Internal waves propagating on density variations, such as the 

 thermocline, have their own current field. Surface waves interact with 

 these currents. The interactions provide a surface trace of internal 

 wave motions, and also provide a system convenient for analysis and 

 experiment . 



Experiments performed in laboratory flumes give another class of 

 currents which are only represented on a prototype scale by flows in 

 artificial cuts or channels. The level of turbulence and the magnitude 

 of secondary circulations are aspects of these flows which are rarely 

 considered but can affect experimental results. 



3. Typical Examples of Wave-Current Interactions. 



The bulk of this review considers rather idealized problems such as 

 unidirectional currents, inviscid and laminar flows, etc. This is 

 because a complex natural situation can be interpreted with the help of 

 simple examples, each of which makes some contribution either toward an 

 observer's physical intuition or toward a mathematical model which 

 combines these simple elements to form a more complete picture. 



Waves are usually generated by the wind. Currents change the 

 effective wind because the relative velocity between the air and moving 

 water differs from that between the air and the fixed bottom. Once 

 formed and freely propagating, the waves are refracted by currents they 

 meet as well as by variations of water depth. Near coasts, where 

 current gradients often increase, refraction may be stronger. The scale 

 of currents can become so small that refraction may be an inadequate 

 term to describe the interactions. (Diffraction might be a better word 

 but it is not always appropriate.) For example, rip currents are 

 usually no wider than a wavelength, and shear layers shed from obstacles 

 are also relatively thin. 



In all such current systems, the ability to predict basic wave 

 properties (period, wavelength, amplitude, and direction) is desired. 

 For design purposes, these properties are the input for estimating 

 stresses or forces; for example, shear stress at the bed to estimate 

 sediment transport or the stability of bed protection, and forces and 

 moments on structures and vessels to establish design criteria. 



Neglect of a current can lead to inaccuracies in interpreting field 

 data. This is especially true where measurements near the bed are used 

 to predict surface properties or vice versa. 



The stronger currents around headlands or through passages lead to 

 tide rips (tide races with steep irregular waves), a prominent example 

 of wave-current interaction. Navigators have known for centuries that 

 these areas can have extremely rough seas, even in otherwise fair 

 conditions and hence are best avoided. There are numerous recorded 

 examples. An aerial view of the Humboldt Bay Entrance during an ebb 

 current (Johnson, 1947) shows how an opposing current augments wave 



