과학 [MP3 for IELTS] Rip-Roaring Time - Part 1 (ABC - Great moment)

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Rip-Roaring Time - Part 1

A few years ago, I was in the ocean with my son, out past the breakers, when I suddenly realised that I was caught in a rip. The shore kept getting smaller, no matter how hard I swam, and I began to get very anxious.

Like most people, I wrongly believed that rips are rare and unusual events, and that once you get caught in one, you're on your way to a Far Distant Shore.

A rip is a narrow concentrated channel of water, flowing out to sea. Rips are like rivers in the ocean. They are part of the normal water circulation pattern of practically every beach, and are how the water that gets carried in by the waves gets back out to sea. "Water in" means "water out". If the incoming water didn't go out again, it would pile up very rapidly on the sand, and then start pushing inland towards the mountains.

The waves break and bring the water in towards the shore. The water then runs up the sloping sand. It will roll back into the shallow water a little way, and then turn (say) to the north and flow along, parallel to the shoreline. It is now called a feeder current. After a hundred-or-so metres, it will run head-on into another feeder current running (say) south. The two feeder currents will then merge and head out to sea. They have joined to make what the scientists call a "rip current", or rip.

As it heads seawards, the rip quickly consolidates to make a narrow fast-moving neck. The neck is usually about 10 metres wide, but it can vary between three and 20 metres across. All the water in the rip is moving seawards - from the surface down to the sandy bottom, although it's slowest near the sand. The rip keeps its structural integrity and shape until it gets past the breakers and out into the unbroken rolling waves. The neck then widens into a broad "rip head", and the moving water gradually slows down until only the water at the surface is moving seawards - and then, it too finally stops. The water in the head spreads sideways, and is carried in towards the shore by the next wave - and the cycle starts up again. In some cases, the rip will actually carry you back to shore. So the fear that you'll get carried to a distant shore is totally unfounded - but in a panic situation, you'll believe anything.

If you have a fixed structure like a cliff, reef or pier, you can have a permanent rip. Rips are less permanent on sandy beaches, but they have been known to persist at the same location for weeks and even months, although they will become strong and dangerous only occasionally. The locations of the channels through which the rip flows out to sea can expand and contract with the surf conditions, the tide and the weather. It usually takes a really large storm to physically shift the channel in the sand bar to a different location. If the beach is long enough, there can be many rips, each roughly the same distance apart. Coastal scientists have been studying rips for three-quarters of a century, and so there are many theories as to why a rip channel should form in any particular location - but none of the theories have all the answers.

There are different ways to spot a rip. It's a good idea to spend a few minutes looking at the surf before you plunge in. The easiest way is to spot the big fat Rescue boards - at patrolled beaches, the life savers usually set them on the sand at a rip. But this method works only at patrolled beaches! You can also look for persistent bands of dark water heading off-shore. Rips tend to go out in deeper channels in the sand - so the water is darker, because it's deeper. Fewer waves break there, so there is less whitewater. This situation is particularly tricky - the water might look safer because there are fewer waves breaking there, but it's actually more dangerous. The surface of a rip is often choppy and disturbed, because the rip is heading out to sea - against the direction of the incoming waves. Finally, you can sometimes spot a rip because it carries stuff out to sea - sand, seaweed, jellyfish, people, pets etc.

And sometimes, you get mega-rips, and I'll worry you with that, next time…

© Karl S. Kruszelnicki Pty Ltd 2006.