Wind generated waves are created by an imbalance of pressure and friction forces, caused unsurprisingly by wind. The waves are generated from energy transferred from the air to the water and, as current theories suggest, have their speed controlled by the relationship between wavelength and water depth, and despite this maths being pretty solid wind waves remain fairly unpredictable hence why the BBC does not include sea state in its shipping forecasts.
Swell on the other hand is akin to the ripples made when a stone is dropped in water. They are initially created by winds at its source, but depending on how strong those winds are they can persist long after the wind has ceased, whilst the swell remains fast moving and extending far deeper than the waves originally produced.
Wind waves can often be superimposed on swell too; which poses a big issue to yachtsman in open seas as it makes handling difficult and dangerous even cargo ships take a pounding!
Areas where the relationship between swell and wind waves, as well as ocean area, bottom depth, and the degree of land enclosure is most volatile can be exceptionally dangerous, with the Raz de Sein, the Alderney Race, the Swinge and Portland Bill being just a few of the nearest rough seas created by this aquatic cocktail.
Knowing Whether to Weather a Storm In one of our recent blog posts we talked about having the "Weather Eye" and learning to watch the weather at sea. Whilst oceanic phenomena like huge wave height, current and water depth, weather is an unpredictable beast that can turn even calm waters into something vicious. When the difficulty of rip tides is matched with the loss visibility and the high winds of a squall, the rapidly building seas can cause panic and very rash reactions.
Whilst predicting the weather is hardly a perfected art, knowing which oceans will be at their worst when you're out on their waters is rather easy to grasp. I saw this once when entering Dover harbour in a not very seaworthy motor boat in a force 6 wind. Very unpleasant. The picture here, taken in Bayona, shows a line of waves coming in from left to right.
At the bottom, waves are reflecting off the wall and crossing the incoming waves. Of course, this is a mini example but the same happens on large scales and can create very rough and disturbed seas.
The first complication is that swell and wind waves, being caused by winds of varying strength are of varying sizes travelling at different speeds. Faster moving waves catch up smaller, slower ones and either combine to produce larger waves or cancel each other out. There are statistics that give some support to the old rule of thumb about every seventh wave being a big one. The second complication is that wind waves are often superimposed on swell. The combination can be critically important to cargo vessels.
Certain directions of swell or waves and size of total sea can cause deck cargo to be washed overboard or grain to be shifted dangerously. A ship in ballast in a big sea can suffer damage due to pounding. For a yacht, such effects can make handling difficult and dangerous.
The larger the wave, the deeper down will the wave action occur. As waves move from deep water to shallower the depth of the sea bed starts to restrict the wave action. The waves slow down so that the energy of movement kinetic energy is reduced. The energy released causes a steeper sea ie higher waves relative to the wavelength. A marked increase in roughness and much breaking of waves is likely to occur.
Similarly, with waves running into a narrower stretch of water. Where there is a very marked change in water depth or where there is a very uneven sea bottom then, with some tidal states, overfills and whirlpools can occur.
Such areas are marked on navigation charts, Obviously, they should be treated with extreme care and respect. Near headlands, such as Start Point or through narrows such as the Needles Channel, wave energy over a wide front is forced to go through a narrower one.
Coupled with a shallowing of the water, the result can be some very rough and unpleasant conditions. There are various ways to view the physics of this phenomenon. The scientifically correct way is to look at the movement of the water particles within the wave.
Perhaps an easier, more intuitive, way is to realise that a stream or current going in the same direction as the wave train will increase the wavelength relative to the height of the waves. The effect will be a smoother sea. If there is a stream in the opposite direction to the waves, then the wavelength will be decreased relative to the wave height. The waves will become steeper and they will start to break.
Perhaps, more correctly, it is a question of wave direction with or against the tidal stream or current. The three effects of tidal stream, coastal topography and sea bottom can come together. Such well known danger areas as the Raz de Sein, the Alderney Race, the Swinge, Start Point and Portland Bill are well known to British sailors as places to be avoided in wind against tide conditions.
The combination of very strong tidal current and rough water can render a small vessel dangerously unmanageable. Anyone who has spent the night in even a gentle swell will know how unsettling it can be. That nicely sheltered bay seemed fine when you dropped the anchor in a gentle wind. You knew that it would drop at night but you forgot the swell.
A little swell that might have been formed some distance away can become very obvious when the local wind has gone and the boat swings across the swell. What is so surprising sometimes is how it can come around headlands due to refraction. If the bay is small, then reflection off the sides can become all too obvious as well. We have found this to be a particular problem in the Mediterranean around the Balearics. Here there are many delightful small coves seemingly well sheltered from wind and swell.
But, the Mediterranean is a sea very prone to a confused swell. This is probably due to two factors. First there is the geography with many coast lines and islands. Secondly, there are the various wind regimes. A swell running along one coast of Mallorca can bend right around the corner and affect the adjacent coast in a most surprising manner. A diver hovering underwater moves in a vertical circle as each wave passes. But when the water is moving bodily in the opposite direction, the wind-driven waves are effectively slowed so the energy transforms into shorter, steeper, higher waves — this is the wind-over-tide situation.
When the wave height to wavelength ratio is around , the waves break, forming overfalls. The converse is also true, but we tend not to notice. In water shallower than half the wavelength, the seabed starts to interfere.
It slows the waves down, and again they pile up, getting shorter, steeper and higher. An underwater reef offshore can cause a bigger wave than usual to rear up, apparently out of nowhere, and break, sometimes with tremendous violence. Even the edge of the continental shelf, metres down, is less than half the wavelength so it affects the sea state metres horizontally is only a good golf shot, after all and this is one reason for the reputation of the Bay of Biscay.
As the River Arun flows out of Littlehampton against a gentle southerly onshore breeze, the wind-over-tide effect is clear. A river mouth bar provides the setting for the worst of both worlds — outgoing stream meets onshore waves over a shallow patch — while the combination of an irregular bottom, strong tide and exposure to heavy seas can be spectacular.
In places like Corryvreckan, the Pentland Firth and Portland Bill, even in the absence of any wind or swell, the tide by itself creates a disturbed sea — a race — and there may be standing waves, which rear up continuously in the same place and can be almost wall-like. In the foreground we can see a clapotic wave, which has reflected off the wall of Dawlish train station and collided with an incoming wave. Waves impinging on cliffs with deep water at their foot tend to bounce back, and the result is a jumbled and chaotic sea state of dancing peaks and hollows.
Rough and choppy water, strong currents such as those that can occur during bad weather and spring tides and dumping waves inspire thoughts of adventure, but they can quickly sap even the most experienced sea users of energy. If you feel conditions change while in the water, err on the side of caution and get out until they are calm enough to go in again. Home Safety Know the risks Waves.
Waves are great fun, but they can be dangerous. Understanding how they work will keep you safer. Understand waves Powerful breaking waves have the potential to bring out the big kid in all of us.
Size and power The size and power of a wave is influenced by three main factors: how strong the wind is how long it has been blowing how far the wave has travelled known as the fetch.
So as a basic rule of thumb: south-east winds have a shorter fetch and cause smaller waves south-west winds have a longer fetch and cause larger and more powerful waves. Spilling waves Spilling waves are softer and more consistent waves that break gradually as they approach the shore. Dumping waves Dumping waves break powerfully in shallow water and should be avoided.
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