Waves are among the most familiar features in the ocean. All waves work similarly, so although we are talking about ocean waves here, the same information would apply to any other waves you might discuss in science classes.
Ocean waves transport energy over vast distances, although the water itself does not move, except up and down. This may surprise you, but if you think about it, once you are past the breakers on your raft, you pretty much just bob up and down. (You might drift up the beach….we’ll get to that.) This orbital motion is explained in the figure below:
There are waves of all sizes and shapes rolling into the beach at any given time. If they’re not stopped by anything, waves can travel across entire ocean basins and so the waves at your beach might be from a storm half a world away. The most familiar ocean waves are caused by the wind. These are wind-driven waves. This sort of motion is set up anytime two fluids rub together, and remember that the atmosphere is essentially fluid. Waves caused by underwater disturbances such as earthquakes, landslides, or volcanic eruptions are called tsunamis. These waves are typically tens to hundreds of kilometers long. The gravitational pull of the sun and moon on the earth causes the tides which are actually tidal waves. We’ll get back to that.
So we can talk about waves, it’s important that you know some of their parts. The following list refers to the figure below:
crest-the very top of the wave
trough-the hollow between two crests
wave height-the vertical distance between the top of one wave crest and the bottom of the next trough
wavelength-the horizontal distance between any one point on one wave and the corresponding point on the next
wave steepness-the ratio of height to length
amplitude-the maximum vertical displacement of the sea surface from still water level
(half the wave height)
period-the time it takes for one complete wavelength to pass a stationary point
wave speed-the velocity with which waves travel
deep water waves-waves that are in water that is deeper than half their wavelength
shallow water waves-waves that are in water that is shallower than 1/20 their wavelength (the important difference on these last two is whether or not the sea floor influences the motion of the wave)
One waves motion is completely independent of any other wave motion. When two groups of waves meet, they pass right through each other. This is obvious if you consider light and sound waves. When two people talk or your child has both the TV and the stereo on, you can hear both. One set of sound waves doesn’t garble the other. Likewise you can see two objects at the same time. What does happen, though is that waves can either add up or cancel each other out as they pass through one another. This property is calledsuperposition. If a crest from one wave happens to line up with the trough of another, they cancel each other out. This is called destructive interference. If two waves line up crest to crest or trough to trough, they add up. This is called constructive interference. This is why waves at the beach are all different sizes. There are lots of different wave groups coming in, and they’re interfering with each other in different ways.
Standing waves result when two equal waves are going in opposite direction and in this case you get the usual up/down motion of the water surface but the waves don’t progress. These are common in coastal areas where waves reflect off seawalls, ship’s hulls, or breakwaters. They’re also common in swimming pools. A special type of standing wave is aseiche. You can observe this by sloshing around in your bathtub (or, if you’re less adventurous try walking with coffee). When you get just the right steady wave frequency going in your tub or your cup, the motion quickly builds up and water or coffee sloshes all over the place. When harbors are designed, care has to be taken to give water built up in seiches some way out other than sloshing up into the first floor condos.
Waves Hitting Things
When a wave hits a hard vertical surface it is reflected. In other words, the wall pushes the water back just as hard as it got pushed, and sets up waves in the other direction. With constructive interference, you end up with bigger and therefore stronger waves. This is why, in the long run, solid seawalls are not good for saving property from the ocean. You end up creating stronger waves that cause even more erosion.
Waves are also refracted. When you’re at the beach, it appears as if the waves are mostly coming ashore in a straight line. If those waves were generated all over the place out at sea, how is it they’re all heading the same direction? Here is an example of shallow water waves (waves getting steered by the seafloor). They may come in at an angle, but the side that hits shallow water first gets slowed down by friction and the other side "catches up" bending around until they’re parallel with shore. This is shown in the figure.
You have no doubt noticed when you swim in the ocean that you tend to drift down the beach. This is called longshore drift and is a consequence of these refracting waves. Along with you, a lot of sand is getting moved along, and this is one way that our barrier island migrate up and down the coastline.
Interestingly, because tsunamis have such long wavelengths, they are shallow water waves and so the seafloor steers them around. This is one reason it is so difficult to predict where these waves will have an impact, even if you know what started them and where. The other amazing thing is that they typically travel about 750 kilometers per hour (or 500 miles per hour)! Because they’re so long and low, it’s hard to identify one until it’s close to shore and by then it’s too late to warn coastal residents.