Van Oovtmevssen 



tures, the maximum wave condition is important; such a condition 

 usually is an irregular sea-state, consisting of high waves, among 

 which sometimes even breaking waves will occur. 



High regular waves are not sinusoidal any longer, the distance 

 of the crest to the still water level becomes greater than the distance of 

 the trough to the still water level. However, a steep regular wave can 

 always be split up into a number of harmonic components. From various 

 experiments the experience was gained, that the forces andpressures in 

 high waves can be found by summation of the forces andpressures, as cal- 

 culated for the different components according to the potential theory for 

 sinusoidal wave s of low amplitude. 



In non-periodic waves, as far as the linear phenomena are 

 concerned, force and pressure spectra can be calculated, departing 

 from the wave energy spectrum and the force and pressure response 

 functions. In such a statistic approach, no data can be obtained with 

 regard to drift forces. Since the magnitude of the drift force is pro- 

 portional to the square of the wave height and also dependent on the 

 wave frequency, this force is no longer constant in irregular seas 

 and is thus known as the slowly oscillating drift force which has a pe- 

 riod of oscillation in the order of magnitude of ten times the mean 

 wave period. For an estimation of the drift force a deterministic ap- 

 proach can be applied (see Hsu and Blenkarn [_8j and also Remery 

 and Hermans 9 J ). In this approach the point of departure is not 



the energy spectrum of the waves, but a record of the wave height 

 to a base of time, which can be obtained either by field measurements, 

 or by calculations, in which case one of the possible realizations of 

 a spectrum is generated by a computer. 



The wave record can be regarded as a sequence of separate 

 wave crests and troughs, each with its own period and amplitude. For 

 every part of the wave record the drift force can be calculated, re- 

 sulting in a record of the drift force to a base of time. The drawback 

 of this method is, that no indication is obtained about the chance of 

 exceeding a certain force. The maximum force, encountered in a 

 certain wave train, will differ from the maximum force in an other 

 wave train with the same energy distribution. 



No theoretical approach is available for the determination 

 of peak loads, which can occur in breaking waves. In j_10] Wiegel 

 gives a review of experimental work performed on this topic. Most 

 of the investigations were related to the phenomena which occur when 

 a wave breaks against a vertical barrier; a smaller part was concer- 

 ned with cylinders in breaking waves. From the laboratory tests with 

 vertical barriers it appeared, that when a breaking wave hits the 



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