wavelengths, the distance depending on the velocity gradient. It is 

 thus likely that circumstances in which currents are stopping waves, 

 some appreciable reflection may occur. Waves breaking under conditions 

 in which a partially reflected wave exists tend to have steeper slopes 

 on the back side as breaking occurs and to project water with a greater 

 vertical component of velocity. 



Wave breaking has a major effect on currents. All the momentum 

 that is lost from the wave motion is gained by the mean current. This 

 can be described more precisely for a quasi-steady breaking wave like a 

 spilling breaker (Peregrine and Svendsen, 1978). The momentum lost by 

 the wave in the breaking region spreads like a turbulent wake behind the 

 wave. This has been confirmed by measurements (Stive 1980; Battjes and 

 Sakal, 1981). In relatively shallow water, this soon becomes a uniform 

 change to any preexisting current. In deeper water, or where the 

 breaking is weak, it leads to a velocity shear in the upper layers (see 

 Fig. 10). Shear in the surface layers of water has a strong influence 

 on wave breaking. 



Phillips and Banner (1974) and Banner and Phillips (1974) examine 

 the effects of a surface wind drift layer on wave breaking. Wind drift 

 is referred to as that part of the wave drift which is not Stokes drift 

 due to the wave motion. Much of this wind drift is due to wave breaking 

 from capillary waves up to the largest waves. Phillips and Banner show 

 that if the drift is in the same direction as wave propagation, as is 

 usually the case, then the surface layer cannot ride smoothly over large 

 waves. There must be some breaking, at least on a small scale, well 

 before waves reach the maximum height predicted by irrotational theory. 



It is worth noting the results of calculating a nonlinear solution 

 for periodic traveling waves on a current with a uniform shear in the 

 vertical (Tsao, 1959; Dalrymple, 1974a; Brink-Kjaer, 1976; Brevik, 

 1979). If the shear is positive, i.e., the greatest current in the wave 

 direction at the surface, then the waves have sharper crests and flatter 

 troughs than the corresponding waves on a uniform current. This is the 

 case for the wind drift example above and might well indicate a lower 

 maximum amplitude and greater tendency to break (see Fig. 11). 



If the shear is in the opposite direction, then the waves become 

 more rounded than waves on a uniform current. Such waves may grow 

 larger and are less likely to break (see Fig. 12). An extreme example 

 is the large-amplitude "surface shear wave" described by Peregrine 

 (1974), which occurs on fast flowing streams, in a form also known as a 

 "wave" hydraulic jump, and in backwash on beaches. 



The extrapolation to breaking properties of these waves is tenta- 

 tive, but the work of Banner and Phillips and the example of the surface 

 shear wave, which can have a vertical face without breaking, support 

 these suggestions. A possible partial explanation for the relative lack 

 of breaking over rip currents is provided. The velocity shear delays 

 breaking while the onshore drift and breaker wakes along the rest of 



53 



