without reef growth, but have similar overwash regimes. The mean overwash discharge across the island crest plays a key role in the ability of islands to keep up with SLR and maintain freeboard, with a value of (10 l m s) separating island construction from destruction. Islands, therefore, can grow vertically to keep up with SLR via flooding and overwash if specific forcing and sediment supply conditions are met, offering hope for uninhabited and sparely populated islands. However, this physical island response will negatively impact infrastructure and assets on developed islands.

Erin Victoria King

and 3 more

Waves and tidal currents resuspend and transport shelf sediments, influencing sediment distributions and bedform morphology with implications for various topics including benthic habitats, marine operations, and marine spatial planning. Shelf-scale assessments of wave-tide-dominance of sand transport tend not to fully include wave-tide interactions (WTI), which non-linearly enhance bed shear stress and apparent roughness, change the current profile, modulate wave forcing, and can dominate net sand transport. Assessment of the relative contribution of WTI to net sand transport requires computationally/ labour intensive coupled numerical modelling, making comparison between regions or climate conditions challenging. Using the Northwest European Shelf, we show the dominant forcing mode and potential magnitude of net sand transport is predictable from readily available, uncoupled wave, tide and morphological data in a computationally efficient manner using a k-Nearest Neighbour algorithm. Shelf areas exhibit different dominant forcing modes for similar wave exceedance conditions, relating to differences in depth, grain size, tide range, and wave exposure. WTI dominate across most areas in energetic combined conditions. Over a statistically representative year, meso-macrotidal areas exhibit tide-dominance, while shallow, finer grained, amphidromic regions show wave-dominance, with WTI dominating extensively >30m depth. Seabed morphology is strongly affected by sediment transport mode, and sand wave geometry varies significantly between predicted dominance classes with increased length and asymmetry, and decreased height, for increasing wave-dominance. This approach efficiently indicates where simple non-interactive wave and tide processes may be sufficient for modelling sediment transport, and enables efficient inter-regional comparisons and sensitivity testing to changing climate conditions with applications globally.