5 . State-of-the-Art for Coastal Engineers . 



This review shows gaps in the knowledge of wave-current 

 interaction, but there are significant areas of understanding, 

 particularly with regard to refraction. 



For engineering design, the wave height is usually the most 

 important wave variable. For sites with sandy beaches or small- craft 

 harbors, wave direction may be equally important. If the currents are 

 known seaward of a particular shore, it is not possible to say for the 

 general case whether wave heights reaching that shore will be increased 

 or decreased by their interaction with the currents. There is limited 

 experience in calculating wave refraction across any but the simplest 

 unidirectional currents. 



To the extent that field conditions satisfy the assumed conditions 

 of large-scale, parallel, shearing flows treated in Section II, 5, the 

 corresponding solutions in Section II, 5 can be used to predict wave 

 behavior. For the nonshearing case where current velocity is constant 

 across the flow and the waves travel in the same direction as, or 

 opposite to, the currents, the effect is straightforward: following 

 currents reduce wave heights, opposing currents increase them. But for 

 shearing currents over constant depth with waves oblique to the flow, 

 the wave will have a minimum height while on the current when the 

 incident angle between wave direction and current direction is approxi- 

 mately 45°. Any refraction away from that direction increases wave 

 height. Minimum wave steepness occurs when this angle is approximately 

 30°. Refraction away from this 30° direction increases steepness. This 

 increase in steepness may be sufficient to cause the wave to break while 

 it is on the current, in which cases the wave will emerge from the other 

 side of the current with a lower height. However, if it does not break, 

 the wave will emerge from the other side of the current with the same 

 height and direction it had on entering the current, only displaced 

 downsteam from the path it had on entering the current. For a synoptic 

 picture of what happens to a wave entering obliquely to the current, see 

 Figures 7 and 8 of Peregrine (1976). 



In most cases, the currents of engineering interest in wave-current 

 interaction are due to the tides. Tidal currents are usually relatively 

 uniform with depth, which simplifies somewhat the complicated 

 possibilities in the interaction. Since tides are usually reversing 

 currents, it is likely that one direction will be more critical than the 

 other for the engineering project. Conservative design requires that 

 this more critical case be identified and used to establish design 

 criteria, but there is not now any general rule on establishing this 

 critical direction. In this context, it would be useful to examine 

 time series of wave measurements from nearshore sites adjacent to 

 substantial reversing tidal flows. 



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