Drawing On Science For Kids
According to the reports two young girls were caught in a rip tide. Coincidentally, this was the topic I chose for the present column. Rip tides or undertows are actually rip currents. Most people are familiar with them, but not how and why they form.
Rip currents are very powerful "rivers" of water flowing away from the shoreline.
They may reach speeds more than three miles per hour. They can carry a lot of sand and shells along with them. Rip currents can also trap and carry away a swimmer.
Rip currents form as waves approach the shoreline where they begin to break forming the surf. The breaking water rises up on the beach's
surf zone and continues on to the upper beach. This moving water and sand are called swash. At some point gravity or some sort of barrier slows down the forward movement of the swash. The water will then move straight back toward the sea in a
backwash. This backward flow can be either weak or strong as it moves below another wave coming up onto the beach.
But, of course, waves rarely approach the shore head-on. They will approach at an angle due to the fact that the winds that generated them move randomly in any direction out at sea.
When waves approach at an angle and break on the shore the swash will move up-beach at an angle, too. But when its movement is slowed it will, again, return straight back. Now, consider the next series of waves, beyond the line of breakers. These waves are, naturally, pushing water toward the shore. But backwash is also pulling this water back into the ocean. This results in water moving somewhat parallel to the beach in what is called a longshore current. Now if the surf brings in more water than the backwash can return, a buildup of water can occur. This is the key to a rip current.
If too much water builds up in the surf zone, and the backwash increases in speed, a portion of the longshore current may begin to move rapidly seaward by overcoming the incoming waves. This forms the rip current. So rip currents typically extend from the shoreline out past the incoming breakers. Any beach may experience rip currents whether it is located on the ocean or a lake.
When you're on the beach you may be able to spot a rip current. Since they carry a great deal of sand they may appear whiter than the surrounding water. Also the waves in the vicinity of a rip current are usually steeper than the other waves. Once caught in a rip current it's best to "ride" it out. Allow it to carry you seaward and as it slows down begin to swim parallel to the shore. Once it stops swim directly back to the beach.
Since rip currents can occur at any time it is not advisable to go swimming unless a lifeguard is on duty, and obey all of his/her warnings. Check out the United States Lifesaving Association, NOAA's National Weather Service, and the National Sea Grant for information on rip current safety and awareness.
Questions/comments? E-mail Steve: Drawingonscience @ aol. com