Free Surf 19

How Waves are Generated

Apart from special waves such as tsunamis, the only thing that produces the waves we see on our coasts is the action of the wind blowing over the sea surface. Waves arriving on a coast can be generated by local wind, in ‘real time’, in which case the waves are called windsea, or they can be the result of a wind that blew over the surface of the ocean thousands of kilometres away, up to several days before, in which case they are termed swell or groundswell.

To produce waves, the air moving over the surface of the water has to somehow transmit its energy to the water. Just how this happens is a very complicated process, still not well understood. The most accepted theory is the one proposed in the 1950s by J.W. Miles and O.M. Phillips" the Miles-Phillips theory. The theory describes how waves are generated from a flat sea using two mechanisms; the first of which produces tiny ripples called capillary waves, and the second of which produces bigger waves called gravity waves (those we ride).

According to the Miles-Phillips theory, capillary waves first begin to grow from an entirely flat sea, and then gravity waves are subsequently formed from a sea already containing capillary waves. Gravity waves and capillary waves are named as such because the restoring force (the force that returns the sea to an equilibrium position after the wind has lifted it up) is gravity in the case of a gravity wave and capillary action, or surface tension, in the case of a capillary wave.

The initial generation of capillary waves is due to perturbations in the surface wind, causing irregularities in the water surface. The wind does not blow completely horizontally all the time; it will naturally contain random disturbances that give it small vertical motions as well. Sometimes, these vertical motions are enough to create tiny up and down motions on the surface of the water itself. This is the vital beginning which triggers off further reactions and facilitates the flow of energy between wind and water.
Once the sea contains capillary waves, there is an increase in surface roughness, which allows the moving air to ‘grip’ the surface of the water. There is no longer any need for small vertical perturbations in the air flow; the horizontally-moving air will now push up the existing bumps in the water surface. This second mechanism is self-perpetuating; the rougher the surface the more ‘grip’, the more grip the bigger the waves, the bigger the waves the rougher the surface, and so on. While the first mechanism causes the waves to grow at a rate which is linear with time, the second mechanism is exponential with time; the bigger they are the quicker they grow.

The restoring force of these bigger waves is now gravity, not surface tension. Eventually a point will be reached where the wind can’t lift up the surface of the sea any more" the force of gravity pulls the water back down again at the same rate as the wind lifts it up. This natural limit is reached for a given windspeed, so, if the wind gets stronger, the waves will get higher.