The Life History of a Wave
By Farwell Forrest
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
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
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.
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 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
Sometimes the seabed rises to meet waves far beyond the surf zone. That's
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
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
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