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SKYSAIL NAVIGATION AND SEAMANSHIP SKILLS CHARTS - including Weather, Course to Steer, EP, Tides, etc
Wave behaviour - what affects Wave height?
|Hokusai - The Wave||Portland Race on a bad day||Chichester Bar|
Five factors affect the growth of wind waves. First, the wind speed must be blowing faster than the transfer of energy from wave crest to wave crest. The second factor is the amount of time the wind blows, or wind duration. The third factor is the fetch, the uninterrupted distance over the sea for which the wind blows without a change in direction. In the Solent, for instance, the fetch is limited by the surrounding coast of the mainland and the Isle of Wight and is rarely more than 10 miles. At sea it can be thousands of miles.As waves enter shallow water their speed decreases, wavelength decreases, and height increases. Waves therefore tend to break in shallow water, for example over a bar at the entrance to a harbour. If the tide direction is against the wind, this will also increase wave height and decrease wavelength. Shallow estuaries and harbours such as Salcombe, Chichester and Carteret will experience large waves in an strong onshore wind, particularly with a Spring ebb tide, and must be avoided in such winds.
So in total, wave height is affected by:
Plus Swell (see below)
Surfers take a big interest in waves - lots of good background and up to date wind and wave conditions here: http://magicseaweed.com
Yacht 'Noblesse' entering Dunbar harbour on a calm day. The swell creates a breaking wave as the water shallows. For a yacht of beam 3 metres, a breaking wave only need to be 3 metres high to capsize the boat if it is beam on to the wave.
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The famous sequence of the Golden Gate capsize
Huge waves in the Raz de Sein - Mozart's Requiem is appropriate....!
The swell may also be severe more than a full day after a strong wind. Swells are waves produced by distant wind and come in at a higher period (longer wave length) than waves produced by the local wind. Usually they will also be at an angle to the local waves. Swells may be high but are usually not steep - unless they encounter shallows or a sea wall. The combination of swell and waves can be a very confused sea. After the passage of a cold front, the big veering wind shift can produce such a confused sea as the new wave train interferes with the old one.
Waves in the Eastern end of the English Channel where the Channel becomes more narrow and shallow, are steeper and shorter than in the West. This is known as a 'chop', also present in the Solent.
Overfalls and Tide Races
A strong wind against a strong tide can create large steep waves known as overfalls, marked on charts with wavy lines. They are most common off headlands where the tide accelerates, in narrow channels such as the Needles and the Little Russel, and over shallow rough seabeds such as off Start Point and Portland Bill. Where these factors combine, waves are particularly violent and confused with no pattern, and such areas must be given a very wide berth. Pilot books will give good advice.
Approaching St Albans Race In the middle
St Albans Race - on a calm day, motoring, no wind, spring tide
of FETCH on Wave Height
Fetch = distance of open sea for the wind to blow unaffected by land.
posted 20th March 2011