Offshore storms generate winds which blow on the surface of the sea and create ripples. The stronger and longer the wind blows, the more effect it has on these ripples and the larger they become. Initially the waves will just be small chop, but these will soon increase in size. As the wind continues to blow and the waves generated remain under the influence of the wind, the smaller waves will increase in size. The wave size is dependent on the wind speed generating it. A certain wind speed will only be able to generate a wave of certain size. Once the largest waves that can be generated for a given wind speed have formed, the seas are fully developed. Waves that are no longer affected by the wind that generated them can be referred to as ground swell.
There are three main factors that affect the size of a wave in open sea:
Laboratory studies with small wave age c/u* (c is the wave phase speed and u* is the wind friction velocity) have shown that wind-wave generation is reduced in the presence of swell (e.g., Phillips and Banner 1974;Donelan 1987, 18–23). Near the shore (within 0.5 km of the coast or at < 10-m depth) the wave age is reduced as the phase speed of the shoaling waves decreases, and increased coupling between the waves and the wind is expected.
In SWAN Model the white-capping (which is taken from WAM Cycle III, WAMDI, 1988) is scaled with the overall steepness of the waves. When swell is added to wind sea, the overall steepness reduces and consequently, the white-capping reduces. This implies that the net growth increases. However, it is hard to see why swell would reduce white-capping. Moreover, since this effect in the WAM-III formulation is direction-independent, it occurs whether the swell is opposing the wind sea or not.
Observations of Donelan (1987) and Mitsuyasu and Yoshida (1989) show that a following swell reduces the growth of wind waves whereas an opposing swell enhances this growth.
In 1989, Mitsuyasu suggested that the wind waves would be more exposed to the wind near the crests of the swell than in the trough (Mitsuyasu, 1989). This, in combination with a longer residence time near the crest and a shorter residence time near the trough (due to the orbital motion of the swell) would result in an enhancement of the growth of the wind waves in the presence of an opposing swell. The modified formulation is,
There are three main factors that affect the size of a wave in open sea:
- Wind speed - The greater the wind speed is, the larger the wave will be.
- Wind duration - The longer the wind blows, the larger the wave will be.
- Fetch - The greater the area the wind affects the wave, the larger the wave will be.
Laboratory studies with small wave age c/u* (c is the wave phase speed and u* is the wind friction velocity) have shown that wind-wave generation is reduced in the presence of swell (e.g., Phillips and Banner 1974;Donelan 1987, 18–23). Near the shore (within 0.5 km of the coast or at < 10-m depth) the wave age is reduced as the phase speed of the shoaling waves decreases, and increased coupling between the waves and the wind is expected.
In SWAN Model the white-capping (which is taken from WAM Cycle III, WAMDI, 1988) is scaled with the overall steepness of the waves. When swell is added to wind sea, the overall steepness reduces and consequently, the white-capping reduces. This implies that the net growth increases. However, it is hard to see why swell would reduce white-capping. Moreover, since this effect in the WAM-III formulation is direction-independent, it occurs whether the swell is opposing the wind sea or not.
Observations of Donelan (1987) and Mitsuyasu and Yoshida (1989) show that a following swell reduces the growth of wind waves whereas an opposing swell enhances this growth.
In 1989, Mitsuyasu suggested that the wind waves would be more exposed to the wind near the crests of the swell than in the trough (Mitsuyasu, 1989). This, in combination with a longer residence time near the crest and a shorter residence time near the trough (due to the orbital motion of the swell) would result in an enhancement of the growth of the wind waves in the presence of an opposing swell. The modified formulation is,