NONRANDOM BEHAVIOR IN FIELD WAVE SPECTRA 

 AND ITS EFFECT ON GROUPING OF HIGH WAVES 



by 



Edward F. Thompson 



1. INTRODUCTION 



1. Hypotheses. 



This study is aimed at several basic assertions about the physical char- 

 acteristics of ocean waves. The study is designed to test the following six 

 hypotheses about the nature of waves : 



(a) Spectral components are sometimes discrete and are not 

 smeared over a broad continuous spectrum. 



(b) Spectral components are sometimes related in a deterministic, 

 nonrandom way. 



(c) The detailed spectral shape may be partially explained by the 

 theory of Benjamin and Feir (1967). 



(d) Waves in deep water tend to be organized so that high waves 

 occur in groups. 



(e) The modulation period of wave groups is sometimes related to 

 the period and steepness of the waves. 



(f) The extent of grouping in each time series and the modulation 

 period are related to certain features of the spectrum. 



Most of the hypotheses are in conflict with commonly held conceptions 

 of ocean waves, including the notion of a random wave field represented by a 

 continuous spectrum with random-phase relationships between components. 



2. General . 



Groups of high ocean waves are an important warning signal to an engineer. 

 Grouped waves represent a more severe condition than ungrouped waves for many 

 coastal and ocean engineering endeavors. Wiegel (1964) observed that "It is 

 the groups of several periodic waves, which are almost always the highest 

 waves in a wave system, that are the most effective in causing structural 

 damage." Groups of high waves may be a major cause of ship capsizing. 



The spectacular collapse of part of the massive breakwater at Sines, 

 Portugal, during attack by waves lower than the design condition (Zwamborn, 

 1979) has recently stimulated interest in the effect of high wave groups on 

 the stability of breakwaters. A laboratory study has shown that groups of 

 high waves are more damaging to a rubble-mound structure than ungrouped waves 

 of the same heights (Johnson, Mansard, and Ploeg, 1978). 



Wave groups have considerable engineering importance beyond the implica- 

 tions of several high waves occurring in succession. The existence of groups 



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