Nearshore Circulation Research Articles

Wave-driven current and vortex patterns at an open beach: Insights from phase-resolving numerical computations and Lagrangian measurements

Wave-driven currents have a substantial impact on local circulation patterns in and across the surf zone, and are responsible for cross-shore and longshore exchange of mass and momentum over a broad range of spatial and temporal scales. Nearshore currents may drive sediment transport, lead to beach erosion, and also affect the spread of bacteria and other marine microorganisms, as well as the distribution of pollutants such as chemicals and microplastics. In addition, surf zone currents can cause hazardous conditions for beach-goers in the form of rip currents.It is known from previous work (Chen et al., 2003; Feddersen et al., 2011; Hally-Rosendahl and Feddersen, 2016) that Boussinesq-type models in combination with appropriate boundary conditions and wave breaking capabilities can function as powerful tools for the analysis of circulation patterns in the surf zone. In the present work, data from a recent field campaign reported on in Bjørnestad et al. (2021) are used to further validate the capability of Boussinesq systems to simulate nearshore dynamics.The numerical model is then used to study the influence of tidal elevation, peak direction and directional spread of the incoming wavefield on the quantity, extent, and circulatory magnitude of the nearshore circulation. In addition, fundamental features such as horizontal eddies are investigated, and comparisons are made to solid-body rotation and irrotational vortices.Overall, it is observed that local variations in the bathymetry across the surf zone are the controlling factor regarding the size of these circulations, and an increasing tidal level, which can be seen as a uniform offset to the bathymetry, favors the generation of larger vortex patterns. For a given tidal stage, the directional spread of the incoming wavefield has the most pronounced influence on the size and strength of the nearshore eddies while the peak direction has the strongest effect on the total number of circulations.

On the Bruun Rule suitability for modelling shoreline change

Modelling and understanding shoreline response to sea-level rise over decadal to centennial timescales is crucial for managing sandy coasts. Globally, the Bruun Rule is one of the most common shoreline model applied to facilitate this. However, there are several limitations to the Bruun Rule, which can compromise coastal management decisions. Here, we provide a novel assessment of the suitability of the Bruun Rule for application in sandy coasts by applying it to hindcast and project shoreline change in three morphologically different sandy beaches over yearly and decadal timescales. We also compare the Bruun Rule predictions against associated observations, published predictions from a hybrid shoreline model, and historical shoreline changes. Results show that the Bruun Rule fails to accurately hindcast and provide realistic projections of future shoreline change in sandy coasts. We attribute this to the Bruun Rule's assumption of linearity between shoreline retreat and relative sea-level rise and its inability to account for nearshore circulation. We, therefore, recommend limiting the use of the Bruun Rule for informing policy and the management of sandy coasts.

Vertical Distribution of Rip Currents Generated by Intersecting Waves in a Sandbar–Groin Systems

To analyze the impacts of groins, sandbars, and channels on the three-dimensional features of rip currents, we conducted experimental investigations on the vertical distribution of rip currents under intersecting waves along barred beaches with channels. This study employed ADV flow velocity measurements at two distinct locations: within channel and on a sandbar. The results indicate that in nodal sections within channel and on a sandbar, the rip head region manifests surface flow characteristics, characterized by high velocities near water surface. In a rip neck location, the vertical distribution of rip currents on a sandbar exhibits greater variability, whereas within channel, the distribution is more homogeneous. The vertical distribution of rip currents in nodal sections within channel aligns with the logarithmic distribution law. The vertical distributions of the alongshore velocity of rip currents display a consistent pattern of higher at the top and lower at the bottom. However, this pattern varies slightly depending on the wave period. The presence of groins influences the fluctuation characteristics of the alongshore velocity of rip currents by regulating the nearshore circulation system. This results in the alongshore velocity of rip currents in channel pointing towards the groin. In contrast, the alongshore velocity of rip currents on sandbar, situated farther away from groin, exerts a weaker effect on the alongshore velocity of a rip current.

ADVANCES IN UNSTRUCTURED WAVEWATCH III AND APPLICATIONS TO NEARSHORE WAVES

The spectral wave generation and propagation model WAVEWATCH III (WW3) (WW3DG 2019) is undergoing rapid community development to extend capability and nearshore modeling applicability. An option for unstructured grids and implicit solution provides WW3 with the flexibility and efficiency to resolve complex shorelines and high-gradient wave zones to drive nearshore circulation, wave setup, and wave-driven sediment transport with multi-scale spatial coverage over approximately three orders of magnitude. A hybrid approach to parallelization uses spectral partitioning for advection in geographical space and domain decomposition for spectral advection and the source term integration. The advection part of wave action equation is integrated fully implicitly, and a new convergent action limiter derived from Komen et al. (1994) and Hersbach and Janssen (1999) is applied. The model is compatible with community-based coupling infrastructure to facilitate two-way coupling with circulation models for simulating hurricane storm surge and waves. Abdolali et al. (2020) evaluated WW3 enhancements on a large-scale numerical domain employing the new parallelization algorithm and implicit solver for unstructured grids and compared to the existing parallelization algorithm, domain decomposition, and robust explicit numerical solver of WW3 on both structured and unstructured grids. These new capabilities in the wave model push the limitations of the model, including minimum resolution (10s of meters), maximum number of model grid points (~2 million), and computational scalability, as well as better accuracy in contrast to the explicit scheme. This paper describes model enhancements and validation.

Wave forced vorticity and dissipation scaling on a rip channeled beach

Rip-currents, commonly observed on natural beaches, are vorticity induced and part of large scale near-shore circulations. The questions arise: how do bathymetric gradients magnitudes relate to rip velocities? how does rip current vorticity scale with wave characteristics and dissipation? What is the dynamics of the large scale 2D vorticity? To address these questions, we utilize a Non Linear Shallow Water model with a shock-capturing scheme. It is validated with preexisting experiments of wave induced rip-currents on uneven bathymetries generated by irregular waves. To do so the enstrophy (spatially averaged square of the vorticity) is shown to be a relevant metric to calibrate the bottom friction coefficient of the model. The numerical study based on a large number of simulations with monochromatic wave forcing shows that the more non-uniform the bathymetry is, the stronger the gradients in wave dissipation are and the stronger the enstrophy is. The rip current velocity is shown to linearly increase with the square root of the local enstrophy. The wave-averaged shallow water vorticity equation terms are evaluated. It is suggested that large scale 2D vorticity dynamics mainly result from an equilibrium between vorticity production, vorticity advection by the circulation and dissipation by bottom friction.

Phase‐Resolved Modeling of Wave Interference and Its Effects on Nearshore Circulation in a Large Ebb Shoal‐Beach System

AbstractA time‐domain Boussinesq model was applied to modeling wave interference and its effects on nearshore circulation in San Francisco Bar and the adjacent Ocean Beach, CA. The model predicted the wave interference phenomena caused by the ebb shoal, with interference scales consistent with the radar observation and are persistent with nodal lines unchanged with time. Nearshore circulation predicted by the model shows small‐scale flow structures tied with the wave modulation patterns. However, the small‐scale modulation in the wave field seems not to generate alongshore variation in wave setup at similar scales. Therefore, in a large‐scale view, the alongshore currents predicted by the Boussinesq model still keep the general features shown in a wave‐averaged model, such as the flow divergence caused by the pressure gradient force associated with the alongshore variation of wave setup. The further analysis based on idealized short crest waves on a plane beach suggests that the alongshore pressure gradient is mainly balanced by the gradient of radiation stresses in the antinodal region, while by the flow advection in the nodal region. The rip current behavior is forced locally near breakers, rather than by gradients in wave setup close to shore. The time‐domain Boussinesq model predicted the spatial variability of wave‐induced processes. The alongshore‐varying wave breakers caused by wave interference are the source of the vorticity generation, inducing energetic vortex eddies nearshore.

Wave-induced mean currents and setup over barred and steep sandy beaches

Wind-generated surface waves breaking in the nearshore cause an increase in mean water levels, the wave setup, which can represent a significant fraction of storm surges developing both along open coasts and over sheltered areas such as coastal lagoons and estuaries. A common way to simulate the wave setup is to assume a balance between the barotropic gradient and the divergence of the depth-integrated wave-averaged momentum flux (radiation stress) associated with breaking waves in the surf zone. Field observations collected at several sandy beaches revealed that this depth-integrated approach could largely underestimate the wave setup close to the shoreline. The present study builds on Guérin et al. (2018) and further investigates how representing the depth-varying wave forcing in modelling systems can improve the prediction of wave setup across the surf zone. We use data collected during two major field campaigns at Duck, N.C., combined with simulations with SCHISM, a three-dimensional (3D) phase-averaged modelling system employing the vortex-force formalism to represent the effects of waves on currents. The ability of SCHISM to reproduce the surf zone circulation is first assessed with data collected during October 1994 (Duck94), which serve as a classical benchmark for 3D hydrostatic oceanic circulation models. The wave setup dynamics are then analysed during a storm event that occurred during SandyDuck. Consistent with the results of Guérin et al. (2018), we find that resolving the depth-varying nearshore circulation results in increased and improved wave setup predictions across the surf zone. At the shoreline, depth-integrated approaches based on the vortex-force formalism or the radiation stress concept underestimate the maximal wave setup by 10%–15% and 30% on the 1:14 foreshore slope, respectively. An analysis of the 3D cross-shore momentum balance reveals that the vertical mixing is the second most important contributor (10%–15% across the surf zone) to the simulated wave setup after the wave forces (80%–90%), followed by the vertical advection whose contribution increases with the beach slope (up to 10% at the shoreline). Simulations performed with a phase-resolving numerical model suggest that the largest discrepancies observed at the shoreline in past studies likely originate from swash-related processes, highlighting the difficulties to disentangle wave and swash processes on steep foreshores in the field.

Primary drivers of multidecadal spatial and temporal patterns of shoreline change derived from optical satellite imagery

Understanding and predicting shoreline change along sandy coasts requires continuous (in both time and space) long-term (decades) shoreline data at good spatial (e.g. 100 s of metres) and temporal (e.g. months) resolution. Publicly available satellite imagery can now provide such time series. However, satellite-derived shorelines (SDS) are associated with uncertainties, particularly at high-energy meso-macrotidal coasts, which challenge the assessment of long-term trends and interannual variability. In this paper we address the 1984–2020 time- and space-evolution of 269 km of high-energy meso-macrotidal sandy coast in southwest France using uncertain (no tide and runup correction) SDS data. The shoreline trends are validated with field data collected over the period 2008–2019. Over 1984–2020, the shoreline eroded by 0.55 m/yr with maximum erosion (accretion) reaching 15.61 m/yr (6.94 m/yr), with the largest changes observed along coasts adjacent to the inlet and estuary mouths. We show that, away from the presence of ebb-tide deltas and swash bars affecting offshore wave transformation and nearshore circulation, the long-term shoreline trend is well explained by the gradients in longshore drift computed from a regional wave hindcast and an empirical longshore transport formula. By averaging the yearly SDS along the entire coastline, we find that interannual shoreline variability is well correlated with the winter West Europe Pressure Anomaly (WEPA), which outscores the other conventional teleconnection pattern indices. WEPA even explains >80 % of the space-averaged shoreline variability over the recent period 2014–2020 when more and higher quality satellite images are available. A more local assessment of the links between climate indices and shoreline response shows that correlation with all climate indices dramatically drops downdrift of the large-scale estuary mouths and inlets. This suggests that along this 10–20 km stretch of downdrift coast, shoreline response is controlled factors internal to the estuary mouth/inlet system. The rest of the coast is mostly controlled by factors external to the system, which are primarily the variability in winter-mean wave height correlated to winter WEPA index. Overall, we demonstrate that an adapted space-averaging of uncorrected (noisy) SDS dataset can allow addressing the time- and space variability of shoreline change and their primary drivers including large-scale climate patterns of atmospheric variability. We also advocate that such SDS analysis can be performed along any coastline in the world in order to guide future model development and application.

Tidal and wave modulation of rip current dynamics

Rip currents have become the primary coastal hazard globally with direct implications for surf-zone recreational safety. Among 101 China's recreational beaches, headland bay with large tidal range and wave-dominated sandbar shorelines are identified with high occurrence risk of rip currents by the morphodynamic assessment. The variability of rip current under tidal and wave modulation is explored by the case simulations. At macro-tidal pocket beach, the strongest rip currents occur in ebb, 20%–50% faster than that during flood, while offshore flows are barely developed at low tide. This asymmetry depends on the quasilinear superimposition of wave-driven and tidal currents. Nearshore circulations are found not sensitive to the wave direction in a macro-tidal environment. For sandbar morphology with weak tidal variations, rip currents are best developed towards low tide with maximum topographic effect on wave dissipation and show high sensitivity to the incident wave angle. The magnitude of vortex increases with wave angle but is not explicitly related to wave height. Rip currents are mostly suppressed by oblique incident waves while the offshore flow may get strengthened as incoming waves slightly deviate from normal incidence when the wave height is small. The decay of rip currents with wave angle did not show clear dependence on channel width which was found in other literatures. The incident wave angle should be incorporated into existing statistical methods or probabilistic models to improve the accuracy of rip current prediction.

Drowning incidents and conditions due to hidden flash rips in Lake Michigan

Flash rips are episodic bursts of water jetting offshore, which can lead to drowning incidents by sweeping swimmers offshore without warning, thus posing a hidden and unrecognized danger to beachgoers. This study reveals hazards of flash rips by investigating a series of drowning incidents along coasts of Lake Michigan during a series of storm events on July 18–21, 2019. Occurrences and causes of flash rips were depicted through webcam image observations, storm features of atmospheric disturbances, hydrodynamic circumstances of wind waves and meteorologically induced water level fluctuations, and model-reconstructed nearshore circulations. Results shows that flash rips were generated during or after storms through nearshore processes of storm-induced wind waves and meteorologically induced water level fluctuations. With small wind waves, low water level fluctuations, and a timing delay of rip occurrences relative to the causative convective storms, flash rips pose a hidden hazard to unaware swimmers. Historical observations for incidents in Lake Michigan between 2002 and 2019 further show that dry conditions or fair weathers and a calm water signature at the beach can likely generate unexpected hidden flash rips, resulting in the highest drowning risks. There is an urgent need for communication, education, and prediction/forecast of hidden flash rips to the Laurentian Great Lakes and worldwide coastal communities.

A convolutional neural network based model to predict nearshore waves and hydrodynamics

This study proposes a convolutional neural network (CNN) based model to predict waves and hydrodynamics in the nearshore zone. Specifically, we address the so-called next-frame prediction of wave and hydrodynamic conditions using the previous-step conditions. A laboratory experiment on unsteady rip currents is simulated using the non-hydrostatic model SWASH. The simulated wave and hydrodynamic results are divided into two groups. The first group of data is used to train the CNN model, and the second group of data is used to evaluate the trained model by comparing the simulated water surface elevation, cross-shore velocity, and longshore velocity by SWASH with the corresponding prediction by the trained CNN model. The result shows that the CNN model accurately predicts the wave propagation over the offshore slope, wave breaking over the sandbar, and bore propagation over the shore including detailed wave crest bending and separation. Time-averaged water level comparison further confirms that the CNN model captures the wave setup induced by wave breaking. The CNN model also correctly predicts the cross-shore velocity and longshore velocity driven by wave breaking over sandbars and wave–current interaction. Finally, the low-frequency nearshore circulation pattern predicted by the CNN model is also evaluated by time-averaging the velocity field. The comparison shows that the CNN model correctly predicts the distribution of circulation cells and the number of cells, as simulated by the SWASH model. This work serves as a paradigm to integrate traditional coastal modeling with machine learning to study coastal processes.

Observation of nearshore crescentic sandbar formation during storm wave conditions using satellite images and video monitoring data

Crescentic nearshore sandbars (CNSBs) that are observed in the shallow waters (< 10 m) of sandy beaches are important for understanding coastal dynamics because of their strong interaction with nearshore circulation. However, their formation, originating from shore-parallel straight nearshore sandbars (SNSBs), has rarely been observed in the field because their occurrence is typically short (less than a week). In this study, a process in which a nearly SNSB changed into a fully developed CNSB was observed using satellite images, field surveys, and video monitoring data at a sandy beach in South Korea. Freely available Sentinel-2 satellite images and the bathymetry data measured by echosounders were used to find out formation of an SNSB system after attack of Typhoon Tapah in September 2019 and that the SNSB changed to a CNSB in February 2020. The process was narrowed down using video monitoring data and hydrodynamic measurements, observing that two storm waves with maximum wave heights of >3 m developed in the site over a one-month period in January 2020 when the CNSB formed. The first storm wave had a sharp peak wave height that reached ~5 m and lasted ~2 days. The second storm wave had lower wave energy but several peak waves of ~4 m height and a total storm period of ~6 days. During both storm periods, the infragravity wave energy increased and strong (>0.5 m/s) offshore and longshore (northwest) currents developed for ~1 day and ~ 3 days for the first and second storm respectively. The results from video monitoring data show that a nearly developed SNSB system transformed into a weakly developed CNSB system after the first storm when alongshore variability on the nearly SNSB was intensified to become horns and bays of the weak CNSB. During the second storm, the CNSB system fully developed as the horns moved further onshore and the bays further offshore shaping the clear horn and bay pattern. This indicates that positive feedback between the flows and sediments played a key role for the formation of the CNSB and, therefore, the self-organization mechanism might be appropriate to describe the process during the two storm periods. Specifically, the high infragravity wave energy and strong quasi-steady currents were important because they could trigger the development of rip channels during the first storm whereas the channels were further strengthened by the long consistency of currents during the second storm.

Long-chain alkyl diols as indicators of local riverine input, temperature, and upwelling in a shelf south of the Yangtze River Estuary in the East China Sea

Sedimentary processes on continental shelves are affected by complex interactions between a variety of riverine and oceanographic processes. In the shelf of the East China Sea (ECS) south of the Yangtze River estuary, we investigated source-to-sink processes relating to local riverine input as well as the effects of the Kuroshio intrusion by examining the distribution of long-chain alkyl diols in surface and core sediments. The results show that at present the main source of 1,14-diols is most likely diatoms belonging to the genus Proboscia. Most C32 1,15-diol is produced in situ in local rivers (rather than originating from the Yangtze River) and its distribution is mainly restricted to the front of local river mouths along the coast (water depth < 50 m) due to the hydrodynamism of the nearshore circulation system. Although the sources of the 1,13- and 1,15-diols remain uncertain, principal component analysis and their distribution patterns suggest that 1,14-, 1,13- and 1,15-diols have different biological sources, and that their producers may bloom in different seasons. The long chain diol index (LDI), a proxy for sea surface temperature (SST), was used in this study to reconstruct records of SST (with a bias towards autumn temperatures) in different parts of the ECS shelf, except for shallow water areas (water depth < 50 m), which are more affected by riverine input. The diol index (DI), based on 1,14-diols, worked well as a upwelling tracer in our study area, and our results show that upwelling prevails in the ECS shelf and that zones of strengthened upwelling are present to some extent at ~28.8°N–29.6°N and ~ 27.0°N–27.8°N during the summer due to the interaction of geomorphology, monsoons, and different water masses (i.e., the Kuroshio branch and the Yangtze River diluted water). The nutrient diol index (NDI) shows potential as an indicator for nutrient concentration in the surface waters in this study area during the warm season, but further work is needed.

Surf Zone Waves at the Onset of Breaking: 1. LIDAR and IR Data Fusion Methods

AbstractThis is the first of a 2‐part series concerning remote observation and wave‐by‐wave analysis of the onset of breaking in the surf zone. In the surf zone, breaking waves drive nearshore circulation, suspend sediment, and promote air‐sea gas exchange. Nearshore wave model predictions often diverge from in situ measurements near the break point location because common parameterizations do not account for the rapid changes that occur near the onset of breaking. This work presents extensive methodology to combine data from a line‐scanning LIDAR and thermal infrared cameras to detect breaking, classify breaker type, and measure geometric wave parameters on a wave‐by‐wave basis, which can be used to improve breaker parameterizations. Over 2,600 non‐breaking and 1,600 breaking waves are analyzed from data collected at the USACE Field Research Facility in Duck, NC, including 413 spilling and 111 plunging waves for which the onset of breaking was observed. Wave height is estimated using a spatio‐temporal method for wave tracking that preserves the sea surface elevation maximum and overcomes field of view limitations. Methods for estimating instantaneous wave speed are refined by fitting a skewed Gaussian function to each wave profile before tracking the peaks. Wave slope is estimated from a linear fit to the upper 80% of the wave face, which provides a robust metric and strong correlation with geometric wave slope defined relative to mean sea level. Finally, breaking wave face foam coverage is analyzed to assess common model assumptions about roller length for wave energy dissipation parameterizations.

The Effect of Wave-Induced Current and Coastal Structure on Sediment Transport at the Zengwen River Mouth

The mechanisms that control estuarine sediment transport are complicated due to the interaction between riverine flows, tidal currents, waves, and wave-driven currents. In the past decade, severe seabed erosion and shoreline retreat along the sandy coast of western Taiwan have raised concerns regarding the sustainability of coastal structures. In this study, ADCPs (Acoustic Doppler Current Profiler) and turbidity meters were deployed at the mouth of the Zengwen river to obtain the time series and the spatial distribution of flow velocities and turbidity during the base flow and flood conditions. A nearshore circulation model, SHORECIRC, has been adapted into a hybrid finite-difference/finite-volume, TVD (Total Variation Diminishing)-type scheme and coupled with the wave-spectrum model Simulating Waves Nearshore (SWAN). Conventional finite-difference schemes often produce unphysical oscillations when modeling coastal processes with abrupt bathymetric changes at river mouths. In contrast, the TVD-type finite volume scheme allows for robust treatment of discontinuities through the shock-capturing mechanism. The model reproduces water levels, waves, currents observed at the mouth of the Zengwen River reasonably well. The simulated residual sediment transport patterns demonstrate that the transport process at the river mouth is dominated by the interaction of the bathymetry and wave-induced currents when the riverine discharge was kept in reservoirs. The offshore residual transport causes erosion at the northern part of the river mouth, and the onshore residual transport causes accretion in the ebb tidal shoals around the center of the river mouth. The simulated morphological evolution displays significant changes on shallower deltas. The location with significant sea bed changes is consistent with the spot in which severe erosion occurred in recent years. Further analysis of morphological evolution is also discussed to identify the role of coastal structures, for example, the extension of the newly constructed groins near the river mouth.

Quantifying Optically Derived Two-Dimensional Wave-Averaged Currents in the Surf Zone

Complex two-dimensional nearshore current patterns are generated by feedbacks between sub-aqueous morphology and momentum imparted on the water column by breaking waves, winds, and tides. These non-stationary features, such as rip currents and circulation cells, respond to changing environmental conditions and underlying morphology. However, using fixed instruments to observe nearshore currents is limiting due to the high costs and logistics necessary to achieve adequate spatial sampling resolution. A new technique for processing surf-zone imagery, WAMFlow, quantifies fluid velocities to reveal complex, multi-scale (10 s–1000 s meters) nearshore surface circulation patterns. We apply the concept of a wave-averaged movie (WAM) to measure surf-zone circulation patterns on spatial scales of kilometers in the alongshore and 100 s of meters in the cross-shore. The approach uses a rolling average of 2 Hz optical imagery, removing the dominant optical clutter of incident waves, to leave the residual foam or water turbidity features carried by the flow. These residual features are tracked as quasi-passive tracers in space and time using optical flow, which solves for u and v as a function of image intensity gradients in x, y, and t. Surf zone drifters were deployed over multiple days with varying nearshore circulations to validate the optically derived flow patterns. Root mean square error are reduced to 0.1 m per second after filtering based on image attributes. The optically derived patterns captured longshore currents, rip currents, and gyres within the surf zone. Quantifying nearshore circulation patterns using low-cost image platforms and open-source computer vision algorithms presents the potential to further our understanding of fundamental surf zone dynamics.

MODELING RAPID BEACH CHANGE SURROUNDING A COASTAL STRUCTURE IN OBLIQUE WAVES

Sandy beaches are typically in equilibrium with the wave climate, and changes occur when the system is perturbed. However, changes to nearshore morphology can occur when coastal structures are built and the system adjusts to a new equilibrium. An example of this is the construction of a shore-perpendicular groin that is designed to trap sediment . We investigate the capability of Delft3D model to simulate wave transformation, nearshore circulation and morphology change around a structure, driven by relatively small breaking waves at a very high incident angle.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/q_rnSiaP7WM

IMPLICATIONS OF CORAL REEF MINING PITS ON 2D HYDRODYNAMICS

Small Island Developing States (SIDS) are among the most vulnerable areas to the impact of natural hazards and climate change. Flooding due to storm surges and extreme waves, coastal erosion, and salinization of freshwater lenses are already a serious threat and could lead to irreversible consequences in the coming decades. On the other hand, reef flat mining is one of the most common practices to source the required material for the implementation of coastal protection measures on SIDS. There are strong concerns about this practice, because partial removal of the protective reef could increase wave impacts on the islands. However, the available data and knowledge on the effects of these mining pits are currently very limited. Klaver et al. (2019) studied the effects that pits may have on nearshore hydrodynamics and wave runup. Results obtained via numerical modeling indicate that excavation pits cause a decrease in infragravity wave energy around the fundamental mode of the reef, which is partly caused by reduced wave transmission. Similarly, changes in sea and swell (SS) wave energy are attributed to reduced transmission, as well as a decrease in wave dissipation and (triad) wave-wave interaction. Earlier studies by Ford et al. (2013) and Yao et al. (2016) point out that mining pits may lead to additional two-dimensional effects. In this presentation, we present the potential 2D effects of reef flat excavation pits on nearshore hydrodynamics. More specifically, we aim to assess the impact of excavation pits on nearshore circulation patterns, which could be an indicator of coastal morphological changes.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/85-IMM66eWE

ADVANCES IN THE UNSTRUCTURED WAVEWATCH III AND APPLICATION TO HURRICANE DORIAN

The spectral wave generation and propagation model WAVEWATCH III (WW3) is undergoing rapid development to extend capability and applicability. An option for unstructured grids and implicit solution provides WW3 with the flexibility and efficiency to resolve complex shorelines and high-gradient wave zones to drive nearshore circulation, wave setup, and wave-driven sediment transport with multi-scale spatial coverage over approximately three orders of magnitude. The model is compatible with community-based coupling infrastructure to facilitate two-way coupling with circulation models for simulating hurricane storm surge and waves. Unstructured WW3 is applied for 2019 Hurricane Dorian and validated with National Data Buoy Center buoys and nearshore gauges at the US Army Corps of Engineers Field Research Facility.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/kz9G46xUD0k

Beach State Recognition Using Argus Imagery and Convolutional Neural Networks

Nearshore morphology is a key driver in wave breaking and the resulting nearshore circulation, recreational safety, and nutrient dispersion. Morphology persists within the nearshore in specific shapes that can be classified into equilibrium states. Equilibrium states convey qualitative information about bathymetry and relevant physical processes. While nearshore bathymetry is a challenge to collect, much information about the underlying bathymetry can be gained from remote sensing of the surfzone. This study presents a new method to automatically classify beach state from Argus daytimexposure imagery using a machine learning technique called convolutional neural networks (CNNs). The CNN processed imagery from two locations: Narrabeen, New South Wales, Australia and Duck, North Carolina, USA. Three different CNN models are examined, one trained at Narrabeen, one at Duck, and one trained at both locations. Each model was tested at the location where it was trained in a self-test, and the single-beach models were tested at the location where it was not trained in a transfer-test. For the self-tests, skill (as measured by the F-score) was comparable to expert agreement (CNN F-values at Duck = 0.80 and Narrabeen = 0.59). For the transfer-tests, the CNN model skill was reduced by 24–48%, suggesting the algorithm requires additional local data to improve transferability performance. Transferability tests showed that comparable F-scores (within 10%) to the self-trained cases can be achieved at both locations when at least 25% of the training data is from each site. This suggests that if applied to additional locations, a CNN model trained at one location may be skillful at new sites with limited new imagery data needed. Finally, a CNN visualization technique (Guided-Grad-CAM) confirmed that the CNN determined classifications using image regions (e.g., incised rip channels, terraces) that were consistent with beach state labelling rules.