For anybody who makes their living on the sea or those who use the sea for recreation, waves are very important. Waves can damage off-shore structures and determine the speed that commercial vessels can travel. Small boats can be swamped by waves and surfers live for them. For marine organisms that live along the coastline, waves have had a strong influence on how they have evolved.
Wind, landslides, other waves and earthquakes transfer their energy to the water, the more energy they have, the bigger the waves can become. In most cases waves are created by the wind and they move away from the source of the energy.
Properties of Waves
The top of the wave is called the crest and the bottom, the trough. The wavelength is the distance between each following crest of trough, the wave height or amplitude is the height from the top of the crest to the bottom of the trough and the period of the wave is the time it takes between the crest or trough and the next crest or trough. At the centre of the energy source, the peaks are sharp but as they move away they become more rounded forming a swell. When waves are long and low they can travel for many thousands of miles.
The classification of waves is based on the period between troughs or crests. Ripples have periods of less than 0.5 seconds whilst tsunamis and tides have periods that are measured in minutes and hours and have wavelengths of hundreds, even thousands, of miles.
The most common types of surface waves that we are all familiar with are the chops and swells which have periods in between ripples and tsunamis. Ocean waves behave in the same way as light waves when they hit an obstacle such as islands, they are reflected or refracted. When different wave groups meet they interact with each other, either increasing or decreasing the force of the others group.
Generation of Waves
The energy of the wind is passed to the surface of the sea by friction and pressure. The stronger the wind, the rougher the surface of the water becomes, first ripples are formed and then larger waves. The size of wind-generated waves is determined by three factors. The first is the length of time of the contact with the energy source, the second is the velocity of the wind and the third is the distance over which the wind is in contact with the sea. The factors are called the fetch.
Propagation of Waves
There are many different groups of waves with differing wavelengths generated in the fetch and these groups interfere with each other. As they move away from the fetch the waves develop to become more regularly spaced and sized. The speed of the wave in open water is closely connected to its wavelength, so the groups of waves are sorted by wavelength, with the biggest, fastest waves moving to the front. The only waves to be affected by the depth of the water are tsunamis, as they have very long wavelengths. Otherwise, waves continue at the same speed until they reach the shallow water at the shore line.
As waves reach the shore, their motion starts to react with the sea floor. The waves are slowed down and the crests bunch up to together, called shoaling. The wave period stays the same but they get taller as their energy is squashed into a shorter distance horizontally and they finally break on the shore.
Breakers can be characterised into two main types, spilling breakers and plunging breakers. As the waves reach a flat shore, the crests break and pour down the front, the energy is dispersed slowly and this is the spilling breaker. On steeper shores, plunging breakers have crests which curl and fall over in front of the advancing wave and the whole wave falls at once. Waves can also refract as they reach the shore, concentrating the wave energy onto headlands and the shape of some beaches is caused by this energy.