What Makes a Wave Move?
Objectives
Students will be able to:
- Identify the parts of a standard wave
- Explain what happens to a wave when the wave height, wavelength, or wave period changes
- Explain the difference between a standard wave and a tsunami
Materials
Computers with Internet access
Copies of the Student Worksheet
Background
Tsunamis act very differently from typical surf swells; they move deeply through the water (often several kilometers deep) rather than just the surface, so they contain immense energy, propagate at high speeds and can travel great transoceanic distances with little overall energy loss. A tsunami can cause damage thousands of kilometers from its origin, so there may be several hours between its creation and its impact on a coast, arriving long after the seismic wave generated by the originating event arrives. Although the total or overall loss of energy is small, the total energy is spread over a larger and larger circumference as the wave travels.
In open water, tsunamis have extremely long periods (the time for the next wave top to pass a point after the previous one), from minutes to hours, and long wavelengths of up to several hundred kilometers (compare to the typical wind-generated swell one sees at a beach, which might be spawned by a faraway storm and rhythmically roll in, one wave after another, with a period of about 10 seconds and a wavelength of 150 m). The actual height of a tsunami wave in open water is often less than one meter. This is often practically unnoticeable to people on ships. The energy of a tsunami passes through the entire water column to the sea bed, unlike surface waves, which typically reach only down to a depth of 10 m or so.
The wave travels across the ocean at speeds from 500 to 1,000 km/h. As the wave approaches land, the sea shallows and the wave no longer travels as quickly, so it begins to 'pile-up'; the wave-front becomes steeper and taller, and there is less distance between crests. While a person at the surface of deep water would probably not even notice the tsunami, the wave can increase to a height of 30 m or more as it approaches the coastline and compresses. The steepening process is analogous to the cracking of a tapered whip. As a wave goes down the whip from handle to tip, the same energy is deposited in less and less material, which then moves more violently as it receives this energy.
A wave becomes a 'shallow-water wave' when the ratio between the water depth and its wavelength gets very small, and since a tsunami has an extremely large wavelength (hundreds of kilometers), tsunamis act as a shallow-water wave even in deep oceanic water. For example, in the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (720 km/h or 450 mi/h) with little energy loss, even over long distances. At a water depth of 40 m, the speed would be 20 m/s (about 72 km/h or 45 mi/h), which is much slower than the speed in the open ocean but the wave would still be difficult to outrun.
Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe. However, tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India ). They also need not be symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography. (Wikipedia)
Procedure
There is a big difference between normal ocean waves and tsunamis. To be able to understand some of the differences, you will need to know the parts of a normal or standard wave:
1. On the Student Worksheet, label the parts of the wave on the diagram.
2. Use the Volvo Ocean Race wave simulation to experiment and actually be able to see what happens when the wave height, wavelength or period of a surface wave changes.
3. Answer the questions on the Student Worksheet about the simulation.
4. A typical surface wave is caused by wind and has a period of about 10 seconds and a wavelength of 150 m and reaches down in the water to a depth of about 10 meters.
Can you model a typical surface wave on the simulation? Explain how or why not.
5. Tsunamis are caused by earthquakes, volcanoes or landslides, have extremely long periods, from minutes to hours, and long wavelengths of up to several hundred kilometers. The actual height of a tsunami wave in open water is often less than one meter. This is often practically unnoticeable to people on ships. The energy of a tsunami passes through the entire water column to the sea bed.
Can you model a tsunami on the simulation? Explain how or why not.
6. Click on the links to view the following animations of the December 2004 Indian Ocean tsunami. Due to the distances involved, the tsunami took anywhere from fifteen minutes to seven hours to reach the various coastlines.
The December 2004 Indian Ocean tsunami traveled all over the world.
7. Answer the questions on the Student Worksheet about the animations.
Assessment
Based on the information in the lesson and the animations, in your own words, explain the difference between a surface wave and a tsunami.
Do you think a surface wave could be as powerful as a tsunami? Explain your reasoning.
