3.5.1: Locally generated sea
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Waves are generated by local wind fields. At the area of wave generation these waves are relatively steep and short-crested. The latter means that there are no distinct wave fronts because the waves are irregular and directional. An example is given in Fig. 3.9. In this photo white-capping can also be seen, which is the steepness induced wave-breaking when the wave height becomes too large as compared to the wavelength (\(H/L > 0.14\)).
The wave characteristics (height, period, propagation direction) and their duration depend on the characteristics of the wind field (speed, duration and direction), the fetch and the local water depth. Fetch is the maximum length of open water over which the wind blows, which is determined by meteorological and geographical conditions. Generally, the higher the wind speed and duration the larger the wave height and period. But only for ideal cases, wave heights can be estimated on the basis of wind velocity, duration and fetch.
Although wind conditions cannot be predicted accurately long in advance, wind conditions can be described statistically. The wind climate consists of both velocity data and directional data. Velocities can be expressed by wind speed (when measured) or as a certain number on the Beaufort scale (when visually observed, see Intermezzo 3.2). These data can be found in meteorological yearbooks and in various atlases. As to the latter, reference is made to specific hydrographical atlases that contain data collected at sea.
The Beaufort wind speed scale (ranging from 0 to 12) relates wind speed to the local sea state using descriptors as wave height, wavelength, white capping, amount of foam and spray. Beaufort, a British naval officer, introduced the Beaufort wind scale in 1805. For tactical reasons the scale was intended to exchange objective information between sailing vessels of the British Navy. The lower scales (2 to 4) refer to sailing speeds of the common naval vessel of that time (man-of-war) under full sail. The intermediate scales (5 to 9) refer to conditions that required reefing of sail. The higher scales (10 to 12) deal with survival of ship and crew. The Beaufort scale is summarised in Table 3.2 in the form that is used at present. Bold printed expressions refer to the official terms of the (WMO). Pictures of typical sea states at various Beaufort wind speeds are available to assist observers on board of sea-going vessels (see also http://en.Wikipedia.org/wiki/Beaufort_scale).
Parameterised wave spectra have been formulated which relate the wind field to spectral density and can therefore be used to hindcast (estimate past events) wave parameters from known wind fields. An example is the Joint North Sea Wave Observation Project (JONSWAP) spectrum which is characteristic for (developing) wind sea in oceanic waters. A typical JONSWAP spectrum was shown in the middle panel of Fig. 3.7. It can be determined from the wind speed and the fetch. For a fully developed sea (not limited by either fetch or duration, which in reality will hardly happen) the so-called Pierson-Moskowitz spectrum is valid. This is a broader spectrum and only dependent on the wind speed.