the beach accelerate breaking. The surface shear caused by winds may 

 explain why onshore winds are said to promote breaking and offshore 

 winds to "hold up" the waves. 



III. ENGINEERING APPLICATIONS 



The preceding sections deal either with the interaction between 

 waves and currents, or with the effect of depth changes on that 

 interaction. However, for engineering applications, interest lies in 

 the effect of the wave-current interaction on its surroundings: 

 structures, vessels, and seabed. The wave and current motions are 

 assumed given — by forecasting, tide tables, field or experimental 

 studies, simple prediction methods, or experienced judgment. The fact 

 that the waves and currents are not independent of each other, as 

 emphasized in preceding sections of this report, must be considered in 

 determining these given conditions. 



In addition to the interaction between waves and currents, there 

 may be an interaction between the flow and the structure itself. This 

 additional interaction is often overlooked, and even if observed, may 

 lie beyond the practical capability of present-day analysis. Cases 

 where the triple (wave-current-structure) interaction is important 

 include flow-induced vibrations, forces on large structures where 

 diffraction effects are important, and flow over loose beds, where the 

 flow determines the bed roughness which, in turn, affects the flow 

 itself. Fortunately, however, there are many cases where the added 

 interaction of the structure is of minor importance, and it suffices to 

 take only the wave-current interaction into consideration. 



1. Computer Programs for Wave-Current Interaction. 



In Section II, 7, general methods of solution of wave-current 

 interaction problems are described, but in most cases, the referenced 

 literature deal only with the depth refraction solution. Even where 

 wave-current interaction is covered, the reference usually concerns 

 either theoretical examples, e.g. Ebersole and Dalrymple (1980), or an 

 interpretation of one particular set of observations, e.g. , Forristall, 

 et al. (1978). 



To date , there are only two general-purpose computer programs which 

 are potentially suitable for computing wave propagation over currents In 

 an engineering project, and neither Involve the ray theory approach. 

 These programs are the Dutch Rijkswaterstaat program based on recent 

 work by Boolj (1981), and the Danish Hydraulic Institute's short wave 

 program (Abbott, Petersen, and Skovgaard, 1978). Both programs use 

 finite differences to approximate differential equations. 



The Dutch program (CREDIZ) is based on the parabolic approximation 

 of Booij's (1981) linear wave-current equation. It takes waves of one 

 frequency, and accounts for both refraction, by depth and current, and 



57 



