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Anatomy Of A Wave
Waves
There is little actually very little distance travelled by the water particles in waves, despite the energy and the force with which they move forward. Waves are simply the movement of energy.
Waves breaking near the shore are the end result of energy that has traveled potentially thousands of miles across the ocean to culminate in a perfect peeling face to ride, or an exploding bomb ready to take your head off. By learning about waves and their formation you will know more about which of these to expect.
The top of any wave is the crest and the bottom is the trough. The distance between two successive crests or two successive troughs is the wavelength. And in scientific terms, the vertical distance between a crest and a trough is the wave height.
Wave Speed and Wavelength
The speed of a wave is simply how far it moves in a given amount of time, which sounds like a physics lesson but is important in why waves hit the shores as they do. Wavelength has a large effect on wave speed. Small waves tend to move slowly at a few knots (1 knot = 1 nautical mile per hour). Medium-size waves can move faster at tens of knots; large waves can move at 50 knots and more. The biggest waves of all, tsunamis, can reach speeds of up to 450 knots.
Wave Interval
Wave interval is the time taken by successive crests to pass a static point and is determined by wavelength and wave speed. This can contain a lot of information about surf conditions. Waves separated by large intervals tend to be larger and stronger, while short-period waves are smaller and less powerful. Measures of wave interval are used by surfers to gauge what kind of swell is on its way, often by checking the wave interval information gathered by buoys posted out at sea.
Wave Frequency
Wave frequency refers to how many waves are made per time interval, usually per second, and is defined as the number of waves passing a static point in a given amount of time.
Wave Height
Surfing definitions of wave height can be confusing, and the first thing to understand is that there is a difference between the size of a swell as it travels through the ocean, and the size of the wave produced when the swell reaches land.
If you took two people and asked them the height of the same wave they could give you totally different answers, and both be right. It’s a matter of perspective. Often surfers will estimate the height of the wave face and report it in feet accordingly. Others will give a measure of the wave face in terms of their body length and tell you that its ‘shoulder high’, ‘overhead’, ‘double-overhead’ and so on. This is common in America and Australia. Others will give you the size of the swell rather than the face.
In Hawaii, waves are measured from the back of the wave and not the face. This is a measure of the swell, and a 6 foot wave in Hawaii will have a face much bigger than that and will be somewhere up to double-overhead to a Californian. Factors including the angle of the swell when it hits the shore, the depth of the water, the speed of the swell, the direction of the wind, the strength of the wind and the wavelength will all affect the size of the wave face, and so an estimation of the face is all that can really be taken from the size of the swell.
In the UK, you get a confusing mix of all three measurements. You may have to clarify. People may say ‘3 foot Hawaiian’ – when you hear this, always nearly double the figure given to get an estimate of wave face. As surfing was born in Hawaii, most surfers respect this and regard the Hawaiian measurement system as the ‘real’ system.
What this all really means is that you shouldn’t take what other people say too seriously. As you stand open-mouthed, listening in admiration to the person who has been surfing for only 6 months tell you how he was out ripping in 6 foot surf, remember, 6 foot to him is likely to be very different from what you would call 6 foot. Taking into account ego and exaggeration, his 6 foot could be as little as what you would regard as being 2 foot. Not quite so impressive.
As a general rule, the bigger the swell, the higher the wave face. But there are bizarre, unexplainable phenomenons. Teahupoo, near Fiji, is a wave which is a terrifying exception to every rule and is the stuff of either dreams or nightmares. This wave is reported never to reach heights greater than the wave produced by an 8-10′ swell, so no matter how enormous the swell is, the face is never bigger than 20 foot high. Instead of growing in height, it gets fatter. The lip of each breaking wave thickens to unthinkable proportions, and when a huge swell hits the reef at Teahupoo it creates a horrifying, sucking monster and a lip ten foot thick can hit the impact zone. Imagine the lip that curls above you as you paddle for a wave at your local break. Now imagine that lip being ten foot thick. Would you stay out?
Waves can travel through water from all directions and at all angles. When waves travelling in different directions meet, they can either cancel each other out (destructive interference) or join and form a single, bigger wave (constructive interference, or in surf-speak, a double-up). You can see this when two distinct and different swells meet at one peak – the result is an unpredictable lottery of big monsters and ripples. When two swells meet close to shore, the result can be lethal. A double-up shorebreak can slam down onto solid ground and break boards and worse, backs. The shorebreak at Waimea Bay in Hawaii is probably the deadliest double-up on earth.
When the sea is messy, choppy and rough, the surface texture you can see is just a mishmash of countless waves of different sizes from different directions meeting and combining to produce a big choppy mess. If you could separate them out you would see distinct peaks and troughs. When you find perfect lines heading shoreward, you are seeing the result of a single swell produced thousands of miles away hitting the shore with no interference. Magic.
The Formation of Waves
With the exception of tsunamis, all waves are formed by wind in one way or another, so it is important to first understand wind.
Why the Wind Blows
Wind is formed by the sun heating up areas of the earth unevenly. This uneven heating – of the equator more than the poles – causes changes in the density of the air (air pressure) in different areas, resulting in air moving about to try to keep the air density equal. This is termed convection. When air is heated it expands and rises, and cooler air descends from the poles to fill the gap. The result? Wind.
It’s not quite as simple as that, or there would be a constant wind heading in one direction. The land and seas heat up and cool down at different rates. When the sun goes down the sea will cool much more quickly than the land, which causes localised convection, and different wind patterns.
Wind and Waves
Waves are caused by wind energy being transferred to the water. Wind blowing across the surface of water firstly causes tiny ripples, which have tiny walls against which the wind can push. The more wind blowing, the bigger the walls of the waves get, providing a bigger surface area to push against.
The size of the waves depends on three factors: wind speed, wind duration, and fetch, which is the area over which the wind blows. Fetch is an important factor in wave generation, and the biggest waves are not produced by intense hurricanes and typhoons as you might think but by large storm systems blowing winds over hundreds and thousands of miles. Hurricanes generate by far the fastest wind speeds, but their power is not as far reaching as the large and consistent swell generated by large storm systems. A stronger wind blowing over a bigger area will provide bigger waves
Wave energy generated by wind activity out in the oceans radiating outward until it meets an obstacle. This is usually a shore, and culminates in breaking waves for us to ride.
Groundswell and Wind swell
All waves are created by wind, but the waves hitting the coast can be divided into two types: groundswell and wind swell.
Wind swell is produced by local wind conditions, forming choppy and irregular surf. Waves produced by wind swell tend to be messy and lack the power of waves formed by groundswell, and so create less than perfect surf conditions.
Groundswell has been produced by a storm far out in the ocean, and is what surfers are looking for. As groundswell travels towards the shore it has time to become much cleaner, more defined and more powerful, and so if localised wind conditions are right and don’t interfere, what we are treated to is strong, regular waves with defined sets and lulls, ideal for surfing.
Sets and Lulls
Wind speed within a storm system is never consistent, whipping the water up into a heaving mass of waves of all different sizes. But waves don’t stay where they are created, they radiate out from their point of origin in swells, and since waves with a long wavelength move faster than short waves, the waves sort themselves out as they travel and group together according to size. For this reason, groups of waves of similar size hit the shore together. We know these groups of waves as sets. The intervals between the sets are lulls.
The sorting process is never complete, and sets are never entirely reliable. The ocean is studied by scientists, but nobody told it that it’s supposed to be scientific so strange things happen all the time, which is what makes surfing so exciting. Waves in a set are never exactly the same size and there are always oddities, rogue waves springing up out of nowhere, little tiddlers sneaking through, and why waves tend to hit the shore in sets of +/- 7 is open for debate. But knowing the basics will make your surfing life a whole lot more fun – trying to paddle out in the middle of the biggest set of the day will not help you under any circumstance – the knowledge that a lull is on its way can prove the difference between success and disaster!
Breaking Waves
Water depth determines when waves break. As a wave moves through the ocean, the water particles aren’t carried along with the energy but move in a circular motion directly underneath the surface.
When the depth of the water is less than half the wavelength of the wave, waves begin to “feel” the bottom and two things happen – the friction created by the contact between the circling water particles and the bottom slows the wave down, and the energy within the wave takes the path of least resistance and as the bottom gets closer it pushes the wave up. The waves slow down and grow taller.
The bottom of the wave feels the bottom first, and so it’s the bottom that slows down first, elongating the motion of the water particles from circular to oblique. This results in the wave stretching and starting to peak. The wave eventually peaks so high that it can no longer keep its shape and it breaks.
Intense studies of wave buoy readings have shown researchers in Hawaii an interesting phenomenon. A buoy can read the same size swell on two separate occasions but produce breaking waves of different heights, and the reason for this difference is wavelength. The energy is more concentrated in swells with a longer wavelength, so that when the waves break, they break with more force and so are bigger and more powerful. This could be because swells with longer intervals have had time to iron out the bumps and chops in between the peaks, so all the energy is gathered in the one spot ready to explode as the wave breaks. A 4 foot groundswell with an interval of 10 seconds won’t produce waves as big as a 4 foot swell with an interval of 20 seconds.
Wave Shape
But what causes the differences in the shape of waves? Why do some waves spill gently to shore while others break in lethal top to bottom barrels?
If the ocean floor slopes gently, as it does around the coastline in the UK, the waves lose most of their energy to friction as they approach the shore. Each wave has a long journey up an incline and slows down before it breaks. But, if the bottom rises suddenly as it does on volcanic islands such as Hawaii and the Canaries, the wave meets the land at the last minute and doesn’t have time to slow down. All the energy is pitched upwards and forwards in one great burst, creating big, powerful waves like those pounding the northern Hawaiian shores in winter.
Large waves break in deeper water. This is because they feel the bottom sooner than small waves. Waves also bend to fit the contours of the bottom. If a swell hits a horseshoe shaped beach, the wave will be shaped by this and bend into a horseshoe to travel into the bay.
If you watch a wave arrive on a beach and start to break, it will be clear that the whole wave doesn’t break at the same time – if it did, we would have close-out after close-out and surfing would come to a grinding halt. Luckily, the bottom of the ocean is uneven, and thanks to sandbars and reefs and dips in the ocean floor, waves break in peaks. This is what you need to watch for when you are about to paddle out. Surfers can tell the shape of the bottom by watching how the waves are breaking.
Deep water channels cause saddles in the surf – places where waves don’t break. Don’t fall into the trap of sitting in a saddle, proud that you have found a quiet spot and wondering when a wave will break for you – it won’t. The water is too deep in that spot and you will wait forever, unless a huge beamer comes through, and then you will probably be caught inside anyway. You’ll have to head for one of those more crowded peaks and get in amongst it.