When wind waves break in the nearshore, free long (infragravity) waves are generated through two mechanisms: breakpoint forcing and bound wave release. Previous studies have highlighted that lower frequency groups breaking on steep slopes favor long wave generation by breakpoint forcing, whereas higher frequency groups breaking on mild slopes favor bound wave release. However, no study has established and demonstrated quantitative thresholds for when each mechanism will dominate. In this paper, we use a one-dimensional linear numerical model to quantify each mechanism and assess their relative dominance in free long wave generation, as a function of wave and bathymetry parameters. The results show that the individual contributions of breakpoint forcing and bound wave release do not add up linearly to the total long wave energy generated from both mechanisms due to the phase differences between free long waves generated from each mechanism. The combination of mechanisms always results in a net smaller long wave amplitude. The normalized bed slope parameter is shown to be effective in differentiating between the dominance of the breakpoint-forced and bound wave release mechanisms. The efficiency of breakpoint forcing is found to be independent of bed slope, wave group frequency and short-wave period. Therefore, the relative importance of each mechanism to long wave generation depends mainly on the efficiency of the bound wave release, which varies with these parameters. Overall, the bound wave release mechanism tends to dominate under most conditions, except for cases with low infragravity frequencies that occur on steep slopes (e.g., typical of reef environments).

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.

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.

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 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.

An alongshore array of pressure sensors and a cross-shore array of current velocity and pressure sensors were deployed on a barred beach in southwestern Australia to estimate the relative response of edge waves and leaky waves to variable incident wind wave conditions. The strong sea breeze cycle at the study site (wind speeds frequently > 10 m s−1) produced diurnal variations in the peak frequency of the incident waves, with wind sea conditions (periods 2 to 8 s) dominating during the peak of the sea breeze and swell (periods 8 to 20 s) dominating during times of low wind. We observed that edge wave modes and their frequency distribution varied with the frequency of the short-wave forcing (swell or wind-sea) and edge waves were more energetic than leaky waves for the duration of the 10-day experiment. While the total infragravity energy in the surf zone was higher during swell forcing, edge waves were more energetic during wind-sea periods. However, low-frequency (0.005–0.023 Hz) edge waves were found to be dominant in absence of wind-sea conditions, while higher-frequency (0.023–0.050 Hz) edge waves dominated when wind-sea conditions were present.

Using the breakpoint forcing model, for long wave generation in the surf zone, expressions for the phase difference between the breakpoint-forced long waves and the incident short wave groups are obtained. Contrary to assumptions made in previous studies, the breakpoint-forced long waves and incident wave groups are not in phase and outgoing breakpoint-forced long waves and incident wave groups are not out of phase. The phase between the breakpoint-forced long wave and the incident wave group is shown to depend on beach geometry and wave group parameters. The breakpoint-forced incoming long wave lags behind the wave group, by a phase smaller than /2. The phase lag decreases as the beach slope decreases and the group frequency increases, approaching approximately /16 within reasonable limits of the parameter space. The phase between the breakpoint-forced outgoing long wave and the wave group is between /2 and and it increases as the beach slope decreases and the group frequency increases, approaching 15/16 within reasonable limits of the parameter space. The phase between the standing long wave (composed of the incoming long wave and its reflection) and the incident wave group tends to zero when the wave group is long compared to the surf zone width. These results clarify the phase relationships in the breakpoint forcing model and provide a new base for the identification of breakpoint forcing signal from observations, laboratory experiments and numerical modeling.

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.

Wave energy is a potential emerging contributor to future global low-emission energy needs. If wave energy is to become a significant part of renewable energy portfolios, then wave energy converters (WECs) will need to be installed in large numbers in array configurations. The environmental effects of wave energy extraction to date has relied on physical (i.e., tank) and numerical modelling studies. In this study, a network of in situ wave measurement devices were deployed around an array of three submerged point absorber WECs, operating intermittently, each with a nominal 240 kW peak capacity. The study site was 3 km offshore (in approximately 24 m water depth) and the field campaign was conducted over approximately one year, with a primary goal of studying 'down-wave' effects of the WECs. The observations were used to calibrate and validate a numerical spectral wave model which can represent frequency-dependent absorption by WECs within the model. For the purpose of a straightforward analysis, we focus on a period when only one WEC was operating. Measurements show a decrease in wave height between locations up-wave and down-wave of the WEC, for the period with only one operating WEC. The numerical model demonstrates that the observed wave height attenuation is due to WEC absorption and exceeds the natural variability of wave height at the site. Impacts on the wave field are particularly strong directly down-wave (40 m) of the WEC with reduction of the wind-sea wave height up to 20%.

The presence of large bottom roughness, such as that formed by benthic organisms on coral reef flats, has important implications for the size, concentration, and transport of suspended sediment in coastal environments. A 3 week field study was conducted in approximately 1.5 m water depth on the reef flat at Ningaloo Reef, Western Australia, to quantify the cross-reef hydrodynamics and suspended sediment dynamics over the large bottom roughness (∼20–40 cm) at the site. A logarithmic mean current profile consistently developed above the height of the roughness; however, the flow was substantially reduced below the height of the roughness (canopy region). Shear velocities inferred from the logarithmic profile and Reynolds stresses measured at the top of the roughness, which are traditionally used in predictive sediment transport formulations, were similar but much larger than that required to suspend the relatively coarse sediment present at the bed. Importantly, these stresses did not represent the stresses imparted on the sediment measured in suspension and are therefore not relevant to the description of suspended sediment transport in systems with large bottom roughness. Estimates of the bed shear stresses that accounted for the reduced near-bed flow in the presence of large roughness vastly improved the relationship between the predicted and observed grain sizes that were in suspension. Thus, the impact of roughness, not only on the overlying flow but also on bed stresses, must be accounted for to accurately estimate suspended sediment transport in regions with large bottom roughness, a common feature of many shallow coastal ecosystems.

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.

Histograms of break force frequency at the study sites in Marmion (L2, H2, and DH1).

Breakpoint forcing is one of the major mechanisms for generation of free infragravity waves in the surf zone. Steep bed slopes, normalised to the incident sea-swell wavelength, are recognized to favor breakpoint forcing, but no study assesses the infragravity energy generated by breakpoint forcing in real conditions. We extend the breakpoint forcing model from bichromatic to spectral forcing and dimenzionalize the model output, and we estimate the sensitivity of the model to various spectral parameters on a plane beach. Steep beaches and short period waves (wind-sea) are favourable to breakpoint forcing, as expected. We also find that more energy is generated with real, multipeak, spectral forcing than with Jonswap spectra. The predicted infragravity response is consistent with observations on a barred beach suggesting breakpoint forcing is a major mechanism for the generation of free infragravity waves in the surf zone.

Temperatures within shallow reefs often differ substantially from those in the surrounding ocean; therefore, predicting future patterns of thermal stresses and bleaching at the scale of reefs depends on accurately predicting reef heat budgets. We present a new framework for quantifying how tidal and solar heating cycles interact with reef morphology to control diurnal temperature extremes within shallow, tidally forced reefs. Using data from northwestern Australia, we construct a heat budget model to investigate how frequency differences between the dominant lunar semidiurnal tide and diurnal solar cycle drive ~15-day modulations in diurnal temperature extremes. The model is extended to show how reefs with tidal amplitudes comparable to their depth, relative to mean sea level, tend to experience the largest temperature extremes globally. As a consequence, we reveal how even a modest sea level rise can substantially reduce temperature extremes within tide-dominated reefs, thereby partially offsetting the local effects of future ocean warming.

3 School of Earth and Environment, University of Western Australia, Crawley, Australia, ARC Centre of Excellence for Coral 4 Reef Studies, University of Western Australia, Crawley, Australia, The UWA Oceans Institute, University of Western 5 Australia, Crawley, Australia, Western Australia Marine Science Institution, Floreat, Australia, School of Civil and 6 Construction Engineering, Oregon State University, Corvallis, Oregon, USA, CSIRO, Oceans and Atmosphere Flagship, 7 Floreat, Australia AQ1

Large bottom roughness is a characteristic of most coral reef environments and this has been shown to have a substantial impact on hydrodynamic processes in these environments. In this paper, we evaluate suspended sediment concentration (SSC) data as well detailed hydrodynamic data over a coral reef flat in Ningaloo Reef, Western Australia, to understand how this bottom roughness affects these processes. A well-developed logarithmic velocity layer consistently developed above a canopy layer during the experiment. Estimates of bottom stresses from these logarithmic profiles were comparable with estimates obtained directly from turbulent Reynolds stresses, and an order of magnitude greater than those typically reported for sandy beach environments having similar flow. Nevertheless, the sediment grain size distribution of the suspended load was very fine relative to what should be mobilized by these stresses, indicated the large roughness substantially suppressed sediment transport.