used a laser Doppler anemometer. Their interpretation of the measure- 

 ments, as they apply to sediment transport, is repeated verbatim: 



The entrainment of sediment under flat bed condi- 

 tions can be related to the predicted instantaneous 

 shear stress. However, although the entrainment of 

 material from the bed can be considered to show a 

 considerable increase under the combined action of 

 waves and currents, the distribution of turbulence 

 intensities suggests that the zone of diffusion 

 would not increase. In fact, the results indicate a 

 reduction in boundary layer thickness [when waves 

 are added to a current.] One might expect, there- 

 fore, that there would be an increase in sediment 

 concentration in the near bed region. In the light 

 of Nielsen's (1979) observations, this distribution 

 would change dramatically under spilling breakers, 

 the material rapidly dispersing over the whole depth 

 of flow. 



The greater turbulent stresses found when waves are 

 superimposed on a current are likely to result in a 

 considerable increase in sediment pickup from a 

 rippled bed. While the increase in turbulence is 

 limited to a region within 6 or 7 roughness heights 

 of the bed with a tendency for this zone to decrease 

 with wave height for a constant wave period, it is 

 to be expected that sediment brought into suspension 

 by the nearbed vortex action will be diffused over 

 the zone of the current-induced turbulence. This 

 could result in significantly higher transport rates 

 as long as the increased bed shear stress is not 

 such as to prevent the formation of high bed 

 ripples . 



In the case of waves alone, the shear stresses at 

 the bed are of the same order as for combined wave 

 and current flow, but the vortex-dominated layer 

 extends only approximately four roughness heights 

 above the bed, and the only means of transporting 

 sediment is by relatively weak wave-induced mean 

 velocities. The limited thickness of the wave- 

 induced vortex layer over a rippled bed has 

 previously been noted by Tunstall and Inman (1975). 

 This suggests that sediment would be concentrated in 

 this near-bed layer. 



It has long been recognized that sediment transport by waves is 

 enhanced by currents generated by these waves. The interaction of the 

 waves with a mobile bed can lead to complicated systems of currents, and 



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