What Waves Know - Understanding the Sea

Collection: Field Notes — Old Fashioned Seamanship

Series Hub: Reading the Sea the Old Fashioned Way

Subject: Wave behaviour, refraction, reflection, diffraction, and how to read disturbed water ahead

Understanding Wave Behaviour at Sea — Refraction, Reflection, Diffraction, and How to Read Disturbed Water

There is a patch of rough water ahead. Not directly ahead. Off the headland, half a mile out, where the sea is throwing itself around in a way that has nothing obvious to do with the wind currently blowing. You can aim at it and find out, or you can read it first. This is about reading it first.

A wave does not carry water. It carries energy. The water itself moves in a small orbital loop, up and forward and down and back, and returns almost exactly to where it started as the wave passes. This sounds obvious until you start using it.

Gooley’s image is shaking out a bedsheet. The wave you see travelling along the sheet is real. The sheet itself has gone nowhere. At sea, the proof is any piece of floating weed or a seabird sitting on the water. A wave passes under it, the bird bobs up and forward and down and slightly back, tracing a small oval. It does not travel with the wave. Only the energy does.

So rough water ahead is a concentration of energy, not a mass of water moving at you. The disturbed patch off the headland is not coming from somewhere and going somewhere else. It is expressing what the energy arriving there is doing. Once you understand what is causing the energy to concentrate or collide at that spot, you can predict where else it is doing the same thing. And where it is not.

Ripples, waves, and swell

All three are waves in the technical sense, but they behave differently. The dividing line is period. The time between successive crests passing a fixed point.

Ripples have a period of less than a second or two. Surface tension governs them. They die within moments of the breeze that created them. They tell you about conditions right now. In a small boat the arrival of a patch of rippled darker water is a warning that a gust is on its way, with perhaps ten seconds to act.

Waves proper have periods of a few seconds to ten. Gravity waves, large enough to have broken free from surface tension. They persist for a while after their generating wind dies and tell you what the wind has been doing for the past hour or few.

Swell has a period of ten seconds or more, often much more. Atlantic swell arriving on the Irish coast might have a fifteen or eighteen second period and have originated in a storm near Newfoundland three days earlier. It travels thousands of miles without losing its direction and crosses everything else on the surface essentially uninterrupted. It tells you what the weather has been doing hundreds of miles away, days ago. The longer reading is in The Ocean’s Long Memory.

You are usually reading three layers at once. Ripples telling you the immediate wind. Waves telling you the recent local conditions. Swell underneath both telling you the larger picture. This note is mostly about the middle layer.

Fetch

Wave height is set by wind strength, the length of time the wind has been blowing, and the distance of open water the wind has crossed. The fetch. All three need to be above a certain threshold for waves to develop, and increasing any of them makes the waves bigger.

Gooley puts this directly. A Force 5 with hundreds of miles of Atlantic behind it is a completely different sea from a Force 5 across a few miles of sheltered water. He says a Force 7 in the Faroes worries him more than a Force 9 in Dover. Not the latitude. The exposure. The full Beaufort-and-fetch reasoning is in The Beaufort Scale and What It Actually Looks Like.

The North Sea is shallow and relatively enclosed compared to the Atlantic. A westerly Force 5 off the East Anglian coast produces short, steep chop because the westerly fetch is limited. Turn the wind north-easterly and suddenly there is fetch all the way from Norway and the same Force 5 builds a much more uncomfortable sea. The Baltic builds a nasty steep chop in strong winds simply because it is shallow. Waves build quickly but do not have the depth to take on a long, rolling character. I sailed Finnish waters in a 25-knot southwesterly that, by Beaufort number, should have been routine. The chop was confused and close-spaced and harder work than larger Atlantic seas I had been in. The geometry mattered more than the number.

Refraction

When waves move from deep water into shallow water they slow down. The mechanism is not friction. The orbital motion of water within the wave gets cramped by the seabed once the water depth is less than half the wavelength, and the wave decelerates.

This produces a piece of geometry that matters. If the sea floor broadly mirrors the coastline, deeper in bays, shallower off headlands, then parts of a wave crest approaching a headland reach shallow water first and slow down, while the parts still in deeper water continue at their original speed. The crest bends. It steers itself toward the shallow water. Toward the headland.

So wave energy concentrates at headlands. Waves converge from both sides, bent by shallowing water on either flank. The sea off a headland is rough partly because the tidal race is there. It is also rough because the basic physics of wave propagation in shallow water is directing energy toward precisely that point, from a wide arc of sea. The Long Beach breakwater incident in 1930 illustrates what this can do at an extreme. Wave energy was refracted by an underwater hump into a concentrated beam that destroyed a structure the offshore sea state had no business damaging. The oceanographers took seventeen years to work out why.

Bays are calmer than headlands by the same mechanism. Waves entering a bay fan out, diluting their energy over a wider area. Any crescent-shaped beach is a product of this effect. The waves have been doing the same thing long enough and consistently enough to move sand into the shape that matches the refraction pattern.

Portland Bill on a spring ebb with a westerly swell is everyone’s textbook example. Duncansby Head at the northeast corner of Scotland. Rattray Head on the Aberdeenshire coast. Every significant headland on the Brittany coast. In Orkney, where I had a limited but instructive acquaintance with the waters during a tall ships race from the islands to Denmark, the headlands do not just have races. They focus swell from several directions into one unpleasant place at once, and the chart gives only a partial warning of where that place will be.

Reflection

Waves bounce. When a wave meets a sufficiently vertical surface in sufficiently deep water it reflects back out to sea with most of its energy intact. Similar to light off a mirror.

Where the reflected wave meets the incoming wave, crests from both directions arrive at the same point at the same time, stack on top of each other, and produce momentarily a crest twice the height of either wave alone. Troughs do the same in reverse. The resulting pattern is called clapotis, from the French for “lapping”, and in its purest form it creates standing waves that appear to rise and fall in place rather than travelling anywhere.

Perfect clapotis, where incoming and reflected waves are exactly the same size and meet head-on, is rare. What you see more often is what Gooley calls clapotis gaufré, waffled clapotis, which forms where reflected waves meet incoming waves at an angle and create a cross-hatched, confused sea. This is the rough, anarchic water close to a cliff or harbour wall that seems to have no particular direction and is worse than the conditions further out. It is not random. It is the superposition of two wave trains from different directions.

The rough water extends some distance out from the reflecting surface. The wall is not the problem zone. The sea immediately in front of it, where reflected energy meets incoming energy, is. The steeper the obstacle and the deeper the water at its base, the more faithfully the reflection occurs. A gently shelving cliff with a reef at its base absorbs most of the incoming wave energy. A sheer granite cliff dropping into forty metres of water reflects almost everything.

Stand at the top of any sea cliff in a moderate sea and you can see the confused water a couple of boat-lengths out from the cliff face is worse than the open sea fifty metres further offshore.

Diffraction

Waves passing the end of a breakwater, a headland, or any obstacle comparable in size to their wavelength bend around the end into the sheltered area behind it. They do not simply stop dead at the edge of the obstacle.

Gooley’s analogy is sound around a tree. You can hear someone speaking on the other side of a tree even though you cannot see them, because sound waves diffract around the trunk. Light waves are too short and do not diffract around a trunk, which is why you cannot see around it.

Ocean waves diffract around headlands and breakwaters this way. Energy fans out into the shadow zone, progressively diminishing the further into shelter you go, but never disappearing. An anchorage described as “sheltered from the west by a headland” will still have some swell from the west. Much reduced, but not zero. This is the source of my mild ongoing annoyance with pilot books that describe shelter without qualifying it. Behind the headland is not flat.

Gooley tells the story of his own wedding anniversary sail along the lee side of the Isle of Wight in a Force 7 southerly. He knew theoretically that refraction and diffraction would bring swells around both ends of the island before he had finished passing it. He was vindicated. The boat was sold shortly after.

Shallowing

As waves enter shallow water on a coastal approach they steepen before they break. The orbital motion within the wave gets cramped by the rising seabed. The crest narrows and heightens. The trough broadens and flattens. The wave face becomes steeper, the wave slows, and it bunches up. Gooley calls the quality of the sea at this point treacly, a useful word for the heaviness experienced sailors detect before the breaking point.

In areas of scattered rocks, reefs, or offshore sandbanks, the Thames Estuary approaches, the Goodwin Sands, the East Anglian offshore banks, any unexpected change in wave character in what should be open water is worth attending to immediately. The wave is telling you the depth has changed. Ibn Majid, the fifteenth-century Arab navigator whose work Gooley quotes, noted a patch of unexpectedly choppy water in otherwise calm conditions, mentally filed it as a shoal, and returned years later to find an island had formed there. This is the principle made flesh. The water had been telling him the truth years before the chart caught up.

Breakers come in three kinds depending on seabed gradient. Spilling breakers, where the crest tumbles forward in foam, on gentle slopes. Plunging breakers, the classic photogenic arch where the crest overtakes the base and the wave pitches forward, on steeper gradients. Surging breakers, where the wave runs up the beach without quite completing its break, on very steep beaches.

The breaker type at a harbour entrance tells you something useful about the approach. Steep plunging breakers in the mouth suggest a bar with a sharp gradient. The gentler spilling foam of a shelving beach is a different proposition. The foreshore-side reading is in What the Foreshore Tells You.

Surf beat

Waves arrive in groups. A set of larger waves, typically five to ten, arrives at the shore. Then a period of relative calm. Then another set. This oscillation has a period of several minutes.

The mechanism is that waves generated by a storm are not uniform in size. The larger waves within a group travel slightly faster than the smaller ones, so by the time a storm’s output reaches a distant coast it has partly sorted itself. Larger waves arrive together. Smaller waves arrive together. Pulses. The timing is not perfectly predictable but consistent enough that watching a surf or harbour entrance for ten minutes before attempting entry in marginal conditions tells you considerably more than a single glance does.

Back to the rough water ahead

The patch off the headland. The questions you are now asking are these.

Where is the swell coming from, and does refraction around this headland put the focus point in my path? If the headland has a shallow bank extending from its tip, the refraction focus is further out to sea than the headland itself.

Is there a cliff or wall here that could be producing a reflected wave train? If so, the rough water extends offshore.

Is there an island or headland providing shelter that might be delivering diffracted swell into the lee?

Has the wave character changed in the last mile, steeper or heavier or shorter period, suggesting a shoal even where the chart shows nothing?

Is there a timing rhythm to the disturbance? Sets arriving and passing with a few minutes of relative calm between them are a different problem from a continuous, confused sea.

None of this replaces the chart. None of it replaces the tidal atlas, the pilot, or sober weather assessment. What it adds is the ability to read in real time what the chart describes in averages and generalities. The chart shows where the headland is. The sea shows what the headland is doing today, in this swell, with this wind. The current side of the equation is in What Moving Water Tells You.

What is still slow for me is the combination case. Refraction at a single clean headland in moderate conditions is straightforward. Refraction plus diffraction plus reflection on a complex coastline in a rising sea, while also handling the boat, that is the part I am still building. I suspect it is mostly built by repetition in the same waters until what was complex becomes legible.

For somewhere to start the reading without anything at stake, the Hithe Finder is a community register of slipways, hards, and beaches suitable for small boats. A breakwater extension in moderate swell is one of the more instructive places to spend an hour on a falling tide.

References

Gooley, T. (2016). How to Read Water: Clues and Patterns from Puddles to the Sea. Sceptre. The source for almost everything in this note. The bedsheet analogy alone is worth the cover price, and the chapter on wave physics is the best plain-English explanation I have read.

Lewis, D. (1994). We, the Navigators: The Ancient Art of Landfinding in the Pacific, 2nd edition. University of Hawaii Press. The Pacific tradition of reading refraction, reflection, and intersecting swell patterns to detect islands beyond the visible horizon.

Lane, C.D. (1942, reprinted 2011). The Boatman’s Manual: A Complete Manual of Boat Handling. 

Series index at Reading the Sea the Old Fashioned Way.

At VAKA I design and build boats that don’t destroy the environment. Find the plans as they are finalised at VAKA Plans and the full field notes here.

VAKA. Traditional craft and natural materials. Nottingham. 2026.