Climate change is amplifying the intensity of severe weather events, with coastal regions such as coral reefs facing heightened vulnerability to cyclonic wave forces. Structural models to predict bending stress and breakage of corals have been developed for coral colonies to enhance comprehension and prediction of the effects of hydrodynamic disturbances on coral reefs. However, there is scope for improving these predictions by evolving the methodology for quantifying complicated and variable coral morphologies. This study aims to predict breakage thresholds for two of the most prevalent coral morphologies: branching and plate corals (using Acropora muricata and Acropora hyacinthus as study species). Laboratory and field measurements were taken to assess coral morphologies and material characteristics. Morphological features of 47 branching colonies and 100 plate colonies were surveyed at the study site (Heron Reef, southern GBR) and the tensile strength of 80 coral samples was obtained by in situ and laboratory testing. Three-dimensional structural models of branching and plate coral colonies were developed, encompassing multiple coral colonies with varying morphological patterns from relatively shallow (5-7 m) to deep (9-12 m) zones. Model results were calibrated and verified with existing data, revealing that velocity thresholds of 1.7 m/s and 5.0 m/s would destroy 90% of the simulated branching coral structures growing in the deep and shallow parts of the forereef zone, respectively. In contrast, the plate corals have sufficient margins of safety even in extreme flow conditions (7 m/s). Additionally, skeletal strength and structural performance were adjusted based on varying degrees of bioerosion inside the coral skeleton. A higher probability of breakage was observed as the extent of bioerosion increased. The laboratory experiments of hydrodynamic loads on coral colony show that the sheltering effect due to one or two neighbouring colonies in the upwave direction is negligible. These models can be easily adjusted to provide predictions for other coral species, shapes, levels of bioerosion, and locations (e.g., sheltered or exposed areas). Comprehensive predictions about the level of expected damage and rubble generation in different areas can be used in reef management planning and restoration prioritization.

Substrate stabilisation can be used to enhance coral recruitment where mobile rubble beds have formed postdisturbance. We trialled gabion-like 'reef bags', biodegradable coconut-fibre coir mesh bags filled with coral rubble at Pinnacle Bay and Bait Reef on the Great Barrier Reef. Most coir remained intact during the first 12 months, but had completely biodegraded after 2 years, leaving behind the experimental rubble mounds. After approximately 2 years, fish abundance was higher above mounds compared to surrounding rubble. After approximately 3 years, rubble stability and binding was also higher in rubble mounds than in surrounding rubble at Pinnacle Bay, but not at Bait Reef (although binding did increase in Bait Reef mounds over this time). The increased stability and binding did not, however, translate to significantly higher coral recruitment on rubble mounds in either location. The placement of these reef bags in terms of depth, reef zone, sediment load and competition appears crucial. Future trials should consider the size and interlocked-ness of rubble, the size of reef bags, larger mesh hole sizes, the number of layers of coir, and include unbagged mounds.

Coral reef systems are facing unprecedented pressures due to climate change, and stable coral rubble substrates are crucial for facilitating large-scale coral regeneration. This study integrates the Sixth Phase of the Coupled Model Intercomparison Project climate models, sea-level rise projections from the Intergovernmental Panel on Climate Change Sixth Assessment Report, Shared Socioeconomic Pathway scenarios, and applies machine learning techniques to assess the risk of coral rubble instability in the Great Barrier Reef under future wave climate and depth change scenarios. Using the EC-Earth climate model under the SSP5-8.5 scenario-calibrated with data from 41 synoptic stations-we estimated various climate data for 2031-2100 and examined the impact of key factors such as wave climate and depth changes on the risk of coral instability. Coral rubble instability risk depends on future wave climate and depth changes from sea-level rise and deposition. Future changes in wave climate are expected to increase the risk of instability, while increased depth mitigates these destabilizing effects. Over the next 70 years, most areas of the Great Barrier Reef are projected to experience stable or decreasing risk of coral rubble instability. The proportion of no-risk areas is higher in the northern regions, whereas the far southern regions have fewer no-risk areas and more high-risk zones. High-risk and very high-risk areas are mainly concentrated along reef edges, reefs facing the Pacific Ocean, and shallow waters near the shoreline. The transition between high-risk and low-risk areas is gradual rather than abrupt. Annual projections align with long-term trends: coral rubble remains relatively stable in the northern and central regions, which is more conducive to future coral recovery. However, the persistent presence of high-risk and very high risk areas poses significant challenges to coral recovery in the far southern region. By providing insights into the spatial and temporal evolution of coral rubble instability risk, this study aims to support decision-makers, environmental scientists and researchers in formulating appropriate interventions to enhance the resilience of coral reefs under changing environmental conditions.

Tropical cyclones generate destructive waves that cause large-scale yet patchy structural damage to corals through dislodgement and breakage. Such damage can impede the effectiveness of active management and interventions. Here, we used a process-based spectral wave model combined with over 1500 synthetic cyclone tracks to estimate high-resolution (20-200 m) near-bottom wave velocity on more than 3000 reefs across the Great Barrier Reef (GBR). We then applied a statistical model with likelihood inference to predict damage given cyclone strength and reef spatial arrangement, and calibrated the model using field observations from five cyclones. This enabled us to define effective model-based velocity thresholds of 2.5 m/s for nearshore reefs and 3.1 m/s for offshore reefs to predict coral damage. These thresholds exceed the mechanical strength of branching and tabular corals to withstand wave energy. Reef vulnerabilities to cyclone damage vary across the GBR shelf. Although offshore reefs are more wave-tolerant compared to nearshore reefs, the central outer-shelf reefs have a higher predicted probability of damage given a cyclone (11 %), potentially because these small and sparse reefs are less effective in dissipating wave energy. Across the GBR, we identified the top 10 % most exposed cyclone hotspots as well as the top 10 % least exposed refugia with relatively high probabilities of experiencing high and low cyclonic wave velocities, respectively. Our model provides a predictive tool and risk maps to assess reef vulnerability to cyclones, highlighting natural disturbance refugia to inform management strategies for reef resilience.

Unstable coral rubble hinders coral recruitment and recovery of coral reefs after damage from cyclones and bleaching events. If coral rubble remains unstable under typical everyday environmental conditions, areas of coral rubble will not be able to recover. Evaluating the probability of rubble instability over regional scale reef systems can assist the optimization of coral reef restoration efforts. Currently, robust and verified models for such applications do not exist. This paper presents a comprehensive assessment method to predict the probability of coral rubble instability, which combines a fluid-structural interaction approach with a statistical regional wave climate model. The hydrodynamic model employs non-linear wave theory to determine near-bed velocity, pressure gradients, and the corresponding drag and inertia forces acting on the coral rubble. The instability model assesses when overturning or sliding forces exceed resisting forces, considering thousands of combinations of different coral sizes and densities to calculate the proportion of instability under a given wave forcing. The model was calibrated and validated using prior laboratory experiments as reported by Kenyon et al. (2023b). The hydrodynamic and instability models use an extensive dataset of non-cyclonic wave climates (hindcast from over 30 years of wind measurements) specific to the region around Heron Reef, Great Barrier Reef, Australia, enabling a comprehensive evaluation of the probability of rubble instability in this area. Results indicate that the overall probability of rubble instability ( Pr 3 ) reaches 0.74 in water depths less than 2 m (typical of reef crests or reef flats), while it declines to below 0.21 at a depth of 12 m (typical deeper parts of the fore reef). Coral rubble on reef crests near Heron Reef, which are sheltered by surrounding formations, demonstrates low probability of instability. Thus, coral rubble instability is influenced by both its specific location within the reef and the position of the reef relative to other nearby reefs. By integrating the rubble instability model with non-cyclonic wave climate data, a map of the probability of rubble instability was generated for eight reefs in the Capricorn and Bunker Group (CBG). This map provides valuable guidance for coral reef restoration efforts, significantly reducing the need for extensive field-based data.

Climate change is amplifying the frequency and intensity of severe weather events, with coastal regions such as the Great Barrier Reef (GBR) facing heightened vulnerability to cyclonic wave forces. Structural models have been developed for coral colonies to enhance comprehension and prediction of the effects of hydrodynamic disturbances on coral reefs. However, the methodology for quantifying complicated and variable coral morphologies remains inadequate, and the corresponding data remains considerably limited, thereby impeding structural analysis and hindering the broader-scale forecasting of coral breakage. This study focuses on two of the most prevalent coral morphologies within the GBR, namely branching and plate corals. Both laboratory and field measurements have been executed to assess coral morphologies and material characteristics. Based on the collected data, 3D structural models of branching and plate coral colonies have been developed, demonstrating the capability to generate multiple coral colonies with varying morphological patterns from different forereef zones. These models are combined with hydrodynamic modelling to predict the probability of failure under different wave climates. As part of the GBR Reef Restoration and Adaption Program, the results are used to create risk maps of coral failure and rubble generation at reef and GBR scales.

The stability of coral rubble is crucial for coral reef recovery since a stable coral rubble base is essential for coral regeneration. To enhance the effectiveness of coral reef restoration and to model the recovery and connectivity of coral reef ecosystems, it is crucial to predict coral rubble stability over extensive areas and long timeframes. The assessment of rubble instability on individual small-scale coral reefs can be calculated using physical-based models. However, these models are time consuming, limiting their ability to predict coral rubble instability over large spatial scales with high resolution. Here, the Random Forest machine learning algorithm is used to efficiently process large databases containing information on coral reef hydrodynamics and related coral rubble instability and provide estimates on the importance of specific variables in the classification of rubble as stable or unstable. Adopting a Random Forest model provides a significant advantage in addressing the non-linear problems inherent in risk assessment and significantly reducing the model calculation time. In this study, physics-based and Random Forest-based assessment models were adopted to evaluate coral rubble instability risk across the Great Barrier Reef (GBR), with the performance of the trained Random Forest model evaluated against the physics-based model. Seven hydrodynamic characteristics were selected, and 1.2 million physics-based model samples were utilised in the Random Forest model for training (70 %), validation (20 %) and testing (10 %). These samples include the southern, central, and northern reef areas of the Great Barrier Reef, ensuring that the model is comprehensively trained and capable of producing more accurate and reasonable prediction results. Results show that the Random Forest model reduces the prediction time by several order of magnitude compared to the physics-based model: assessing rubble instability in the entire GBR takes only two minutes with the Random Forest model. Additionally, the trained Random Forest model eliminates the resolution limitation: it can rapidly assess new cases irrespective of data resolution changes, without the need for recalculations, unlike physics-based models. Moreover, the Random Forest model enables broader application for assessing coral rubble instability beyond just the GBR. This is because the Random Forest model only requires relevant key factor data to assess coral rubble instability. Among all seven factors, mean depth has the greatest impact on the prediction results, with a Gini decrease index of 34 %. The Gini decrease index measures the importance of a factor by indicating how much it decreases the impurity in the data split; a higher value means greater importance. Other factors have indices around 10 %. Therefore, even in regions with limited data, rubble instability can be effectively assessed with only depth measurements and wave climate information. This study demonstrates a novel and highly successful approach to predicting rubble instability on large scales, offering value guidance for coral reef recovery efforts in the GBR and significantly reducing the required field-based data.

Nature-based solutions (NbS) present a promising approach to coastal protection, leveraging the natural capacity of coastal ecosystems to mitigate hazards. Despite their potential, the practical implementation of NbS faces obstacles, including a lack of clear guidance for design and implementation. In this study, we conducted interviews with 34 practitioners involved in NbS projects for coastal protection to (1) identify key perceived barriers and suggested/or realised solutions and (2) assess how these varied among practitioner groups, spanning Coastal Engineers, Coastal Engineering Scientists, Ecologists, and/or Project Managers. During the interviews, practitioners identified 34 distinct challenges to NbS implementation, falling into 14 categories and 345 solutions into 15 categories. Both challenges and solutions varied between practitioner groups. While all groups identified unfamiliarity with NbS as a key challenge, Coastal Engineers and Coastal Engineering Scientists had more design-focused views about NbS challenges (e.g. risk, technical guidelines and data deficits) and solutions (e.g. hybrid solutions). In contrast, Ecologists and Project Managers typically had more implementation-focused challenges (e.g. cost/lack of funding) and opportunity-driven solutions (e.g. community acceptance and education). The solutions most suggested by Coastal Engineers were for hybrid solutions, whereas Coastal Engineering Scientists suggested interdisciplinary teams. The anthropocentric-ecocentric gap between engineers and ecologists highlights the need for NbS teams to be interdisciplinary and utilise standardised language. Overcoming challenges to NbS will also require advocacy for government support and policy reform, along with early, meaningful engagement and capacity building with Indigenous people, which was identified as a crucial solution to current NbS challenges.

Retrogressive breach failures (RBF) are submarine landslides that result in a nearly vertical sand wall above and below the water surface. Previous studies suggest a four-phase mechanism of RBFs, including triggering, propagation, termination, and recovery phases. There have been both laboratory and field studies on the later three phases, while the triggering mechanism of RBFs remain unknown given the event occurrence is unpredictable both spatially and temporally. Amity Point on North Stradbroke Island, 37 km Northeast from Brisbane, Australia is a valuable coastal flow slide study site with frequent occurrence of approximately every two weeks (Beinssen et al., 2014) of them at a fixed location. Aiming at revealing the triggering mechanism of flow slides, an eight-meter-tall underwater tripod was manufactured and anchored on the seabed. The tripod location was designed to be fixed at a point with minimum bathymetry change during flow slide events referring to existing studies (Beinssen et al., 2014) with between 1.5 m and 2 m deep helical anchors. The tripod collapsed overnight by the undermining of two of the three helical anchors, including the 2 m deep anchor, by a significant underwater flow slide while there was minimal flow slide evidence on the beach. The short lifespan of the tripod presents the complexity of RBFs (especially underwater) and the limitations of existing research. Underwater RBF events that do not propagate to the shore (or slightly erode the shore) occur more frequently than previous research reported (Beinssen et al., 2014). While it does not propagate to the shore, the event still erodes a significant amount of sand underwater. It indicates the triggering occurs at 10 m underwater or even deeper.

Wave attenuation provided by coastal ecosystems, including seagrass, mangroves, and saltmarsh, has been well studied in coastal engineering literature, but results are often not comparable due to differences in experimental methods, reporting units and a lack of complete published datasets. Many coastal ecology and conservation studies aim to qualitatively quantify the value of these ecosystems for coastal protection. However, without data reported in a comparable format, quantitative values are not finding their way into the ecology literature, contributing to the misconception that ‘rules of thumb’ exist for predicting coastal protection. To alleviate the challenges of non-comparable data and incomplete datasets, the drag coefficient (CD) is a useful tool to quantify and compare wave attenuation provided by different ecosystems. Using seagrass as a case study, we conducted a meta-analysis of 119 seagrass-wave attenuation studies both in the field and in laboratory experiments, of which 11 were eligible for inclusion in our study (Twomey et al., 2020).

Coastal flow slides have been reported around the world (Mastbergen et al., 2019; Nédélec et al., 2022), often at over-steepened banks subject to strong tidal flows, pose threat to coastal communities and infrastructure (Lulla, 2017). With sea level rise caused by climate change, these flow slides are expected to occur more frequently in more sites worldwide. Amity Point Beach, North Stradbroke Island (27o 23’ 35’’S, 153o26’23’’E), about 40 km from Brisbane, Australia, have experienced regular coastal flow slides for more than 40 years (Eberhardt, 1978). Together with observations at Amity Point beach field site, small-scale flow slides are simulated in laboratory by tilting metal box filled with sand and fluid.

The proportional cover of rubble on reefs is predicted to increase as disturbances increase in intensity and frequency. Unstable rubble can kill coral recruits and impair binding processes that transform rubble into a stable substrate for coral recruitment. A clearer understanding of the mechanisms of inhibited coral recovery on rubble requires characterisation of the hydrodynamic conditions that trigger rubble mobilisation. Here, we investigated rubble mobilisation under regular wave conditions in a wave flume and irregular wave conditions in situ on a coral reef in the Maldives. We examined how changes in near-bed wave orbital velocity influenced the likelihood of rubble motion (e.g. rocking) and transport (by walking, sliding or flipping). Rubble mobilisation was considered as a function of rubble length, branchiness (branched vs. unbranched) and underlying substrate (rubble vs. sand). The effect of near-bed wave orbital velocity on rubble mobilisation was comparable between flume and reef observations. As near-bed wave orbital velocity increased, rubble was more likely to rock, be transported and travel greater distances. Averaged across length, branchiness and substrate, loose rubble had a 50 % chance of transport when near-bed wave orbital velocities reached 0.30 m s−1 in both the wave flume and on the reef. However, small and/or unbranched rubble pieces were generally mobilised more and at lower velocities than larger, branched rubble. Rubble also travelled further distances per day (∼2 cm) on substrates composed of sand than rubble. Importantly, if rubble was interlocked, it was very unlikely to move (< 7 % chance) even at the highest velocity tested (0.4 m s−1). Furthermore, the probability of rubble transport declined over 3 d deployments in the field, suggesting rubble had snagged or settled into more hydrodynamically stable positions within the first days of deployment. We expect that snagged or settled rubble is transported more commonly in locations with higher-energy events and more variable wave environments. At our field site in the Maldives, we expect recovery windows for binding (when rubble is stable) to predominantly occur during the calmer north-eastern monsoon when wave energy impacting the atoll is less and wave heights are smaller. Our results show that rubble beds comprised of small rubble pieces and/or pieces with fewer branches are more likely to have shorter windows of recovery (stability) between mobilisation events, and thus be good candidates for rubble stabilisation interventions to enhance coral recruitment and binding.

The development of slosh suppression blocks is an important step forward for mitigating sloshing in floating closed containment fish tanks and makes the tanks suitable for operating in a broader range of wave conditions. This experimental study investigates the motion and sloshing response of a scaled floating rigid containment tank model under regular wave action in a wave flume. The tank is fitted with slosh suppression blocks of different dimensions to investigate their effectiveness in mitigating sloshing while considering the state of the internal fluid for fish well-being. This study shows that by sacrificing moderately higher sloshing amplitudes at lower excitation periods, slosh suppression blocks are able to reduce the overall sloshing amplitudes and mitigate the build-up to large sloshing amplitudes. By letting fluid overtop onto the slosh suppression block disturbances to the main fluid volume are minimised, thereby preserving a calm living environment for the fish below. Effective slosh mitigation allows for deploying these closed containment systems in higher energy fish farming sites as the aquaculture industry expands operations into more spacious offshore sites.

The Sixth Assessment report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) states with high confidence that most sandy coasts around the world will experience an increase in coastal erosion over the twenty-first century. An increase in long term coastal erosion (coastline recession) along sandy coasts can translate into massive socio-economic impacts, unless appropriate adaptation measures are implemented in the next few decades. To adequately inform adaptation measures, it is necessary to have a good understanding of the relative importance of the physical processes driving coastline recession, as well as of linkages between consideration (or not) of certain processes and the level of risk tolerance; understandings that are hitherto lacking. Here, we apply the multi-scale Probabilistic Coastline Recession (PCR) model to two end-member sandy coastal types (swell dominated and storm dominated), to investigate where and when coastline recession projections are dominated by the differential contributions from Sea Level Rise (SLR) and storm erosion. Results show that SLR substantially increases the projected end-century recession at both types of coasts and that projected changes in the wave climate have only a marginal impact. An analysis of the Process Dominance Ratio (PDR), introduced here, shows that the dominance of storm erosion over SLR (and vice versa) on total recession by 2100 depends on both the type of the beach and the risk tolerance levels. For moderately risk-averse decisions (i.e. decisions accounting only for high exceedance probability recessions and hence do not account for very high amounts of potential recession—for example, the placement of temporary summer beach cabins), additional erosion due to SLR can be considered as the dominant driver of end-century recession at both types of beaches. However, for more risk-averse decisions that would typically account for higher potential recession (i.e. lower exceedance probability recessions), such as the placement of coastal infrastructure, multi-storey apartment buildings etc., storm erosion becomes the dominant process. The results of this study provide new insights on which physical processes need to be considered when and where in terms of numerical modelling efforts needed for supporting different management decisions, potentially enabling more streamlined and comprehensive assessments of the efficacy of coastal adaptation measures.

Model term LR χ 2 df p-value (>χ 2 ) calculated.velocity_corrected 1012.400 1 <0.001 size 8.692 3 0.034 substrate 0.732 1 0.392 branched 191.305 1 <0.001 calculated.velocity_corrected:size92.921 3 <0.001calculated.velocity_corrected:substrate33.668 1 <0.001Table S3 Pairwise comparisons between branched and unbranched rubble for 4 rubble size categories and 4 levels of continuous variable 'velocity' (0.01 m/s, 0.2 m/s (mean), 0.3 m/s, 1 0.4 m/s) for model described in Table S2. 2 Rubble size Near-bed wave orbital velocity (m/s) Odds Ratio SE df z-ratio p-value

Marine aquaculture in floating closed containment tank systems is a promising proposition for the seafood farming industry as it addresses many farming problems associated with open net pens. Current floating closed containment systems typically have a relatively large free surface area and the contained fluid makes up most of the total mass. A major challenge is sloshing of water in the tanks when they are sited at more exposed farming sites. Significant sloshing affects the structural integrity of the containment tank, the day-to-day operation and fish well-being. As fish benefit from a calm and controlled environment with a free surface for air and sunlight, it is necessary to find an engineering solution to mitigate sloshing while live fish are in the tanks. A novel solution developed by University of Queensland (UQ) researchers is to install a slosh suppression block at the top end of the tank. These suppression blocks may be fabricated from HDPE material, and they form an annular slab with its top surface in line with the internal water surface. These suppression blocks contract the free water surface and interfere with resonance by letting sloshing fluid overtop onto them. Experiments were carried out on a floating containment tank model fitted with slosh suppression blocks in UQ’s wave flume. The experimental results demonstrated that appropriately designed slosh suppression blocks can mitigate resonant sloshing and sloshing amplitude. The sloshing flow associated with slosh suppression blocks reduces interference with the main fluid volume where the fish reside to facilitate a calm and controlled environment for fish well-being.

Coral reefs are likely to be exposed to more intense cyclones under climate change. Cyclone impacts are spatially highly variable given complex hydrodynamics, and coral-specific sensitivity to wave impacts. Predicting reef vulnerability to cyclones is critical to management but requires high resolution environmental data that are difficult to obtain over broad spatial scales. Using 30m-resolution wave modelling, we tested cyclonic and non-cyclonic wave metrics as predictors of coral damage on 22 reefs after severe cyclone Ita impacted the northern Great Barrier Reef, Australia in 2014. Analyses of coral cover change accounting for the type of coral along a gradient of vulnerability to wave damage (e.g., massive, branching, Acroporids) excluded cyclone-generated surface wave metrics (derived from wave height) as important predictors. Increased bottom stress wave environment (near-bed wave orbital velocity) due to Ita (Ita-Ub) explained spatial patterns of 17% to 46% total coral cover loss only when the initial abundance of Acroporids was accounted for, and only when exceeding 35% cover. Greater coral losses occurred closer to the cyclone path irrespective of coral type. Massive and encrusting corals, however, had losses exacerbated in higher non-cyclonic bottom-wave energy environments (nc-Ub). The effect of community composition on structural vulnerability to wave damage was more important predicting damage that the magnitude of the cyclone-generated waves, especially when reefs are surveyed well beyond where damaging waves are expected to occur. Exposure to Ita-Ub was greater in typically high nc-Ub environments with relatively low cover of the most fragile morphologies explaining why these were the least affected overall. We reveal that the common surface-wave metrics of cyclone intensity may not always be able to predict spatial impacts and conclude that reef vulnerability assessments need to account for chronic wave patterns and differences in community composition in order to provide predictive tools for future conservation and restoration.

A modelling framework for using regional climate projections to assess flooding hazard has been developed and applied to the Gwydir River (catchment 26 600 km2 and floodplain 8100 km2), NSW, Australia. The model framework uses NSW and ACT Regional Climate Modelling version 1.5 projections combined with computationally efficient hydrologic and hydraulic models. Although it required model management and high-performance computing resources, the modelling framework successfully processed 18 regional climate projections into flood projections. Specifically, a six-member set of climate model combinations simulating a historical period (1951–2005) and a future period (2006–2100) under two global emission pathways (RCP4.5 and RP8.5) were used to predict flood depth and speed. In total, 1470 continuous years were simulated at hourly time steps. These flood (depth and speed) projections were analysed to assess the flood hazard changes under future climate scenarios by estimating changes in the annual probability of occurrence of a range of flood hazard classes. The six-member ensemble indicates that the flood hazard in the Gwydir Valley will decrease in the short, medium and long term. There are also cases within the ensemble, which includes increases in all non-safe flood hazard classifications while decreasing the safe flood hazard classification.