Wind Waves and Swell

Abstract

Winds blowing over the water surface generate waves. In general the higher the wind velocity, the larger the fetch over which it blows, and the longer it blows the higher and longer will be the average waves . Waves still under the action of the winds that created them are called wind waves, or a sea. They are forced waves rather than free waves. They are variable in their direction of advance (Arthur, 1949). They are irregular in the direction of propagation. The flow is rotational due to the shear stress of the wind on the water surface and it is quite turbulent as observations of dye in the water indicates. After the waves leave the generating area their characteristics become somewhat different, principally they are smoother, losing the rough appearance due to the disappearance of the multitude of smaller waves on top of the bigger ones and the whitecaps and spray. When running free of the storm the waves are known as swell. In Fig. 1 are shown some photographs taken in the laboratory of waves still rising under the action of wind and this same wave system after it has left the windy section of the wind-wave tunnel. It can be seen thati-the freely running swell has a smoother appearance than the waves in the windy section. The motion of the swell is nearly irrotational and nonturbulent, unless the swell runs into other regions where the water is in turbulent motion. Turbulence is a property of the fluid rather than of the wave motion. After the waves have travelled a distance from the generating area they have lost some energy due to air resistance, internal friction, and by large scale turbulent scattering if they run into other storm areas, and the rest of the energy has become spread over a larger area due to the dispersive and angular spreading characteristics of water gravity waves. All of these mechanisms lead to a decrease in energy density. Thus, the waves become lower in height. In addition, due to their dispersive characteristic the component wave periods tend to segregate in such a way that the longest waves lead the main body of waves and the shortest waves form the tail of the main body of waves. Finally, the swell may travel through areas where winds are present, adding new wind waves to old swell, and perhaps directly increasing or decreasing the size of the old swell.

Expected Distribution of Surfing Days in the Iberian Peninsula

“…For example, in the Mediterranean, local surfers call low periods those smaller than four seconds, medium between four seconds and eight seconds and high periods are those larger than eight seconds. This contrasts with the typical values for the Atlantic, where low periods are those smaller than eight seconds, medium range between 8 s and 13 s and high, larger than 13 s [7].…”

Evaluation of Wave Contributions in Hurricane Irma Storm Surge Hindcast

“…The ADCIRC-2DD1 model is used for the simulation of tides and surges. The model uses the continuous Galerkin finite element method to solve shallow-water equations to model hurricane storm surges on unstructured meshes [11,26]. The wind drag formulation of Powell, with a cap of 0.0028, is adopted for the calculation of wind stress.…”

“…The size of waves depends on the wind speed and wind duration, and is directionally dependent on the area over which the wind is blowing [11][12][13][14]. From earlier studies, such as [11,12], the relationship between waves and fetch is well understood. The larger the fetch over which the wind blows, the longer the average waves will be [11].…”

“…From earlier studies, such as [11,12], the relationship between waves and fetch is well understood. The larger the fetch over which the wind blows, the longer the average waves will be [11]. The study [12] found that wave height is proportional to the square root of the fetch.…”

Modelling swell propagation across the Pacific

“…Whilst the phase speed of the swell remains greater than the local wind, it is generally assumed that there is little interaction between winds and the propagating swell (Young and Sobey, 1988;Young et al, 2013). However, should this difference decrease, swells can reconnect with local wind along their propagation paths and increase or decrease the energy of the waves (Wiegel, 1960;Ardhuin et al, 2009;Babanin et al, 2019). Since the wave climate of the world's oceans is significantly dependent on swell wave conditions, studying the properties and the behavior of swell is important.…”