European Coasts - An Introductory Survey Chapter 2: The impact of the sea 2.2 Waves (2) |
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Irregular waves
Though a useful instrument to describe the physics of water waves, the sinusoidal wave theory does not account for the irregularity of the real sea surface. This irregularity is described with statistics: stochastic
variables represent the water surface. Stochastic wave descriptions play an important role in coastal
engineering.
The most common stochastic variable used in coastal engineering is Hs, the significant wave height. It is the mean height of the highest 1/3 of the waves. One single value of Hs represents the measured wave heights in an area during several hours of similar weather conditions. It characterises short term conditions and may be used to estimate the short term distribution of individual wave heights. A theoretical distribution which often adequately gives the short term distribution of the wave heights is the Rayleigh distribution (see Fig. 39).
Apart from this short term distribution of the wave heights, a second statistical instrument is needed which describes the long term distribution of the waves. It gives the probability of a sea state characterized by a single Hs value. Examples of observed long term occurrences are given in Table 2.
The large impact of storm waves on the coastal area is illustrated in Fig. 40 and Fig. 41.
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