Analysis of Interfering Fully Developed, Colinear Deepwater Waves

The sea surface is normally irregular as a result of dissimilar waves generated in different areas. To describe such a sea state, various methods have been proposed, but there is no general consensus as to the best characterizing parameters of the interwaves. Three simple methods are proposed here to calculate a characteristic interwave period, length, and height for fully developed, colinear deepwater waves. The results of this study indicate that the interwave period and length are equal or very close to the period and length of the dominant component wave, irrespective of the periods of the subordinate waves. In cases where the dominant wave period is double or more than double the periods of the subordinate waves, the wave period, length and height are within 4% of the dominant wave parameters, so that such interfering, irregular waves have virtually the same characteristics as monochromatic waves. Secondary, individual interwaves propagate at the velocity of the component wave with the shortest period, that is, slower than the primary interwaves which have the same celerity as the dominant component wave. 1. Introduction Waves are generated by wind in different areas and may be dissimilar in period, length, height, and celerity. Where such waves converge, interference takes place so that their crests are reinforced or diminished depending on whether crest-crest or crest-trough interference occurs. For ocean and coastal engineers, it is important to understand such interactions, because the design of structures naturally has to consider the peak conditions arising from them. Wave interference also influences sediment transport as well as coastal and coral reef erosion. Complex wave fields arise because of different stages of wave development and water depths (both of which result in significant changes in the individual wave profiles), as well as the fact that waves may come from different directions. Modelling all the possible combinations resulting from these differences would be a very demanding task, so that this paper focuses instead on the relatively simple situation of interfering colinear waves with Airy characteristics. Such sinusoidal waves are typical of deepwater conditions where the wind has blown for a sufficient period of time over a long enough fetch for them to become fully developed. This would be the case for synoptic-scale (100–1000？km) pressure systems developed over the deep ocean, where there is no interference from bathymetric and land effects and where waves generated upwind have longer wavelengths than downwind waves, thus