11/16/2014 -- What causes ocean waves to crash on the shore?

posted Nov 16, 2014, 6:31 PM by Patrick Poole   [ updated Nov 16, 2014, 8:47 PM ]
Surfs up!
Going to the beach is fun for most people because of the relaxation and entertainment you can find there. It can also be fun because of the interesting natural phenomenon that go on here: there’s a unique opportunity for sunset viewing, for observing the change in water level due to the tide, and for watching ocean waves roll in, which serve as probably the most accessible form of a wave encountered in daily life. All of these will make good posts, and today we’ll talk about what goes into an ocean wave.

Water waves are caused by a number of effects, but the most common is wind far from the shore

Like all waves, ocean waves are the physical manifestation of energy moving through a medium. Imagine you are holding one end of a jump rope, and the other end is tied in place to a fencepost. If you move your arm up and down, you will send a wave moving along the rope.

The energy that initially makes the wave can come from a number of sources. In the jump rope case it comes from your arm. Most ocean waves come from a bit of wind blowing near the water surface, and big waves can originate from the powerful winds moving near off-shore storms. Huge waves like tsunamis start from earthquakes that happen near underwater tectonic plates, or from volcanic eruptions underwater.

Water is just the medium through which the wave travels, so a given water molecule doesn’t actually move along with the wave

In the jump rope analogy, the rope itself doesn’t go anywhere because of the waves your arm starts. If you watch one section of jump rope it will just rise and fall as the wave passes by. Water wave are just the same: as the wave passes one section of water will move up and down, but it won’t actually move forward as the wave passes. Now it’s definitely possible for things to move with the wave—that’s what’s happening when a surfer rides the wave in towards the shore—but most of the water molecules aren’t going much of anywhere, especially if the wave is small and far from the shore.

Lonely wave making

Tsunamis are sometimes called tidal waves, but in fact they have nothing to do with the tides! Tides are caused by moon gravity pulling on the Earth, but that’s another post.

Water molecules move roughly in orbits, and this occurs under the water surface as well

You can image the water molecules in an ocean wave doing the same up-and-down motion that you’d observe in your jump rope—in fact they move in little oval patterns, or orbits. This has to do with how freely the water is able to move: you get a sort of sloshing effect as the wave passes by. Importantly, water below the surface also moves in this up and down orbit motion, just not quite as strongly. All oceans waves have a sort of depth related to them, where eventually there is no water motion below the surface. Check out this awesome gif by Krainnest showing circular water motion from a wave:

Awesome wave gif

As the wave approaches the shore, the sea bed pushes the wave up, which causes a crest

The nature of the ground under the ocean can be complicated, but near the shore it behaves about like you would think: there is a gradual decrease in depth as you travel closer to the shore line. As the wave is travelling towards land, the depth of the water will get close to the depth of the wave. Ocean waves will lose some energy because of this interaction with the sea floor, but not all of it, and that conservation of energy will cause the wave to rise. This is why you will see waves grow as they approach the shore: that energy has to go somewhere, since it can’t move water as deep as it would like due to the sea floor, so instead the water goes up.

As the wave gets higher it will start to curve over at its crest due to its forward motion, and a certain height it will fall over on itself, or crest. The fluid dynamics associated with wave breaking are actually complex and apply to a number of other fluid phenomena—for example, you can read about (at least) four different types of wave breaking here.

Look at that shore

Great Wave Off Kanagawa
Bonus physics—How big can waves get?
In Christopher Nolan’s recent movie Interstellar, there is a planet with ocean waves as high as a mountain. How big can waves get on Earth? The biggest waves reported in our oceans have been around 30 meters high, or as tall as a 10-story building. It’s tough to make such a wave from the wind speeds we typically get on Earth (60 mph wind typically gives you 15 meter high waves or so for example), but if you had a planet with no land to break up the wave, it could be possible that extreme winds or deep undersea disturbances could make a wave this huge.

Now, in the movie it's implied the huge gravity from a nearby black hole is causing these waves. Is that plausible? Find out in an upcoming post when we talk about tides!

Thanks to Brocken Inaglory (wikipedia) for the wave picture, and Kraaiennest (wikipedia) for the awesome wave simulation gif.