Wave-current Interaction Research Articles

Wave-current interactions during extreme weather conditions in southwest of Bohai Bay, China

Wave-current interactions widely occur in shallow waters. However, the dominant factors for the modulation differ over different coastal regions and the relevant generation mechanisms have not been fully addressed. In this study, a Delft3D wave-current coupled model is applied to the southwestern Bohai Bay, China, which is a shallow-water coast with a mild slope, to estimate the local wave-current interactions during extreme weathers. The results indicate that the wave set-up is strongly influenced by local bottom slope, water depth and radiation stress gradient. The mean current velocity is decreased by wave by up to 15% due to enhanced bottom roughness, and the impact of momentum transfer from wave to current is non-negligible. Both tidally-varying depth and current are able to modulate significant wave height (SWH), and their modulating abilities are increased and decreased onshore respectively. Furthermore, the sensitivity studies suggest that the depth-induced SWH modulation is primarily due to the shoaling effect which is related to bottom slope. The wind speed plays an important role on the depth- and current-induced SWH modulation, as it could affect the state of wind-wave and wind input energy. Whereas, wind direction change from onshore to offshore only results in remarkable changes of current-induced modulation.

Wave Energy Resource Assessment for Exploitation—A Review

Over recent decades, the exploitation of wave energy resources has sparked a wide range of technologies dedicated to capturing the available power with maximum efficiency, reduced costs, and minimum environmental impacts. These different objectives are fundamental to guarantee the development of the marine wave energy sector, but require also refined assessments of available resource and expected generated power to optimize devices designs and locations. We reviewed here the most recent resource characterizations starting from (i) investigations based on available observations (in situ and satellite) and hindcast databases to (ii) refined numerical simulations specifically dedicated to wave power assessments. After an overall description of formulations and energy metrics adopted in resource characterization, we exhibited the benefits, limitations and potential of the different methods discussing results obtained in the most energetic locations around the world. Particular attention was dedicated to uncertainties in the assessment of the available and expected powers associated with wave–climate temporal variability, physical processes (such as wave–current interactions), model implementation and energy extraction. This up-to-date review provided original methods complementing the standard technical specifications liable to feed advanced wave energy resource assessment.

Variability in infragravity wave processes during estuary artificial entrance openings

ABSTRACTIntermittently open/closed estuaries (IOCE) are wave‐dominated estuaries with entrances that temporarily close to the ocean. Wave–current interactions play a major role in estuary entrance morphodynamics and influence the degree of energy transfer from the ocean into the lagoon. This study utilizes artificial entrance openings of multiple ICOE in Victoria, Australia, to capture continuous hydrodynamic and geomorphic data throughout the opening cycle. We illustrate that water level oscillations in the infragravity (IG) band) are present in the basin during open entrance conditions. IG waves were observed to propagate up to 1.8 km upstream of the mouth while the entrance was open. Our work identifies that changes in cross‐sectional area, bed depth at the berm position, and offshore wave height control the magnitude of IG waves within the estuary basin. IG wave magnitude is also tidally modulated and increases with high tides when the nearshore water level is higher. Late during the drainage phase, waves were observed to track the margins of the channel, away from the thalweg, and reach the basin. IG wave energy was highest immediately after the basin had ceased draining and while channel dimensions at the mouth were within 10% of their maximum value. As the entrance aggrades, IG wave magnitude decreases in the absence of energetic offshore wave conditions. We relate the changes in IG wave magnitude and frequency to a six‐stage conceptual model of the opening–closure sequence. Within the basin, IG wave energy, height and frequency were also consistently highest closer to the mouth and decreased with distance upstream. Our findings indicate that water level oscillations in the IG band are a persistent feature in IOCE and may be the norm rather than the exception in these systems. As IG waves were captured serendipitously as part of a larger field campaign, future work will focus on instrumenting IOCE to gain high‐resolution data to quantify IG wave processes during entrance openings. © 2020 John Wiley & Sons, Ltd.

Statistical analyses of eddies in the Western Mediterranean Sea based on Synthetic Aperture Radar imagery

We analysed 7188 Sentinel-1A Synthetic Aperture Radar (SAR) images acquired in a 3-year period, between October 2014 and September 2017, in the Western Mediterranean Sea and found imprints of about 14,300 oceanic eddies with diameters ranging from 0.4 km to 160.1 km. Those eddies manifest on the SAR imagery either through the accumulation of surfactants or through wave-current interaction, and considering the favourable wind-speed ranges for both mechanisms we performed statistical analyses to gain insight into the eddies' spatio-temporal distribution, their typical sizes and shapes, and into the predominant mechanisms that are responsible for their formation. The vast majority of the detected eddies is elliptical, with a tendency for only large mesoscale eddies towards a circular shape. We found no general influence of the wind on the eddies' orientation, although mesoscale eddies tend to orient into the main current direction in each subregion. Submesoscale eddies do not show such tendency. Submesoscale eddies in the Western Mediterranean Sea are predominantly cyclonic (>90%), and with increasing eddy diameter there is a transition from cyclonic to anticyclonic sense of rotation. In general, we found highest densities of submesoscale eddies in autumn, when the biological activity is still high and when mean regional winds are already strong, but variable. In addition, instabilities of thermal fronts during summer and autumn and instabilities of the main and secondary currents influence the formation of submesoscale eddies. Some of our results confirm those previously obtained for other regions.

Effect of Breaking Waves on Near-Surface Mixing in an Ocean-Wave Coupling System under Calm Wind Conditions

Estimating wave effects on vertical mixing is a necessary step toward improving the accuracy and reliability of upper-ocean forecasts. In this study, we evaluate the wave effects on upper-ocean mixing in the northern East China Sea in summer by analyzing the results of comparative experiments: a stand-alone ocean model as a control run and two ocean–wave coupled models that include the effect of the breaking waves (BW) and of the wave–current interaction (WCI) with a vortex-force formalism. The comparison exhibits that under weak wind conditions, the BW effect prescribed by wave dissipation energy significantly enhances near-surface mixing because of increased downward turbulent kinetic energy (TKE), whereas the WCI has little effect on vertical mixing. Increased TKE results in a mixed-layer depth deepened by ~46% relative to the control run, which provides better agreement with the observed surface thermal structure. An additional experiment with local wind–based BW parameterization confirms the importance of nonlocally generated waves that propagated into the study area upon near-surface mixing. This suggests that under calm wind conditions, waves propagated over distances can largely affect surface vertical mixing; thus, ocean–wave coupling is capable of improving the surface thermal structure.

Sediment exchange between channel and sand ridges in the southern Yellow Sea: The importance of tidal asymmetries

The radial sand ridges found off the coast of Jiangsu (China) are major morphological features in the southern Yellow Sea and undergo rapid changes in morphology. Despite extensive observational and modeling efforts, the physical processes controlling sediment transport and associated morphological evolution over the ridges remain not well understood. Here we analyzed data collected from systematic hydrodynamic and sedimentary surveys including records from moorings and bottom-mounted tripods deployed at channels and adjacent ridges. We explored the role of advection and local resuspension processes in controlling morphological changes and how they are modulated by tidal asymmetry. Bottom shear stresses due to mean tidal currents, waves, and wave-current interactions were estimated to examine the effects of tidal asymmetry, advection and resuspension processes on suspended sediment concentration (SSC) variability. The simplified depth-averaged model of Bass et al. (2002) was used to explain the phase relationships between fine sediment suspensions and tidal currents. We found that resuspension is the major controlling factor of bottom SSC during mean and spring tides, while advection is the dominant process during neap tides. The asymmetries in water level and tidal current lead to a net lateral sediment transport directed from the channel to the sand ridges, which cause erosion in channels and deposition on the sand ridges. This sediment transport pattern likely contributes to the evolution of the sand ridges under fair weather conditions (i.e., excluding storm events).

MaRINET2 Tidal Energy Round Robin Tests—Performance Comparison of a Horizontal Axis Turbine Subjected to Combined Wave and Current Conditions

This Round Robin Test program aims to establish the influence of the combined wave and current effect on the power capture and performance of a generic tidal turbine prototype. Three facilities offering similar range of experimental conditions have been selected on the basis that their dimensions along with the rotor diameter of the turbine translate into low blockage ratio conditions. The performance of the turbine shows differences between the facilities up to 25% in terms of average power coefficient, depending on the wave and current cases. To prevent the flow velocity increasing these differences, the turbine performance coefficients have been systematically normalized using a time-average disc-integrated velocity, accounting for vertical gradients over the turbine swept area. Differences linked to blockage effects and turbulence characteristics between facilities are both responsible for 5 to 10% of the power coefficient gaps. The intrinsic differences between the tanks play a significant role as well. A first attempt is given to show how the wave-current interaction effects can be responsible for differences in the turbine performance. In these tanks, the simultaneous generation of wave and current is a key part often producing disruptions in both of these flow characteristics.

A FVCOM study of the potential coastal flooding in apponagansett bay and clarks cove, Dartmouth Town (MA)

A high-resolution Finite-Volume Coastal Ocean Model (FVCOM) inundation model has been developed for Dartmouth Town near Apponagansett Bay and Clarks Cove. Series of modeling experiments were conducted for the purpose of: (1) Assess the potential impacts of the climate-induced Sea Level Rise (SLR) on the storm-induced coastal inundation in Dartmouth Town; (2) Compare the current patterns, wave fields and surge distributions under different dynamic forces including winds in different directions and wave-current interaction; (3) Evaluate the impact of the bank on the flooding protection. Results show that under the hundred-year nor’easter storm condition, the climate-induced SLR could significantly enlarge possible flooding areas with the percent area enlargement of approximately 60% per foot of SLR. The directions of wind essentially determine the feature of the current patterns, wave and surge distributions. The northeasterly and easterly winds mainly threaten the western coast of the bay and the estuarine areas, and the southerly and southeasterly winds endanger the regions around the inner part of the bay. Wave-current interaction can change the current pattern nearshore, including formation of eddies and narrow alongshore currents, greatly enhancing the strength and complexity of the currents near the mouth of the bay. In addition, wave-induced surge tends to accumulate in the bay and near the estuary and coastal regions. The bank blocks a large amount of flooding current and waves into the bay and improves the local current and wave condition effectively near the mouth and in the bay.

Experimental and numerical study of wave-current interactions with a dumbbell-shaped bridge cofferdam

Recently, a new structure type of a vertical truncated dumbbell-shaped cylinder has been widely used in bridge cofferdams in many sea-crossing bridges in China. Wave-only and wave-current interactions with this new structure type are studied experimentally and numerically. The modified Morison equation is applied to derive the drag coefficients CD and inertia coefficients CM. The influence of the incident flow directions, current velocity, static wave height and wave periods on the hydrodynamic coefficients is explored. The study shows that the hydrodynamic coefficients are strongly correlated with the sine values of the incident flow directions. In addition, a 3D numerical model using a multilayer σ-coordinate model and an immersed boundary method based on the Navier-Stokes equations (Kang et al., 2015) is established. Based on the validated numerical model, flow patterns around the structure, including the free surface and 3D vorticity distributions, are analyzed numerically. The results presented herein provide important guidance to better understand the hydrodynamic performance of this new structure type.

Morphological responses of unsheltered channel-shoal system to a major storm: The combined effects of surges, wind-driven currents and waves

The storm impact on coastal environment is complex and systematic. With a validated numerical model, this study quantitatively investigates the effects of a historical storm process on the morphodynamic evolution of an unsheltered Nanpu channel-shoal system (NS) in the Bohai Sea, China. Results show that the storm event in October 2003 induced a destructive erosion in the tidal flats during the entire storm period. The channel also suffered erosion, however, it rapidly restored after this storm. Numerical estimations also confirm that wave and wind-driven current dominate the erosion of tidal flat and channel, respectively. Wind-driven current can further enhance the erosion on the tidal flat by intensifying the wave-current interaction, while surge plays a minor role in the channel-shoal evolution. The morphological evolution pattern of NS varies as a function of wind direction and wind speed, where three morphological evolution patterns appear as the wind speed growth in the investigated NE and NW wind directions. In the study area, NE strong wind usually produces flooding disasters and erosion of NS, while NW strong wind can induce severe tidal flat loss and channel accretion. Although this study is based on a site-specific model, the findings will provide valuable insights into the morphological feedbacks under destructive storm and hence assist in making efficient coastal management strategies in unsheltered coasts.

Analysis of microwave backscattering from nonlinear sea surface with currents: doppler spectrum and SAR images

AbstractElectromagnetic scattering from the sea surface is of great significance in radar detection, target recognition, ocean remote sensing, etc. By introducing the action spectrum, the modified spatio-temporal variation wave spectrum is used to establish a nonlinear sea surface with currents in this paper. Traditional capillary wave modification facet scattering model (CWMFSM) can only calculate the backscattering from the wind-driven sea surface. By using the new spatio-temporal variation wave spectrum to modify the scattering amplitude of every facet, the new CWMFSM can be used to calculate the nonlinear sea surface scattering with surface currents. Therefore, the model simultaneously considers the modulation of sea surface wind and currents to the radar back echo. The dependence of backscattering coefficient from nonlinear sea surface on the incident angle and the polarization are discussed. The results verify that the nonlinear model is more consistent with the measurement data. This paper also investigates the Doppler spectrum characteristics of the sea with currents. It is found that the effect of wave–current interaction on Doppler spectra is weaker than that of wave–wave interaction. The SAR images of nonlinear sea surfaces are also simulated and different bands, polarizations, and baseline length effects on sea current detection performance of along-track interference SAR are analyzed.

Tropical Cyclone Induced Wave Setup around New Caledonia during Cyclone COOK (2017)

Jullien, S.; Aucan, J.; Lefevre, J.; Peltier, A., and Menkes, C.E., 2020. Tropical cyclone induced wave setup around New Caledonia during cyclone COOK (2017). In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1454-1459. Coconut Creek (Florida), ISSN 0749-0208.Tropical cyclone (TC)-induced coastal flooding arises from a combination of atmospheric surge, and energetic wave-induced setup. TC-generated waves in deep water considerably evolve in shallow water before reaching the coast. Islands surrounded by a barrier reef and lagoon are a particular case of steep and irregular bathymetry over which waves undergo various transformations, and induce complex wave-current interactions. The present study investigates the TC-induced coastal impact on the reef environment of New Caledonia in the tropical southwest Pacific. An unstructured wave-current model simulation encompassing New Caledonia and Vanuatu is used to describe the complete TC-induced waves impact, from wave generation to propagation and transformation in the coastal area. Comparison with in-situ measurements are performed during cyclone COOK in 2017. The results show that coastlines with a double protection (barrier and fringing reefs) are relatively sheltered from the waves induced by TCs (significant wave height is 3 m at the coast vs. 13 m offshore) while shores fronting the reef passes are more severely exposed with waves reaching 9 m close to the coast. Setups and currents that are associated to wave breaking on the barrier reef induce strong lagoon currents and localized sea level anomalies that does not necessarily reach the coast. In more sheltered bays, wave breaking at the entrance of the bay acts against outgoing currents. This phenomenon leads to an additional elevation of the mean water level in the bay.

Importance of Wave Non-linearity for 3D Morphodynamic Modelling

Mengual, B.; Bertin, X., and Martins, K., 2020. Importance of wave non-linearity for 3D morphodynamic modelling. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1201-1205. Coconut Creek (Florida), ISSN 0749-0208.The effect of wave non-linearity on morphological changes of a sandbar is investigated through a realistic application at Duck Beach (North Carolina, USA) of a 3D state-of-the-art process-based morphodynamic model, which couples sediment transport, currents and waves (vortex force formalism). From simplified 1D/2DH models, previous studies highlighted that acceleration skewness of non-breaking waves over a sandbar could promote its progressive onshore migration. This process can counterbalance the offshore migration occurring under breaking waves through the development of strong offshore-directed “undertow” currents near the seabed. Based on the existing literature, an additional bedload flux associated to acceleration skewness of waves is implemented in the 3D model. Numerical experiments with and without this additional term clearly demonstrate the need to account for this supplementary wave-induced transport to reproduce onshore migration phases of the sandbar. Effectively, even a model integrating wave asymmetry effects on bedload flux estimates and 3D wave-current interactions fails to reproduce the observed onshore migration of the sandbar.

Wake Characteristics of a Tidal Stream Turbine under Combined Wave and Current

Lin, X.; Zhang, J.; Guan, D.; Zhang, J.; Zhang C., and Gan, M., 2020. Wake characteristics of a tidal stream turbine under combined wave and current. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1558-1562. Coconut Creek (Florida), ISSN 0749-0208.Tidal stream energy has been considered as one of the most promising renewable and clean resources to ease energy shortage. This study experimentally investigates wake characteristics of a tidal stream turbine subject to combined wave and current, using an advanced acoustic Doppler velocimeter to measure its wake properties. The phase-space filter is used to remove spikes in raw data, and the phase-averaged method is adopted for further data analyses. The experimental results indicate that the mean wake characteristics under combination of wave and current share a similar trend with that under pure current. However, it's found that the wave-current interaction fastens the wake recovery and increases the turbulence intensity level. Besides, the supporting structure is found to play an important role in the near wake. A region with low velocity and high turbulence intensity is observed in the lee side of the supporting structure. Furthermore, the wake zone is found to shift upward to the free surface due to wake merging of the turbine rotor and the supporting structure.

Recent Advances in Tidal Inlet Morphodynamic Modelling

Bertin, X.; Mengual, B.; de Bakker, A.; Guerin, T.; Martins, K.; Pezerat, M., and Lavaud, L., 2020. Recent advances in tidal inlet morphodynamics modelling. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1016–1020. Coconut Creek (Florida), ISSN 0749-0208.Tidal inlets connect the ocean to inner water bodies and are present worldwide. Complex interactions between tides, waves and shallow bathymetry often drive fast morphological changes but the underlying processes remain only partly understood. To better understand these processes, the development and application of morphodynamic models represents a unique perspective. This paper evaluates the impact of recent developments in the modelling system SCHISM, which include: a WENO method to solve the Exner Equation, a 3D coupling between waves and currents using a vortex force formalism, an adaptive parameterization for the dissipation of short waves by breaking and improved representations for wave-induced sediment transport. In order to evaluate the relevance of these developments, SCHISM is applied to the Maumusson Inlet, a mixed-energy inlet located on the Western Coast of France. Model-data comparison reveals firstly that complex wave-current interactions take place over the inlet ebb-delta, that include partial wave blocking during the ebb. Compared to classical 2DH approaches, our improved modelling system better reproduces the dynamics of adjacent beaches, inlet migration under oblique waves and sediment infilling of the main channel under storm waves. The relevance of these developments is demonstrated at the mixed-energy Maumusson Inlet (France).

Storm Surges in the Bohai Sea: The Role of Waves and Tides

A storm surge is a complex phenomenon in which waves, tide and current interact. Even though wind is the predominant force driving the surge, waves and tidal phase are also important factors that influence the mass and momentum transport during the surge. Devastating storm surges often occur in the Bohai Sea, a semi-enclosed shallow sea in North China, due to extreme storms. However, the effects of waves on storm surges in the Bohai Sea have not been quantified and the mechanisms responsible for the higher surges that affect part of the Bohai Sea have not been thoroughly studied. In this study, we set up a storm surge model, considering coupled effects of tides and waves on the surges. Validation against measured data shows that the coupled model is capable of simulating storm surges in the Bohai Sea. The simulation results indicate that the longshore currents, which are induced by the large gradient of radiation stress due to wave deformation, are one of the main contributors to the higher surges occurring in some coastal regions. The gently varying bathymetry is another factor contributing to these surges. With such bathymetry, the wave force direction is nearly uniform, and pushes a large amount of water in that direction. Under these conditions, the water accumulates in some parts of the coast, leading to higher surges in nearby coastal regions such as the south coast of the Bohai Bay and the west and south coasts of the Laizhou Bay. Results analysis also shows that the tidal phase at which the surge occurs influences the wave–current interactions, and these interactions are more evident in shallow waters. Neglecting these interactions can lead to inaccurate predictions of the storm surges due to overestimation or underestimation of wave-induced set-up.

Dynamic response of spar-type floating offshore wind turbine in freak wave considering the wave-current interaction effect

In this paper, the dynamic responses of a Spar-type floating offshore wind turbine (FOWT) are simulated and analyzed under the scenarios with freak wave superimposed a uniform current. The wave-current interaction is considered in the process of freak wave generation. An improved phase modulation algorithm is used to generate the free surface profile of the freak wave. Compared with the deterministic wave modeling method, this method can realize the randomness of the sea state and also achieve high efficiency. The effect of the presence of current on the wave energy spectra is firstly investigated. Then the OC3 Hywind Spar-type FOWT in the parked condition is chosen as an example to study the dynamic responses under the extreme wave scenarios. An in-house MATLAB code based on a nonlinear aero-hydro-mooring coupled model is adopted to perform these numerical investigations. The simulations are conducted in the time domain and then the results of wave forces and platform motions in the surge, heave and pitch are summarized and analyzed considering the presence of current with different velocities. The results of mooring lines tension and the acceleration at nacelle are also investigated. Through the simulation results, we can have a clear understanding of the mechanism of the wave-current interaction phenomenon for extremely high waves in Gaussian seas. Furthermore, it also allows identifying the severest wave conditions which are important for the analysis of the floating structure ultimate design conditions.

A data set of sea surface stereo images to resolve space-time wave fields

Stereo imaging of the sea surface elevation provides unique field data to investigate the geometry and dynamics of oceanic waves. Typically, this technique allows retrieving the 4-D ocean topography (3-D space + time) at high frequency (up to 15–20 Hz) over a sea surface region of area ~104 m2. Stereo data fill the existing wide gap between sea surface elevation time-measurements, like the local observation provided by wave-buoys, and large-scale ocean observations by satellites. The analysis of stereo images provides a direct measurement of the wavefield without the need of any linear-wave theory assumption, so it is particularly interesting to investigate the nonlinearities of the surface, wave-current interaction, rogue waves, wave breaking, air-sea interaction, and potentially other processes not explored yet. In this context, this open dataset aims to provide, for the first time, valuable stereo measurements collected in different seas and wave conditions to invite the ocean-wave scientific community to continue exploring these data and to contribute to a better understanding of the nature of the sea surface dynamics.

On the model formulations for the interaction of nonlinear waves and current

We study the model formulations of wave–current interactions in the framework of Euler equations. This work is intrigued by a recent paper from Wang et al. (2018) (hereafter WMY), which proposes such a model for the evolution of nonlinear broadband surface waves under the influence of a prescribed steady and irrotational current without vertical shear. We show that WMY’s model can be derived from a more general model accounting for an arbitrary steady and irrotational current. Under further assumption of scale separation between waves and current (i.e. horizontally slowly-varying current), WMY’s model is equivalent to an earlier model, in contrast to WMY’s claim that their model includes additional higher-order effects in wave steepness. We demonstrate the usefulness of such models in a numerical study on wave blocking by opposing current, where the nonlinear effect on the caustic location and wave amplitude amplification is elucidated. We further show that the model formulation in the framework of Euler equations form a Hamiltonian system conserving the total energy of waves and current, justifying the theoretical significance of the model equations. Finally, we generalize the formulation to nonlinear wave evolution in the presence of a rotational current with constant vorticity, which overcomes a limitation of such models that has been overlooked in previous work.

Influence of surface waves on the hydrodynamic performance of a horizontal axis ocean current turbine

It is known that surface waves have significant influence on the hydrodynamic performance of ocean current turbines which locate near the water surface. In order to quantitatively analyze the wave influence and reveal the interaction mechanism between the wave and the turbine flow, this paper proposes a three-dimensional transient computational fluid dynamics (CFD) model which can accurately predict the hydrodynamic performance of ocean current turbines under current-wave interaction flow conditions. The influences of two key wave parameters, the wave height and the submerged depth of the turbine, on the hydrodynamic forces and flow structures of a three-bladed horizontal axis ocean current turbine are discussed in depth. It is found that the both the average value and the oscillation amplitude of the torque on the turbine increase with the increased wave height, but decrease with the increase of the submerged depth. It is also found that in the cases of shallow submerged depth, the wake structures of the turbine are affected by the surface wave.