Waves
Breaking wave from Satan Cruz, CA (Source: Wikipedia Commons)
What is a wave?
A wave is the repeating and periodic disturbance that travels through a medium (e.g. water) from one location to another location. In the ocean most waves are generated by wind, but Tsunami waves can also be generated by landslides, and undersea earthquakes. Waves in the ocean don't just happen, they are born. In the case of wind waves, when the wind starts to blow, tiny wavelets are born -- just like the ripples that streak across your coffee when you blow on it to cool it down. The wavelets move in the same general direction as the wind, and if the wind is strong enough (faster than about two miles per hour) they'll form into stable waves that travel along with the wind.
Wave Energy
Waves do not move water, they move through water. More specifically, the energy of the wave passes through the water, causing individual water particles to trace vertical circles within the shape of the wave. Each particile will return to its original location after the wave has passed by.
The Anatomy of a Wave
- Wave crest: The highest part of a wave.
- Wave trough: The lowest part of a wave.
- Wave height: The vertical distance between the highest (crest) and lowest (trough) parts of a wave.
- Wave Period: The time required for one wave to pass
- Wavelength: The distance from a certain point on one wave to the same point on the next wave (e.g. distance between two consecutive wave crests or between two consecutive wave troughs). (image source: NOAA)
Factors affecting the size of wind waves
The height of a wind-generated wave from the peak, or crest, to the bottom, or trough, is determined by three factors: the speed of the wind, how long the wind has blown in the same direction, and the width of the open water, or fetch, over which it is blowing.
Winds that blow strong and steady for great distances, such as the winds that circle Antarctica will create steeper, more powerful waves than will brief gusts. The average height of an ocean wave is about 12 feet; but far larger waves are sometimes born of raging storms in the open sea. Waves have been documented that exceed 100 feet from crest to trough.
Breaking Waves
Waves travel with little change across the vast expanses of the open ocean, but eventually all waves must reach shore. As a wave approaches the coast, it becomes shorter in length and more abrupt, increasing its height. Friction with the bottom causes the trough of the wave to disappear, the crest to slow its movement, and, when the depth causes the wave height to become 1.3 times the water depth (or reaches a wave height that is 1/7 the wavelength), the crest falls, forming a breaker. Breaking waves may take the shape of a tube (these are called plungers) or they may simply collapse and surge forward (these are called spilling waves).
Longshore Currents
The
energy in crashing waves can create massive erosion and dramatically
shape coastlines, but they aren't the only force that shapes
coastlines. As waves break on shore, a churning froth of turbulent
water, called swash, moves as a sheet up the slope of the beach toward
shore. Once it runs out of energy, it flows back toward the surf zone,
as backwash. Depending on the strength of the surf, swash can scour
sand, pebbles, and even rocks off the surface of the beach; backwash
deposits the debris back onto the beach, slightly displaced from its
original position. Even more sand and gravel is transported by a type
of flow called a longshore current. When waves -- particularly steep
waves -- approach a straight beach at an angle, alternating swash and
backwash can transport water along the beach in addition to sand and
gravel. The water flows parallel to the shore, in the form of a
longshore current. Longshore currents can haul vast amounts of sand and
gravel along the shore, destroying beaches in some spots, and creating
new ones in others.
(image from: Wikipedia Commons)
Tsunami's
Tsunami’s are very different than wind waves. Tsunami waves may travel thousands of miles, moving very rapidly on the sea surface. Contrary to popular belief (and the movies), the tsunami waves on the high seas have very little height (0.3 to 0.6 m / 1 to 2 ft.) and very long wave lengths (50-250 miles) with periods of more than 15 minutes. Given these characteristics and the ever-present swells, the wave is imperceptible. Thus, tsunamis at sea are rarely noticed. They race by ships at sea, which remain totally unaware of their presence. Indeed, tsunami characteristics are very difficult to measure at sea, even with sophisticated instrumentation and advance knowledge of their approach.
Questions:
- From the reading above, what three factors affect a
wave's size (or height).
- From the reading above, a wave’s height is measured from its ______________ to its ________________.
- When a wave becomes ______ times the depth of the shoreline, the wave breaks. What does that mean?
- Go to the wave simulator (PBS) and use it to see how these three forces will generate larger and smaller waves. You could also go to the wave simulator (National Geographic) which is a similar simulator that will allow you to adjust the wavelength, wave height, and wave period. Explain what principals of waves these two simulations teach you.
Breaking waves will take one of two shapes. Identify the two
pictures below. Use your reading to help you.
A
B
- Click on Storm Surf,
once there select "sea" and watch what happens. This is an animation for predicted wave heights for the
next 120 hours. Describe what you see in the animation.
Where are the largest waves this week, what direction to the
move, what happens to them over time.
- Occasionally large storms generate waves that are larger than physics dictates they should be. These are called Rogue or Freak waves. Go to Savage Seas or Rouge waves revealed by satellites and describe the evidence we have for these waves.
- Go to back to the Savage Seas and
read the side bar on Tsunami’s.
a. How fast do Tsunami’s travel?
b. Are they easily visible at sea?
c. Why is the word tidal wave not appropriate for a Tsunami? - Go to Wikipedia: 2004 Asian Tsunami, provide a brief description of the forces that causes the 2004 Asia Tsunami.
- Examine the picture to the right. In your words explain why this picture is not a real image of a Tsunami. The following link may be of some help: Anatomy of a Tsunami