European Coasts - An Introductory Survey
Chapter 2: The impact of the sea

2.2 Waves (2)

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Sea area Hs range chance of occurrence
Baltic

North Sea

Irish Sea

Atlantic Ocean,
west of Ireland
Gulf of Biscay

Atlantic Ocean,
west of Spain
Medeterranean W

Medeterranean E
 2-3 m
5-6m
2-3 m
9-10 m
2-3 m
9-10 m
3-4 m
11-12 m
3-4 m
10-11 m
3-4 m
11-12 m
2-3 m
7-8 m
2-3 m
7-8 m

185/1000
3/1000
211/1000
2/1000
246/1000
2/1000
205/1000
2/1000
160/1000
1/1000
197/1000
1/1000
178/1000
3/1000
171/1000
2/1000
Table 2: Significant wave heights at different locations in Europe. Both a common wave height range and an extreme wave height range have been given (From Global Wave Statistics, 1986) Fig 40: High wave attack and wave run-up on a rocky coast 911/1/87, Amalfi, Italy) Fig 41: Wave attack on the Boulevard at Vlissingen (Flushing, the Netherlands)

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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