We’ve all experienced waves but have you ever stopped to think about what causes these breaks in the water? Well, waves have been studied for many years and they are more fascinating than you may think.
The breaking of waves is studied by fluid dynamics. This is a sub-discipline of physics that specializes in the science behind gases and liquids. Scientists have actually discovered why waves break and we are going to share this information with you today.
Read on to discover what waves are, when they tend to break, and why they break in the first place. After this article, you may never see waves the same again!
What is the top of a wave called?
Before we go into the science of why waves break, we need to find out what a wave is. A wave is actually made up of different parts. There is the top, the bottom, and the face. Okay, we all know several things about waves. We know that they break, that they travel, and have varying heights.
All waves are of different sizes. Some are more powerful than others and they usually come in sets of three or six. When surfing, the best waves tend to come from groundswells. But, even the waves we experience on the shoreline may have traveled thousands of miles before breaking up into whitewater. Their energy simply dissipates and it’s the end of that wave’s story.
When we break a wave down, there are three components:
- The crest
- The trough
- The face
The crest is the top of the wave. This is simply the highest point of any wave.
The trough is the bottom of the wave or the lowest region of it. The trough is generally constant for those waves that travel in the open ocean. As soon as a wave is about to break, it will generate deeper troughs.
The face of a wave is its front. In other words, it’s the vertical distance (height) between the crest and the trough. The wavelength is measured by the distance between the adjacent crests or troughs while the wave period is the time period between the crest of a wave and the crest of the next one along.
Have you ever noticed objects or people bobbing up and down on the passage of different waves? This is due to the orbital motion of the water. For example, a floating object will complete a circuit every time a new wave passes by. Therefore, it will always end up in more or less the same spot as before.
If the object is in deeper water, there is no net movement found at the surface of the wave. But, when in shallower water, there is a light net forward movement known as Stokes drift. There is far more to waves than meets the eyes!
When do waves break?
After years of research, scientists concluded that waves tend to break when their amplitude reaches a certain, critical level. In turn, this causes huge amounts of wave energy to transform into kinetic energy. This energy is extremely turbulent. Think of a football rolling down a steep hill and this is a similar motion.
In simpler terms, when waves get to shallow waters, such as coastlines, they will increase in height. Therefore, their crest (top of the wave) will meet the law of gravitation. The result? The waves break.
Waves begin to break when the ratio of wave height and wavelength exceeds 1/7. Remember, the wavelength is the distance between two crests or troughs. The wave steepness, or height, controls the effect of shoaling (the deformation of incident waves found on the lower shoreface) but this is based on the wavelength of the incoming waves. Waves then become steeper by increasing in amplitude and decreasing in wavelength.
For instance, if a 15-foot wavelength reaches a height of just two feet, the wave will break apart. From a visual perspective, the wave’s profile becomes too thin before it breaks.
Offshore winds are usually responsible for holding a wave up and slowing it down as it builds and moves toward a shoreline. Onshore winds do the opposite and actually accelerate the process of breaking a wave apart.
Energy and the ocean floor
As well as offshore and onshore winds affecting waves, ocean floor topography can have a significant impact on how wave energy is transformed into whitewater. As waves reach the shoreline, any energy in front of the waves tends to slow down. This is because of the friction with the shallow bottom.
Behind the wave, the energy will be moving at a full speed and channeling upwards. This climbs the back of a growing wave. Eventually, the wave will break, usually in water depth that is 1.3 times the height of the wave.
Types of breaking waves
When it comes to the breaking of waves, there are four basic types:
- Spilling
- Plunging
- Collapsing
- Surging
Spilling waves are the gentlest of the waves. Their crest breaks softly toward the shore and they usually break when the ocean floor has a gradual slope.
Plunging waves will break when the ocean floor is much steeper or has a sudden change in depth. These can be in the form of very powerful barrels (tubular or curved waves) or gigantic close-outs (a wave that closes all at once).
A collapsing wave is a combination of both spilling and plunging waves.
Surging waves are usually the result of a long period of swells. This results in a slow wave with smooth, oblique faces. These are more unique as the crest hardly exists and they may not even break at all. Breaking waves always have a deep trough while surging waves do not.
In Summary
Many factors can determine why and how a wave breaks. It could be the wind direction or the slope and features of the seafloor. This is why no two waves are the same.
While you will get similar kinds of waves in a particular area, the ever-changing weather conditions and wind swells can see waves change form as they get closer to shorelines. Just remember that the next time you see a wave break up at the beach, it may have traveled a very long way for that very moment.
