As waves interact with the slopes of coral reefs and other steep bathymetry profiles, plunging breaking usually occurs where the free surface overturns and violent water motion is triggered. Resolving these surf zone processes pose significant challenges for conventional mesh-based hydrodynamic models, due to the rapidly-deforming nature of the free surface and associated flows. Yet the accurate prediction of these surf zone hydrodynamics is critical for predicting a wide range of nearshore processes driven by wave breaking (e.g., wave dissipation and energy transfers; mean water levels and currents; and wave runup). In this study we assess the ability of the mesh-free, Lagrangian particle-based numerical modelling approach Smoothed Particle Hydrodynamics (SPH) based on DualSPHysics, to simulate the fine-scale hydrodynamic processes driven by irregular wave transformation over a fringing reef profile, by comparing results against detailed experimental observations from a physical modelling study. To greatly improve the computational efficiency, the SPH model was coupled to the mesh-based multi-layer nonhydrostatic wave-flow model SWASH. With this coupled approach, SWASH was used to efficiently simulate the evolution of non-breaking waves from the wavemaker up to the fore reef slope, with the SPH model then used to simulate the detailed hydrodynamic processes over the reef from just offshore of the breakpoint to the shoreline. The SPH model was able to accurately reproduce the complex free surface deformations during plunging breaking, the spectral evolution of waves across the reef flat (including nonlinear wave shape), the mean water levels and currents, and wave runup at the shoreline. Using the long duration simulations (>400 wave periods), the model was able to reproduce the full range of wave motions over the reef (from sea-swell to infragravity frequencies), including the increasing dominance of low frequency waves towards the shoreline and the large cross-reef standing wave motions excited by the reef geometry.

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. Re-entrainment 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.

Wave-driven water level variability (and runup at the shoreline) is a significant cause of coastal flooding induced by storms. Wave runup is challenging to predict, particularly along tropical coral reef-fringed coastlines due to the steep bathymetric profiles and large bottom roughness generated by reef organisms, which can violate assumptions in conventional models applied to open sandy coastlines. To investigate the mechanisms of wave-driven water level variability on a reef-fringed coastline, we performed a set of laboratory flume experiments on an alongshore uniform bathymetric profile with and without bottom roughness. Wave setup and waves at frequencies lower than the incident sea-swell forcing (infragravity waves) were found to be the dominant components of runup. These infragravity waves were positively correlated with offshore wave groups, signifying they were generated in the surf zone by the oscillation of the breakpoint. On the reef flat and at the shoreline, the low-frequency waves formed a standing wave pattern with energy concentrated at the natural frequencies of the reef flat, indicating resonant amplification. Roughness elements used in the flume to mimic large reef bottom roughness reduced low-frequency motions on the reef flat and reduced wave runup by 30% on average, compared to the runs over a smooth bed. These results provide insight into sea-swell and infragravity wave transformation and wave setup dynamics on steep-sloped coastlines, and the effect that future losses of reef bottom roughness may have on coastal flooding along reef-fringed coasts.

Abstract Tropical cyclones generate extreme hazards along coastlines, often leading to losses of life and property. Although coral reefs exist in cyclone‐prone regions globally, few studies have measured the hydrodynamic conditions and morphological responses of reef‐fringed coastlines to tropical cyclones. Here, we examine the impact of Tropical Cyclone Olwyn on a section of Australia's largest fringing reef (Ningaloo Reef) using in situ wave and water level observations, topographic surveys, and numerical modeling. Despite forereef significant wave heights reaching 6 m and local winds of 140 km h−1, average beach volume change was only −3 m3 m−1. The results indicate that this erosion was due to locally generated wind waves within the lagoon rather than the offshore waves that were dissipated on the reef crest. A comparison of these volume changes to observations of tropical cyclone impacts along exposed sandy beaches quantitatively demonstrates the substantial coastal protection reefs can provide against extreme storms.

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.

The comparative role of subtidal water level and wave height variations on seasonal shoreline changes was investigated at a reef‐fringed beach in southwestern Australia. The data set consisted of continuous sea level and wave records, monthly topographic beach surveys over a 2 year period, and bimonthly high‐resolution aerial images over a 7 year period. Shorelines were extracted from images and topographic surveys, and then an empirical orthogonal function (EOF) analysis was applied to both data sets. The temporal amplitudes of the first EOF mode of the image‐derived shoreline data set (∼60% of the variance) were most correlated with 30 day averages of the subtidal water level (variations up to ±0.2 m) driven by geostrophic adjustment of the Leeuwin Current. The geostrophic response of the Leeuwin Current was found to be further correlated with the phasing of the El Niño–Southern Oscillation, leading to higher water levels during La Niña and lower levels during El Niño, which as a consequence resulted in interannual variations in the shoreline behavior reflected in the first EOF mode. The temporal amplitudes of the second EOF mode (∼20% of the variance) were most correlated with 50 day averages in offshore wave height. Our results indicate that the seasonal beach response was primarily influenced by seasonal variations in offshore water level rather than by wave heights as has been generally observed in exposed beaches. A simple EOF‐based model is presented, which reproduces the alongshore‐variable shoreline response and its seasonal and interannual modulation due to El Niño–Southern Oscillation events over the study period.

Rocky reef coastlines typically feature highly variable and often abrupt cross-shore and alongshore changes in bathymetry. The effects of this irregular rocky bathymetry on the dynamics of infragravity waves are largely unknown. Most models of infragravity wave dynamics have been developed and validated on smooth alongshore-uniform bathymetries, which may break down over these highly variable bathymetries. A 2 week field experiment was conducted on a rocky reef-fringed beach to investigate how the variable bathymetry affects the spatial and temporal variability of infragravity waves. The height of short (sea-swell) waves decreased over the shallow reef due to breaking, whereas the height of infragravity waves increased toward the shoreline. Both during a storm event (H-m0 = 2.3 m) and under moderate wave conditions (H-m0 = 1.0-1.8 m), the infragravity waves formed a persistent cross-shore standing wave pattern along the entire shoreline, despite the irregular bathymetry. In addition, the alongshore components of infragravity waves refracted by the presence of the nearshore reef were observed to propagate in opposite directions up and down the coast resulting in a local alongshore standing wave pattern. Thus, the presence of highly variable nearshore bathymetry, which commonly occurs along rocky reef coastlines, may produce both cross-shore and alongshore standing wave patterns.

Four years of beach elevation surveys at Ocean Beach, San Francisco, California, are used to extend an existing equilibrium shoreline change model, previously calibrated with fine sand and moderate energy waves, to medium sand and higher-energy waves. The shoreline, characterized as the cross-shore location of the mean high water contour, varied seasonally by between 30 and 60 m, depending on the alongshore location. The equilibrium shoreline change model relates the rate of horizontal shoreline displacement to the hourly wave energy E and the wave energy disequilibrium, the difference between E and the equilibrium wave energy that would cause no change in the present shoreline location. Values for the model shoreline response coefficients are tuned to fit the observations in 500 m alongshore segments and averaged over segments where the model has good skill and the estimated effects of neglected alongshore sediment transport are relatively small. Using these representative response coefficients for 0.3 mm sand from Ocean Beach and driving the model with much lower-energy winter waves observed at San Onofre Beach (also 0.3 mm sand) in southern California, qualitatively reproduces the small seasonal shoreline fluctuations at San Onofre. This consistency suggests that the shoreline model response coefficients depend on grain size and may be constant, and thus transportable, between sites with similar grain size and different wave climates. The calibrated model response coefficients predict that for equal fluctuations in wave energy, changes in shoreline location on a medium-grained (0.3 mm) beach are much smaller than on a previously studied fine-grained (0.2 mm) beach.

Shoreface nourishment can be an efficient technique to feed sediment into the littoral zone without the order of magnitude cost increase incurred by directly nourishing the beach. An erosion hot spot at Ocean Beach in San Francisco, California, USA, threatens valuable public infrastructure as well as safe recreational use of the beach. In an effort to reduce the erosion at this location, a new beneficial reuse plan was implemented in May 2005 for the sediment dredged annually from the main shipping channel at the mouth of San Francisco Bay. From 2005 to 2007, approximately 230,000 m(3) of sand was placed annually at depths between 9 and 14 m, in a location where strong tidal currents and open-ocean waves could potentially feed sediment onto the section of beach experiencing critical erosion. The evolution of the disposal mound and adjacent beach were monitored with 12 multibeam bathymetric surveys, and over 40 high-resolution beach topographic surveys. In addition, sediment transport processes were investigated using sediment grab samples, acoustic Doppler profilers, and two separate models: a cross-shore profile model (UNIBEST-TC) and a coastal area model (Delft3D). The results of the monitoring and modeling demonstrate that the disposal mound may be effective in dissipating wave energy striking this vulnerable stretch of coast with negligible shadowing effects, but a positive shoreline response can only be achieved by placing the sediment in water depths less than 5 m.

Understanding how sandy beaches respond to storms is critical for effective sediment management and developing successful erosion mitigation efforts. However, only limited progress has been made in relating observed beach changes to wave conditions, with one of the major limiting factors being the lack of temporally dense beach topography and nearshore wave data in most studies. This study uses temporally dense beach topographic and offshore wave data to directly link beach response and wave forcing with generally good results. Ocean Beach is an open coast high-energy sandy beach located in San Francisco, CA, USA. From April 2004 through the end of 2008, 60 three-dimensional topographic beach surveys were conducted on approximately a monthly basis, with more frequent "short-term" surveys during the winters of 2005-06 and 2006-07. Shoreline position data from the short-term surveys show good correlation with offshore wave height, period, and direction averaged over several days prior to the survey (mean R-2=0.54 for entire beach). There is, however, considerable alongshore variation in model performance, with R-2 values ranging from 0.81 to 0.19 for individual sections of the beach. After wave height, the direction of wave approach was the most important factor in determining the response of the shoreline, followed by wave period. Our results indicate that an empirical predictive model of beach response to wave conditions at Ocean Beach is possible with frequent beach mapping and wave data, and that such a model could be useful to coastal managers.