Coastal region and small island papers 19 |
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Wave characteristics
Background Waves are the main source of energy that causes
beaches to change in size, shape and
sediment type. They also move
marine debris between the beach and
offshore zone. Waves are generated
by the wind blowing over water. Waves formed where the wind is blowing
are often irregular and are called
wind waves. As these waves
move away from the area where the wind is blowing, they sort themselves
out into groups with similar speeds and form a regular pattern known as
swell. |
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Measuring waves |
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Wave height is measured by having an observer with a graduated staff or a ranging pole (pole with measured sections in red and white) walk out into the sea to just seaward of where the waves are breaking, and then to have the observer record where the wave crest and the following wave trough cut the staff; the difference between the two is the wave height. Alternatively, an estimate may be made of the wave height. Such estimates can be made in imperial or metric units, whichever the observer feels most comfortable with. Often it is best to have two observers independently estimate wave height and then to compare their results. The height of at least five separate waves should be estimated and the average taken. Wave period is the time in seconds for eleven wave crests to pass a fixed object, or if no such object exists, the time for eleven waves to break on the beach. Use a stopwatch if available, or a wristwatch with a seconds hand. Start the timing when the first wave passes the object or breaks on the beach, and stop it on the eleventh. Divide the total number of seconds by ten to get the wave period.
Wave direction is the
direction from which the waves approach and is
measured in degrees. This can be measured with a compass, standing high
up on the beach and sighting the compass along the direction from which
the waves are coming, which
will be at right angles to the wave crests (see Figure 18). |
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When to measure |
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This will depend on the time available and the
nature of the monitoring activity. Waves change from day to day, so
daily measurements are the most useful. However, if time is not
available for daily measurements, weekly measurements or even
twice-monthly measurements can still provide useful data. |
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Watching out for a tsunami | ||||||||
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During tsunamis, low-lying coastal
areas, those below 20 ft (6 m) in height, may be flooded. However,
because of the speed at which tsunami waves travel (500 mph or 800
km/hr) an earthquake off the Venezuelan coast might result in a tsunami
reaching some Caribbean islands within minutes. Thus, knowing the
warning signs
could result in saving lives. One of the best warning signs is the
earthquake itself, though it should be noted that not every earthquake
generates a tsunami. A second warning sign is when the sea recedes –
before the arrival of the tsunami wave(s), the sea recedes a
considerable distance leaving a significant portion of the seabed dry.
If you are at the beach or near the
shore, and you see either or both of
these warning signs, run inland for higher ground and alert as many
people as possible. |
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Discussion |
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