Effect of wave-current interaction on waves and circulation over Georges Bank during storm events

The coupled spectral wave and circulation model SWAN+ADCIRC was applied to investigate the wave-current interaction during storm events over Georges Bank, a large shallow submarine bank on the eastern seaboard of North America that separates Gulf of Maine from the North Atlantic Ocean. The current over the Georges Bank displays a rotary feature over a tidal cycle. The wave-induced current is in the same order as the wind-driven current and generally in the same direction as the depth-averaged tidal current, indicating strong nonlinear wave-current interaction. The magnitude of wave-induced current reaches 0.07 m/s at low tide and 0.2 m/s at the other three tidal phases. The effect of wave-current interaction on waves at the four tidal phases is also analyzed. The role of Georges Bank in dissipating wave energy is most significant at rising mid-tide and high tide, which is close to the storm peak. At rising mid-tide, the wave height is decreased by 0.3 m to 0.5 m over the majority of the bank when the wave propagates in the same direction as the current. At falling-mid tide, the wave height is increased by 0.5 m at the southern flank and decreased by 0.5 m at the northern flank of the bank.</jats:p

Wind and current effects on extreme wave formation and breaking

AbstractWind and current effects on the evolution of a two-dimensional dispersive focusing wave group are investigated using a two-phase flow model. A Navier–Stokes solver is combined with the Smagorinsky subgrid-scale stress model and volume of fluid (VOF) air–water interface capturing scheme. Model predictions compare well with the experimental data with and without wind. It was found that the following and opposing winds shift the focus point downstream and upstream, respectively. The shift of focus point is mainly due to the action of wind-driven current instead of direct wind forcing. Under strong following/opposing wind forcing, there appears a slight increase/decrease of the extreme wave height at the focus point and an asymmetric/symmetric behavior in the wave focusing and defocusing processes. Under a weak following wind, however, the extreme wave height decreases with increasing wind speed because of the dominant effect of the wind-driven current over direct wind forcing. The vertical shear of the wind-driven current plays an important role in determining the location of and the extreme wave height at the focus point under wind actions. Furthermore, it was found that the thin surface layer current is a better representation of the wind-driven current for its role in wind influences on waves than the depth-uniform current used by previous studies. Airflow structure above a breaking wave group and its link to the energy flux from wind to wave as well as wind influence on breaking are also examined. The flow structure in the presence of a following wind is similar to that over a backward-facing step, while that in the presence of an opposing wind is similar to that over an airfoil at high angles of attack. Both primary and secondary vortices are observed over the breaking wave with and without wind of either direction. Airflow separates over the steep crest and causes a pressure drop in the lee of the crest. The resulting form drag may directly affect the extreme wave height. The wave breaking location and intensity are modified by the following and opposing wind in a different fashion.</jats:p

Modelling storm surge wave overtopping of seawalls with negative freeboard

A Reynolds-averaged Navier-Stokes based wave model (RANS) is used to simulate storm surge wave overtopping of embankments. The model uses a wave generating boundary condition that accepts a wave time history as an input and reproduces the time history in the model. This allows a direct wave by wave simulation of recorded data. To investigate the success of the model at reproducing the wave generation, transformation and overtopping processes the model is compared with experimental laboratory data. A wave-by-wave comparison is performed for overtopping parameters such as discharge, depth and velocity. Finally the overtopping discharge predicted by the model is compared against design formulae.</jats:p