Why do Waves Break?

wave beginning to crest on the beach

Waves in the ocean, or on another body of water, are generally started by wind. The wind blows across the surface of the water, pushing the water as it goes. Once the wave reaches a certain size it no longer needs the wind to sustain it. This is similar to pushing a cart along a slight slope. It may take work to get the movement started, but once it is moving its energy helps to keep it moving.

Anyone who has been in a boat on rougher water has seen that while waves will break near the shore, we can also get whitecaps, basically waves breaking, in the middle of a water body as well. The intensity of these breaking waves is dependent on a lot of factors but it seems like no matter how much time a person spends on the water two things remain undeniable. First, waves have a lot of power and second, it is nearly impossible to predict what wave will break when and where.

Looking at the power of the waves is a pretty significant area of research right now. As climate change results in warming waters and changing and strengthening wind patterns, the potential of water to dissipate increased energy may become more and more important. Is there a way to cause waves to break to reduce the risk to oceanside communities? Or perhaps, we can harness this power as a source of renewable energy? For a great review of the challenge of using waves to generate electricity check out this Minute Earth video.

Onto the predictability, or lack there of, of waves. Waves do all sorts of things that lend to their unpredictability. To follow the science of this everyday occurence we need to cover a little terminology.

Wave terminology

This is a standard diagram of a wave. It shows some of the key components. One thing to note is that a wavelength doesn’t have to be measured from crest to crest; it just needs to be measured between the same points on the wave. So it could be measured between two troughs or between two other points at the wave.

Typically, unless there is an external force acting at the wave the amplitude will be the same, as will the slope in and out of the peak of the crest. This is one aspect that makes a breaking wave different. In a breaking wave, the slope is steeper on the front face compared to the trailing side. In addition, while waves tend to have equal amplitude for the trough and the crest, a breaking wave has a greater amplitude for the wave crest above the average height of the water’s surface, compared to the trough below the surface. According to Babanin, Chalikov, Young, and Savelyev (2007), the determining factor is how the vertical the wave gets. Regardless of other factors it is this that makes a wave break. Despite this information, surface waves breaking is still pretty unpredictable because there are so many different factors that can make the wave vertical.

Waves breaking at shore is a little more known, although it isn’t that much more predictable when watching. As a wave moves into shallower water it slows down. At the same time, their height and thus the steepness increases. The waves will break when the water depth is about 1.3 times the wave amplitude (Biello, 2006). However, as wind, bottom topography, and other factors still affect the height of the waves, it can still be difficult to predict which waves are going to break.

If you would like additional resources on the types of waves that break on shore, check out this page from a course at UBC.

References

Babanin, A., Chalikov, D., Young, I., & Savelyev, I. (2007). Predicting the breaking onset of surface water waves. Geophysical Research Letters, 34(7). https://doi.org/10.1029/2006GL029135

Biello, D. (2006). Why do waves always break in odd-numbered groups? Scientific American. https://www.scientificamerican.com/article/why-do-waves-always-break/