Waves of any form—be it ocean waves, sound waves or seismic waves—are a way of transferring energy from point A to point B. There are two types of waves in the universe, electromagnetic (e.g., light waves) and mechanical (e.g., sound waves). The waves we’re interested in, ocean waves, are a type of mechanical wave. This means they have to travel through something (a medium) which, in the case of ocean waves, is water.
Ocean waves (normally) form when the wind blows across an open body of water, giving them the alternate title of wind waves or wind generated waves. We’ll just call them ocean waves here though. They travel along the surface of the water, at the interface (a fancy word for boundary) between the air and the water. Waves are important for coastal systems because they’re a method of moving energy, energy which ultimately erodes and shapes coastlines. As a result, understanding how they develop and how to describe them is super helpful.
When the wind blows across the ocean, its speed varies with height being slower near the surface of the ocean. This is because there is friction acting between the water and the air. This friction produces turbulence in the air near the surface of the water while also transferring some of the air’s kinetic energy into the water. The transfer of energy into the water combined with the turbulence produces perturbations in the surface of the water that eventually become waves.
Initially these waves are small. How they grow is controlled by how much energy the wind can transfer to them. The obvious control on this is wind speed. Faster winds have more kinetic energy and can therefore produce larger waves. Another control is the length of time the wind blows for. The longer it blows for, the more energy can be transferred into the water, producing stronger waves. A less obvious control on the strength of generated waves is the fetch. This is the length of water the wind has blown over. A longer fetch means the water’s had more opportunity to transfer kinetic energy and so we get stronger waves.
The wind isn’t the only way of generating ocean waves though. Anything which disturbs the surface of the ocean has the potential to generate waves. Cataclysmic events like earthquakes, submarine volcanic eruptions or landslides generate immense waves known as tsunamis1. These don’t look like2 the normal ocean waves we’re used to and have far more kinetic energy. Because of the infrequent occurrence of these waves, we’re not going to worry about them in this topic. Just be aware that they are still a type of ocean wave.
As ocean waves are just another type of wave, they have a bunch of properties that should be faimiliar to you if you’ve ever studied basic Physics3.
The crest and trough of a wave are the highest and lowest points of a wave respectively. If you imagined the surface of the water was flat and overlayed a wave on top of it, the amplitude of the wave would be the height of the crest above the stationary water. Note that a wave’s amplitude is not the same thing as it’s height. A wave’s height is the vertical distance between a crest and a trough.
The wavelength of a wave is the horizontal distance between two crests or troughs. For wind generated ocean waves, it can vary from a few tens of centimetres to hundreds of metres. The wavelength is sometimes referred to using the lowercase Greek letter lambda (λ). This isn’t too common in Geography but it’s used all the time in Physics.
The time period of a wave (sometimes called the wave period) is the time it takes for a wave to travel through one wavelength. That’s another way of saying it’s the time between two crests or troughs passing a stationary point. The time period is normally measured in seconds.
The time period is linked to another property of a wave, its frequency. The frequency is the number of crests or troughs passing a stationary point per second. Mathematically, it’s the inverse of the time period or one divided by the time period. If the time period were in seconds per wave (or just seconds), the frequency would be in waves per second. For ocean waves, this is normally a small number so the frequency is more commonly given in waves per minute.
Finally, the velocity of a wave is the speed at which it moves in a certain direction. For wind generated waves, the velocity has to be less than the velocity of the wind, above it and the wind can’t impart energy anymore.
Particle Motion in Waves
When we talk about the motion of waves, what we’re really talking about is the motion of particles as waves move through them. After all, that’s what we can see. The motion of particles is related to the ratio of the wave’s wavelength to the water’s depth.
In water that is much deeper than the wavelength of the wave, particles follow an almost closed circular path. The radius of the path depends on the wavelength of the wave and the depth of the particle, decreasing with increasing depth. At a depth of around one wavelength, the radius is so small the particles effectively don’t move. In water that isn’t much deeper than the wavelength of the wave, the particles follow an almost closed elliptical path. This increase in eccentricity happens because the sea bed starts to interfere with the motion of the particles.
Now, notice that I said for both shallow and deep water waves, the particles follow an almost closed path. Those of you that paid attention during Physics will remember that particles in a wave oscillate around a central position, but they always return to the same position once the wave has passed. In other words, there is no net movement of particles. This isn’t the case for ocean waves due to a phenomenon called Stokes Drift. This is a complicated piece of fluid mechanics that you don’t need to understand. You just need to know that it’s responsible for water wave particles having non closed paths that causes a net movement of particles in the direction of the wave’s propagation.
Waves & Coastlines
So far we’ve only talked about waves in the context of a deep ocean but the place where we really see waves in action is near the coast. Here, the water gets shallower, the waves get bigger and some of that energy the waves have been carrying finally makes it to the land.
On the run-up to a coastline, the depth of the water becomes shallower and a wave is forced to slow down. As it slows, its wavelength decreases and its height increases4. This entire process is called shoaling. The wave’s height can only increase so far however. Above a height of around one seventh the wavelength of the wave5, the wave becomes unstable and it breaks.
Depending on the properties of the wave, when it breaks on a coastline it can be classified as either constructive or destructive (also referred to as surging or surfing). They differ in the strength of their swash (rush of water up a beach) and backwash (rush of water down a beach).
Constructive waves have a long period, a long wavelength and a low amplitude. When they break on a beach, they have a strong swash and a weak backwash. This means they deposit more material on a beach than they remove. Over time, they build up gentle beaches. The repeated action of pushing material up a beach eventually leads to the development of berms.
Destructive waves have a short period, short wavelength and a high amplitude. They tend to be steep and form during storms. When they break on a beach, they have a weak swash but powerful backwash. As a result, they remove material from a beach and produce a steep beach with breakpoint bars. In particularly stormy weather, destructive waves can be powerful enough to throw material to the back of a beach producing a storm beach—a ridge of coarse material.
Don’t ever, ever, ever, call these tidal waves. Never. I’m serious. Don’t you dare. They’ve got nothing to do with the tides.↩
In the open ocean, they have wavelengths in the hundreds of kilometres range while their amplitude is only a few tens of centimetres. They’re barely noticeable. It’s when they get to shallower water that they grow in size. ↩
This is again because of some complicated fluid mechanics. In shallow water, the group velocity of a wave is proportional to the square root of the water’s depth. As the depth decreases, the group velocity decreases but the wave’s frequency remains constant. To ensure a constant energy flux, the wave’s wavelength must decrease which in turn means its height must increase. You absolutely do not need to know this but I’m sure you found it interesting. ↩
I’ve seen some sources say breaking occurs at one eighth the wave’s wavelength. ↩