I saw this once when entering Dover harbour in a not very seaworthy motor boat in a force 6 wind. Some remains to bounce back and create a very confused sea. Not all the energy is dissipated when the waves hit the wall. Reflection occurs when waves meet a solid barrier such as a harbour wall. The result is that the whole line changes direction. Those nearest the left hand of the line take short steps and, therefore, march slower than those further out who take longer steps. The effect is rather like a line of soldiers given the command to "Wheel Left". The result is a rotation of the wave train. ![]() The waves nearer the shore in shallow water travel slower than those further out in deeper water. The most obvious example is when, on the beach, we see waves approaching the beach nearly parallel to the shore when the wind creating the waves is along the coast. Rather like light and sound waves, water waves can be subject to refraction. However, large, slow sea level rises and falls occur as the very flat waves arrive. By that time the swell wavelength is so long that wave crests cannot be detected. Strong winds down the east coast of South America can create swell that reaches Nigeria. ![]() They are particularly evident around western coasts of the British Isles, France and Spain. Many of us are familiar with the long rolling swells that come in from the Atlantic. ![]() The very large amounts of energy transferred to the sea results in swell waves being fast moving and extending far deeper than the waves produced locally by the wind. Experienced officers of the watch on the bridge of a ship can often report swells from two or even three directions, of differing periods and amplitudes. Trains of swell waves can arrive from more than one direction. The wind waves will have been caused by the local wind ahead of the front, while the swell is being caused by the wind behind the front. A swell coming from a different direction to the waves and swell generated by the local wind may indicate that a front is approaching. The wind may cause wind waves superimposed on the swell but, often, the wind will have died away or changed direction so that wind waves may well be across the swell. Thus, the wavelength of swell increases with distance away from the source but the height, or amplitude, decreases relatively slowly.īecause of their length in relation to their height, swell waves do not break even if the wind is still blowing over the swell. This cascading energy effect will be recognised by those with a background in physics. The longer that the wind blows in the source area, the longer will the swell persist, even long after the wind has ceased or changed direction.Īs these waves move away from the source area, energy is transferred from the short wavelength, high frequency waves to longer and longer, lower frequency waves. The stronger the winds at the source area, the bigger will be the swell and the further will it travel. These waves that move away from their source are known as swell. This energy travels downwind away from the source area, rather like ripples in a pond when a stone is dropped in, or from the bow wave from a ship. Wind waves are a local manifestation of the energy that has been transferred to the sea from the wind. It can be a hazard at some harbour entrances and, in those cases, swell forecasts for the coasts affected are valuable. Swell originates beyond the areas of local forecasts and varies depending on its source, so it is hard for a sailor to predict. ![]() It is my belief that sea state near the coast cannot usefully be predicted on the space/time scales of an inshore waters or offshore forecast. As in all of sailing, predictions will improve with experience gained and lessons learned. A sailor should be able to predict, qualitatively, how rough the sea is likely to be using a local wind forecast, tidal atlas and charted warnings of overfills and eddies. Sea state depends on wind, current, coastal topography, the depth and nature of the sea bed. Here, I try to explain how waves and swell are formed and why sea state can be so variable in space and time. I have been asked why the UK does not include swell height in its marine forecasts. There are requests from time to time for more detailed forecasts of sea conditions.
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