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Rip Currents & Undertows by Ray McAllister  


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more info & printable posters at www.ripcurrents.noaa.gov

Rip Currents or "Undertow"
by Ray McAllister

There have been a number of incidents of swimmers caught in "undertows" this year. We oceanographers call them "rip currents" and many lifeguards call them "run-out", but by any name they can be terrifying to the uninitiated or weak swimmer. They consist of a rapidly flowing current running from the near beach area, inside the breaking waves, out to sea. In most cases they flow faster than a person can swim, even with swim fins on, and they carry one out well beyond the surf. But this is only part of the problem!

Imagine that you are playing in 3 to 5 foot breakers, diving thru them and body surfing back to the beach, generally unconcerned because you are only a few feet from the warm sand. Suddenly your feet don't touch bottom any more and you notice you are farther from the beach. "No sweat." You turn and swim for the beach, only a few feet away, where the sunbathers are so close you can watch them applying suntan lotion to each other. A quiet, almost pastoral scene. But they are getting further away! You put on a burst of speed but they still get further away. Now panic, the great killer, appears. You are scared and put everything you've got into one last spurt for the beach. You are, typically, out of shape and soon, gasping, take in a mouthful of salt water and sink. The rip current has claimed another victim. Because you reduced your buoyancy by inhaling a lot of seawater, replacing air in your lungs, your body sinks. It was not the undertow pulling you under, it was the inhaled water, making your body negatively buoyant, that took you under. Either way it was fatal!

Another factor helps with the panic. Safety is only a few yards away and one is sure that another short spurt will get you to the beach where kids are playing in the sand and lovers are necking in the sun! That final desparate spurt often does the swimmer in.

The terrible thing is, it was so unnecessry. A very simple rule will save your life if you are ever in this position. SWIM PARALLEL TO THE BEACH. Refer to the drawing to see how and why this helps. The rip is a narrow stream and, once out of it, you can turn toward the beach in perfect safety. It has saved many lives and could save yours. Remember this rule. And now the reasons for rips.

There are two main cases for rips on open coasts. One is for a coast or beach with no underwater obstructions. The scheme works like this: As waves cross the ocean in deep water (defined as being deeper than half of the distance between two wave crests) the water under the wave moves in circular orbits, with very little forward motion of the water. Only the wave shape advances. When this wave gets into water shallower than half the distance between the crests, it begins to drag the bottom and changes occur in the wave characterists. It shortens as the forward crest slows down and the following crest catches up. As it shortens it gets higher, exactly what the surfer looks for on a surfing beach.

Instead of orbital water motion the wave breaks and the water now moves forward and up onto the beach. Each succeeding wave brings more water onto the beach and only a portion of it goes back. The result is a buildup of water; a pile of water getting more unstable with each succeeding wave. It is held on the beach by the breakers. Finally a part of one breaker is a bit lower than the rest and the "wall" of water piled up inside breaks thru and flows out to sea as a rip. This lowers the pile in this spot and the pileup both up and downbeach from the spot flows into the low spot and out to sea. These currents feed the rip and so are called feeder currents. In extreme cases they are strong enough to knock children down and roll them into the rip current. As long as the waves bring water to the beach about as fast as it flows out to sea, the rip continues. When the waves die down, the rip ceases.

The second case results in an even more pronounced rip. It occurs when a sand bar (or nearshore reef) acts as a barrier, over which waves break. The barrier holds the water on the beach and it finds its way back out to sea thru a gap in the bar or reef. Obviously this should be a strong rip and in the case of the reef, usually a permanent rip. In California we used them to transport SCUBA divers out beyond the surf, but we came back in almost anywhere else but in the rip.

I've tried to swim against a rip with swim fins and found myself going feet first out to sea. Just don't panic - swim parallel to the beach. You can often tell a rip because the water flowing out to sea beats down the incoming waves and a surface track of white foam often indicates its position. Sometimes the sand being carried out makes the rip muddier than the water around it. Watch for these tell tale signs and pay attention to the lifeguards. They are very savvy about runouts and post RIP CURRENT warnings on their bulletin boards. Learn the simple rule and save your life or one of your friend's.

Ray McAllister, Prof (Emeritus) Ocean Eng., FAU, Boca Raton, FL 33064
Diving Dinosaur, Geologist/Oceanographer/Ocean Engineer, 44 years SCUBA
mcallist@gate.net (954) 426-0808, Author Diving Locations, Boynton/Dania

This article is used with the kind permission of the author, Ray McAllister. If you wish to use it in whole or in part please seek the same authorization.

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The following articles are postings to the newsgroup sci.geo.oceanography
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Subject: Re: What is an undertow?
From: baum@stommel.tamu.edu (Steve Baum)
Date: 1996/12/12

In article <850366940.13421@dejanews.com>, wrote: Hello, Hope it's okay to ask these questions here. I've never understood what an undertow is. We used to go to islands when we were little, and our parents warned us to watch out for the undertow. The picture I have is that I could go in the ocean and be pulled underneath the island. Is this possible? And if it is, wouldn't a person be able to see an opening, or wouldn't there be a big drop on the ocean floor?

The other question is about rip tides. Is a rip tide when someone is say standing in the water or swimming and the water that is returning to sea is so strong that it carries the person out with it? If the person is standing, couldn't he dig in and hang in there? Anyway, thank you for answering these questions -

A rip *current* is a narrow return flow caused by the piling up of water in the surf zone. This piling up is caused by waves breaking and transporting water into the surf zone. While the piling up occurs on scales on the order of the width of the beach, the return of this piled up water seaward occurs in narrow bands due to bathymetric irregularities, i.e. if there is a transverse trough or ditch in the bathymetry the water will preferably flow downhill towards this trough and then seaward due to the hydraulic head of the piled up water. These things can drag you seaward with them, but seeing how they're fairly narrow the best strategy for getting away from them is to swim sideways rather than madly thrashing towards the shore.

An undertow is a broader return flow at depth, i.e. at the bottom. It is another way for the water piled up at the shore to return seaward, although it only occurs in the bottom portion of the water column while the rip current occurs throughout the entire water column. As such, the logical way to avoid being deposited in Davey Jone's locker by this would be to swim towards the shore at the surface.

There are other currents in deeper water unrelated to the action in the surf zone that have also been called undertows. These are usually due to strong local currents induced by the tides. One doesn't want to even have to attempt to avoid the effects of very strong tide-induced currents.

skb

========
Newsgroups: sci.geo.oceanography
Subject: Re: What is an undertow?
From: baum@stommel.tamu.edu (Steve Baum)
Date: 22 Dec 1996 11:29:23 -0600

In article <593red$31i$2@nntpd.lkg.dec.com>,
Neil Savage wrote:
In article <58pe6j$bqu@stommel.tamu.edu>, baum@stommel.tamu.edu (Steve Baum)
writes...

An undertow is a broader return flow at depth, i.e. at the bottom. It is another way for the water piled up at the shore to return seaward, although it only occurs in the bottom portion of the water column while the rip current occurs throughout the entire water column.

You have evidence for this?

Why is it that Willard Bascomb could never find this phenomenon? In his book, "Waves and Beaches" he writes of his quest to find the elusive "undertow". In his career-long search, he never did satisfy his curiously about this lay term. He concluded (as do I) that people using the term hadn't any clue as to what they were observing.

- neil (the skeptic)

A poster I saw at AGU last week featured measurements of the undertow. The main surprise according to the author was that it didn't feature a maximum near the bottom. Rather, the maximum was a bit above the bottom. The poster also explicitly stated that this was indeed undertow rather than a rip current phenomena since the beach profile was uniform in the longshore direction. I am a great admirer of Bascom and, indeed, have read both of his books many times, but "Waves and Beaches" was written well over 20 years ago, and measurement techniques have improved greatly since then. A casual perusal of any year's JGR-Oceans since then will yield several articles on the measurement of various phenomena in the surf zone. As an aside, Henry Stommel provides a good example of how certain phenomena must wait for measurement technology to catch up in order to be properly investigated in his 100+ page autobio in the recently publisheed "Works of Henry Stommel" collection. He wanted to establish a measurement program in 1962 that later became the MODE program, and states in no uncertain terms that there was no way he could have done MODE with 1962 measurement technology.

skb


Subject: Re: What is an undertow?
From: savage@tle.enet.dec.com (Neil Savage)
Date: 1996/12/13

In article <850366940.13421@dejanews.com>, ghillier@concentric.net writes... Hello, Hope it's okay to ask these questions here. I've never understood what an undertow is. We used to go to islands when we were little, and our parents warned us to watch out for the undertow. The picture I have is that I could go in the ocean and be pulled underneath the island. Is this possible? And if it is, wouldn't a person be able to see an opening, or wouldn't there be a big drop on the ocean floor?

"Undertow" is a technically inaccurate term that does not describe any observable natural phenomenon. The 'layman's' use of 'undertow' is muddled at best. It may refer to the return flow of water from a breaking wave that has run up on to the beach. The law of gravitation says that what runs up the slope must run down to the edge of water again. The downrunning of water from off the beach due to a large breaking wave can often be powerful enough to knock a small child of his or her feet and into harms way in the surf.

Note that there is no "under" to this particular "tow". Once the return flow meets the spill-and-runup from the following breaking wave, the back run of the previous wave halts, and the flow goes up the beach once again. Still, the dynamic actions of waves of 4 or 5 feet or more can be disconcerting to the parent of a wave-tossed child and have been known to lead to many drowings and near drownings. So when you hear 'undertow' from someone who chooses not to communicate using scientifically accurate terms, stay away from the water's edge anyway. FWIW: the correct generic term is normally, "dangerous surf conditions". If the top of a large wave falls over on you, it may be 15 seconds or more before you can get a breath of air again. That's the "under" part (but no "tow").

The other question is about rip tides. Is a rip tide when someone is say standing in the water or swimming and the water that is returning to sea is so strong that it carries the person out with it? If the person is standing, couldn't he dig in and hang in there? Anyway, thank you for answering these questions -

You are probably referring to 'rip current'. When water gathers at the nearshore region of a beach, the volume buildup has to be relieved All that water piling up is going to get back to sea somehow; rip currents are how nature restores the hydraulic balance.

If you are standing on shore observing a line of powerfully breaking waves, you may notice a place where the water surface is much flatter. That's where the rip current is. It can carry a swimmer out beyond the nearshore surf zone in a matter of seconds. An unsuspecting swimmer may be surprised by the rapidly receeding shoreline and panic.

The first rule is DON'T PANIC. If you are a weak swimmer, though, you are in a lot of trouble. If you are a strong swimmer, remember rule #2: always try to swim back to shore through the rough part of the surf, not the smooth, flat area from whence to you came to be towed 'out to sea' as much as the length of a football field or more.

Note: If you are on a lifeguarded beach, do not be tempted to hitch a ride on a rip current. I know from personal experience that the guard's good humor is not helped by pulling such a stunt - even though it is lots of fun for a good strong swimmer to do.

- neil

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