However, for many problems, other considerations lead to different 

 time and length scales. For waves, their coherence could be relevant; 

 i.e., the time scale of a group of waves may be appropriate. In 

 propagation problems, the length of the wave's path and its duration may 

 be more important. For example: the time scale of a current might be 

 represented by: 



T^ = l'^maxl/|9'^/9t|max <1) 



where u is the current velocity. For semidiurnal tides T^ = 12/27rs2 

 hours. Thus, if waves are propagating over tidal currents for more than 

 an hour, the unsteadiness of the current needs to be considered. 



A current is large scale if 



T, » T and L, = lu^^J/IVu^^^ » L (2) 



This is often the case. The term small-scale currents will be used for 

 the cases T^ =T and L^, = L, as well as T^, << T and L^, << L. Little 

 work has been done on small-scale currents, so the bulk of this review 

 covers large-scale currents. 



In detailed applications concerning flow past structures or over bed 

 forms, other scales become important — in particular the amplitude of 

 water particle excursion due to the wave motion compared with a typical 

 length, or the magnitude of wave-induced water velocities compared with 

 currents . 



In some applications, the knowledge of water wave properties in the 

 absence of currents is still inadequate. This is particularly true of 

 sediment transport and wave forces, the applications of greatest concern 

 to coastal engineers. Because of the balance of the present 

 understanding, this review is weighted toward wave properties rather 

 than their effects. 



2. Effects of a Horizontally and Vertically Uniform Current. 



If a current is perfectly uniform, i.e., if it has the same 

 direction and magnitude over a wide area and at all points from a 

 horizontal bed to the surface, then the current is equivalent to still 

 water viewed from a reference frame moving with the current velocity. 

 If there are water waves on the uniform current, then the apparent speed 

 of the waves will depend on the motion of the observer's reference 

 frame. Proper choice of the reference frame can simplify the analysis 

 and improve interpretation of observations, without changing the 

 physical properties of the waves. As an analogy, the transient passage 

 of a ship viewed from shore suggests a complex series of waves, but when 

 viewed from the deck of the moving ship, the wave pattern becomes 

 stationary and simpler to understand. None of the wave's physical 

 properties are affected, but perception of the wave field changes. 



13 



