Wave-induced Nearshore Currents Research Articles

Analysis of Measured Surface Nearshore Currents Using Multiple Drifters on Anmok Beach, South Korea

Lim, H.S.; Kim, M.J.; Kim, D.H., and Lee, H.J., 2019. Analysis of measured surface nearshore currents using multiple drifters on Anmok beach, South Korea. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 46-50. Coconut Creek (Florida), ISSN 0749-0208.To understand the mechanism of sediment transport and beach erosion processes in the surf zone near the coast of the Korean Peninsula, the development of coastal erosion control technology, an R&D project supported by the Korean government, has been conducted since 2013. Anmok Beach, located on the eastern coast of the Korean Peninsula, has been studied by measuring currents and sediment transport using bottom-mounted acoustic sensors and offshore long-term wave observation since February 2015. Moreover, for measurement of surface currents, multiple Lagrangian drifters were developed and successfully deployed nine times near the surf zone and submerged breakwaters close to Kangneung northern breakwaters. The variation of the measured bottom currents was induced directly by seasonal wave height and direction, depending on the incoming waves from the NNE in winter, ENE in summer, and NE, normal to the shoreline, in the case of high waves. However, the measured surface currents were affected not only by the incoming waves in the presence of high-energy waves, but also by wind speed and direction in the presence of low-energy waves. In this study, the observed surface nearshore currents measured by small Lagrangian GPS drifters near the surf zone and submerged breakwaters at Anmok were analyzed to investigate the variability of the wave-induced currents and nearshore surface currents generated by different wave and wind environments. The drifter experiments showed that offshore surface currents in the surf zone were predominantly developed, especially by strong seaward winds that opposed the direction of the weak waves. However, onshore surface currents were developed by wave-induced currents generated by the interaction of waves from offshore accompanied by sea winds. This measurement is now used to calibrate and validate the coastal sediment modeling system for the eastern coast of the Korean Peninsula.

Modeling two-dimensional infragravity motions on a fringing reef

This study investigates infragravity motions on a two-dimensional fringing reef with a concave topography using a wave-resolving nonlinear Boussinesq wave model. The effect of bathymetric nonlinearity on spatial variations of infragravity waves is studied from shore-normal incident random waves. The model is first verified from cross-shore/alongshore significant wave height, wave setup and infragravity wave height measured in laboratory experiments. The modeled velocity is further examined to elucidate the infragravity-wave current field. Wavenumber-frequency velocity spectra show that cross-shore infragravity motions are controlled by the leaky waves, while alongshore infragravity motions exhibit edge waves characteristics. Additionally, high-mode edge waves prevail on the entire infragravity frequency band while low-mode edge waves are dominant at very low frequencies. Furthermore, the model results illustrate the spatial and temporal variability of wave-induced nearshore currents, alongshore and cross-shore flows accompanying with circulation cells and rip currents. The infragavity-wave current field is nearly identical to the mean current field and the circulations develop at the same spatial scale as the reef flat.

Coupling of wave and current numerical model with unstructured quadtree grid for nearshore coastal waters

This paper presents 2D wave-current interaction model for evaluating nearly horizontal wave-induced currents in the surf-zone and coastal waters. The hydrodynamic model is the two-dimensional depth-averaged nonlinear shallow water equations by using an unstructured non-staggered and multiple-level quadtree rectangular mesh, this mesh information is stored in simple data structures and it is easy to obtain a locally high resolution for important region. The intercell fluxes are computed based on the HLL (Harten-Lax-van Leer) approximate Riemann solver with shock capturing capability for computing the dry-to-wet interface of coastal line. The effects of pressure and gravity are included in source term in the model, this treatment can simplify the computation and eliminate numerical imbalance between source and flux terms. The wave model readily provides the radiation stresses that represent the shortwave-averaged forces in a water column for the hydrodynamic model and the wave model takes into account the effect of wave-induced nearshore currents and water level. The coupling model is applied to verify different experimental cases and real life case of considering the wave-current interaction. The calculated results agree with analytical solution, experimental and field data. The results show that the modeling approach presented herein should be useful in simulating the nearshore processes in complicated natural coastal domains. wave-current interaction, unstructured quadtree grid, finite volume method, HLL approximate Riemann solver, wave action spectral model

NUMERICAL STUDY ON RANDOM WAVES AND WAVE-INDUCED LONG-SHORE CURRENTS AT LEADBETTER BEACH

Shallow coastal zone is extremely dynamic region, where the hydrodynamics are complicated as evident in the refraction, diffraction, shoaling and breaking of water waves, as well as the presence of wave-induced near-shore currents. In this paper, the distribution of waves and wave-induced long-shore currents formed by the breaking of obliquely incident random waves at Leadbetter Beach, Santa Barbara, USA are numerically studied. In the present numerical models, the random water waves are simulated based on the parabolic mild slope equation, and so the wave radiation stresses exerted on waveinduced currents are calculated based on variables in the parabolic mild slope equation, and the wave-induced longshore currents are simulated based on these. The numerical results have also been validated by field data and show good agreement with field data. In the process of numerical modeling, it is found that the bathymetry gradient is determinant to the distribution of wave transformation and wave-induced long-shore current in this coastal zone. In the areas where the bathymetry gradients are greater, the break line is closer from the shoreline, and the radiation stresses and radiation stress gradients are both greater than else areas associating with the presence of the relatively greater maximum wave heights, which result in slightly greater wave-induced long-shore currents, whereas, in the areas where the bathymetry gradients are smaller, the distribution of the break line, wave heights and wave-induced long-shore currents are just opposite. On the side, the wave-induced long-shore current deflects with the variation of bathymetry gradients.

LOCAL CONCENTRATION OF SLOWLY VARYING WAVE AND CURRENT FIELDS AROUND THE ABRUPTLY CHANGING BOTTOM SLOPES ALONG THE SHORE

This study aims to investigate physical mechanisms of locally concentrated nearshore hydrodynamic forces under interactive fields of waves and currents on abruptly changing coastal bathymetry. Laboratory experiments were first performed to represent such phenomena and a newly developed monitoring system based on image processing techniques successfully captured the local concentrations of wave fields as quantitative high-resolution data sets. Numerical experiments were then performed to investigate the physical mechanisms of the observed features. It was found that, among the other various possible factors, slowly varying wave-induced nearshore currents have one of the most significant impacts on local concentrations of waves around the abruptly changing bathymetry.

Modelling coastal barrier breaching flows with well-balanced shock-capturing technique

This paper presents a coastal hydrodynamic model for simulating coastal barrier breaching flows through an inlet which are mostly induced by extreme hydrological conditions such as storm/hurricane surges, waves and tides. In order to simulate wave field and wave-induced flow field in a coast, a wave action spectral model is coupled with a hydrodynamic model. The Godunov-type shock-capturing technique is used in the hydrodynamic model to simulate the supercritical flows and shocks driven by the extreme storm conditions. The hydrodynamic model is based on the solution of depth-averaged non-linear shallow water equations with all physical forcings common to coastal hydrological conditions so that it is capable of simulating multiple flow regimes, in which subcritical, transcritical, or supercritical flows may happen. The bed slope terms in the system of equations are treated in such a way that exact balance between flux gradient and bed slope terms is achieved under still water condition. The wave model readily provides the radiation stresses that represent the shortwave-averaged forcings in a water column and take into account wave-induced nearshore currents. In the coupled system, the models are operated systematically. The coastal hydrodynamic model is shown to accurately reproduce analytical and benchmark numerical solutions. To further test the accuracy of the model, flow through a coastal inlet with a storm surge is simulated and the results are compared with an established coastal flow model. Finally, the model is examined to simulate a severe storm surge that develops supercritical flows and the results are found to be encouraging.

Concentration of Slowly-Varying Nearshore Waves and Currents around the Edge of Two Different Bottom Slopes

This study aims to investigate physical mechanisms of locally concentrated nearshore hydrodynamic forces under interactive fields of waves and currents on sharply changing coastal bathymetries. Laboratory experiments were first performed to represent the focusing phenomena and a newly developed measuring system based on image analysis techniques successfully obtained quantitative high-resolution data of locally concentrated wave fields. Numerical analysis were finally performed to further investigate the physical mechanisms of the observed features and it was found that, among the other various possible factors, the slowly varying wave-induced nearshore currents have one of the most significant impacts on local concentrations of waves around the sharp edge of bathymetry.

Non-seagrass meadow sedimentary facies of the Pontinian Islands, Tyrrhenian Sea: A modern example of mixed carbonate–siliciclastic sedimentation

The soft bottom of the Mediterranean continental shelf is characterized by a heterozoan skeletal assemblage ( sensu [James, N.P., 1997. The cool-water carbonate depositional realm. In: James, N.P., Clarke, J. (Eds), Cool-water Carbonates. Spec. Publ. Soc. Sediment. Geol., vol. 56, pp.1–20.]). Although the contemporary presence of terrigenous and skeletal carbonate sediments has been well established [Tortora, P., 1996. Depositional and erosional coastal processes during the last postglacial sea-level rise: an example from the Central Tyrrhenian continental shelf (Italy). J. Sed. Res. 66, 391–405.; Fornós, J.J., Ahr, W.M., 1997. Temperate carbonates on a modern, low-energy, isolated ramp: the Balearic Platform, Spain. Journal of Sedimentary Research , 67, 364–373.; Fornós, J.J., Ahr, W.M., 2006. Present-day temperate carbonate sedimentation on the Balearic Platform, western Mediterranean: compositional and textural variation along a low-energy isolated ramp. In: Pedley, H.M., Carannante, G. (Eds.) 2006, Cool-water Carbonates: Depositional Systems and Palaeoenvironmental Controls. Geological Society, London, Special Publications, 255, pp. 121–135], the interactions between carbonate and terrigenous–siliciclastic sedimentation has not been documented well enough. A total of 33 surface sediment samples from the Pontinian shelf (Tyrrhenian Sea, central Mediterranean) have been analysed. Sampling stations range from 15 to 250 mwd (meter water depth) and are located along five transects (PonzaW, PonzaNW, Ponza NE, Ponza E, Zannone), plus four samples collected around Palmarola Island. Sectors colonized by seagrass meadows have not been sampled. A total of 6 sedimentary facies (F) and 10 microfacies (mf) have been recognized by using component analyses, grain size percentage, sorting, carbonate content and authigenic mineralization rate. These facies and microfacies represent the Pontian Islands shelf sedimentation, in the interval between the upper infralittoral and the epibathyal zones that represent shelf-break and upper slope sedimentation. The Maerl facies (F4a,b; mf4a,b) and the skeletal sands (F2a,b; mf2a1, mf2a2, mf2b) fall within the circalittoral zone. The circalittoral zone in the water depth interval between 82 m and 112 m display relict facies (F6, mf6). Finally facies F5 (Siliciclastic sands) includes subfacies F5b (mf5b), located in the circalittoral zone at depths of 49 to 101 mwd and restricted to the western and eastern sectors of Ponza, and subfacies F5a in the upper infralittoral zone (15 mwd/25 mwd) where erosional processes prevail. Carbonate content analyses indicate that maximum carbonate production on the Pontinian shelf took place in the 60–80 mwd interval. Facies F4 (Maerl) represents the environment characterized by the highest carbonate production rates. In the Pontian area siliciclastic–carbonate mixing took place in the infralittoral zone and in the lower circalittoral zone. In the infralittoral zone erosional processes on the rocky shoreline produced lithoclasts and vulcanoclastic deposits that were reworked by wave-induced near-shore currents. In the lower circalittoral zone the prolific production by photic biota (red algae) ends, while skeletal remains of the aphotic environment mixes with planktonic sediments characterized by low carbonate values. Sand (63 μm–2 mm) is the dominant grain size class, however gravel-dominated facies (F4 Maerl) are present in water depths (50 to 112 mwd) which are significantly below the storm wave base. Glauconite mineralization appears on the Pontinian shelf from 50 mwd and increases in abundance along the deeper bathymetries. The compositional characteristics of relict facies F6 shows the concurrence of biota assemblages of the infralittoral and circalittoral zones, likely representing the record of the last Holocene transgressive event (18 ky) and expressed by the overlapping of components of different environments.

Mathematical balancing of flux gradient and source terms prior to using Roe’s approximate Riemann solver

An algebraic technique is presented for balancing flux gradients and source terms when applying Roe’s approximate Riemann solver in finite volume schemes. The numerical imbalance is eradicated by reformulating the governing matrix hyperbolic system of conservation laws in terms of deviations away from an unforced but separately specified equilibrium state. Thus, balancing is achieved by the incorporation of this extra physical information and bypasses conventional numerical treatments of the imbalance. The technique is first applied to the shallow water equations. Simulations of benchmark flows including wind-induced flow in a two-dimensional basin, transcritical flow in a one-dimensional channel and wave propagation over a two-dimensional hump are in close agreement with analytical solutions and predictions by alternative numerical schemes. The technique is then applied to a more complicated coupled pair of equation sets, the hyperbolic period- and depth-averaged ray-type wave conservation and modified shallow water equations that describe wave current interaction in the nearshore zone at the coast. Reasonable agreement is obtained with laboratory measurements of wave diffraction behind a submerged elliptical shoal [Coastal Engrg. 6 (1982) 255] and of wave-induced nearshore currents at a half-sinusoidal beach [Wave-induced nearshore currents, Ph.D. Thesis, Liverpool University, UK, 1981].

Wave-induced nearshore currents at a tri-cuspate beach in the UKCRF

This paper describes an investigation into the surf-zone hydrodynamics at a fixed tri-cuspate beach installed in the UK Coastal Research Facility (UKCRF). Measurements are presented of the wave height field, horizontal wave-induced current patterns and detailed structure of rip and meandering longshore currents. For waves normal to the beach, the flow patterns consist of repeated primary circulation cells driven by longshore currents feeding into seaward rip currents. Each rip current is a narrow time-varying jet. From the measurements, the rip currents reached a maximum non-dimensional speed of (below the threshold of a stopping current), where |V| is the magnitude of current velocity, g is acceleration due to gravity and h is still water depth. The vertical profile through a typical rip current is reasonably uniform above a thin wave-induced oscillatory boundary layer, indicating that the rip current is predominantly a two-dimensional flow feature. Within the surf-zone, peak seaward-directed velocities occur at the top of the bed boundary layer, and incorporate the undertow in addition to the rip current. For oblique regular waves, a stable meandering longshore current forms, of magnitude above 0·5 m/s (corresponding to , with a current-induced logarithmic near-bed profile. The data should be useful in validating numerical models of nearshore wave–current interaction.

Wave-induced nearshore currents at a tri-cuspate beach in the UKCRF

This paper describes an investigation into the surf-zone hydrodynamics at a fixed tri-cuspate beach installed in the UK Coastal Research Facility (UKCRF). Measurements are presented of the wave height field, horizontal wave-induced current patterns and detailed structure of rip and meandering longshore currents. For waves normal to the beach, the flow patterns consist of repeated primary circulation cells driven by longshore currents feeding into seaward rip currents. Each rip current is a narrow time-varying jet. From the measurements, the rip currents reached a maximum non-dimensional speed of [Formula: see text] (below the threshold of a stopping current), where |V| is the magnitude of current velocity, g is acceleration due to gravity and h is still water depth. The vertical profile through a typical rip current is reasonably uniform above a thin wave-induced oscillatory boundary layer, indicating that the rip current is predominantly a two-dimensional flow feature. Within the surf-zone, peak seaward-directed velocities occur at the top of the bed boundary layer, and incorporate the undertow in addition to the rip current. For oblique regular waves, a stable meandering longshore current forms, of magnitude above 0·5 m/s (corresponding to [Formula: see text], with a current-induced logarithmic near-bed profile. The data should be useful in validating numerical models of nearshore wave–current interaction.

Wave-induced nearshore currents at a tri-cuspate beach in the UKCRF

This paper describes an investigation into the surf-zone hydrodynamics at a fixed tri-cuspate beach installed in the UK Coastal Research Facility (UKCRF). Measurements are presented of the wave height field, horizontal wave-induced current patterns and detailed structure of rip and meandering longshore currents. For waves normal to the beach, the flow patterns consist of repeated primary circulation cells driven by longshore currents feeding into seaward rip currents. Each rip current is a narrow time-varying jet. From the measurements, the rip currents reached a maximum non-dimensional speed of (below the threshold of a stopping current), where |V| is the magnitude of current velocity, g is acceleration due to gravity and h is still water depth. The vertical profile through a typical rip current is reasonably uniform above a thin wave-induced oscillatory boundary layer, indicating that the rip current is predominantly a two-dimensional flow feature. Within the surf-zone, peak seaward-directed velocities occur at the top of the bed boundary layer, and incorporate the undertow in addition to the rip current. For oblique regular waves, a stable meandering longshore current forms, of magnitude above 0·5 m/s (corresponding to , with a current-induced logarithmic near-bed profile. The data should be useful in validating numerical models of nearshore wave–current interaction.

Quadtree grid numerical model of nearshore wave–current interaction

This paper describes an adaptive quadtree-based 2DH wave–current interaction model for evaluating nearly horizontal wave-induced currents in the surf-zone. The model accounts for wave breaking, shoaling, refraction, diffraction, wave–current interaction, set-up and set-down, mixing processes (turbulent diffusion), bottom frictional effects, and movement of the land–water interface at the shoreline. The wave period- and depth-averaged governing equations, which conserve mass, momentum, energy and wave action, are discretised explicitly by means of an Adams–Bashforth second-order finite difference technique on adaptive hierarchical staggered quadtree grids. Grid adaptation is achieved through seeding points distributed according to flow criteria (e.g. local current gradients). The model is verified for nearshore circulation at a sinusoidal beach and nearshore currents at a multi-cusped beach. Reasonable agreement is obtained with experimental data from da Silva Lima [da Silva Lima, S.S.L., 1981. Wave-induced Nearshore Currents. PhD Thesis, Department of Civil Engineering, University of Liverpool] and Borthwick et al. [Borthwick, A.G.L., Foote, Y.L.M., Ridehalgh, A., 1997. Nearshore measurements at a cusped beach in the UK Coastal Research Facility, Coastal Dynamics '97, Plymouth, 953–962]. The modelling approach presented herein should be useful in simulating nearshore processes in complicated natural coastal domains. Of particular value is the local grid enrichment capability, which permits refined modelling of important localised flow behaviour such as rip currents and surf-zone circulation systems.

Three-dimensional dispersion of momentum in wave-induced nearshore currents

In this paper we consider the circulation induced by waves breaking near a coast. We show that the vertical nonuniformity of the wave-averaged horizontal velocities leads to mixing-like terms for the horizontal velocity in the depth-integrated equations of momentum. The mechanism is analogous to Taylor's (1953, 1954) shear-dispersion mechanism for solutes in a shear flow. The results presented here are an extension of the results found by Svendsen & Putrevu (1994) to the general case of unsteady flow over an arbitrary bottom topography.

A NUMERICAL MODEL OF NEARSHORE CURRENTS BASED ON A FINITE AMPLITUDE WAVE THEORY

A numerical model of wave-Induced nearshore currents taking into account the finite amplitude effect is developed, with a cnoidal wave theory used for the estimation of wave characteristics. The model is applied to the computation of wave transformation and nearshore currents on uniformly sloping beaches and on two-dimensional model topographies. The comparison with the results obtained by a linear model shows that wave nonlinearity has a strong influence on wave transformation in shoaling water and in the surf zone and on the strength of nearshore circulation, but that it does not have much effect on the longshore current profile. Moreover, the validity of the present model is supported by the quantitative agreement with the experiment for wave height variations, and the qualitative correspondence with the experiment for mean water level variation and longshore currents and the observation for nearshore currents.