Date: Sun Mar 25 16:42:52 2001
Posted By: Jason Goodman, Postdoctoral Fellow, Department of Geosciences
Area of science: Earth Sciences
I'll answer your last question first, since it will help with the others.
"Why do ocean waves only form breakers near the shore"?
For most ocean waves in deep water, only the upper part of the ocean is
sloshing back and forth. Below about 50 meters, the water is calm.
Imagine a wave moving toward the coast. When the water depth becomes
shallow enough, the sloshing water starts to bump into the bottom. The
sloping bottom forces the water upward when it tries to move toward shore.
At the same time, it turns out that this bumping against the bottom slows
the wave down -- waves in shallow water move slower than waves in deep
water. This means that water from offshore and from below pile up in a
sort of "traffic jam", making the height of the wave greater. Eventually
the wave gets so tall that it can't support itself, and crashes over onto
There are several major kinds of large uncommon ocean waves. I'll briefly
describe each type, and how it is created.
- Storm waves
These are just bigger versions of the waves you see at the beach on an
ordinary day, and are not very unusual. They're created by the storm winds
which push the water surface around. They can be dangerous on the open
ocean, but they usually break before doing much damage on shore. Storm
waves occasionally sink ships,
and can cause property damage and drownings, but they're usually not very
catastrophic on their own. Radar
instruments have measured storm waves under hurricanes with heights of
18 meters (60 feet).
- Storm surge
In addition to creating ordinary storm waves, when a hurricane or winter
storm strikes land, it can create a storm surge.
Basically, the wind just pushes the water against the land, piling it up
and raising the sea level for as long as the storm lasts. This is more
like a flood than a wave, so it can travel much farther inland than even a
large wave, and can do much more damage to buildings. Storm surges from a
large hurricane can be as much as 10 meters (30 feet) above sea level.
Millions of people on the East Coast of the U.S. live in areas at risk from
storm surges. Historically, 9 of 10 people killed by hurricanes are
claimed by storm surge.
- Freak Ocean Waves
On occasion, a ship sailing across the ocean will encounter a "freak
wave". Though the sea may be fairly calm, suddenly the ship will find
itself struck by a wall of water up to 30 meters (100 feet) high! These
damage several large ships every year, and occasionally sink them. They
don't cause a huge amount of damage on shore, because they are very rare,
and break far offshore before reaching land. Scientists aren't even sure
where they come from: the best guess is that they are an interaction
between ordinary storm waves and a very strong ocean current. PBS has a
some more information on freak waves.
are commonly known as "tidal waves", but oceanographers try to avoid that
name because tsunamis have nothing to do with the tides. They are caused
by earthquakes, volcanic eruptions, underwater landslides, or asteroid
impacts -- any huge, sudden release of energy within a large area of
ocean. Unlike freak waves, tsunamis are not very high in the open ocean --
often only half a meter or so. But the crest can be 150 km (100 miles)
wide, and many thousand km wide. Also, in a regular wave (even a "freak"
wave) only the top 50 meters or so of the water is moving, but a tidal wave
pushes the whole ocean, up to 6 kilometers deep. A tsunami moves at speeds
of 800 km/hour! (500mph)
When a tsunami moves into shallow water, the deep part of the wave gets
pushed upward by the sloping bottom. The wave slows down, and its crest
gets narrower. As a result, the wave gets taller and taller. By the time
it reaches shore, it can be up to 20 meters (70 feet) high, large enough to
flood everything lower than 12 meters (15-40 feet) above sea level. The
most destructive tsunami in history killed 100,000 people in Japan in 1703.
Tsunamis are probably the most dangerous of large waves because they strike
with almost no warning, and can affect large areas. A single earthquake in
the Aleutian Islands can cause a tidal wave which affects the entire
website at University of Washington has lots of good information about
tsunamis, including a
survey of large tsunamis in history.
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