2. Types and Locations of Currents . 



Currents important in wave-current interaction include tidal 

 currents, ocean currents, local wind-generated currents, river currents, 

 and wave-generated currents, including currents associated with wind 

 waves and internal waves. In addition, some special laboratory currents 

 are important for developing insight and testing predictions of theory. 



The most regular and predictable currents are the tides, and on most 

 areas of the Continental Shelf, and in many coastal inlets, these are 

 the most significant currents. Regularity of the tides means that in 

 most areas observations already exist for predicting the current regime. 

 Even in a poorly documented area, 1 month's observation can give a 

 reasonable basis for the prediction of currents. The unsteadiness of 

 these currents has a significant effect on waves propagating over them. 



Some ocean currents and riverflows are as regular as tides in their 

 behavior; currents generated by local winds are less regular. Surges 

 caused by severe storms have surface elevations and currents similar to 

 tides. For all these cases, reasonably satisfactory estimates of large- 

 scale current fields can be made with numerical models; e.g. , Peregrine 

 (1981b) describes work on surges and other currents off Northwest 

 Europe. 



It is important to note what information may be required about 

 currents. If the prediction of wave properties is to include refraction 

 through tidal currents, as for example in Barber (1949), then the 

 current field is required in the region across which the waves 

 propagate. A numerical model of the current field is of value in this 

 type of example. On the other hand, if local wave conditions are 

 already known, and it is desired to predict local forces, then only the 

 local current is needed. 



The variation of current with depth is important in many applica- 

 tions. A vertical velocity profile arises both from friction at the 

 bottom and from wind stress at the free surface. 



Often the most important currents are those local to the site in 

 question. These can include strong nonuniformit ies, such as thin shear 

 layers and eddies behind headlands, breakwaters or other projecting 

 structures; the flow around a structure such as a pile or floating 

 vessel; or rip currents from a beach. The last example is a wave- 

 generated current; such currents are not discussed in detail here al- 

 though they are related to the subject of this report. 



In almost all currents there is a greater or lesser degree of 

 turbulence. The "turbulence" of oceanic eddies clearly has an effect on 

 waves different from the bottom-induced turbulence of a shallow current 

 because of the large difference in scale. 



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