2004-07-23 / Columnists

Drawing On Science

by Stephen Yaeger


You are standing in the water watching a large wave approach. You get ready to dive. The wave rises up and just as it begins to roll over on itself you plunge into the wall of water. The churning water tumbles your body and you soon stand up saying, “That was awesome. Gee, I wonder how waves form.”

OK. So the ‘gee’ part you’d never say, but it’s a good lead-in to today’s topic.

A wave has a crest or top, trough or bottom, length or the distance between two crests or troughs, and height or the distance from trough to crest. (Drawing 1). The time it takes for a two successive crests to pass a given point is known as the wave’s period . There are different types of waves including wind-generated waves, catastrophic waves, and standing waves. Just about anyone who has seen the ocean (or a lake) is familiar with wind-generated waves. These waves are the result of wind energy being transferred to the water’s surface. When wind moves across the surface of the ocean the wind sort of pushes and lifts the water. This action causes the formation of a sea wave . Sea waves, which are most directly affected by the wind, are the first stage in wind-generated waves. Such waves have no uniform pattern. The waves have different heights and periods. They move in different directions, but travel, more or less, away from the area in which they were first formed. This area is called the storm region and as these waves travel away from this area they become more and more uniform in height, period, and velocity. The period depends on three things: wind velocity, how long the wind is blowing, and the wind’s fetch or distance over which the wind travels. Wave height and length generally increase due to wind velocity and duration. Fetch also affects a wave’s wavelength. When the fetch is large, the wavelength is longer. As the sea waves travel they form a regular pattern of waves, which is the second stage of wind-generated waves known as swell . As the waves in the swell advance, the length of the waves remains the same, but their height increases.

Once a wave reaches the continental shelf and approaches the shoreline, a number of changes occur.

The most obvious change is when the wave shoals or breaks— where the wave falls over on itself. Shoaling occurs when the wave enters shallow water and its velocity and length decreases. This decrease in speed and length is due to friction between the wave’s trough and the bottom sand where the circular motion of the wave’s bottom is distorted.

As a wave slows down each successive wave behind does the same and tends to pile up on the one in front. As the wave travels up onto the beach it will reach a point where the depth of the water is about one-half the height of the wave. This is the site at which it will break.

As each wave enters the same position, they will break too. The shoaling of waves on the beach is called the surf, the third and most familiar stage of all wind-generated waves.

For surfers the slope of the shore is everything. When the slope is just right, waves will tend to break slowly and travel up the shore for a good distance. Also the height of the wave will contribute to a good ride. The higher the wave, the bigger is the ‘tunnel’ in which surfers do their thing.

So, what exactly is a wave? We see what appears to be rough or gently moving water depending on the wind’s energy in creating the waves.

But what happens when a wave forms? In deep water the waveform (you know, crests, troughs, etc.) is advancing, but the water itself moves very little. Why? Well, the individual particles of water in a wave move in a circular pattern (Drawing 2).

The circles that the particles form at the surface have large diameters and the particles move very fast. As we get deeper in the water, the diameters get smaller, and the particles’ speed decreases. So because water particles in a wave move in circular patterns it is actually the waveform itself that is moving forward, not the water. That’s why you may see a floating object bobbing up and down in a rough sea, but it doesn’t move forward.

Try this: the next time you take a bath place your rubber ducky, toy boat, or whatever in the water. Sit very still and with your open hand gently push water toward the object. Watch it bob up and down, but not really move away from you as fast as you might expect.

Waves entering shallow water at an angle to the beach or meeting irregular changes on the shore’s bottom will change direction. This change in direction is known as refraction . Refraction occurs because one part of the wave reaches the shallow water before the rest of the wave. This first contact causes the wave to slow down and the entire wave turns, changing direction toward the beach.

After the wave breaks some water, called backwash , returns to the ocean along the bottom of the surf zone. Water also moves along the beach because of the angular approach of incoming waves. When this current of water or longshore current increases and overcomes the water flowing seaward a rip current is the result (Drawing 3).

During severe storms such as hurricanes, high winds push large amounts of water toward the shore creating catastrophic waves. The water gradually piles up and moves up the beach. This is a storm surge and, if the waves are high enough, may result in loss of life and property.

Storm surges that occur during high tides are the most dangerous. Another catastrophic wave is the tsunami (tsu-NAH-me) or seismic sea wave , which is incorrectly called a tidal wave. Such waves may be the result of a disturbance on the ocean bottom such as an earthquake, massive landslide, or some volcanic island that blew itself up when it burped.

Because of the ocean’s depth, tsunamis are not very noticeable on the open ocean; being no different from any other wave except for the fact that the wave’s length may reach 300 miles. A tsunami has been known to travel over 7,000 miles! And to make matters worse, the speed of such waves can reach 500 miles per hour.

So the next time you are basking in the sun on our beautiful Rockaway beaches and you see the surf begin to move away from the shore exposing the bottom mud, don’t stop to admire the clams, flopping fish, seastars, etc., which have now been exposed. It would be to your advantage to make a dash for the Queens landmass—if you can run, swim, or drive fast enough.

If you use the bridges, perhaps you should not stop for the tolls! Not to worry, however, since there is an international warning system in place to warn coastlines of any tsunami danger.

Questions? E-mail Steve: Drawing onscience@aol.com

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