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Clapotis—

The Sound of Two Waves Clapping

By Tamia Nelson
tamia@paddling.net

May 13, 2008

What, exactly, is the sound of two waves clapping? Well, before I begin exploring this somewhat different take on the classic Zen koan, I’d like to revisit my childhood for a minute. When I was a little kid, I loved to jump on my bed. Trampolines were all the rage back then, but I didn’t have a trampoline. So I made do with what I had. Sometimes my brother and sister would join in, and the three of us would bounce so hard that we lost control, rocketing off the mattress in all directions. As you can imagine, my mother wasn’t too keen on this, and in the time-honored fashion of mothers everywhere, she tried to put a stop to it. “Keep fooling around,” she warned us, “and somebody’s going to get hurt.” But we kids ignored her good advice. You can guess the result, I’m sure. We continued our unsanctioned trampoline exercise on the sly—until the day when my little brother misjudged a “wave” and was thrown right across the bedroom. His flight stopped when his face hit the corner of a heavy oak table. Hard.

That was the end of my brother’s quest for “air time.” My sister soon developed other interests, too. Facial scars had no place in her game plan for life. But I was more stubborn. Despite the fact that my mother had proven herself an excellent prophet, I kept up my clandestine trampolining. I relished the feeling of being on the edge, the exhilarating sense of weightless flight that came with every spring-assisted bounce. And long after I outgrew my mattress trampoline, the memory of my early airborne sorties stayed with me.

Fast-forward several decades to a warm summer day on the Flow. Memorial Day, to be exact. The gentle morning breeze had dwindled down to nothing, but that didn’t mean the Flow was calm. Traffic was heavy. Jet-skis zigzagged aimlessly, party barges plowed up and down the main channel, and waterski tow boats weaved among the slower craft with reckless abandon. There were a few kayaks out and about, too. Farwell and I were returning from an afternoon circumnavigation of our home waters. Ever mindful of the Gross Tonnage Rule, and conscious of the fact that our 14-foot kayaks were no match for a ton or more of sprayed fiberglass mat, driven by 200 horsepower and piloted by a six-pack-fueled skipper, we hugged the rocky, eroded shoreline. We couldn’t avoid the big boats’ wakes, however. Multiple wave trains—some of them several feet high—crisscrossed the water, crashing ashore at random intervals and then bouncing back in strict obedience to the dictates of the law of reflection.

It was getting close to suppertime, and we were ready to pack it in. One obstacle remained: landing. The wake-eroded shoreline in front of our cabin rose steeply for two or three feet, and boulders lurked just below the water’s surface. Hauling out gracefully was tricky enough when it was calm. Coming or going, squat-and-scoot was usually the order of the day. Now, however, with wave trains moving in from every angle and ricocheting out again, we opted for discretion. So we waited a couple of boat lengths offshore, hoping for a brief respite in the traffic, our kayaks bobbing up and down in the swells. So far, so good. The easy rise and fall of my boat had a lullaby quality, and I soon fell into a pleasurable reverie. Then two wave crests collided—under my kayak. One was going out. One was coming in. Suddenly, I was heaved up nearly four feet. A quick brace kept me dry, but I plummeted down just as suddenly as I’d shot up, and I felt my boat’s hull slam onto a rock, newly exposed in the extra-deep trough that followed the extra-high wave that had tossed me skyward.

 

All in all it wasn’t much of a homecoming, though my Seda’s Kevlar® hull came through its ordeal with nothing more than a scratch. And I had another reason to be grateful. I’d gotten a relatively benign introduction to a phenomenon that can do a lot more damage to paddlers who venture out on exposed seacoasts:

Clapotis

To my ear, this sounds like a dread disease, or maybe an exotic restaurant entrée, but clapotis is actually a wave phenomenon, the product of not-so-chance meetings between wave trains moving in opposite directions. (Clapotis are a type of standing wave. In fact, clapotis means “standing wave” in French.) When the wave trains are in sync, when trough coincides with trough and peak with peak, the result is bigger than either parent wave—wave peaks are higher and troughs are deeper. This is bad enough on a smallish reservoir. On an exposed coast, when deep-water storm waves are reflected back out to sea by a high breakwater, pier or cliff, only to collide just offshore with the next set of inbound waves, the resulting giant haystacks can be spectacular. Sometimes the colliding wave trains smash together with such force that they explode upwards in what amounts to a marine train wreck. Then the sound of two waves clapping can be deafening. Needless to say, it’s not a good place to be in a small boat.

Thanks to self-similarity over changes of scale, you can demonstrate this phenomenon in your bathtub. All the essentials are there. Waves (just paddle with your hands to get things started), a near vertical iron-bound coast (the sides of the tub), and deep water close inshore (of course, “deep” is relative here). Experiment a bit, and you’ll see several patterns emerge. The most spectacular collisions occur when wave trains strike the coast head on. Then the incoming and outgoing waves meet along a broad front—a genuine train wreck. On the other hand, when swells come in at an angle, peak-to-peak and trough-to-trough encounters will be limited in scope and much less dramatic. (The resulting checkerboard of peaks and troughs has been called clapotis gaufré, or “honeycomb clapotis.”) Of course, in the world beyond your bathtub, size does matter. The clapotis that Farwell and I encountered on the Flow only gave us a bit of a bounce. Less fortunate paddlers might have capsized, I suppose, and their boats could easily have suffered more damage than the scratch I came away with, but that would have been it. This can’t be said of any boater unlucky enough to find herself paddling just offshore from a sea wall when heavy, wind-driven swells roll in, however. She’s betting her life against formidable house odds.

Clearly, then, while daredevils may enjoy the challenge of pitting themselves against “honeycomb clapotis,” no paddler, however expert, wants to be smashed in a train wreck. More cautious boaters—and all novices—have even less reason to equivocate. For them, the best rule is total avoidance. Happily, this usually isn’t hard. As we learned in the bathtub, clapotis requires three primary ingredients: a wave train, a steep coast (the harder the coast, the better), and relatively deep water close inshore. Absent any of these, the risk falls to zero. Well, maybe not quite to zero. There are a couple of other possible scenarios. Clapotis can also form where rivers flow into the sea against an onshore swell, or when wave trains are deflected around islands, only to come together with a bang in the lee. That pretty much exhausts the possibilities.

 

Conclusion? If there’s any sort of a sea running, don’t venture close inshore near sea walls or piers or cliffs. And be careful whenever you’re paddling in traffic, even on inland waters, especially if the traffic includes big, slow-moving vessels like tugs. Canals are a particular hazard. Barge trains moving at speed can raise towering wakes, and the resulting wave trains often bounce straight back from the canal walls. Kayaks and canoes beware! All in all, there’s just no substitute for local knowledge. While guidebooks can sometimes help, expert companions are your best sources of information.

But what happens if, despite your efforts, you find yourself in harm’s way? If it’s just a bit of bouncy water on a busy inland lake, you can get by with your standard repertoire of boat-handling skills. (A timely brace has saved many a padder from an unwelcome swim.) If, on the other hand, you’re caught up in a real train wreck, you’re in a world of hurt. Get out before you go in, if you can. If this proves impossible, however, and if you go over and can’t roll up, try to swim your boat to seaward, away from the collision zone. This may be your only hope. Self-rescue is likely to be impossible where large waves are smashing together, and you can’t expect much help from your companions in such places, either. They’ve got their own necks to save, after all.

Does this sound unpleasant? It is. All the more reason to avoid trouble in the first place. If in doubt, don’t go out. Choose another destination, or return on another day when no sea is running. There’s wisdom—tinged by the bitter realism of the working waterman—in the words of the Aran Island boatmen, as recorded by John Millington Synge:

A man who is not afraid of the sea will soon be drownded,… for he will be going out on a day he shouldn’t. But we do be afraid of the sea, and we do only be drowned now and again.

That’s very good advice, despite the melancholy twist at the end. And we’re luckier than the Aran Island boatmen. Few of us are working watermen. We paddle for pleasure. We can choose when and where we go out. So there’s no reason for any of us to be drownded, is there? Nope.

Clapotis can make for a lively, if challenging, ride. But (sorry, Mae!) you can get too much of a good thing. The place where wave trains crash together is no place for a paddler: the sound of two waves clapping can be the splintering noise of a shattered boat—or a broken body. So if you’re tempted to play where the waves go both ways, reflect on my Mom’s cautionary words: “Keep fooling around, and somebody’s going to get hurt.” Mom was right. (No surprise there.) And the body in question could be yours. ’Nuff said?

Copyright © 2008 by Verloren Hoop Productions. All rights reserved.









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