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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. Plain Language Summary: Small‐scale nearshore rocky reefs are found worldwide along a variety of sandy and rocky coastlines. Wave breaking over small reefs drives mean alongshore circulation patterns in their lee that may cause shoreline accretion or erosion. In this study, we apply a wave‐flow model to investigate the physical drivers of the mean currents in the lee of small reefs. The alongshore circulation was primarily driven by the differences of the mean water levels between the lee and the adjacent nonreef fronted beaches. Mean water levels increased by wave breaking; however, the onshore‐directed mean flows over the reef created offshore‐directed bottom stresses that reduced the mean water levels in the reef lee. If the shoreline mean water levels in the lee were less than the adjacent beach, alongshore currents that converged from the adjacent beaches toward the reefs were developed. If the shoreline mean water levels in the lee of the reef exceeded the adjacent beach, alongshore currents that diverged from the reef toward the adjacent beach occurred. The improved understanding of the circulation drivers developed in this study enhances our ability to characterize and predict wave‐driven flows in small‐scale nearshore reef systems. Key Points: Phase‐resolved model simulations were used to understand circulation patterns that occur in the lee of small‐scale reefsTwo‐cell and four‐cell circulation patterns were primarily driven by alongshore pressure gradients, which depend on the setup dynamicsA four‐cell pattern generally developed if the shoreline wave setup in the lee of the reef was less than the adjacent nonreef fronted beach [ABSTRACT FROM AUTHOR]

Wave breaking on the steep fore‐reef slopes of shallow fringing reefs can be effective at dissipating incident sea‐swell waves prior to reaching reef shorelines. However, wave setup and free infragravity waves generated during the sea‐swell breaking process are often the largest contributors to wave‐driven water levels (wave runup) at the shoreline. Laboratory flume experiments and a two‐dimensional vertical phase‐resolving nonhydrostatic wave‐flow model, which includes a canopy model to predict drag forces generated by roughness elements, were used to investigate wave‐driven water levels for along‐shore uniform fringing reefs. In contrast to many previous studies, both the laboratory experiment and the numerical model account for the effects of large bottom roughness. The numerical model reproduced the observations of the wave transformation and runup over both smooth and rough reef profiles. The numerical model was then extended to quantify the influence of reef geometry and compared to simulations of plane beaches lacking a reef. For a fixed offshore forcing condition, the fore‐reef slope controlled wave runup on reef‐fronted beaches, whereas the beach slope controlled wave runup on plane beaches. As a result, the coastal protection utility of reefs is dependent on these slopes. For our examples, with a fore‐reef slope of 1/5 and a 500 m prototype reef flat length, a beach slope of ∼1/30 marked the transition between the reef providing runup reduction for steeper beach slopes and enhancing wave runup for milder slopes. Roughness coverage, spacing, dimensions, and drag coefficient were investigated, with results indicating the greatest runup reductions were due to tall roughness elements on the reef flat. Plain Language Summary: Wave breaking and bottom friction are effective at reducing incident sea‐swell waves before they can reach the shoreline of reef‐fronted coastlines. However, globally, mass mortality of coral reef‐building organisms is causing coral reefs to become structurally flatter, a process that may lower wave dissipation. Laboratory flume experiments and a multilayer phase‐resolving wave‐flow model that includes drag forces generated by roughness elements (representing coral reef forms) were used to investigate wave‐driven water levels in the lee of fringing reefs and assess the influence of reef geometry and roughness characteristics. The numerical model was able to reproduce laboratory observations of waves and water levels over both smooth and rough reef profiles. Wave runup, the vertical extent of wave uprush on a beach, was sensitive to the presence and characteristics of roughness. Roughness coverage, spacing, and dimensions were investigated with results indicating the greatest runup reductions were due to tall roughness elements on the reef flat. Key Points: SWASH reproduced laboratory observations of waves and runup over both smooth and rough fringing reefsFore‐reef slope controls wave runup on reef‐fringed coasts as opposed to beach slope on plane open coast beachesWave runup was sensitive to bottom roughness with tall roughness elements on the reef flat generating the greatest runup reduction [ABSTRACT FROM AUTHOR]

A fully nonlinear non-dispersive energy balance for surfzone waves is derived based on the nonlinear shallow water equations to study the nearshore dynamics of infragravity (IG) waves. Based on simulations of waves on a relatively moderate and mild beach slope with a non-hydrostatic wave-flow model (SWASH), the new theory shows that spatial gradients in IG energy flux are nearly completely balanced by the combined effect of bottom stresses and predominantly nonlinear triad interactions. The new balance confirms many features of existing weakly nonlinear theories, and yields an improved description in the inner surfzone where waves become highly nonlinear. A gain of IG energy flux throughout the shoaling and outer surfzones is driven by triad interactions between IG waves and pairs of sea-swell (SS) waves. The IG energy flux decreased in the inner surfzone, primarily through an energy cascade to the swell-band and superharmonic frequencies where wave energy is ultimately dissipated. Dissipation by bottom friction was weak on both slopes. The IG wave breaking, characterized by triads between three IG or two IG waves and one SS wave, was significant only deep inside the surfzone of the mild slope. Even though IG waves broke on the mild slope, nonlinear interactions between IG waves and pairs of SS waves were responsible for at least half of the net IG flux loss. [ABSTRACT FROM AUTHOR]

Understanding directional spectra of infragravity (IG) waves composed of free and bound components is required due to their impacts on various coastal processes (e.g., coastal inundation and morphological change). However, conventional reconstruction methods of directional spectra relying on linear wave theory are not applicable to IG waves in intermediate water depths (20–30 m) due to the presence of bound waves. Herein, a novel method is proposed to reconstruct directional spectra of IG waves in intermediate depth based on weakly nonlinear wave theory. This method corrects cross‐spectra among observed wave signals by taking account of the nonlinearity of bound waves in order to reconstruct directional spectra of free IG waves. Numerical experiments using synthetic data representing various directional distributions show that the proposed method reconstructs free IG wave directional spectra more accurately than the conventional method. The method is subsequently applied to observations of severe sea‐states at two field sites. At these sites, free IG waves are not isotropic and have clear peak directions. Numerical modeling of the wave fields shows that these peak directions correspond to the reflection of IG waves from the shore and/or coastal structures. Additionally, the validity of the underlying weakly nonlinear wave theory of the present method is assessed by a newly proposed method employing bispectral analysis. The bound wave response generally agrees with the theory at the field sites but deviates slightly for energetic sea states. The applicability of the present method on a sloping bottom is further discussed by an analytical solution. Plain Language Summary: Infragravity (IG) waves, long waves whose wave period is much longer than sea and swells, are known to play important roles in coastal inundation and beach topographic change during high wave conditions. However, the magnitude of IG waves propagating to beaches is not well understood. This is because conventional methods to estimate directional distributions (wave energy propagating to each direction) of sea and swells are not applicable to IG waves that are composed of "free" and "bound" components. In this study, a novel method to estimate directional distributions of IG waves is proposed. The method estimates directional distributions of free IG waves by considering bound IG waves in observed wave data based on their theoretical solution. This method is tested in numerical experiments using synthetic wave data, and the results demonstrate its high applicability and superiority over the conventional method. Applying this method to field measurement data reveals that free IG waves are directionally focused owing to the reflection from the shore and coastal structures. These findings violate the assumption of uniform directional distributions of free IG waves implemented in recent numerical models. The new method will help future studies to elucidate the magnitude of IG waves propagating to beaches. Key Points: A new method to reconstruct directional spectra of free infragravity waves based on weakly nonlinear wave theory is proposedThe method is verified to accurately reconstruct directional spectra of free infragravity waves from synthetic dataDirectional distributions of free infragravity waves vary diversely owing to wave reflection from nearby beaches and coastal structures [ABSTRACT FROM AUTHOR]

Infragravity waves are low-frequency surface waves that can impact a variety of nearshore and oceanic processes. Recent measurements in the North Sea showed that significant bursts of infragravity energy occurred during storm events. Using a spectral wave model, we show that a substantial part of this energy was radiated from distant shorelines where it was generated by the incident sea-swell waves. These radiated infragravity waves can cross the North Sea basin and reach distant shorelines. The origin of the infragravity wave energy varied between the different storms, and particularly depends on where largest sea-swell waves made landfall. Along the coastlines of the North Sea, shoreward directed infragravity waves that originate from a remote source were non-negligible during storm events. This suggests that radiated infragravity waves can potentially contribute to coastal dynamics and hazards away from their region of generation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 phaseresolved 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. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Aquatic plants that grow in estuaries and coastal oceans (such as seagrass, kelp, and mangroves) have a considerable influence on water flow (currents) and on waves propagating toward the shore. However, mean flows generated by the waves interacting with aquatic vegetation (with time scales much longer than the individual wave periods) have not been comprehensively studied. This study provides a description of how submerged vegetation alters the mean wave‐driven flow structure. A combination of detailed experiments conducted in a wave flume and numerical simulations are used to show that the mean flow just above the vegetation is relatively strong (up to 20% to 50% of the maximum wave velocity above the canopy), while it is considerably weaker inside the vegetation. We identified three forces that govern the mean current profile: the wave Reynolds stress gradient, the turbulent Reynolds stress gradient, and the vegetation drag force. These forces are usually not accurately described in larger‐scale computer simulations of coastal processes. However, reliable simulation of processes in the coastal ocean such as sediment transport and nutrient exchange requires an accurate prediction of mean flows, and thus these forces need to be properly incorporated in computer models when applied to regions with aquatic vegetation. Key Points: Physical and numerical modeling is used to investigate the wave‐induced mean flow profiles in the presence of submerged vegetationThe multilayered phase‐resolving wave‐ flow model was able to accurately reproduce the depth variation of the wave‐driven mean flow profilesWave‐driven mean flows inside vegetation canopies are driven by vertical gradients in the wave and turbulent Reynolds stresses [ABSTRACT FROM AUTHOR]

Currents can affect the evolution of waves in nearshore regions through altering their wavenumber and amplitude. Including the effect of ambient currents (e.g., tidal and wind-driven) on waves in phase-resolving wave models is not straightforward as it requires appropriate boundary conditions in combination with a large domain size and long simulation duration. In this paper, we extended the non-hydrostatic wave-flow model SWASH with additional terms that account for the influence of a depth-uniform ambient current on the wave dynamics, in which the current field can be taken from an external source (e.g., from observations or a circulation model). We verified the model ability by comparing predictions to results from linear theory, laboratory experiments and a spectral wave model that accounts for wave interference effects. With this extension, the model was able to account for current-induced changes to the wave field (i.e., changes to the wave amplitude, length and direction) due to following and opposing currents, and two classical examples of sheared currents (a jet-like current and vortex ring). Furthermore, the model captured the wave dynamics in the presence of strong opposing currents. This includes reflections of relatively small amplitude waves at the theoretical blocking point, and transmission of breaking waves beyond the theoretical blocking point for larger wave amplitudes. The proposed model extension allows phase-resolving models to more accurately and efficiently simulate the wave dynamics in coastal regions with tidal and/or wind-driven flows. • Extension of non-hydrostatic model to account for effect ambient currents on waves. • Comparison to linear theory, spectral wave models and laboratory experiments. • Extended model captured current-induced changes to wave kinematics. • Improved modelling of wave dynamics in coastal regions with tidal/wind-driven currents. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • Extension to non-hydrostatic approach to accommodate submerged floating objects. • Model results compare well with analytical solutions, CFD model, and lab experiment. • The model efficiently captures the wave-structure interactions and body response. • First step towards simulating regional WEC impacts with a non-hydrostatic model. [ABSTRACT FROM AUTHOR]

This paper presents a numerical model to simulate the evolution of waves and their interactions with a restrained ship that is moored in coastal waters. The model aims to be applicable at the scale of a harbour or coastal region, while accounting for the key physical processes that determine the hydrodynamic loads on the ship. Its methodology is based on the non-hydrostatic wave-flow model SWASH, which provides an efficient tool to simulate the nonlinear dynamics that govern the nearshore wave field. In this work, we propose a new numerical algorithm that accounts for the presence of a non-moving floating body, to resolve the wave impact on a restrained ship. The model is validated through comparisons with an analytic solution, a numerical solution, and two laboratory campaigns. The results of the model-data comparison demonstrate that the model captures the scattering of waves by a restrained body. Furthermore, it gives a reasonable prediction of the hydrodynamic loads that act on a restrained container ship for a range of wave conditions. Importantly, the model captures these dynamics efficiently, which demonstrates that it retains this favourable property of the non-hydrostatic approach when a floating body is included. The findings of this study suggest that the model provides a promising new alternative to simulate the nonlinear evolution of waves and their impact on a restrained ship at the scale of a realistic harbour or coastal region. [ABSTRACT FROM AUTHOR]

Predictions of the wave-induced response of floating structures that are moored in a harbour or coastal waters require an accurate description of the (nonlinear) evolution of waves over variable bottom topography, the interactions of the waves with the structure, and the dynamics of the mooring system. In this paper, we present a new advanced numerical model to simulate the wave-induced response of a floating structure that is moored in an arbitrary nearshore region. The model is based on the non-hydrostatic approach, and implemented in the open-source model SWASH, which provides an efficient numerical framework to simulate the nonlinear wave evolution over variable bottom topographies. The model is extended with a solution to the rigid body equations (governing the motions of the floating structure) that is tightly coupled to the hydrodynamic equations (governing the water motion). The model was validated for two test cases that consider different floating structures of increasing geometrical complexity: a cylindrical geometry that is representative of a wave-energy-converter, and a vessel with a more complex shaped hull. A range of wave conditions were considered, varying from monochromatic to short-crested sea states. Model predictions of the excitation forces, added mass, radiation damping, and the wave-induced response agreed well with benchmark solutions to the potential flow equations. Besides the response to the primary wave (sea-swell) components, the model was also able to capture the second-order difference-frequency forcing and response of the moored vessel. Importantly, the model captured the wave-induced response with a relatively coarse vertical resolution, allowing for applications at the scale of a realistic harbour or coastal region. The proposed model thereby provides a new tool to seamlessly simulate the nonlinear evolution of waves over complex bottom topography and the wave-induced response of a floating structure that is moored in coastal waters. • New numerical tool to predict the wave-induced response of moored floating structures. • Comparison to solutions to the potential flow equations for a variety of sea-states. • The model captures the first and second-order response induced by irregular waves. • Model can predict the motions of a moored structure based on an offshore sea state. [ABSTRACT FROM AUTHOR]

Abstract: The non-hydrostatic wave model SWASH is compared to flume observations of infragravity waves propagating over a plane slope and barred beach. The experiments cover a range of infragravity wave conditions, including forcing by bichromatic and irregular waves, varying from strongly dissipative to strongly reflective, so that model performance can be assessed for a wide range of conditions. The predicted bulk wave parameters, such as wave height and mean wave period, are found to be in good agreement with the observations. Moreover, the model captures the observed breaking of infragravity waves. These results demonstrate that SWASH can be used to model the nearshore evolution of infragravity waves, including nonlinear interactions, dissipation and shoreline reflections. [Copyright &y& Elsevier]

Coastal safety assessments with wave-resolving storm impact models require a proper offshore description for the incoming infragravity (IG) waves. This boundary condition is generally obtained by assuming a local equilibrium between the directionally-spread incident sea-swell wave forcing and the bound IG waves. The contribution of the free incident IG waves is thus ignored. Here, in-situ observations of IG waves with wave periods between 100 s and 200 s at three measurement stations in the North Sea in water depths of O (30) m are analyzed to explore the potential contribution of the free and bound IG waves to the total IG wave height for the period from 2010 to 2018. The bound IG wave height is computed with the equilibrium theory of Hasselmann using the measured frequency-directional sea-swell spectra as input. The largest IG waves are observed in the open sea with a maximum significant IG wave height of O (0.3) m at 32 m water depth during storm Xaver (December 2013) with a concurrent significant sea-swell wave height in excess of 9 m. Along the northern part of the Dutch coast, this maximum has reduced to O (0.2) m at a water depth of 28 m with a significant sea-swell wave height of 7 m and to O (0.1) m at the most southern location at a water depth of 34 m with a significant sea-swell wave height of 5 m. These appreciable IG wave heights in O (30) m water depth represent a lower bound for the expected maximum IG wave heights given the fact that in the present analysis only a fraction of the full IG frequency range is considered. Comparisons with the predicted bound IG waves show that these can contribute substantially to the observed total IG wave height during storm conditions. The ratio between the predicted bound- and observed total IG variance ranges from 10% to 100% depending on the location of the observations and the timing during the storm. The ratio is typically high at the peak of the storm and is lower at both the onset and waning of the storm. There is significant spatial variability in this ratio between the stations. It is shown that differences in the directional spreading can play a significant role in this. Furthermore, the observed variability along the Dutch coast, with a substantially decreased contribution of the bound IG waves in the south compared to the northern part of the Dutch coast, are shown to be partly related to changes in the mean sea-swell wave period. For the southern part of the Dutch coast this corresponds to an increased difference with the typically assumed equilibrium boundary condition although it is not clear how much of the free IG-energy is onshore directed barring more sophisticated observations and/or modeling. [ABSTRACT FROM AUTHOR]

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 5–25 s), infragravity wave heights (periods 25–600 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. [ABSTRACT FROM AUTHOR]

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. • A phase-resolved model simulates hydrodynamic interactions with submerged structures. • Waves can drive 2-cell or 4-cell mean circulation and sediment transport patterns. • Bottom shear stresses presented an overall similar pattern to the mean currents. • We develop a predictor for the flow patterns based on wave and structure parameters. • The submerged structures had limited influence on shoreline setup and wave runup. [ABSTRACT FROM AUTHOR]