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Long waves play an important role in coastal inundation and shoreline and dune erosion, requiring a detailed understanding of their evolution in nearshore regions and interaction with shorelines. While their generation and dissipation mechanisms are relatively well understood, there are fewer studies describing how reflection processes govern their propagation in the nearshore. We propose a new approach, accounting for partial reflections, which leads to an analytical solution to the free wave linear shallow-water equations at the wave-group scale over general varying bathymetry. The approach, supported by numerical modeling, agrees with the classic Bessel standing solution for a plane sloping beach but extends the solution to arbitrary alongshore uniform bathymetry profiles and decomposes it into incoming and outgoing wave components, which are a combination of successively partially reflected waves lagging each other. The phase lags introduced by partial reflections modify the wave amplitude and explain why Green’s law, which describes the wave growth of free waves with decreasing depth, breaks down in very shallow water. This reveals that the wave amplitude at the shoreline is highly dependent on partial reflections. Consistent with laboratory and field observations, our analytical model predicts a reflection coefficient that increases and is highly correlated with the normalized bed slope (bed slope relative to wave frequency). Our approach shows that partial reflections occurring due to depth variations in the nearshore are responsible for the relationship between the normalized bed slope and the amplitude of long waves in the nearshore, with direct implications for determining long-wave amplitudes at the shoreline and wave runup.

Wave energy converters (WECs) will have to be arranged into arrays of many devices to extract commercially viable amounts of energy. To understand the potential coastal impacts of WEC arrays, most research to date has relied on wave-averaged models given their computational efficiency. However, it is unknown how accurate wave-averaged model predictions are given a lack of validation data and their inherent simplifications of various hydrodynamic processes (e.g., diffraction). This paper compares the predictions of coastal wave farm impacts from a coupled wave-averaged and flow model (Delft3D-SNL-SWAN), to a wave-resolving wave-flow model (SWASH) that intrinsically accounts for more of the relevant physics. Model predictions were compared using an idealized coastal bathymetry over a range of wave conditions and wave farm geometries. Both models predicted the largest impacts (changes to the nearshore hydrodynamics) for large and dense wave farms located close to the shore (1 km) and the smallest impacts for the small and widely spaced farm at a greater offshore distance (3 km). However, the wave-resolving model generally predicted somewhat larger impacts (i.e., changes to the nearshore wave heights, mean velocities and mean water levels). We also found that coupling the wave-averaged model to a flow model resulted in more realistic downstream predictions than the stand-alone wave-averaged model.

Nearshore rocky reefs with scales of order 10–100 m are common along the world's coastline and often shape wave-driven hydrodynamics and shoreline morphology in their lee. The interaction of waves with these reefs generally results in either two or four-cell mean circulation systems (2CC and 4CC, respectively), with diverging flows behind the reefs and at the shoreline in the 2CC case and flows that diverge in the lee and converge at the shoreline in the 4CC case. By applying a phase-resolving wave-flow model to conduct a detailed analysis of mean momentum balances for waves interacting with nearshore reefs, we develop an understanding of the drivers of 2CC and 4CC flow dynamics and how they vary for different reef geometries and wave and water level conditions. The 2CC or 4CC patterns were primarily driven by alongshore pressure gradients toward the exposed (nonreef fronted) or reef-fronted beach. These alongshore pressure gradients were dependent on the cross-shore setup dynamics governed by the balance between pressure (i.e., related to the setup) and radiation stress gradients, and mean bottom stresses exerted on the water column. If shoreline wave setup in the lee of the reef was less than the exposed beach, a 4CC pattern developed with convergent flow at the shoreline in the lee of the reef; otherwise, a 2CC emerged with divergent flow at the shoreline. Across the parameter space investigated, reef roughness, distance to the shoreline, and beach slope were the three parameters most likely to change the flow patterns between 2CC and 4CC.

Coastal habitats have faced decades of loss caused by urbanization. Global recognition of the ecosystem services that coastal habitats provide has led to an emphasis on cities to adopt nature-based solutions (NBS). However, a broad assessment of urban areas and their potential to conserve remaining coastal habitat has not been undertaken. Here we apply spatial analytics to investigate 5,096 coastal urban areas in tropical and subtropical regions within the distribution of mangroves, tidal flats, seagrass meadows, and coral reefs, and find

When extracting wave energy, arrays of wave energy converters (or wave farms) may alter surrounding wave and flow fields. This paper studies the modification of hydrodynamic processes at the coastline induced by nearshore wave farms using a recently developed phase-resolving wave-flow model. Changes to nearshore hydrodynamics were assessed for various farm configurations of submerged point-absorbers positioned 1–3 km offshore that were subject to realistic sea-states. In the lee of the farms, wave heights were attenuated and onshore directed flows were generated that extended several hundred meters shoreward but did not impinge the coast. For scenarios in which the wave shadow extended to the coast, the nearshore wave height and setup were reduced resulting in longshore pressure gradients driving longshore flows that converged in the lee of the farms. Changes were largest for compact farms at smaller offshore distances, and conversely, were significantly smaller for wider spaced arrays at greater offshore distances. Based on a bulk longshore sediment transport formulation, the converging flow patterns indicate conditions favourable for the accumulation of sediment in the direct lee and divergence of sediments at locations up/down coast from the farm, suggesting a reorientation of the shoreline in response to the wave farm configurations considered.

Despite Australia having one of the most abundant offshore wave energy resources globally, there remains a lack of understanding of how this offshore resource extends into the coastal zone where most wave energy converters would be deployed. We used the phase-averaged wave model SWAN to simulate 38 years (1980–2017) of wave conditions near Albany, Western Australia, which has been proposed as a future commercial wave energy development site. The nearshore (30 m depth) wave resource varied seasonally and interannually with the wave energy flux and mean wave direction negatively correlated to the phase of the Southern Annular Mode (SAM) and positively correlated to the latitudinal position of the subtropical high-pressure ridge. As a result, the observed positive trend in SAM over recent decades may cause a decrease in nearshore wave energy (including fewer storm events) and an anti-clockwise (more southerly) rotation in wave direction. These changes may facilitate wave energy development and extraction by reducing the number and magnitude of extreme events during which wave energy cannot be extracted and equipment can be damaged. The interannual fluctuations in the wave resource can be significant and should be considered during the site selection for wave energy projects.

The Australian Forum for Operational Oceanography (FOO), started in 2015, established a working group to focus on wind-waves. One of the aims of this working group was to identify the key priorities of wind-waves research. This undertaking has been driven by Australian needs, but Australia is just one part of the larger international waves community; results of this process are also relevant to other marine-focused nations. The process to identify research priorities engaged both researchers and stakeholders in a democratic, collaborative, and iterative process. The main steps were 1) soliciting possible research questions via an online survey, 2) reviewing and editing the questions at a face-to-face workshop, and 3) ranking the research questions. A total of 360 survey invitations were emailed to possible participants, with 69 respondents. Half of these were from research organizations, and the remainder from private industry, service providers, or government. The survey gathered a list of ideas that were consolidated to 155 suggestions, which were further reviewed by participants at a wind-waves research symposium and then ranked via a voting process. A second round of online voting was then undertaken that specifically targeted the industry and stakeholder community. The top five priorities were identified, and are referred to here as “tier 1” priorities. A further 10 priorities were identified, and are referred to here as “tier 2,” providing a total of 15 top-ranked priorities.

Interactions between oceanic and atmospheric processes within coral reefs can significantly alter local-scale ( 60%) over most of this system; however, the bleaching patterns were not uniform. Little is known about the processes governing thermodynamic variability within atolls, particularly those that are dominated by large amplitude tides. Here, we identify three mechanisms at Scott Reef that alleviated heat stress during the marine heatwave in 2016: (1) the cool wake of a tropical cyclone that induced temperature drops of 1.3 °C over a period of 8 days; (2) air–sea heat fluxes that interacted with the reef morphology during neap tides at one of the atolls to reduce water temperatures by up to 2.9 °C; (3) internal tidal processes that forced deeper and cooler water (up to 2.7 °C) into some sections of the shallow reefs. The latter two processes created localized areas of reduced temperatures that led to lower incidences of coral bleaching for parts of the reef. We predict these processes are likely to occur in other similar tide-dominated reef environments worldwide. Identifying locations where physical processes reduce heat stress will likely be critical for coral reefs in the future, by maintaining communities that can help facilitate local recovery of reefs following bleaching events that are expected to increase in frequency and severity in the coming decades.

Engineered and natural submerged coastal structures (e.g., submerged breakwaters and reefs) modify incident wave fields and thus can alter hydrodynamic processes adjacent to coastlines. Although submerged structures are generally assumed to promote beach protection by dissipating waves offshore and creating sheltered conditions in their lee, their interaction with waves can result in mean wave-driven circulation patterns that may either promote shoreline accretion or erosion. Here, we analyse the mean flow patterns and shoreline water levels (wave runup) in the lee of idealised impermeable submerged structures with a phase-resolved nonhydrostatic numerical model. Waves propagating over submerged structures can drive either a 2-cell mean (wave-averaged) circulation, which is characterised by diverging flows behind the structure and at the shoreline, or 4-cell circulation, with converging flows at the shoreline and diverging flows in the immediate lee of the structure. The numerical results show that the mode of circulation can be predicted with a set of relationships depending on the incoming wave heights, the structure crest level, and distance to the shoreline (or structure depth). Qualitative agreement between the mean flow and proxies for the sediment transport using an energetics approach suggest that the mean flow can be a robust proxy for inferring sediment transport patterns. For the cases considered, the submerged structures had a minimal influence on shoreline wave setup and wave runup despite the wave energy dissipation by the structures due to alongshore wave energy fluxes in the lee. Consequently, these results suggest that the coastal protection provided by the range of impermeable submerged structures we modelled is primarily due to their capacity to promote beach accretion.

Significance The growth of coral reefs is threatened by the dual stressors of ocean warming and acidification. Despite a wealth of studies assessing the impacts of climate change on individual taxa, projections of their impacts on coral reef net carbonate production are limited. By projecting impacts across 233 different locations, we demonstrate that the majority of coral reefs will be unable to maintain positive net carbonate production globally by the year 2100 under representative concentration pathways RCP4.5 and 8.5, while even under RCP2.6, coral reefs will suffer reduced accretion rates. Our results provide quantitative projections of how different climate change stressors will influence whole ecosystem carbonate production across coral reefs in all major ocean basins., Ocean warming and acidification threaten the future growth of coral reefs. This is because the calcifying coral reef taxa that construct the calcium carbonate frameworks and cement the reef together are highly sensitive to ocean warming and acidification. However, the global-scale effects of ocean warming and acidification on rates of coral reef net carbonate production remain poorly constrained despite a wealth of studies assessing their effects on the calcification of individual organisms. Here, we present global estimates of projected future changes in coral reef net carbonate production under ocean warming and acidification. We apply a meta-analysis of responses of coral reef taxa calcification and bioerosion rates to predicted changes in coral cover driven by climate change to estimate the net carbonate production rates of 183 reefs worldwide by 2050 and 2100. We forecast mean global reef net carbonate production under representative concentration pathways (RCP) 2.6, 4.5, and 8.5 will decline by 76, 149, and 156%, respectively, by 2100. While 63% of reefs are projected to continue to accrete by 2100 under RCP2.6, 94% will be eroding by 2050 under RCP8.5, and no reefs will continue to accrete at rates matching projected sea level rise under RCP4.5 or 8.5 by 2100. Projected reduced coral cover due to bleaching events predominately drives these declines rather than the direct physiological impacts of ocean warming and acidification on calcification or bioerosion. Presently degraded reefs were also more sensitive in our analysis. These findings highlight the low likelihood that the world’s coral reefs will maintain their functional roles without near-term stabilization of atmospheric CO2 emissions.

Long waves are generated and transform when short-wave groups propagate into shallow water, but the generation and transformation processes are not fully understood. In this study we develop an analytical solution to the linearized shallow-water equations at the wave-group scale, which decomposes the long waves into a forced solution (a bound long wave) and free solutions (free long waves). The solution relies on the hypothesis that free long waves are continuously generated as short-wave groups propagate over a varying depth. We show that the superposition of free long waves and a bound long wave results in a shift of the phase between the short-wave group and the total long wave, as the depth decreases prior to short-wave breaking. While it is known that short-wave breaking leads to free-long-wave generation, through breakpoint forcing and bound-wave release mechanisms, we highlight the importance of an additional free-long-wave generation mechanism due to depth variations, in the absence of breaking. This mechanism is important because as free long waves of different origins combine, the total free-long-wave amplitude is dependent on their phase relationship. Our free and forced solutions are verified against a linear numerical model, and we show how our solution is consistent with prior theory that does not explicitly decouple free and forced motions. We also validate the results with data from a nonlinear phase-resolving numerical wave model and experimental measurements, demonstrating that our analytical model can explain trends observed in more complete representations of the hydrodynamics.

Accurate prediction of coastal flooding requires a detailed understanding of all individual contributions to sea level variability and how they interact to trigger extreme sea level (ESL) events. In this study, we focus on the expansive (∼10,000 km) coastline of Western Australia, a region that experiences large latitudinal gradients in met‐ocean sources of sea level variability, as a case study to investigate the mechanisms responsible for ESLs and trends over the past 54 years (1966–2019). Using long‐term sea level records from tide gauges and satellite altimetry, we explore how different contributions to sea level variability at different time scales (from hourly to interannual) interact to generate ESLs. We observe that all individual, nontidal contributions to ESLs (i.e., atmospheric surge, seasonal and interannual variability) are of similar magnitude (of order 10 cm) along the entire coast and comparable to the tidal variations in the microtidal southwestern region. The results reveal the important role that seasonal and interannual sea level variability plays in generating ESLs, with these low‐frequency contributions being relatively large compared to typical global values. With mean sea level having risen by ∼10 cm over this 54‐year study period, sea level rise was also identified as making an increasingly significant contribution to observed increases in the frequency of ESLs. Overall, due to the comparatively large magnitude of low‐frequency sea level contributions (seasonal and longer), the Western Australia coast provides a useful case study to investigate how sustained periods of elevated sea level will impact coastlines worldwide more broadly in the future.

Coral reefs provide critical ecological and geomorphic (e.g. sediment production for reef-fronted shoreline maintenance) services, which interact in complex and dynamic ways. These services are under threat from climate change, requiring dynamic modelling approaches that predict how reef systems will respond to different future climate scenarios. Carbonate budgets, which estimate net reef calcium carbonate production, provide a comprehensive ‘snap-shot’ assessment of reef accretionary potential and reef stability. These budgets, however, were not intended to account for the full suite of processes that maintain coral reef services or to provide predictive capacity on longer timescales (decadal to centennial). To respond to the dual challenges of enhancing carbonate budget assessments and advancing their predictive capacity, we applied a novel model elicitation and review method to create a qualitative geo-ecological carbonate reef system model that links geomorphic, ecological and physical processes. Our approach conceptualizes relationships between net carbonate production, sediment transport and landform stability, and rates knowledge confidence to reveal major knowledge gaps and critical future research pathways. The model provides a blueprint for future coral reef research that aims to quantify net carbonate production and sediment dynamics, improving our capacity to predict responses of reefs and reef-fronted shorelines to future climate change.

Two-dimensional mean wave-driven flow and setup dynamics were investigated at a reef-lagoon system at Ningaloo Reef, Western Australia, using the numerical wave-flow model, SWASH. Phase-resolved numerical simulations of the wave and flow fields, validated with highly detailed field observations (including >10 sensors through the energetic surf zone), were used to quantify the main mechanisms that govern the mean momentum balances and resulting mean current and setup patterns, with particular attention to the role of nonlinear wave shapes. Momentum balances from the phase-resolved model indicated that onshore flows near the reef crest were primarily driven by the wave force (dominated by radiation stress gradients) due to intense breaking, whereas the flow over the reef flat and inside the lagoon and channels was primarily driven by a pressure gradient. Wave setup inside the lagoon was primarily controlled by the wave force and bottom stress. The bottom stress reduced the setup on the reef flat and inside the lagoon. Excluding the bottom stress contribution in the setup balance resulted in an over prediction of the wave-setup inside the lagoon by up to 200–370%. The bottom stress was found to be caused by the combined presence of onshore directed wave-driven currents and (nonlinear) waves. Exclusion of the bottom stress contribution from nonlinear wave shapes led to an over prediction of the setup inside the lagoon by approximately 20–40%. The inclusion of the nonlinear wave shape contribution to the bottom stress term was found to be particularly relevant in reef regions that experience a net onshore mass flux over the reef crest.

International audience; The rates of water exchange between coastal reef systems and the surrounding ocean are key physical drivers of water quality and reef ecosystems. It is generally assumed that water exiting a reef system through reef channels is predominantly replaced by 'new' water from offshore. However, exiting water may also recirculate back into the reef system reducing the rate of exchange between the reef and the ocean, which has implications for reef water temperatures, nutrient fluxes and population connectivity. To quantify flow re-entrainment at a rocky reef site in southwestern Australia, flow patterns were measured with GPS-tracked drifters during a two-week field experiment. The field observations were extended via a set of idealized numerical experiments to determine the effect of variable oceanic forcing and reef geometry on flow re-entrainment. The observations demonstrate that re-entrainment can vary significantly and the numerical results support the hypothesis that re-entrainment increases with increasing offshore wave height, increasing alongshore currents outside of the reef, and decreasing reef channel spacing but is largely not impacted by reef roughness. Reentrainment was correlated with a predictor variable R, which is a measure of wave forcing versus the total offshore flow cross-section, and alongshore currents outside the reef. For large values of R and strong alongshore currents, flow re-entrainment increases the effective flushing time by a factor of three or more. The results suggest that flow re-entrainment may be particularly important in small-scale reef systems or reefs exposed to an energetic wave climate and/or strong alongshore currents.

Coral reef islands are among the most vulnerable landforms to climate change. However, our understanding of their morphodynamics at intermediate (seasonal to interannual) timescales remains poor, limiting our ability to forecast how they will evolve in the future. Here, we applied a semi-automated shoreline detection technique (CoastSat.islands) to 20 years of publicly available satellite imagery to investigate the evolution of a group of reef islands located in the eastern Indian Ocean. At interannual timescales, island changes were characterized by the cyclical re-organization of island shorelines in response to the variability in water levels and wave conditions. Interannual variability in forcing parameters was driven by El Niño Southern Oscillation (ENSO) cycles, causing prolonged changes to water levels and wave conditions that established new equilibrium island morphologies. Our results present a new opportunity to measure intermediate temporal scale changes in island morphology that can complement existing short-term (weekly to seasonal) and long-term (decadal) understanding of reef island evolution.

Understanding the transport and exchange of water masses both within a reef and between a reef and the surrounding ocean is needed to describe a wide-range of ecosystem processes that are shaped by the movement of material and heat. We show how novel Lagrangian data processing methods, specifically developed to reveal key and often hidden transport structures, can help visualize flow transport patterns within and around morphologically complex reef systems. As an example case study, we consider the wave-driven flow transport within the Ningaloo Reef in Western Australia. We show that a network of attracting, repelling, and trapping flow transport structures organizes the flow transport into, around, and out of the reef. This approach is broadly applicable to coral reef systems, since the combination of well-defined bathymetry and persistent flow-forcing mechanisms (e.g., by wave breaking or tides) is conducive to the existence of persistent Lagrangian transport structures that organize material transport., Fluids, 5 (4), ISSN:2311-5521

The non-hydrostatic wave-flow model SWASH was used to investigate the hydrodynamic processes at a reef fringed pocket beach in southwestern Australia (Gnarabup Beach). Gnarabup Beach is a ~1.5 km long beach with highly variable bathymetry that is bounded by rocky headlands. The site is also exposed to large waves from the Southern Ocean. The model performance was evaluated using observations collected during a field program measuring waves, currents and water levels between June and July 2017. Modeled sea-swell wave heights (periods 5&ndash, 25 s), infragravity wave heights (periods 25&ndash, 600 s), and wave-induced setup exhibited moderate to good agreement with the observations throughout the model domain. The mean currents, which were highly-spatially variable across the study site, were less accurately predicted at most sites. Model agreement with the observations tended to be the worst in the areas with the most uncertain bathymetry (i.e., areas where high resolution survey data was not available). The nearshore sea-swell wave heights, infragravity wave heights and setup were strongly modulated by the offshore waves. The headlands and offshore reefs also had a strong impact on the hydrodynamics within the lagoon (bordered by the reefs) by dissipating much of the offshore sea-swell wave energy and modifying the pattern of the nearshore flows (magnitude and direction). Wave breaking on the reef platforms drove strong onshore directed mean currents over the reefs, resulting in off-shore flow through channels between the reefs and headlands where water exchanges from the lagoon to ocean. Our results demonstrate that the SWASH model is able to produce realistic predictions of the hydrodynamic processes within bathymetrically-complex nearshore systems.

The Australian marine research, industry, and stakeholder community has recently undertaken an extensive collaborative process to identify the highest national priorities for wind-waves research. This was undertaken under the auspices of the Forum for Operational Oceanography Surface Waves Working Group. The main steps in the process were first, soliciting possible research questions from the community via an online survey; second, reviewing the questions at a face-to-face workshop; and third, online ranking of the research questions by individuals. This process resulted in 15 identified priorities, covering research activities and the development of infrastructure. The top five priorities are 1) enhanced and updated nearshore and coastal bathymetry; 2) improved understanding of extreme sea states; 3) maintain and enhance the in situ buoy network; 4) improved data access and sharing; and 5) ensemble and probabilistic wave modeling and forecasting. In this paper, each of the 15 priorities is discussed in detail, providing insight into why each priority is important, and the current state of the art, both nationally and internationally, where relevant. While this process has been driven by Australian needs, it is likely that the results will be relevant to other marine-focused nations.

The physical roughness (canopies) formed by organisms within aquatic ecosystems (e.g., seagrass, kelp, and mangroves) modifies the local wave-driven hydrodynamics within coastal and estuarine regions. In wave-dominated environments, an understanding of the mean wave-driven flows generated within and above canopies is important, as it governs material transport (e.g., of nutrients, sediment, and biota). However, until recently the effect of submerged canopies on wave-current interactions and the resulting mean (wave-averaged) flow dynamics has received relatively little attention. In this study, a combination of wave flume experiments and numerical modeling is used to investigate the wave-induced mean flow profiles in the presence of a submerged canopy. The measured velocities and vegetation forces were used to derive bulk drag and inertia coefficients, and to validate a nonhydrostatic 2DV wave-flow model. The numerical model results were used to conduct an in-depth analysis of the mean horizontal momentum terms responsible for driving the mean (horizontal) flow within and above the submerged canopies. We show that the mean canopy hydrodynamics are driven by vertical gradients in wave and turbulent Reynolds stresses, balanced by the mean canopy drag forces. The wave Reynolds stress gradient is the dominant force driving the in-canopy mean flow and is directly related to the vorticity that is generated when the wave orbital motions become rotational near the canopy interface. This study provides new insight in the mechanisms responsible for wave-driven mean flows within submerged canopies and guidance for how these hydrodynamics can be predicted in coastal wave-circulation models.

Tropical coral reef-lined coasts are exposed to storm wave-driven flooding. In the future, flood events during storms are expected to occur more frequently and to be more severe due to sea-level rise, changes in wind and weather patterns, and the deterioration of coral reefs. Hence, disaster managers and coastal planners are in urgent need of decision-support tools. In the short-term, these tools can be applied in Early Warning Systems (EWS) that can help to prepare for and respond to impending storm-driven flood events. In the long-term, future scenarios of flooding events enable coastal communities and managers to plan and implement adequate risk-reduction strategies. Modeling tools that are used in currently available coastal flood EWS and future scenarios have been developed for open-coast sandy shorelines, which have only limited applicability for coral reef-lined shorelines. The tools need to be able to predict local sea levels, offshore waves, as well as their nearshore transformation over the reefs, and translate this information to onshore flood levels. In addition, future scenarios require long-term projections of coral reef growth, reef composition, and shoreline change. To address these challenges, we have formed the UFORiC (Understanding Flooding of Reef-lined Coasts) working group that outlines its perspectives on data and model requirements to develop EWS for storms and scenarios specific to coral reef-lined coastlines. It reviews the state-of-the-art methods that can currently be incorporated in such systems and provides an outlook on future improvements as new data sources and enhanced methods become available.

Shear stresses on vegetated beds play an important role in driving a wide range of processes at the sediment-water interface, including sediment transport. Existing methods for the estimation of bed shear stress are not applicable to vegetated beds due to the significant alteration of the near-bed velocity profile and turbulence intensities by the vegetation. In addition, bed shear stress distributions are highly spatially variable in the presence of vegetation. In this study, computational fluid dynamics simulations were used to investigate the spatial variability of bed shear stresses in the presence of emergent vegetation (modeled as arrays of circular cylinders) and the variation of bed stress with characteristics of both the bulk flow and the array. A recently proposed model that assumes a linear variation of stress in the viscous layer immediately above the bed is shown to be a reliable tool for estimating the spatially averaged bed shear stress over a wide range of flow conditions and vegetation densities. However, application of this model is found to be restrictive due to the lack of a reliable predictive tool for the thickness of the viscous layer. Based on a balance between turbulent kinetic energy production in the vegetation stem wakes and the viscous dissipation of turbulent kinetic energy at the bed, an enhanced formulation is proposed to predict the thickness of the viscous layer, which significantly improves the accuracy of model predictions. This improved model enhances the predictive capability for important benthic processes (such as sediment transport) in vegetated aquatic systems.

With the increasing interest in wave energy, and when moving towards commercial-scale wave-energy projects, a detailed understanding of the interactions between single and arrays of wave-energy converters (WECs) with the ambient wave and flow field becomes imperative for both design and operational purposes, as well as assessment of their environmental impacts. This work presents a new numerical approach to simulate the nonlinear evolution of the waves and their interactions with a submerged wave-energy converter at the scale of a realistic coastal region. The numerical approach is based on the non-hydrostatic framework, and implemented in the open-source SWASH model, which provides an efficient tool to simulate the nonlinear evolution of waves over realistic coastal bathymetries. Here, we present a numerical extension to the non-hydrostatic approach to account for interactions between waves and a submerged point absorber, and to capture the response of such a wave energy device. Model results are compared with an analytical solution based on potential flow theory, a CFD simulation, and experimental data to validate its capabilities in simulating the wave-WEC interactions for both linear and nonlinear wave conditions. Overall, the results of this validation demonstrate that the model captures the wave-structure interactions and the body response with satisfactory accuracy. Notably, the results also indicate that a coarse vertical resolution was sufficient to capture these dynamics, making the model sufficiently computationally efficient to simulate the interaction of waves and WECs over large scales. As a consequence, this new modelling approach should provide a promising new alternative to simulate the interactions between nonlinear wave fields and submerged point absorbers at the scale of a realistic coastal region.

The distant passage of an intense and large Category 3 Tropical Cyclone (TC) generated highly energetic surface waves in a large region across the Australian North West Shelf (NWS). Two sites on the continental shelf experienced maximum significant wave heights of up to 10 m with near-bed wave orbital velocities up to 0.7 m s−1 at 40 and 74 m depth. Concurrent current profiles were measured between 0.5 and 8.5 m above the seabed at these sites and therefore the observations presented in this study allowed a detailed analysis of the wave-current interactions in the continental shelf bottom boundary layer during highly energetic tropical cyclone conditions. These observations revealed a strong modification of the current profiles during the TC with an increase of the apparent bottom roughness by up to 2 orders of magnitude compared to the typical tide-dominated current conditions. This modification was predominantly due to wave-current interactions; however, the results also suggest that changes in bed forms during the TC conditions likely acted as a secondary mechanism contributing to the enhanced bottom roughness experienced by the mean currents. A comparison of the total mean bed shear stresses as a function of the waves and currents showed a somewhat larger enhancement due to the nonlinear wave-current interactions than predicted by common semi-empirical models. Overall, the enhancement of the mean bed shear stresses during the TC significantly reduced the mean current velocities in the bottom boundary layer, highlighting the importance of including wave-current interactions within ocean circulation models to accurately predict shelf currents during TC conditions.

Owing to their strong forcing of the ocean surface, tropical cyclones strongly modify the hydrodynamics of Australia's North West Shelf (one of the world's tropical cyclone hotspots), which in turn plays a dominant role in its sediment dynamics. Previous modeling studies have focused on describing the short term sediment dynamics during individual tropical cyclones, but have lacked validation of the responses using field observations. As a consequence, the long term cumulative impact of the tropical cyclones on the residual sediment transport pathways at the shelf scale remains unclear. In this study we apply a sediment transport model over the North West Shelf, validate its performance using an extensive field dataset, and implement a 14 year‐long model simulation to assess the sediment fluxes. The model results confirm the overwhelming role tropical cyclones play on sediment transport processes over most of the shelf, despite each cyclone only influencing a small portion of the shelf at a particular time. Overall we identified 19 tropical cyclone events over the 14 year period, which despite accounting for less than 5% of time, were found to drive the majority of both the suspended sediment alongshore and seaward cross‐shore transport. The results revealed significant inter‐annual variability of the tropical cyclone‐induced sediment dynamics with greater suspended transport during the 3 consecutive Ningaloo Niño years (2011‐2013) where sea surface temperatures off northwestern Australia were anomalously warm with elevated tropical cyclone activity.

The accurate prediction of extreme wave run-up is important for effective coastal engineering design and coastal hazard management. While run-up processes on open sandy coasts have been reasonably well-studied, very few studies have focused on understanding and predicting wave run-up at coral reef-fronted coastlines. This paper applies the short-wave resolving, Nonhydrostatic (XB-NH) and short-wave averaged, Surfbeat (XB-SB) modes of the XBeach numerical model to validate run-up using data from two 1D (alongshore uniform) fringing-reef profiles without roughness elements, with two objectives: i) to provide insight into the physical processes governing run-up in such environments; and ii) to evaluate the performance of both modes in accurately predicting run-up over a wide range of conditions. XBeach was calibrated by optimizing the maximum wave steepness parameter (maxbrsteep) in XB-NH and the dissipation coefficient (alpha) in XB-SB) using the first dataset; and then applied to the second dataset for validation. XB-NH and XB-SB predictions of extreme wave run-up (Rmax and R2%) and its components, infragravity- and sea-swell band swash (SIG and SSS) and shoreline setup ( ), were compared to observations. XB-NH more accurately simulated wave transformation but under-predicted shoreline setup due to its exclusion of parameterized wave-roller dynamics. XB-SB under-predicted sea-swell band swash but overestimated shoreline setup due to an over-prediction of wave heights on the reef flat. Run-up (swash) spectra were dominated by infragravity motions, allowing the short-wave (but not wave group) averaged model (XB-SB) to perform comparably well to its more complete, short-wave resolving (XB-NH) counterpart. Despite their respective limitations, both modes were able to accurately predict Rmax and R2%.

The temporal and spatial variations in shoreline position were analysed at a low-energy beach in southwestern Australia fronted by an offshore reef to examine the relative contributions of storm events, megacusps and diurnal sea-breeze cycles to the beach morphologic response. The shoreline position did not exhibit a seasonal sinusoidal erosion/accretion pattern as often observed at open coast sandy beaches, but instead a dynamic morphology dominated by storm and recovery cycles that generated fluctuations of ~10 m in the shoreline position. During low swell recovery periods, the beach rapidly developed megacusps (wavelength ~100 m, amplitude ~5 m), which despite generating local erosive changes within the megacusps embayments, the beach still tended to accrete during these low swell conditions. In most cases, the embayments coincided with locations of rip currents suggesting that rip currents might contribute to the formation of the megacusps. Additionally, high-frequency measurements collected over a series of diurnal sea-breeze cycles typical of the site during summer months demonstrated that the erosive impact due to sea breezes was much smaller than that observed during storms, despite wave heights often being similar during these events. While the beach eroded during intensification of the diurnal sea-breeze, the beach mostly exhibited net daily accretion due to the low-energy swell that dominated the wave spectrum during the overnight and morning hours. The formation and alongshore migration of megacusps during sea-breeze cycles also contributed to beach accretion, overwhelming the erosion due to the consecutive sea-breezes events after a few days.