The study, published in Results in Engineering, was motivated by a growing exposure problem. Climate change is intensifying tropical-cyclone wave hazard and the risk of coastal flooding, compounding the long-term vulnerability of waterfront infrastructure. Coastal cities bordering the South China Sea face particularly severe threats, and Hong Kong, with its densely developed shoreline, frequent south-easterly wave exposure, and a population exceeding seven million, is especially vulnerable. Recent decades have brought recurring impacts, including Typhoon Hato in 2017 and Typhoon Mangkhut in 2018, both of which caused severe coastal damage in Hong Kong and nearby cities. These events have sharpened interest in protection strategies that reduce wave impacts while minimising ecological disturbance and the environmental footprint of construction, precisely the niche that conventional hard defences such as seawalls and breakwaters struggle to fill.

Nature-based and bioinspired solutions have emerged as promising candidates for sustainable coastal protection in urbanised settings. Oyster reef restoration and 3D-printed artificial reef structures can provide simultaneous wave attenuation and ecological co-benefits, such as habitat provision and biodiversity enhancement, that hard defences rarely deliver. In Hong Kong, this prospect is particularly relevant because oyster farming has a long history, and restoration initiatives have recently been undertaken at Lau Fau Shan, Tolo Harbour, and the waterfront of Hong Kong International Airport. When such reef formations are arranged in spatially periodic arrays, they function as submerged bar arrays capable of attenuating wave energy through Bragg resonance, a mechanism in which surface waves are constructively reflected by periodic bottom topography when the bar spacing is approximately half the incident wavelength. Because the reflecting elements remain fully submerged and attenuate waves through interference rather than blocking, such arrays offer a complement to conventional defences with lower ecological and visual impact.

The physics of Bragg reflection on seabed topography has been established across theory, laboratory confirmation, engineering application, and nonlinear simulation. Early theoretical work by C. C. Mei in 1985 laid the foundation, while Davies and Heathershaw provided laboratory confirmation in 1984 that reflection is strongest when bar spacing is approximately half the incident wavelength and increases with the number of bars. Subsequent studies extended the framework to oblique incidence, higher-order resonance conditions arising from bottom and free-surface nonlinearity, and practical applications such as mitigating harbour oscillations induced by long waves and short wave groups. More recent analytical work has examined graded trapezoidal arrays that exploit a rainbow-reflection mechanism to produce broad-band attenuation, in which the spacing increment can be adjusted to broaden the effective frequency bandwidth of reflection. However, most of these studies considered simplified bottom geometries or idealised periodic bar arrays under unidirectional waves over prescribed horizontal, sloping, or reef-type bed profiles.

The key practical question addressed by the new research is whether engineered Bragg arrays remain effective when deployed over natural bathymetry under broadband tropical-cyclone forcing. In a realistic coastal setting, natural bathymetry continuously modifies local wavelength, wave direction, and group velocity through shoaling and refraction, so a spacing strategy optimised on an idealised bed may become detuned. Tropical-cyclone forcing further complicates performance assessment because the incident sea state is energetic, broadband, and directionally spread, requiring attenuation to remain effective beyond a single narrow resonance peak. For typhoon-exposed coastal cities such as Hong Kong, Macao, and Shenzhen, where conventional seawalls and breakwaters are constrained by ecological regulation, visual impact, and high construction cost in deep water, submerged alternatives that can be integrated into existing coastal management frameworks represent an important emerging option.

To answer this question, the team employed a fully nonlinear potential flow model, the Enhanced Spectral Boundary Integral (ESBI) solver, which combines efficient spectral computation with variable-depth capability and is applicable to large coastal domains. The model was first validated against three benchmark problems covering the three capabilities required for the case study: Bragg reflection by a graded trapezoidal bar array under regular waves, Bragg reflection of normally incident irregular waves over sinusoidal bars, and Bragg reflection under oblique wave incidence over sinusoidal ripples. In the first case, the model reproduced both the overall envelope and the local features of the reflection curve for an eight-bar graded array, achieving a root-mean-square error of 0.0255 against the analytical solution. In the second, comparisons against numerical results and analytical estimates for two test configurations yielded RMSE values between 0.0157 and 0.0484. In the third, the model reproduced the angular dependence of oblique Bragg reflection across incident angles from 0 to 45 degrees, with an RMSE of just 0.0084 against perturbation theory.

With the model validated, the researchers applied it to the Junk Bay region of Hong Kong, a semi-enclosed embayment on the southeastern coast of the New Territories whose mouth opens toward the South China Sea. The densely developed Tseung Kwan O district occupies the inner shoreline and is exposed to tropical-cyclone waves. The computational domain was a 19.6 kilometre by 19.6 kilometre square region discretised on a 4096 by 4096 grid with a spacing of approximately 4.8 metres, rotated so that the primary model axis aligned with the dominant incident wave direction. High-resolution bathymetry data were interpolated onto the rotated grid, allowing the natural seabed to be represented directly. The incident wave conditions came from a hindcast of Typhoon Mangkhut at the instant of maximum significant wave height, synthesised at the wave-making boundary as a linear superposition of 36 frequency bins and 24 directional bins, corresponding to a significant wave height of 9.92 metres, a peak period of 15.55 seconds, and a mean wave direction of 129.7 degrees.

The reference bar array consisted of eight trapezoidal bars, each 4 metres high with a 12-metre top width and 20-metre base width, extending approximately 850 metres transversely to cover the principal wave exposure zone of the bay. Bar heights at the transverse edges were tapered smoothly to zero using a Gaussian weighting function to avoid abrupt lateral depth transitions. The target mean spacing was selected from the local wave conditions: a baseline simulation over the natural bathymetry estimated a mean local wavelength of approximately 110 metres in the candidate deployment area, where the mean water depth is 10.5 metres, giving a target spacing of approximately 55 metres for the primary Bragg resonance condition. Performance was quantified by comparing simulations with and without the array under identical forcing, using the wave energy flux ratio, which accounts for the frequency- and depth-dependent propagation speed of wave energy, together with the protected-area fraction, which measures the share of the region of interest achieving at least a 40 percent reduction in wave energy flux.

The results revealed a pronounced attenuation zone shoreward of the bar array, where the energy flux ratio dropped to values between 0.4 and 0.8 over a broad area of the inner bay. Seaward of the array, a striped pattern of alternating enhancement and near-unity flux extended southward, consistent with a partial standing-wave field formed by the superposition of incident and reflected components. The graded array, with a grading ratio of 1.10, substantially outperformed the uniform array of the same mean spacing and bar count: the protected-area fraction increased from 0.291 to 0.655, more than doubling the areal coverage of cells experiencing at least 40 percent reduction, while the mean flux ratio decreased from 0.693 to 0.634. Frequency-resolved spectral analysis along a shore-normal transect showed that the uniform array concentrated attenuation within approximately 0.06 to 0.08 hertz, whereas the graded array extended this band to approximately 0.05 to 0.09 hertz, yielding a wider simulated reflection bandwidth that agrees qualitatively with the rainbow-reflection behaviour reported in earlier analytical work.

Sensitivity analyses revealed important design trade-offs. Increasing the number of bars from 6 to 12 produced a non-monotonic response: the protected-area fraction rose sharply from 0.386 to 0.655 between 6 and 8 bars, changed marginally to 0.661 at 10 bars, and then decreased to 0.590 at 12 bars. Two site-specific effects may explain this reversal. As the array extends seaward, the outermost bars sit in progressively deeper water, where the reduced relative bar height weakens the wave-bottom interaction and diminishes their Bragg resonance contribution. The longer array may also introduce more complex wave interactions at the lateral edges that redistribute energy within the protected region. Bar height, by contrast, showed a monotonic improvement: raising the height from 3.2 to 4.8 metres decreased the mean flux ratio from 0.752 to 0.531 and increased the protected-area fraction from 0.046 to 0.788, with the largest single gain occurring between 3.6 and 4.0 metres. Taller bars reduce crest submergence and strengthen the wave-bottom interaction but require more construction material and provide less navigational clearance.

The authors caution that the physical scope of the model follows from its fully nonlinear potential-flow formulation, which assumes inviscid and irrotational flow, so turbulence and viscous effects are not explicitly represented, and wave breaking is handled through a high-wavenumber filter and dissipation treatment rather than resolved explicitly. Coastlines were represented as wave-absorbing boundaries, so shoreline run-up and overtopping were not simulated, meaning the reported reductions in wave energy flux should not be interpreted as direct predictions of overtopping reduction. Uncertainty from random-phase realisations was assessed with three additional realisations, showing that the mean attenuation was relatively insensitive to phase realisation, whereas the protected-area fraction showed larger variation and should be interpreted with caution. No site-specific wave measurements were available for direct field verification. Nevertheless, for the simulated case, approximately 60 percent of the protected region was predicted to experience at least a 40 percent reduction in wave energy flux under the selected Mangkhut sea state. Because the structures remain fully submerged, they impose no visual barrier above the waterline, and when constructed from biocompatible substrate, such arrays could support natural oyster colonisation over time, potentially combining the simulated wave-attenuation function with reef habitat provision. The findings suggest that coastal managers in Hong Kong and similar South China Sea cities may consider graded submerged bar arrays as a complementary element within integrated coastal defence strategies, particularly for exposed embayments where hard-structure alternatives face regulatory, ecological, or aesthetic constraints, and that effective deployment requires site-specific hydrodynamic design matched to the local wavelength distribution rather than simply maximising array length or height.

Subject of Research: Site-scale simulation of graded submerged bar arrays for Bragg-resonant wave attenuation over natural bathymetry under tropical-cyclone wave conditions in Junk Bay, Hong Kong.

Article Title: Graded submerged bar arrays for nature-based coastal wave attenuation under tropical-cyclone conditions

Article References: Zhang, J., Gao, C., Wang, J., Stocchino, A., Duan, H., & Ning, D. (2026). Graded submerged bar arrays for nature-based coastal wave attenuation under tropical-cyclone conditions. Results in Engineering, 32, Article 113456. https://doi.org/10.1016/j.rineng.2026.113456

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113456

Keywords: Bragg resonance, coastal protection, nature-based solutions, submerged bar arrays, wave attenuation, tropical cyclones, Typhoon Mangkhut, Junk Bay, oyster reefs, rainbow reflection, Hong Kong, coastal resilience

The central insight of the study is that the pace of sea-level rise acts as a master control on how fast oyster reefs grow upward. For roughly 18 centuries before 1800 CE, when relative sea level along this stretch of coast climbed at about 0.9 millimeters per year, the reefs accreted slowly, adding height and sediment at a modest, steady rate. After 1800, as the rate of relative sea-level rise accelerated toward the modern value of approximately 2.4 millimeters per year, the same reefs responded by accreting vertically and accumulating sediment three times faster than before. In other words, healthy reefs did not merely keep pace with rising water; they ramped up their construction in response to it, creating the vertical space they needed as flooding intensified.

The mechanism behind this acceleration is elegantly physical. As sea level rises, it creates what geologists call accommodation space, the vertical room available for sediment to fill in. Faster rise means more room, and the reefs exploited it. The team’s cores show that the accelerated accretion was driven by an increased flux of both shell material and organic-rich sediment into the subsurface. Oysters are prodigious filter feeders, pumping water and packaging suspended particles into biodeposits that settle onto the reef, and their shells form the rigid framework that traps this fine material. The result, visible in the stratigraphy, is a reef matrix rich in shell fragments and organic matter, deposited far more rapidly in the last two centuries than at any time in the preceding 1,800 years.

To build this record, the researchers vibracored 25 natural intertidal reefs across five North Carolina estuarine systems, including Back Sound, the North River, the Newport River, the White Oak River and the Shallotte River. They radiocarbon-dated articulated shells of the eastern oyster, Crassostrea virginica, at multiple depths in each core, calibrated those ages with the Marine20 curve, and constructed age-depth models using the Bayesian software OxCal. From these models they calculated mass accumulation rates and vertical accretion rates through time, correcting for sediment compaction and core shortening. The composition of each subsample, including dry bulk density, percent shell and percent mud, allowed them to separate the organic carbon buried in the reef matrix from the inorganic carbon locked in carbonate shell material.

That separation matters enormously for the carbon story. When the team tallied the organic carbon accumulating in the reefs, they found that faster accretion after 1900 indeed meant more organic carbon being buried per unit time. Oysters actively promote the deposition of organic matter, and the study’s data show that reef sediments are enriched in organic material relative to surrounding estuarine muds. On paper, this looks like a blue carbon success story, the kind of natural climate solution that restoration advocates and policymakers have been eager to bank on. Salt marshes, mangroves and seagrass meadows have earned their reputations as carbon burial hotspots, and oyster reefs have frequently been lumped into the same category.

But the ledger has a second column. Building a shell is a carbon-intensive enterprise. Oysters extract carbonate from seawater to construct their shells, and in doing so they release carbon dioxide; the calcification reaction shifts the carbonate chemistry of the water and liberates CO2 to the atmosphere. When the researchers compared the organic carbon buried in the reefs against the CO2 emitted during shell formation, the two fluxes essentially cancelled each other out. None of the 25 reefs sampled functioned as a net carbon sink. The organic carbon story, real as it is, is offset by the inorganic carbon cost of the very shells that make the reef a reef. This finding aligns with earlier work by some of the same authors, including a 2017 study in Proceedings of the Royal Society B that first flagged oyster reefs as potential carbon sources rather than sinks.

The implications cut in two directions at once, and that tension is what makes the study so consequential for coastal policy. On the positive side, the demonstration that reefs can triple their accretion rate in response to accelerating sea-level rise is a powerful endorsement of their role as living breakwaters. Nature-based infrastructure, from living shorelines to reef-based breakwaters, is increasingly promoted as a way to protect communities from storm waves, and the new data suggest that healthy reefs have the capacity to keep pace with, and continue damping waves under, the higher rates of rise projected for the coming decades. The wave-attenuation service, in other words, appears robust to climate change as long as the reefs themselves remain healthy.

On the carbon side, however, the study delivers a caution against marketing oyster reef restoration as a greenhouse gas mitigation strategy. The authors show that as sea-level rise accelerates further, vertically growing reefs are likely to become larger sources of CO2, not smaller ones, because faster accretion means more shell production and more calcification-driven emissions, even as organic carbon burial increases in parallel. The net change in carbon burial, the study concludes, is minimal. For planners weighing investments in nature-based infrastructure, this means oyster reefs should be valued primarily for their coastal protection, habitat and water-quality benefits, while carbon accounting should be left to marshes, mangroves and seagrasses, whose burial economics are far more favorable.

There is also a sobering caveat about the future. The remarkable responsiveness the researchers documented depends on reefs being healthy, with robust oyster recruitment, adequate shell production and estuarine conditions that support vigorous growth. The authors explicitly caution that the ongoing deterioration of the estuarine conditions needed to sustain healthy oyster reefs warns against projecting these ecosystem service trade-offs into the coming century. Overharvesting, disease, low-oxygen events, sedimentation and ocean acidification have already degraded reefs worldwide, and a stressed reef cannot triple its growth rate to meet a rising sea. Historical evidence, including studies linking large-scale oyster bed harvesting to elevated coastal vulnerability, underscores how quickly this natural infrastructure can be lost.

What emerges from the study is a more honest, more nuanced portrait of the oyster reef as a piece of climate-era infrastructure. It is a breakwater of genuine and possibly growing value, capable of astonishing resilience when given the chance, and it is not a carbon sink, no matter how appealing that label might be. The 2,000-year record extracted from North Carolina’s estuaries shows nature responding dynamically to a changing coast, and it hands restoration science a crucial correction: the services an ecosystem actually provides over decadal to millennial timescales can differ sharply from the ones measured on young, restored reefs less than a decade old. As sea level continues to climb, the reefs will keep building. Whether the estuaries around them remain healthy enough to let them is the question that will determine if this living infrastructure endures.

Subject of Research: The effect of sea-level rise on oyster reef accretion and net carbon burial

Article Title: Sea level-induced oyster reef accretion causes minimal net carbon burial change

Article References: Rodriguez, A. B., Ridge, J. T., Bost, M. C., Nice, N., Eisemann, E., Sharifi, Y., Himmelstein, J. D., & Fodrie, F. J. (2026). Sea level-induced oyster reef accretion causes minimal net carbon burial change. Nature Geoscience. https://doi.org/10.1038/s41561-026-02121-y

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02121-y

Keywords: oyster reefs, sea-level rise, carbon burial, blue carbon, coastal protection, living shorelines, Crassostrea virginica, ecosystem services, calcification, North Carolina estuaries, Nature Geoscience, climate change

The scale of the problem facing oyster reefs provides the backdrop for the new valuation. Worldwide, an estimated 85 percent of oyster reefs have been lost over the past century, with the steepest declines concentrated in estuaries and river deltas, the very environments where these reef-building bivalves thrive. Like all coastal ecosystems, oyster reefs are under growing stress from overexploitation, pollution, and climate change. The United Nations’ Sustainable Development Goal on life below water calls for the conservation and sustainable use of coastal and marine resources to safeguard human wellbeing, yet the habitats that could deliver many of those benefits continue to disappear. The new study was designed to quantify exactly what is being lost, and what could be regained, if restoration were pursued at scale.

To build their global picture, the researchers, led by corresponding author Dr Matteo Convertino, an associate professor at Tsinghua University, began by assessing where suitable habitat for oyster reefs exists today and where reefs could spontaneously form given the right protection and restoration interventions. They imported thousands of geotagged records of oyster reefs from two public databases and then applied a well-established machine learning algorithm to predict geographic distributions and habitat suitability around the world, flagging potential new locations where reefs might be established. The suitability model weighed a suite of environmental variables, including local water temperature, salinity, light availability, nutrient richness, and oxygenation. Any location scoring 65 percent or greater on habitat suitability was classified as suitable for reef formation.

The team then went a step further, combining each area’s suitability score with a measure of the ecosystem services a reef there could provide to calculate what they call the ecopotential of every location. The higher the ecopotential, the greater the potential benefit of restoring or protecting a reef at that site. This portfolio of services extends far beyond oysters for human consumption. Oyster reefs filter enormous volumes of water, protect shorelines from erosion and storm damage, recycle nutrients through the food web, and sequester carbon. They also provide food, shelter, and nursery grounds for a plethora of fish and invertebrates, functioning as foundational habitat that supports entire coastal food webs. Capturing the full value of this portfolio is what pushes the estimated GEP into the trillions.

The modeling results identified ten hotspot nations where predicted ecopotential is greatest: the United States, Australia, Chile, China, Japan, South Korea, Argentina, France, Germany, and Italy. Across these countries, approximately 295,000 square kilometers, or about 6 percent of their coastal zones, were deemed suitable for oyster reefs. Yet reefs are currently known to occupy only 10 percent of that suitable area, mainly in lagoons and bays, estuaries, and river deltas. The gap between what exists and what could exist points to enormous room for expansion. Perhaps more striking is the finding that only 23 percent of the suitable area currently falls under any form of marine protection, meaning the vast majority of remaining and potential reef habitat remains exposed to the pressures that have already erased most of the world’s oyster reefs.

The quantified benefits of full restoration are vivid when expressed in raw ecological terms. If every area with positive ecopotential were occupied by reefs, the authors calculate that these ecosystems could filter prodigious volumes of water: 77.58 quadrillion liters in the United States, 69.92 quadrillion liters in Australia, and 46.58 quadrillion liters in Chile. Water filtration by dense aggregations of filter-feeding oysters improves clarity, removes excess nutrients, and can help suppress harmful algal blooms, cascading through coastal ecosystems in ways that benefit seagrasses, fish populations, and human users alike. The same fully realized reef network could sequester as much as 12.3 million metric tons of carbon per year, adding a meaningful climate mitigation component to the reefs’ portfolio of services.

For a single country, the monetary value of just a subset of these services is already substantial. In the United States alone, the combined potential value of nitrogen removal, carbon sequestration, and water purification by oyster reefs was estimated at 25.1 billion dollars a year. Nitrogen removal matters because coastal eutrophication, driven by agricultural and urban runoff, is one of the most damaging forces in estuarine systems worldwide, and oysters incorporate and process nitrogen as they filter and grow. Carbon sequestration reflects the burial of organic material in reef structures and surrounding sediments. Water purification, meanwhile, underpins fisheries, tourism, and coastal water quality. These three services represent only part of the total portfolio, which is why the global GEP estimate reaches into the trillions when all services and all suitable nations are considered.

The authors are careful to frame these figures as estimates rather than guarantees. The calculations infer ecosystem services from global averages of key parameters, and the actual values could shift with changes in oyster populations over time or with the advancing climate crisis. Significant work on habitat restoration and protection would also be required before any of the potential could be unlocked. We estimated that 90 percent of sites suitable for oyster reefs require restoration, while 80 percent need protection through conservation efforts, Convertino said. In other words, the current oyster situation is very suboptimal, but there is a huge opportunity, considering the benefits. The statement captures the central tension of the study: the ecological and economic potential is enormous, but realizing it demands investment and policy action on a comparable scale.

Climate change adds a further layer of complexity to the outlook. Because global warming generally decreases the habitat quality of oyster reefs, the researchers expect the global GEP to decline from the current potential value of twelve trillion dollars as conditions shift. Yet the picture is not uniformly negative. Some countries may see an expansion of reefs due to more favorable conditions for the growth and dispersal of oysters. The study singles out Australia, Indonesia, South Korea, Japan, and France as places where reef extent could grow under warming conditions, a reminder that the biogeography of coastal ecosystems will be reshuffled rather than simply diminished. For policymakers, the implication is that restoration priorities may need to be dynamic, tracking shifting habitat suitability over decades.

What the study ultimately offers is a new way of seeing an ecosystem that most people encounter only on a dinner plate. By translating the ecological work of oyster reefs into the language of gross ecosystem product, the researchers make a case that resonates in ministries of finance as much as in marine biology laboratories. A reef that filters quadrillions of liters of water, sequesters millions of tons of carbon, buffers shorelines, and nurseries commercial fish stocks is not a luxury but infrastructure, and infrastructure that has been allowed to collapse by 85 percent in a single century. The twelve trillion dollar figure is not a promise but a ceiling, one that depends on restoring nine tenths of suitable sites and protecting four fifths of them. Whether the world’s nations choose to make that investment will determine whether oyster reefs remain a remnant of past coastlines or become a cornerstone of future planetary health.

Subject of Research: Global valuation of the gross ecosystem product of oyster reefs

Article Title: The world’s oyster reefs could add $12 trillion to planetary health each year – if they were better managed

Article References: The world’s oyster reefs could add $12 trillion to planetary health each year – if they were better managed. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: oyster reefs, ecosystem services, gross ecosystem product, marine restoration, coastal ecosystems, water filtration, carbon sequestration, habitat suitability, machine learning, climate change, marine conservation, Frontiers in Marine Science