Andrew W. M. Pomeroy

Globally, shorelines are experiencing widespread recession, promoting greater interest in nature-based solutions (NbSs) for coastal protection. Along tropical and sub-tropical coastlines, mangroves are among the habitat-forming species increasingly targeted for nature-based coastal protection. However, practical implementation requires tools that identify shorelines where hydrodynamic and geomorphic conditions are likely to support successful mangrove establishment and long-term persistence. This study quantifies first-order suitability envelopes (hereafter thresholds) for mangrove presence by linking wave height and bathymetric slope to observed mangrove distributions in subtropical wave-dominated barrier estuarine systems. Mangroves were most likely to be present where the 80 ^th percentile significant wave height ( H _s ) was below 0.14 m and bathymetric slope was gentler than 0.01. Above these thresholds, the probability of mangrove occurrence decreased from approximately 23%–25% to 13%–15%, indicating strong but non-exclusive physical controls. Importantly, the probability did not decline to zero beyond these limits, suggesting that mangrove distribution is better described as a suitability threshold rather than a fixed ecological threshold. Spatial analysis indicated that approximately 26% of currently unvegetated shoreline areas in the study fell within the identified wave-slope thresholds, representing potentially suitable but unrealized mangrove habitat. These findings demonstrate that mangrove distribution is partly governed by hydrodynamic and geomorphic processes. While these potential restoration areas are identified based strictly on hydrodynamic and bathymetric thresholds, other critical drivers, such as tidal elevation, sediment supply, and salinity gradients, may further restrict suitable locations. The proposed methods of deriving thresholds in this study provide transferable, process-based criteria for identifying restoration opportunity areas and informing the design of NbSs for coastal resilience in estuarine environments.

Living shorelines have gained increasing interest as a nature-based solution to manage erosion over conventional engineered infrastructure through wave attenuation and shoreline stabilization. Projects that have integrated oyster reefs into living shorelines have typically deployed reef substrate for oyster colonisation in a linear row, parallel to the shoreline to prioritize wave attenuation. The main design parameters considered in these reef-based living shorelines are the height and width of the reef crest relative to the water depth and wave conditions. In nature, however, oyster reefs exist as spatial arrangements of individual patches with varying morphological characteristics. To expand on the conventional parameters considered in oyster reef living shorelines, a numerical model (XBeach) was used to simulate nearshore hydrodynamics and examine the effects of varying oyster reef spatial configurations considering patch size and spacing, the number of reef rows and their orientation on wave transmission. Results showed that continuous reef configurations produced the greatest wave attenuation for a single reef row, with attenuation further improved by increasing the number of rows and optimising spacing between them. Although larger patch perforations increased wave transmission, attenuation capacity could be partially or fully restored by increasing patch size, staggering patch placement, increasing row number, and optimising row spacing, while simultaneously creating more spatially complex configurations analogous to natural oyster reefs. This study provides a foundation for evaluating how reef patch dimensions and spatial configuration can be strategically designed in oyster reef living shoreline projects to achieve target levels of wave transmission while potentially enhancing ecological complexity.

Living shorelines that comprise oyster reefs within a mosaic of multiple coastal habitats can be a resilient and adaptive coastal protection alternative to conventional engineered structures. The success of an oyster reef living shoreline depends on the evolution of a stable base substrate that provides initial short-term coastal protection, to the growth of a living oyster reef and associated habitats in the longer-term, which can enhance protection and provide other ecosystem services. Interdisciplinary global teams that include ecologists, biologists, engineers, techno-economists, and industry are leading the development and implementation of innovative reef-based coastal protection (“Reefense”) solutions that can be scaled to face the challenges of climate change.

Climate change and anthropogenic stressors are accelerating coral reef degradation, prompting urgent restoration strategies. This study evaluates the performance of two coral nursery types, floating mid-water nurseries (FNs) and bottom-attached table nurseries (TNs), at two contrasting reef environments in Mauritius: the degraded, high sedimentation site of Flic-en-Flac (FEF) and the more pristine Pointe aux Feuilles (PAF). Coral fragments from Millepora sp., Acropora muricata, Acropora selago, and Pocillopora damicornis were monitored over three years for survivorship, growth, and linear extension rate (LER). Survivorship exceeded 88% in all cases, with Millepora sp. in PAF–TN achieving the highest rate (99.8%) and P. damicornis in FEF–FN the lowest (88%). Growth was greatest at PAF–TN, where Millepora sp. reached a mean length of 27.25 cm and LER of 9.66 mm y⁻¹. In contrast, the same species in FEF–TN averaged only 3.64 cm in length and 3.44 mm y⁻¹ in LER. Environmental conditions including higher turbidity, nitrate, and phosphate at FEF, and higher phytoplankton density at PAF significantly influenced coral performance. We propose a site-specific nursery selection framework, including FNs for high-sediment areas and TNs for protected and biodiverse sites, to support more effective coral farming outcomes in island restoration programs.

Existing formulations for predicting wave dissipation by submerged canopies generally fall into three categories where (a) an empirical coefficient (energy dissipation factor) is attributed to the canopy ignoring its physical properties; (b) estimates of canopy drag forces based on a bulk drag coefficient and undisturbed velocities above the canopy are used to estimate dissipation; and (c) canopy flow theory is used to account for how modifications to in‐canopy flows influence canopy forces and associated dissipation. We measured rates of spectral wave dissipation across a dense seagrass meadow comprised of Posidonia australis in southwestern Australia, which also included high‐resolution flow measurements within and above the seagrass canopy. These observations were used to quantify the effectiveness of the three different approaches to predict observed rates of spectral wave dissipation. The results showed that conventional approaches that do not account for canopy flow modifications and/or seagrass flexibility tend to overestimate both bulk and frequency‐dependent wave dissipation. Conversely, approaches that consider frequency‐dependent flow attenuation in canopies were found to improve predictions of wave dissipation, particularly when also accounting for how the deflection of flexible seagrass blades induced by flow modifies the effective canopy height. The results show that the canopy flow velocities induced by short period wind waves were less attenuated than longer period swell, explaining the frequency dependency of rates of wave dissipation, with shorter period wave heights being more efficiently attenuated by the meadow.

Coral reefs are effective natural flood barriers that protect adjacent coastal communities. As the need to adapt to rising sea levels, storms, and environmental changes increases, reef restoration may be one of the best tools available to mitigate coastal flooding along tropical coastlines, now and in the future. Reefs act as a barrier to incoming short-wave energy but can amplify low-frequency infragravity waves that, in turn, drive coastal flooding along low-lying tropical coastlines. Here, we investigate whether the spacing of reef restoration elements can be optimized to maximize infragravity wave energy dissipation while minimizing the number of elements—a key factor in the cost of a restoration project. With this goal, we model the hydrodynamics of infragravity wave dissipation over a coral restoration or artificial reef, represented by a canopy of idealized hemispherical roughness elements, using a three-dimensional Navier–Stokes equations solver (OpenFOAM). The results demonstrate that denser canopies of restoration elements produce greater wave dissipation under larger waves with longer periods. Wave dissipation is also frequency-dependent: dense canopies remove wave energy at the predominant wave frequency, whereas sparse canopies remove energy at higher frequencies, and hence are less efficient. We also identify an inflection point in the canopy density–energy dissipation curve that balances optimal energy losses with a minimum number of canopy elements. Through this work, we show that there are an ideal number of restoration elements per across-shore meter of coral reef flat that can be installed to dissipate infragravity wave energy for given incident heights and periods. These results have implications for designing coral reef restoration projects on reef flats that are effective both from a coastal defense and costing standpoint.

Suspended kelp canopies have the potential to provide a coastal protection service in addition to their primary function of generating a sustainable resource. In this study, the attenuation of incident waves by kelp suspended from the surface was quantified. We adapted an analytical 1D cross-shore wave attenuation model and tested the effect of (1) water depth, (2) vegetation density, and (3) longline density. The results show that as the percentage of vegetation in the water column increases, wave attenuation by the canopy also increases. However, this attenuation is affected by seasonal variations in kelp growth as well as harvesting strategies. Careful choice of the adopted harvesting strategy was found to be important to maintain optimal wave attenuation by kelp aquaculture farms throughout the year. Partial and targeted removal of the vegetation along longlines is preferred to harvesting all laterals on longlines. This study demonstrates that there is an opportunity for the emerging global kelp aquaculture industry to provide a coastal protection service in addition to resource production, which will help to affect how coastal protection is realized and scaled globally.