The accuracy of nearshore hydrodynamic model predictions depends on the quality of the forcing applied at the offshore boundary. Waves are represented statistically in phase-averaged models, while they are represented individually in phase-resolving models. Since XBeach Surf-Beat fully resolves infragravity waves, the frequency distribution of the forcing wave spectra affects the computation of the forced infragravity waves at the offshore boundary. We ran XBeach Surf-Beat simulations with two types of forcing: parametric forcing, where a JONSWAP spectrum spectra is reconstructed internally, and spectral forcing, which utilises the observed full directional spectra. For unimodal wave conditions, both the JONSWAP reconstruction and spectral forcing produced results that closely matched infragravity wave observations. However, under bimodal wave conditions, the parametric forcing (JONSWAP) was less accurate compared to spectral forcing. In regions influenced by multiple wave climates, spectral forcing provides a more accurate representation of nearshore hydrodynamic processes.

Porous artificial reefs are increasingly being used for nature-based coastal protection, given their ability to attenuate waves while providing habitat for marine species. The wave attenuation and ecological functions of porous artificial reefs depend on how wave-driven flows interact with the porous interior structure of a reef; however, these hydrodynamic processes are still relatively poorly understood. To overcome the challenges with resolving the detailed flow-structure interactions within porous artificial reefs at fine (order mm) spatial resolution, this study utilized a mesh-free Computational Fluid Dynamics modelling approach based on Smoothed Particle Hydrodynamics (SPH) using the DualSPHysics solver. The capability of the SPH model to accurately reproduce the reef hydrodynamics (including wave transformation, hydrodynamic forces acting on the structure, and drag and inertia coefficients) was first validated against three independent experimental datasets of wavestructure interactions. The model was then used in a two-dimensional (2D) numerical investigation of wavestructure interactions with porous artificial reefs, where the 3D geometric parameters of the reef structure were adjusted within the 2D model to properly account for the hydrodynamic forces within the reef (i.e., using a quasi-3D approach). The results reveal how the porous reefs modify the dynamics of wave-induced oscillatory flows within the reef structure that are responsible for generating horizontal and vertical drag forces, wave dissipation, turbulent kinetic energy, and mean currents. Drag coefficients decreased with the KeuleganCarpenter number, with vertical drag coefficients typically larger than horizontal values. Wave dissipation across the porous reefs was due to a combination of horizontal drag forces and wave breaking, with vertical drag forces playing only a secondary role. Compared to less porous structures, the enhanced drag dissipation in porous artificial reefs enables them to attenuate waves more effectively over a greater range of water levels. Finally, the findings of this study underscore the potential for SPH models to be used as a cost-effective tool to support the design of porous artificial reefs for coastal protection.

Davenport, C.E.; Barnard, P.L.; Hansen, J.E., and Battalio, R.T., 2026. From symptoms to systems: Addressing the root causes of disappearing beaches through a case study from San Francisco's Ocean Beach. Journal of Coastal Research, 42(5), 955–974. Charlotte (North Carolina), ISSN 0749-0208. Development, flood control, and sediment management have modified urban coastal systems globally. This is especially true in San Francisco Bay and the surrounding coastal system. But instead of focusing on these root causes, more than a century of erosion management at San Francisco's Ocean Beach has focused on armoring shorelines, nourishing beaches, and rebuilding dunes. These strategies focus on the visible symptoms of erosion, particularly the narrowing beach, whereas changes to the actual drivers of this erosion go largely unaddressed. This paper reframes erosion at South Ocean Beach not as a site-specific coastal issue but as the downstream response of a sediment-starved system spanning San Francisco Bay and adjacent watersheds including the Golden Gate inlet, which links San Francisco Bay to the shoreline. This paper traces how watershed damming, in-bay aggregate mining, ebb-tidal delta contraction, and seaward expansion of the city have both directly and indirectly resulted in a narrowing beach on San Francisco's west side. The symptoms—narrowing beaches and coastal infrastructure risk—are the physical manifestations of these anthropogenic changes over the last century. But addressing the symptoms alone will not maintain South Ocean Beach, because both rising seas and changing storm patterns threaten a shifting sediment system. By contrast, a systems approach identifies the levers of recovery: increased sediment inputs, reduced sediment removals, precise placement of dredged material, and landward realignment of infrastructure. This framework shifts coastal management from reactive protection toward geomorphic restoration, recognizing that durable adaptation at San Francisco beaches, and urban coastal systems worldwide, depends on rebuilding the processes that once sustained them.

A comparison of spectral and time-domain wave observations collected by accelerometer-based Datawell Waverider and Global Navigation Satellite System (GNSS) based Sofar Spotter wave buoys is presented over nearly 24,000 h at two sites in the Southern and Indian oceans. All buoy data were processed consistently using the raw displacement time series. Across integrated wave parameters derived from the spectrum, the observations show good agreement; however, based on the computed bias, the Spotter wave heights were slightly smaller at the Southern Ocean site (Datawell Waverider 4) and slightly larger at the Indian Ocean site (Datawell Waverider MkIII). The Waverider buoys showed more energy at frequencies higher than the peak and less at frequencies lower than the peak. The high and low-frequency ends of the spectrum also had different spectral shapes, reflecting the different band-pass filtering methods completed on-board each buoy. The Spotter buoys also consistently reported broader directional spreading. In the time domain, comparison of the significant wave height and mean period is similar to that of the spectral comparison. The Spotter buoys more frequently reported large (unphysical) zero-crossing individual wave heights. This behavior was able to be reproduced when the buoy was momentarily submerged, causing loss of the GNSS satellite signal. Similar to other studies, we also found that at both sites, the Waverider buoys recorded horizontal displacements larger than those recorded by the Spotter and expected by linear theory. Across our complete analysis, the two buoy types reported similar integrated statistics; however, differences emerged when assessing higher-order parameters.

As sea levels rise many coastal areas are predicted to experience beach erosion. However, this erosion can potentially be partially mitigated by natural onshore transport from offshore sediment reserves. While offshore sediment reserves have been recognized as a potential source of sediment, the mechanisms by which this sediment makes its way toward the coast (often up- slope) where it can be further transported onto beach has been less well studied. Further the non-linear wave motions that have primarily been hypothesized to drive onshore transport (e.g. Elgar et al., 2001) are poorly resolved in most phase-averaged (spectral) wave models used to simulate coastal change over management timescales. Here, using co-located and contemporaneous observations of sea floor sediment mobility (via bedform migration) and near-bed velocity, we aim to understand the physical drivers of onshore sediment transport across shallow shore attached sand ridges.

Submerged coastal structures (e.g., submerged breakwaters and artificial reefs) modify shoreline hydrodynamics by altering incident wave fields. Submerged breakwaters are designed to provide coastal protection by decreasing incident wave energy. In addition to providing wave attenuation, nature-based artificial reefs seek to enhance ecosystem services linked to the creation of habitat for marine organisms. However, the use of artificial reefs in coastal protection applications is still relatively limited compared to conventional coastal structures (i.e., low-crested breakwaters) in part due to uncertainty in how reefs modify a range of wave-driven hydrodynamic processes that drive shoreline change. Here we present the findings of a comprehensive series of 3D wave basin experiments. These experiments were performed to examine the detailed wave-driven hydrodynamics around artificial reefs subject to a range of wave conditions, water levels and reef layouts.

Nature-based coastal protection solutions, such as artificial reefs, often consist of porous modules with complex geometries. While the porous designs are due to the requirements of promoting benthic habitat and attenuating incident waves, it is unclear how porosity and novel shapes affect the hydrodynamics near the modules. Meshless computational fluid dynamics (CFD) models are ideal for simulating the detailed hydrodynamic interactions between the waves and novel-shaped porous coastal structures. However, such CFD methods are computationally demanding, constraining the model domains to the immediate vicinity of structures. In comparison, mesh-based wave models can simulate larger scales due to the smaller computational demands but lack the required details to understand the impact of structural porosity. The aim of this work is to present a 3D coupled numerical model of mesh-based and meshless methods for simulating the wave-driven hydrodynamics near porous artificial reef modules to reproduce the physical experiments in a large-scale wave flume.

Porous artificial reefs can be used for coastal protection when they are effective at dissipating incident wave energy. Previous studies have used observations of wave interactions with porous reefs to develop empirical formulations to parameterize wave transmission as a function of reef geometry and hydrodynamic parameters. However, such approaches do not distinguish between the different processes that contribute to dissipation, namely wave breaking and drag-induced dissipation. While drag-induced dissipation can be more significant in porous reefs than in conventional rubble mound structures, the mechanisms that govern wave dissipation by drag forces within porous reefs are not well characterized. As a result, there is limited predictive capacity for describing wave-driven hydrodynamic processes in the interior of porous reefs and how these processes translate into wave dissipation. In this study, physical modelling experiments were conducted in a wave flume to investigate the detailed velocity structure, forces and wave dissipation within multi-row and single-row porous cubic artificial reefs that were exposed to a range of non-breaking regular wave conditions and submergence depths. The results reveal how the porous reef modifies the dynamics of the in-reef flows that are responsible for generating horizontal and vertical drag forces. Drag coefficients for different configurations of single- and multi-row reefs were similar and decreased with a reef Keulegan-Carpenter number (defined as the ratio of the wave orbital excursion to a structural hydraulic radius). Rates of wave dissipation derived from changes in wave energy fluxes across the reef could be explained primarily by the work done by horizontal drag forces, with vertical drag forces playing only a secondary role. Finally, the results from this study were used to develop an analytical model to predict drag-induced dissipation by porous reefs, which was shown to accurately predict wave attenuation across the reef as a function of reef, wave, and depth characteristics.

Observing air-sea interactions on a global scale is essential for improving Earth system forecasts. Yet these exchanges are challenging to quantify for a range of reasons, including extreme conditions, vast and remote under-sampled locations, requirements for a multitude of co-located variables, and the high variability of fluxes in space and time. Uncrewed Surface Vehicles (USVs) present a novel solution for measuring these crucial air-sea interactions at a global scale. Powered by renewable energy (e.g., wind and waves for propulsion, solar power for electronics), USVs have provided navigable and persistent observing capabilities over the past decade and a half. In our review of 200 USV datasets and 96 studies, we found USVs have observed a total of 33 variables spanning physical, biogeochemical, biological and ecological processes at the air-sea transition zone. We present a map showing the global proliferation of USV adoption for scientific ocean observing. This review, carried out under the auspices of the ‘Observing Air-Sea Interactions Strategy’ (OASIS), makes the case for a permanent USV network to complement the mature and emerging networks within the Global Ocean Observing System (GOOS). The Observations Coordination Group (OCG) overseeing GOOS has identified ten attributes of an in-situ global network. Here, we discuss and evaluate the maturation of the USV network towards meeting these attributes. Our article forms the basis of a roadmap to formalise and guide the global USV community towards a novel and integrated ocean observing frontier.

Artificial reefs are often deployed for purposes of habitat creation, yet can also act as a nature-based solution for coastal protection due to their ability to attenuate wave energy. There is, however, a lack of quantitative understanding of wave transformation and dissipation over porous artificial reefs, and how this depends on reef geometry parameters and hydrodynamic conditions. This limited understanding has led to a lack of guidance on how to optimize reef design to maximize coastal protection benefits. Given that artificial reefs have some structural similarities to traditional submerged breakwaters, there may be similar wave transformation characteristics across these two structures. However, relative to submerged breakwaters that typically have relatively low porosities, artificial reefs are usually designed as highly porous structures for habitat provision. The increased porosity of an artificial reef has the potential to modify how waves transform across reefs and dissipate energy. In this study, physical modelling experiments were conducted in a wave flume with both an idealized cubic modular porous artificial reef and an impermeable reef, with the aim to examine how wave propagation, breaking and transmission characteristics differ over artificial reefs with high porosity.

Porous artificial reefs can provide nature-based coastal protection by reducing nearshore wave transmission. Existing approaches to predict wave transmission across porous artificial reefs have relied on empirical formulations to describe bulk wave transmission that aggregate the role of different hydrodynamic processes responsible for wave attenuation, including wave dissipation by both breaking and drag forces and wave reflection from the reef. The lack of an integrated predictive model capable of accurately parameterizing these different hydrodynamic processes adds uncertainty to predictions of wave attenuation by artificial reefs when applied to different reef designs and hydrodynamic conditions. To address this gap, this study develops a parametric phase-averaged modelling approach to predict wave transformation processes across porous artificial reefs by parameterizing the individual contributions of breaking, drag dissipation and wave reflection to the changes of wave energy fluxes across a reef. Observations of wave transformation over an impermeable reef (i.e., in the absence of interior-reef drag forces) were initially used to assess breaking dissipation formulations, while non-breaking wave cases across three different cubic-type porous reefs were used to assess formulations to describe drag-induced dissipation by the reef. The model was further shown to accurately predict wave transformation for porous reef cases where both breaking and drag dissipation simultaneously occurred, for conditions that spanned a wide range of water depths, regular and irregular wave conditions. The validated model was finally applied to investigate the influence of different design parameters, including water depths, wave conditions and reef geometry parameters, on wave transmission and energy balances across a broad range of reef application scenarios. The predictive framework developed in this study is designed to be applicable to other porous reefs when geometry-dependent parameters can be robustly defined, which can be incorporated into phase-averaged wave models to predict wave transformation processes across porous reef structures to aid the design of modular artificial reefs for nature-based coastal protection.

Accurate measurement of ocean wave parameters is essential for marine engineering and environmental monitoring. This study analyses a unique dataset from a Sofar Spotter buoy that detached from its mooring in the Southern Indian Ocean. Originally deployed in 330 m deep water offshore of Perth, Western Australia, the buoy drifted southwest towards the Southern Ocean during the austral winter before returning to the coast, where it was eventually recovered. The complete deep-water displacement record, covering both moored and drifting modes, provides a rare opportunity to evaluate the Spotter buoy's performance across different configurations. Analysing ten months of data, we developed methods to detect and correct signal spikes, assessed how buoy motion affects measurement fidelity, and compared these findings to theoretical expectations. Additionally, we examined variations in buoy performance between moored and drifting modes, providing insights into buoy trajectories and movement patterns. This research advances the understanding of buoy-based wave measurement systems and lays a foundation for improving data accuracy in various oceanographic applications.

Coastal erosion in sandy beaches can be caused by numerous factors. Examples include wave energy, sea level, riverine runoff, terrain subsidence, and climate oscillations. Fringing reefs impose an additional layer of complexity to the morphodynamics of such systems. They act as rigid structures that can both dampen wave energy and induce intricate wave-induced circulation patterns. It has been hypothesized that, when compared to open-ocean beaches, wave-induced erosion at reef- fronted beaches is more sensitive to changes in the sea level. The reason for this is that fringing reefs can act as a physical barrier that shelters the shoreline from incoming wave energy, thereby leading to lower shoreline variability during periods of low sea level and higher shoreline variability during periods of high sea level. It has not been quantified, however, what are the relative contributions of wave power and sea level on sandy beaches fronted by fringing reefs, nor a systematic comparison between reef-fronted and open-ocean sandy beaches has been conducted. The aim of this research is to compare three distinct beaches with increasing abundances of fringing reefs to quantify such relative contributions to test this hypothesis.

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).

Studies of arrays of wave energy converters (WECs) with respect to power absorption and array interactions are often performed using linear models. However, nonlinear effects can be important and may change power estimates and optimal array designs. In this study, we have compared linear predictions of the behaviour of a shallowly submerged, buoyant point absorber with predictions from the nonlinear model SWASH with a WEC incorporated (WEC-SWASH). The latter was first comprehensively validated against 1:20 Froude scaled measured data from physical experiments. WEC-SWASH predictions of body motions and mean power absorption were generally in good agreement with measurements (absolute bias within 25%), although some discrepancies were observed in the body motions, especially when the device exhibited motion instabilities. The validated WEC-SWASH model was then compared with a linear frequency-domain model, as the latter is well-known and widely used because of its computational efficiency. Model comparisons were carried out for both an isolated WEC and small arrays (up to 5 devices). The mean power estimates from the linear model and WEC-SWASH for two representative wave farms showed good agreement for mild waves, with a difference of less than 5%. However, for larger waves, the disagreement increased to about 75 to 85% between the models (for the two wave farms tested). We found that array interactions for these arrays depend on wave amplitude and not just wave frequency. Furthermore, we discovered that the power take-off coefficients optimized using the linear model were not the optimum coefficients for the nonlinear model (WEC-SWASH).

Porous artificial reefs can function as nature-based solutions for coastal protection, due to their ability to dissipate wave energy while providing habitat for marine species. However, due to the lack of quantitative understanding of wave interactions with porous artificial reefs, there is uncertainty in how to optimize design for coastal protection applications. To address this gap, physical modelling experiments were conducted in a wave flume to investigate wave transformation across both porous and impermeable artificial reefs exposed to a range of regular wave conditions and submergence depths. The results highlight how key changes to wave kinematics (e.g., wave celerity, wave breaking) and wave energy (through reflection, dissipation and transmission) differ considerably between impermeable and porous artificial reefs. These differences in wave kinematic properties and energy balances across each reef can be well characterized by consideration of the effective crest depth (i.e., the total height of water at a point on the reef, including that in the voids) that accounts for the porosity of the reef structure, rather than the actual depth of the reef crest. The ratio of the effective crest depth to the incident wave height, termed the ‘relative effective crest depth’, was used to develop robust formulations to accurately predict wave reflection and transmission coefficients across both reefs over the range of wave and water depth conditions. The new formulations were also compared to other datasets reported in the literature, and were found to provide accurate predictions of both wave reflection and transmission for the broad range of submerged porous coastal structures considered (including artificial reefs and submerged breakwaters).

National weather forecasting agencies routinely issue a range of hazard warnings. But to our knowledge, along sandy coastlines where storm waves and storm surge can result in widespread but location-specific beach erosion and beachfront flooding, no national-scale early warning service for these hazards is presently operational. This paper outlines the scientific basis and implementation of a new framework for large area coastal storm hazards forecasting, currently being tested along the southwest (Indian Ocean) and southeast (Pacific Ocean) coasts of Australia. The system provides 7-day rolling predictions of localized beach erosion and/or coastal flooding linked to forecasted extreme weather events. Coastal setting influences the nature and occurrence of these hazards, with sandy beaches along wave-dominated coasts more prone to erosion and at surge-dominated coasts to flooding. An existing nearshore water-level forecasting system and a new inshore wave modeling capability are used to forecast beach erosion and coastal flooding at every 100 m along the shore. At the regional scale O(100-1 000 km of coastline), a threshold-based decision tree model categorises the predicted extent, location, and severity of erosion and flooding. At a more local scale O(100-1 000 m), physics-based modeling using XBeach focuses on vulnerable or high-value locations, providing specific storm hazard indicators tailored to local needs. This twotier approach is feasible for national implementation due to the reduced computational effort, limiting intensive modeling to pre-identified critical locations. Delft-FEWS manages the data and modeling workflow, ensuring scalability and compatibility with existing forecast infrastructure. Initial evaluations of the system are promising, with a detailed 2-year evaluation in progress. Future enhancements could include the use of satellite imagery for real-time beach width and dune topography assimilation and exploring alternative modeling approaches to further improve forecast accuracy.