Wave-current Interaction Mechanisms Research Articles

2-D Characteristics of Wave Deformation Due to Wave-Current Interactions with Density Currents in an Estuary

In this study, numerical simulations were conducted in order to understand the role of wave-current interactions in wave deformation. The wave-current interaction mechanisms, wave reflection and energy loss due to currents, the effect of incident conditions on wave-current interactions, the advection-diffusion characteristics of saltwater, and the effect of density currents on wave-current interactions were discussed. In addition, the effect of saltwater–freshwater density on wave-current interactions was investigated under a hypopycnal flow field via numerical model testing. Turbulence was stronger under the influence of wave-current interactions than under the influence of waves alone, as wave-current interactions reduced wave energy, which led to decreases in wave height. This phenomenon was more prominent under shorter wave periods and higher current velocities. These results increase our understanding of hydrodynamic phenomena in estuaries in which saltwater–freshwater and wave-current pairs coexist.

Mechanistic analysis of the wave-current interaction in the plume region of a partially mixed tidal inlet

In this research, a three-dimensional coupled wave-circulation model, including meteorological forcing, freshwater inflow and time varying open boundary conditions, for New River Inlet is validated. A mechanistic approach is taken to investigate how various wave-current interaction mechanisms affect the nearshore circulation, plume expansion and surface wave field in the plume region of a relatively small partially mixed tidal estuarine system. More specifically, focus is comparing four different modeling cases including: (1) a three-dimensional ocean circulation model (no wave effects), (2) a coupled wave-circulation model, (3) a coupled wave and circulation model including vertical mixing enhancement due to wave breaking, and (4) a wave model without surface current effects. Findings reveal forces are applied by incoming waves due to various wave-current interaction mechanisms. Wave momentum released by incoming waves pushes the outgoing freshwater ebb plume back to the shoreline and prevents the plume from expanding freely towards the open ocean. Findings also reveals that releasing wave-dissipated energy in the expanding plume region enhances vertical mixing, mixes down freshwater, and therefore thickens the plume. These results are congruent with observations at the mouth of the Columbia River.

Ocean feature detection using altimetry backscatter: The Gulf Stream

This study presents a basic analysis of ERS-1 altimeter data, from its 3-day repeat track periods taken over a section of the Gulf Stream. Along track variations of radar backscatter (?0) have been compared with positions of the Gulf Stream derived from the composite maps of sea surface temperature from ship and satellite-sensor observations, produced by NOAA. Variations in ?0 , indicating potential interaction with the Gulf Stream, are found at both high and low wind speed conditions. At high wind speeds, comparison with model wind data indicates that peaks in altimeter-derived wind speeds along the path of the North Wall of the Gulf Stream may be related to real increases in wind speed. A possible reason may be a tendency for winter storm tracks to follow the same path as sections of the Gulf Stream. At low wind speeds, minima in altimeterderived winds are associated with sections of the path of the Gulf Stream but there were no comparable occurrences of low wind speeds in model wind fields, with implied consequences for winds derived from satellite sensor data. Although, for low wind speeds, the cause of these 'anomalously high' backscatter regions is unknown, such associated relatively 'calmer' waters have been previously noted as the result either of a tendency for surface slicks to be confined to the shears associated with local current systems or by wave-current interaction mechanisms causing a local reduction in the sea surface roughness.

On the behaviour of tidal current directions due to the presence of submarine sand waves

Abstract Side-scan sonar records reveal that the crestlines of megaripples in the troughs as well as on both sides of the slopes of sand waves form an angle with the crestline of the sand waves itself. The existence of such an angle between the crestline of megaripples and sand waves indicates that changes of the (tidal) current direction across sand waves can be expected. Direct evidence that such (tidal) current direction changes occur above sand waves is derived from measurements of the Air–Sea Interaction Drift Buoy (ASIB) system at the sea surface and from side-scan sonar records of the sea bed. Both systems were operated from on board research vessels during two C-STAR field experiments in the Hoek van Holland study area off the Dutch coast in April 1996 and in April 1997. Measurements performed by electrical resistance wires on board the ASIB system indicate that variations of the short and moderate wave direction and the wave directional spread are associated with changes of the (tidal) current direction at the sea surface. According to the results of the measurements derived from the ASIB system and the side-scan sonar records it can be summarised that the variation of the direction of short and moderate period water waves as well as changes of the (tidal) current speed and direction across large sand waves must be considered for correct modelling of the wave-current interaction mechanism.

Flow resistance in the coastal zone

The apparent bed roughness, the roughness value experienced by a mean flow outside the wave-boundary layer, is deduced from the physical bed roughness and the wave–current interaction mechanism. Both the physical bed roughness and the wave–current interaction are described by a (combination of) model(s). Modelling of the apparent bed roughness leads to realistic results, however, the final results are rather sensitive to the particular choice of these models. Four bed form models and two wave–current interaction models were implemented in a 1-DV flow model to calculate near-bed velocities. A comparison between measured and predicted velocities shows that reasonable results can be obtained in this way. A constant bed roughness of 0.1 m, however, leads to even better results at this site during all conditions. This can be explained by the reversed influence of the form roughness and the wave–current interaction on the apparent bed roughness value for varying wave conditions.

A case study of Doppler-shifted inertial oscillations in the Norwegian Coastal Current

A case study with emphasis on near-inertial oscillations with frequency below the inertial frequency ƒ has been performed based on current measurements from 40 m depth in the Norwegian Coastal Current. The measurements show evidence of transient features with periods of 15 and 130 h while the local half-pendulum day is 13.9 h. The currents related to the 15-h period show an elliptic pattern in clockwise rotation, while the currents corresponding to the 130-h period show east-west oscillations. By a modal decomposition of the velocity field it has been shown that energetically, the barotropic and first baroclinic mode dominate the field. Based on data analysis and application of a reduced gravity model with an associated uniform geostrophic current, the oscillations are considered with regard to the Doppler shift wave-current interaction mechanism. Perturbations on the basic current as plane waves propagating in the current direction, have been considered. The dispersion relationships obtained show two near-inertial and one low frequency wavemode of the Rossby type related to the sloping interface. The Doppler shift affects the dispersion properties for upstream and downstream propagating waves in different ways. For the downstream propagating waves the dispersion relationships are slightly modified compared with propagation in stationary mediums. For the upstream propagating waves the Doppler shift distorts the dispersion properties, resulting in a dispersion relation with both positive and negative group velocities and thus zero group speed for a certain wavelength with corresponding frequencies below ƒ for that waveband. In accordance with observations, the model shows zero group speed for a wavelength of 45 km with a corresponding frequency of 0.9 ƒ. The velocity components for near-inertial wavemodes show velocities in anti-cyclonic and cyclonic rotations. The pronounced observed oscillations with frequency 0.9 ƒ are explained as a result of the different dispersion properties for upstream and downstream propagating waves, where upstream propagating waves in the waveband of zero group speed are amplified due to accumulation of energy. Also the long periodic velocity field corresponds reasonably well with observations where the model shows eas-twest oscillations for symmetric disturbances.

A Third-Generation Model for Wind Waves on Slowly Varying, Unsteady, and Inhomogeneous Depths and Currents

Abstract A full discrete spectral model for propagation generation and dissipation of wind waves for arbitrary depth, current and wind fields is presented (WAVEWATCH). This model incorporates all relevant wave-current interaction mechanisms including changes of absolute frequencies due to unsteadiness of depth and currents. The model furthermore explicitly accounts for growth and decay of wave energy and for nonlinear resonant wave-wave interactions. The numerical schemes for propagation are basically second-order accurate. Effect of refraction and frequency shifts (due to unsteadiness of depth and current) are calculated on a fixed grid, also using second-order schemes. This paper focuses on the governing equations and the numerical algorithms. Furthermore some results for academic and realistic cases are presented to illustrate some features and merits of the model.