Ebb and flow, the story of waves

When we think of ocean waves we immediately conjure up a vision of rolling water, but everyone has a different response to them.

Some are gentle and some (when kicked up by a mighty wind) can be so majestic and devastating to be capable of destroying solid rock walls and buildings. A surfer spends a lifetime looking for the perfect shape and size, but when we surfed past the rocks at Coffs Harbour in Ard Righ in the aftermath of a strong electric storm a couple of years ago, hangin’ ten was not at the forefront of our emotions!

Waves exist in most fluids; the air of our atmosphere is a fluid and wind is a wave that in turn blows on the surface of the water forming waves. Waves are also transmitted through the earth from subterranean shocks as earthquakes, termed ‘mechanical waves’. Light and radio waves also share some properties with these physical undulations but instead are waves of electrical and magnetic fields.

What causes them and how they interact has been the subject of countless hours of study and this little discussion takes only a tiny look at what they are and how they interact.

Water is in constant motion around the planet as waves, tides and currents. Each of these will affect a yacht one way or another.

We are all familiar with tides which are a slow, relatively regular rise and fall of the water surface generated primarily by the gravitational attraction of the sun and moon. These produce a movement of water that flows in and out of harbours, rivers etc. They are also influenced by the shape of the coast line and the ocean floor, but that is another subject.

We also experience the effects of large movements of water in the form of currents. As you sail around the Australian coast you will experience the effects of three of the world’s major currents: the South Equatorial current along the Western Australian coast; the Southern Ocean Current and the East Australian Current. They move an unimaginable volume of water that has significant influence on the climate on the nearby coast.

Waves in the ocean are broadly classed depending on the direction of movement of the individual particles of the water.

When water particles move up and down they are termed transverse waves; while a forward movement is termed a longitudinal wave. Water particles, in fact any particle that is floating on the sea surface, will follow predominantly a transverse path (up and down) as the wave passes; while the energy in the wave actually moves in a circular motion.

A shock wave is longitudinal; sailors have reported a loud bang when an earthquake occurred nearby. This is due to the sound wave, a longitudinal wave, travelling through the water as particles bump together like a wave running along a coiled spring. The energy runs along the spring, but the metal of the spring does not ‘flow’ from one end to the other. Interestingly,
an earthquake will generate both longitudinal and transverse wave motion through the earth; these are termed P and S waves.

When the wind blows cross the surface of water, turbulence in the wind and friction with the water begins to generate waves. A gentle wind will make little waves (ripples) and as the wind increases in strength increasingly larger waves quickly form.

Another aspect is the direction and distance that the wind travels across the water, termed the ‘fetch’. A wind blowing strongly over a long stretch of the ocean will cause much larger waves to form than one blowing over a shorter stretch of water. These ocean waves can persist for some time after the wind in the area ceases.

So if I want to travel along the east coast a westerly wind, off the land, of moderate strength will give me the most comfortable ride due to the short fetch. These waves do not have time to grow to any significant size if I keep relatively close to the coast.

If however there has been an easterly of the same strength, particularly if it has been blowing for some days, then the waves are likely to be much higher and may cause me to reconsider travelling.

Locally generated wind waves tend to be relatively steep and close together. If they form in shallow water they can be steeper and more uncomfortable than those forming in deeper water.

Another example of local conditions changing the shape of a wave occurs when the wind blows in the opposite direction to the wave or when the wave meets a current or tide running in the opposite direction. These ‘wind against tide’ conditions tend to produce steeper and more uncomfortable waves.

While a wave will travel long distances the water particles themselves are not transported, rather it is the energy of water particles bumping together that moves. This can be seen if you watch the inevitable junk on the sea surface which moves backwards and forwards with each wave that passes, but rarely travels forward very far.

This is because the wave energy is transported as a circular movement with the energy at the crest of the wave moving in the forward direction and the energy in the trough moving backwards.

However, when the waves move into shallow water the rules change as friction with the bottom slows the lower part of the circular movement causing the top of the wave to increase in height and lean forward, eventually tumbling in a mass of foam. This is when it gets serious for a sailor as now the water does move forwards often rapidly and will carry any objects with it, including a yacht, if it is strong enough.

Of course this can also happen in deep water; as the wave reaches its maximum size the top becomes unstable and falls, taking everything with it.

Waves formed by a strong disturbance (an intense low for example) will travel for hundreds of nautical miles as swell into an otherwise calm region. These waves are more rounded with a relatively long distance between the waves, termed the wave length.

The further we are from the source of these waves, the longer the wave length and more rounded the wave. So it is possible to be sailing with little local wind-generated waves but with the boat rising and falling gently due to the passing swell.

All waves interact with each other.

It is possible for a regular sea to suddenly produce a wave that is higher and much steeper than the other regular waves. This is often more pronounced when a local wind-wave meets one or two swell series.

This becomes vitally important when we are preparing to cross a bar; a regular rise and fall can suddenly turn into a dangerously steep wave so it is important to wait and watch several wave sequences before committing ourselves to the crossing.

Water depth also becomes important. When the ratio between the wave height and depth of water reaches 3:4 then we could be in for a bumpy ride. For example: a wave a little over two metres at a bar of three metres at high water could begin to break turning a simple entry into something much more exciting.

Remember that seawater is quite dense, which means that a cubic metre of it weighs roughly one tonne. Taking a large wave on the side of a yacht can consist of several tons of water travelling at sufficient speed to do real damage, as many sailors in round-the-world races have experienced.

Freak waves

There are many stories over the years of massive waves that suddenly appear out of nowhere and overwhelm ships. Yachts have been lost without trace, their disappearance subject to much speculation.

It may be the ‘freak, rogue’ or episodic wave that does the damage. These waves have massive vertical fronts, some have exceeded 30ms, that smash even large ships with little or no warning. While not a lot is known of these they have been reported as occurring in moderate or otherwise seemingly benign conditions.

One infamous area is on the south eastern side of South Africa where the south westerly flowing Agulhas current meets the Antarctic circumpolar current, also called the west wind drift; this and a south-westerly due to a cold front have been put forward as a cause for the formation of freak waves.

Another special case is a tsunami. These are massive shock waves that occur when a major seismic disturbance, earthquake, under water volcano erupting or submarine land slip, causes a surface wave. In very deep water (well off the continental shelf) these waves, which can move at up to 800 kilometres per hour, may pass unnoticed but will do serious damage when they reach the shore. As recently as July 1992 a 15m tsunami killed more than 2,100 people on the north coast of Papua New Guinea.

Tsunamis occur most commonly around the Pacific ring of fire, which runs from Japan through South East Asia along Indonesia and Papua New Guinea and down to New Zealand on the western edge and across from Russia and Alaska down the west coast of both North and South America.

Some special effects

Christiaan Huygens born in April 1629 at The Hague spent much of his scientific career studying waves in a small harbour he could see from his study.

The properties he observed, reflection, diffraction and refraction are well recognised today and affect all waves including radio and light waves. We regularly see these effects as
we sail.

When the edge of the shore is a vertical rock face then much of the energy from an incoming wave is returned to the sea. The returning waves and the incoming waves add and subtract to give a disturbed patch of water. This wave is termed a Clapotis, as it does not strictly travel but stands up and collapses as the incident (or incoming) wave adds and subtracts from the reflected wave. This gives the familiar washing machine effect at the entry to Sydney Heads.

The Zuydhorp Cliffs in Shark Bay in Western Australia are well known for this effect as are many other locations around our coast line where there is a vertical cliff, rock face or sea wall.

In contrast waves running up onto a smooth sandy beach will often dissipate their energy with little of it causing a wave to return to the sea.

Refraction occurs when a wave bends around an object. How often have you hidden behind a small island to get out of the weather only to find that you spent a sleepless night rolling from side to side?

This can often be answered by refraction where the corner of the island causes the wave to bend around the edges.

Diffraction can also occur when a wave enters a harbour or it encounters the gap between two islands. The incoming wave is diffracted by the slot in the rock wall or the gap between the two islands causing it to fan out making the sides of an otherwise sheltered side uncomfortable.

This effect is well known in optics and radio transmission. Radio waves will bend around objects allowing us to receive a signal even when there is an object between the transmitter and the receiving aerial.

High Frequency radio waves can be bent and refracted off the upper atmosphere. A reflected microwave signal is the basis of radar, where the signal is reflected back to the set allowing us to see objects that would otherwise be invisible to us.

We can also experience the effect of a refracted wind wave as we approach a steep island. A wind that is predominantly coming from the direction of the island can suddenly reverse direction as we enter the region where the wind, diffracted by the top of the island, curls back toward the island. As we approach the island we will enter its shelter, but may still be affected by bullets where the wind is accelerated by the terrain and the shape of the island. The rougher the ground, the more disturbed the wind.

Radio waves are reflected and refracted; this allows us to use our mobile devices in the city and inside buildings even when the transmitting base station is not directly in view, termed line-of-sight.

If you listen to FM radio stations in the car, as I do, you will occasionally hear the signal become distorted and go to static, most commonly when a large truck or bus pulls alongside. This is not always the loss of the radio signal due to the bus shielding us, but the result of two waves interfering and cancelling each other out. The direct signal from the radio station (the incident wave) is interfered with by another wave from the same radio station that has been reflected off the large object (the bus or a building), as each radio wave travels a different distance. Usually the reflected wave travels further than the direct wave.

The result is that the signal strength will increase when the waves are added together (in phase) but will subtract from each other when they are out of phase – called destructive interference.

Knowing what waves will do and how they interact allows us to better understand when and where conditions may be unsuitable, or allow us to pick the best time to approach a bar or harbour entrance.

Similarly an understanding of how radio waves interact may explain why we may get reception where it should otherwise not be possible.

John Tylor

Pantaenius Sailing
M.O.S.S Australia
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