Making Waves

The Life History of a Wave

By Farwell Forrest
farwell@paddling.net

November 9, 2004

Waves, like wind, are an inescapable feature of the paddlers' world. In fact, they're often a large part of our fun — until they get too big, that is. Two-foot rollers give novice canoeists a lively ride and nothing more. But ten-foot breakers can make even expert kayakers wish they'd stayed at home.

What's the difference? Energy. Big waves lift a lot of water high in the air, and water is terribly heavy. No paddler in his right mind wants to be sitting under a towering breaker when it topples. But not every wave is a breaker. Many just roll beneath us as we paddle, buoying us up and then setting us down. Gently. Most of the time, rollers simply take us along for the ride and nothing more. There's a downside, of course. Boaters soon discover that their companions play hide-and-seek in big rollers, riding high on a crest one minute and disappearing from view in a trough the next. This can be mighty disconcerting, not to mention dangerous. It's hard to keep a party together when half the paddlers can't be seen. And even when the troughs are too small to hide a kayak, the never-ending roller-coaster can make all but iron-stomached boaters a little queasy. Yet few experienced paddlers fear rollers.

Breaking waves are something else. They can grab a kayak within seconds of launching and fling it back on shore, leaving the dazed paddler with a crumpled boat and a mouthful of sand. Nor are breakers confined to beaches. Far out beyond the surf zone, breaking waves can capsize a boat in an instant, or even catapult its bow over its stern, in the heart-stopping loop-the-loop that sailors call pitchpoling. If you've seen someone toss the caber at the Scottish games, you've got a pretty good idea what a pitchpoled paddler experiences. And that's not all. Should you ever be unlucky enough to be caught between two breakers moving in opposite directions — as can sometimes happen when a wave strikes a seawall or cliff face and then bounces back into the path of the next oncoming wave — the resulting slam-dunk can release enough force to smash both your boat and you. It's a very good idea to be somewhere else.

Then there are the so-called "rogue waves," familiar to anyone who's seen The Perfect Storm. It turns out that rogue waves aren't really rogues at all. In fact, they're not uncommon. You certainly don't need to venture out into the North Atlantic to find one. You can see them much closer to home. Spend a day by a busy lake next summer. Don't paddle. Just relax on shore with a cool drink. But don't fall asleep in your chair. Keep your eyes open. Early in the day, before the first water-skiers have revved their engines, the only waves you'll see are wind-driven rollers. Unless it's blowing a gale, breakers will be few and far between. Once gasoline alley opens for business, though, things will change in a hurry. Except when planing, each powerboat trails a train of good-sized waves behind it. These wave trains bend as they go around obstacles like islands, points, and piers, and they bounce off headlands and seawalls. They also collide with other wave trains. Soon, the laid-back rollers of the early morning will be lost in a chaos of breaking waves. Watch closely. Occasionally — every few minutes if traffic is heavy enough — you'll see one wave suddenly rise up, bigger than all the rest. It towers over its companions, but it doesn't last long. In seconds it will have collapsed in a mass of froth. Congratulations. You've just witnessed the birth (and death) of a rogue wave, though on a much smaller scale than the ship-killing monsters of the open ocean. A warning is in order here, however. Kayaks and canoes are the smallest of small craft. If the lake you've chosen to use as your laboratory has a lot of commercial traffic — barges, tugs, and ferries, say — you'll see rogue waves that are more than big enough to spoil a kayaker's day. That's another good reason to observe the "Gross-Tonnage Rule," arguably the most important of all the Rules of the Road.

Unfortunately, while canoeists and kayakers can usually avoid mixing it up with bigger boats, we can't escape waves. In truth, few of us would want to. That doesn't mean we shouldn't try to understand them better, though. Let's begin at the beginning, then, with…

What Makes a Wave?

Lots of things, as it turns out, some of them pretty exotic. Undersea earthquakes. Even asteroid impacts. No, I'm not joking. Drop a pebble in a fish pond. See how the ripples move out from the point of impact? Now scale them up several billion-fold, climb a nearby mountain, and watch the sea remodel the coastline. It's happened in the past. And it will happen again. Someday. But probably not today. Or even the day after tomorrow.

The list of exotic wave-makers also includes calving glaciers and landslides. Anytime you drop something in the water, you make waves, right? It's only natural. Such wonders are outside the run of most paddler's experience, of course, although calving glaciers aren't that unusual in Alaska, Greenland, and southern Chile, all of them familiar destinations for globe-trekking adventurers. Still, the waves that most of us encounter every day we're on the water owe their existence to one of two far more commonplace causes: gravity and wind. Gravity makes waves in rivers and along the world's seacoasts, where tidal currents create races and overfalls, saltwater counterparts to the standing waves and rapids of whitewater rivers. This is a story for another time, however. I'll return to waves in moving water in a later article.

We're left with the wind. Wind makes waves. The wind blows over the water, dimpling its surface in much the same way as a caressing hand dimples the skin of a lover's arm. Those watery dimples are waves. The harder the wind blows, and the longer it lasts, the bigger the dimples get. Wind speed and duration determine wave height, in other words. But is that all? No. The distance over which the wind blows without meeting any obstacle is known as the "fetch." It, too, contributes something to the making of waves. The greater the fetch, the bigger the waves can grow.

The upshot? Wave size is a function of three variables: wind speed, duration, and fetch. This interlocking, dependent causality will come as no surprise to most paddlers. A gale that topples hundred-foot-tall pines may still leave the surface of a tiny beaver pond unruffled, but even a fifteen-knot (Force 4, or about 17 miles per hour) breeze can raise respectable five-foot seas if it has a hundred miles to work in. When the wind blows hard and long over open water, watch out. The theoretical limit for wind-waves on the open ocean is over 200 feet high. That's one limit that few of us will want to test.

There's more to waves than their height, though. In well-ordered seas, one wave follows another at a more or less predictable interval. That interval — usually expressed as wavelength, or the distance from one crest to the next — increases as waves grow higher. So far so good. But waves also steepen as they grow. Moreover, they build up faster than they stretch out. Then, when the ratio of height to length exceeds one in seven, they break.

This phenomenon underlies the Beaufort scale. In open water, the whitecaps of breaking waves first appear when the wind rises to 10 knots (Beaufort Force 3). At 20 knots (Beaufort Force 5), whitecaps are everywhere. The phrase "open water" is important here. You won't have much luck applying the Beaufort scale in a beaver pond, though it works well enough on big lakes. As you've probably already guessed, fetch is the determining variable. Tamia and I lived on an Adirondack flow for many years. (A flow is the local name for a drowned river. Some flows are natural phenomena, but most are not. Engineers call these unnatural flows "reservoirs.") It stretched nearly three miles from west to east, but was only a quarter mile wide opposite our home. When the wind blew from the west, I could easily use the Beaufort criteria and get a wind speed that matched the reading on the scale opposite the little white ball in my Taylor anemometer. But when the wind veered north, at right angles to the long axis of the flow, all this changed. It took a gale to raise so much as a ripple on the water. Then I had to look to the trees to read the wind.

Now that we know what gives birth to waves, let's take a look…

Beneath the Surface

Like an iceberg, much of a wave is invisible. River waves mostly stay in one place. The water flows through the stationary waves. By contrast, wind-generated waves move across the water. They're driven pell-mell before the wind. The waves move, that is. The water stays put. It's never completely still, however. The wind imparts a vertical motion to the water. Viewed from the side, each leaf or discarded beer can describes an endlessly repeating circle, and the apex of each circle will coincide with the crest of each passing wave. (The circles creep slowly downwind, in fact, but this can usually can be ignored.)

The wind-generated disturbance isn't confined to the surface. It reaches much deeper than that, and as waves come ashore, the round-and-round, up-and-down movement is hobbled by contact with the bottom. That's when the waves start to topple. In effect, a breaking wave is tripped up by the rising seabed (or lake bed). This usually occurs at depths approximately one and one-half times the wave's height: the higher the wave, the farther offshore it breaks. Fishermen and lifeboatmen used to say that waves began to tumble as soon as they "felt the bottom." It's as good a description as any.

Another variable enters the equation here: the slope of the seabed near the shore. Steep foreshores trip waves up and send them smashing down with explosive force. Such "dumpers" create dramatic seascapes, but make for difficult launching and landing. Boaters — and surfers — have an easier time negotiating the "spilling" breakers created by gently sloping bottoms.

Sometimes the seabed rises to meet waves far beyond the surf zone. That's why offshore bars and banks are such lively places in a storm. But waves can also break when they collide with a current going in the other direction, as often happens at the mouths of large rivers, or whenever the tide and wind run opposite ways. Not even the open ocean is safe. Mariners who've braved places like the entrance to the Mozambique Channel — where the southwest-flowing Agulhas current meets giant waves spawned by the westerlies of the Roaring Forties — speak in hushed tones of "holes in the sea" and the towering breakers that follow right behind them. It's a one-two punch that not even supertankers can always survive.

Of course, few kayakers venture out onto the open sea. But as anyone knows who's ever fought to bring a small boat safely home across a big lake in a stiff Force 7 near gale, wind alone can steepen waves until they break, even when the bottom's far below. On days like that, when each passing wave dumps a load of green water on your deck and threatens to twist your boat out from under you, there's a crumb of comfort to be found in the words of an anonymous Aran Island boatman: "The waves have small mercy, indeed, but the rocks now…the rocks have no mercy at all."

Here's to small mercies!

Wind makes waves. Born on open water, waves end in foam and turbulence. Waves are everywhere around us when we paddle. We can't escape them, and we wouldn't want to if we could. That's why it makes sense to understand them. We need to know where they come from, and what they're like beneath the surface. How to use them when we can, and how to avoid them when we must. It's a big subject, and there's a lot more still to be said. Stay tuned.

Copyright © 2004 by Verloren Hoop Productions. All rights reserved.