Do you know why waves move the way they do?

Waves, what are they? A wave transfers energy from one place to another. Whether you're thinking of an ocean wave, a sound wave, or a light wave, they all essentially do the same thing. Waves get some kind of energy from point A to point B through whatever substance, or medium, they are in, be it water, air, or space. Waves oscillate the substance they are in to transfer that energy. What we're interested in for this article are how surface waves in water behave. We are going to demystify two wave things about waves:

• How and where they break
• How they can turn as the move towards the coast

Before we launch into it, let's check out a nice photo of waves. The photo is extra special because it shows some really neat effects of water waves bending light waves and causing bright light ripples in the water.

Nice picture of water waves and light waves interacting from http://oceanleadership.org/

You've probably already developed an intuition for how waves move just by watching. In fact, you might be much more of an expert than you think if you spend your time surfing, sailing, or just hanging out at the water! Experience counts for a lot here, so we'll try to weave a picture between good experience and science.

Let's start with wave breaking. Why do waves break? First, a good rule of thumb to remember is that waves slow down in shallow water. Taking it a step further, when the water is shallow enough, that slow down happens more at the bottom of the wave. The faster moving top of the wave tips over the bottom like a tipping glass of water. Lo and behold ... We have a breaking wave. While the physics is more complicated when you dig into the problem, that's the essential idea!

There is generally a depth of water where waves of a certain height will break.  This depth is cleverly called the "Depth of Breaking". A water depth of approximately one and a quarter (1.25) times the wave height causes a wave to break. So, a 10 ft wave will typically start to break in 12.5 ft of water. While there are a lot of things that can modify this ratio, such as a steep rock reef or very steep beach, its a good rule of thumb. The figure below gives us a conceptual picture of what's happening with waves breaking. You can see the wave marching towards the shore. As the water depth decreases, the wave slows down due to shoaling in the shallow water. When it hits the depth of breaking, the top keeps moving fast and tips over the bottom and we have a breaking wave.

As waves approach the shoreline they feel the bottom which slows them down and
eventually causes them to break at the "depth of breaking" in a depth of water
about 1.25 times the wave height.

The waves change direction by almost 90 degrees in the space of a quarter of a mile!

Curving waves, or wave refraction, is our next topic. Take a moment and look at the picture below. This is Rincon Point in Santa Barbara, California and produces one of the most famous right handed point breaks in the surfing world. Notice the waves at the top of the picture are parallel with the top of the picture frame and the wave at the bottom are facing the right side of the frame. The waves change direction by almost 90 degrees in the space of a quarter of a mile!

Nice aerial showing wave wrapping around the point at Rincon in Santa Barbara , CA. Image from http://www.travelgrom.com/.

So what's happening here? If we think back to our rule of thumb, waves slow down in shallow water, we can put together a picture of what's happening. When we extend this to long wave front, the wave will experience different water depths along its length as it approaches a coast. A point on the coast sticking out into the ocean, like Rincon, provides big depth differences in a small space. In the image below we see the waves coming in from the left and approaching a point.  The portion of the wave near the point hits a shallow water point earlier than the rest of the wave.

So what happens when these waves hit the shallower water near the point? They slow down and eventually break. The shallower bottom is almost like a hand pushing on waves and slowing them down, while in the deeper water the waves keep moving uninterrupted. I drew the  black arrows on the figure below to help visualize this slow down so we can see what happens. The shallower slower moving waves hang back while the waves at the top of the image in deeper water keep marching forward. The wave curves in response and causes the wave to turn, or wave refraction! 

The focusing and curving of energy at coastal points is why point breaks are often such amazing surf spots.  

When we pull this all together at a coastal point we get an interesting phenomenon called wave focusing. Let's marry up the figure above with the other half of a point and plot it out in the figure below. Now we have the waves curving in towards the point from both sides focusing the wave energy at the point. You can see the energy coming from both sides to make the waves bigger. The blue arrows showing wave direction help visualize this "focusing" of energy at the point. The focusing and curving of energy at coastal points is why point breaks are often such amazing surf spots.  

An important parting note is what we started talking about in the post about how sand moves around at the coast. The change in wave energy from the point to the adjacent beaches causes two interesting things to happen.  First, the focusing of energy will pick up sand and even erode cliffs where the energy is high. Second, the material moved at the point is carried down the coast to lower energy areas due to longshore transport. The sand creates our beaches, sand bars, and even fills our harbors as we discussed in the previous articles! Now you're starting to see a picture of how all of this works together.

Please help support our efforts by signing up for our newsletter on the right side of this page or on our website to get regular updates on the world of ocean science, ocean energy, surfing, and other fun topics. Or course you can get the BEST idea as to what the ocean is doing with your very own WaveClock. Great resources like Dean and Dalrymple (2002) provide the detailed wave mechanics behind these processes I have greatly simplified here. Check it out to dive in more.

Enjoy the the beach!
Craig

R.J. Dean and R.A. Dalrymple (2002). Coastal processes with engineering applications. Cambridge University Press. ISBN 0-521-60275-0. p. 96–97.