MadSci Network: Earth Sciences

Re: How do different waves differ from one another? HOw destructive are they?

Date: Sun Mar 25 16:42:52 2001
Posted By: Jason Goodman, Postdoctoral Fellow, Department of Geosciences
Area of science: Earth Sciences
ID: 984502572.Es

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 itself.

There are several major kinds of large uncommon ocean waves. I'll briefly describe each type, and how it is created.

  1. Storm waves
  2. 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).

  3. Storm surge
  4. 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.

  5. Freak Ocean Waves
  6. 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 waves have 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.

  7. Tsunamis
  8. Tsunamis 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 Pacific ocean!

    The tsunami website at University of Washington has lots of good information about tsunamis, including a survey of large tsunamis in history.

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