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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.

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 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%.

A laboratory experiment was conducted to investigate the dynamics of cross-shore sediment transport across a fringing coral reef. The aim was to quantify how a highly bimodal spectrum of high-frequency (sea-swell) and low-frequency (infragravity and seiching) waves that is typically present on coral reef flats, influences the various sediment transport mechanisms. The experiments were conducted in a 55 m wave flume, using a 1:15 scale fringing reef model that had a 1:5 forereef slope, a 14 m long reef flat, and a 1:12 sloping beach. The initial 7 m of reef flat had a fixed bed, whereas the back 7 m of the reef and the beach had a moveable sandy bed. Four seven-hour irregular wave cases were conducted both with and without bottom roughness elements (schematically representing bottom friction by coral roughness), as well as for both low and high still water levels. We observed that the wave energy on the reef flat was partitioned between two primary frequency bands (high and low), and the proportion of energy within each band varied substantially across the reef flat, with the low-frequency waves becoming increasingly important near the shore. The offshore transport of suspended sediment by the Eulerian mean flow was the dominant transport mechanism near the reef crest, but a wide region of onshore transport prevailed on the reef flat where low-frequency waves were very important to the overall transport. Ripples developed over the movable bed and their properties were consistent with the local high-frequency wave orbital excursion lengths despite substantial low-frequency wave motions also present on the reef flat. This study demonstrated that while a proportion of the sediment was transported by bedload and mean flow, the greatest contributions to cross-shore transport were due to the skewness and asymmetry of the high and low-frequency waves.

Aquatic vegetation in the coastal zone dissipates wave energy and provides a form of natural coastal protection against storm waves. The capacity of aquatic vegetation to attenuate incident waves depends on the extent to which wave energy is dissipated by small-scale hydrodynamic interactions within a canopy, which in turn depends on the work done by drag forces exerted by the canopy. Canopy drag forces (and hence rates of wave dissipation) are conventionally parameterised using a drag coefficient ( C d ). Existing empirical models for predicting C d are usually dependent solely on either Reynolds number ( R e ) or Keulegan–Carpenter number ( K C ), and hence neglect the potential effect of vegetation canopy density ( λ ) and the interactions between adjacent stems. This study uses high-resolution numerical simulations to investigate the dynamics of wave-driven oscillatory flow through emergent canopies (modelled as arrays of rigid cylinders). The simulations cover a wide range of K C . The influence of two mechanisms in modifying canopy drag, namely the effects of blockage and sheltering, are evaluated. The blockage effect is found to be the dominant mechanism responsible for increasing the canopy drag coefficients at high K C for medium to high density canopies; conversely, the sheltering effect plays only a minimal role in reducing the drag coefficient of the very sparse canopies. We show that Cd for canopies at high K C can be robustly estimated using a new modified drag formulation for unidirectional flow. Conversely, in the limit of the low K C , Cd is close to that of a single isolated cylinder. The results of this study can be used as a basis for developing new predictive formulations for specifying bulk canopy drag coefficients, and in turn quantifying wave attenuation by vegetation.

Tropical Cyclone (TC) Olwyn travelled along the coast of Ningaloo Reef in northwestern Australia as a Category 3 TC during 12–13 March 2015 with sustained wind speeds of over 130 km h−1 and significant wave heights on the forereef reaching 6 m. Observations from TC Olwyn showed that, contrary to typical wave transformation patterns across a coastal reef-lagoon, under the TC conditions there was a substantial shoreward increase of wave height. This unusual pattern was primarily due to strong local wind wave generation under the extreme wind conditions, despite the lagoon itself being relatively small (order 1 km wide) and shallow (order 1 m depth). Two common types of coastal numerical wave models that include either wave growth (i.e. the phase-averaged model SWAN) or infragravity waves (i.e. the surfbeat resolving model XBeach), but not both processes, were used to assess the capabilities of numerical models to predict the cyclone-induced hydrodynamics. Model results showed that wave conditions in the lagoon and at the shoreline were dominated by locally-generated wind waves, with the reef efficiently dissipating the large incident waves offshore. The infragravity waves in the lagoon were only of secondary importance during the TC compared to the locally-generated sea-swell waves. Although both the phase-averaged (SWAN) and surfbeat-resolving (XBeach) models were able to predict some of the local hydrodynamic responses, for this specific site and storm, the use of a phase-averaged model yielded the most accurate predictions of the hydrodynamics given that it included wind wave growth. However, because the influence of infragravity waves cannot be neglected in reef environments in general, these results emphasize the need for a more general modeling approach that requires the addition of wind wave growth formulations into surfbeat- or fully-phase resolving classes of numerical wave models and/or the addition of an infragravity wave formulation in spectral wave models.

Low-frequency (infragravity) wave dynamics on a fringing coral reef were investigated using the numerical model XBeach ( Roelvink et al., 2009 ). First, the skill of the model was evaluated in one- and two-dimensions based on its predictions of short waves (0.04–0.2 Hz), infragravity waves (0.004–0.04 Hz) and water level measurements (tidal and wave setup) obtained during a 2009 field study at Ningaloo Reef in Western Australia. The model calibration was sensitive to friction coefficients for short waves and current/infragravity bed friction, which were assumed independent in this model study. Although the one-dimensional cross-shore model captured the gradients in the dominant hydrodynamic processes at the site, a high current/IG bed friction coefficient was required. This resulted in an overestimation and a phase lag between the observed and predicted wave setup signal. In the two-dimensional model, a lower (more realistic) current/infragravity wave friction coefficient was required to achieve optimum performance due to the presence of significant reef and lagoon mean flows in the model, which led to reduced setup across the reef. The infragravity waves were found to propagate from the surf zone across the reef in a dominantly cross-shore direction towards the shore, but with substantial frictional damping. The infragravity waves were strongly modulated also over the reef by tidal depth variations, primarily due to the variability in frictional dissipation rates when the total water depth over the reef varied. Two mean wave-driven circulation cells were observed in the study area, with cross-shore flow becoming more alongshore-dominated before exiting the system via the two channels in the reef. The results reveal that short waves dominated bottom stresses on the forereef and near the reef crest; however, inside the lagoon, infragravity waves become increasingly dominant, accounting up to 50% of the combined bottom stresses.