Summary 



A system of four instruments was used for the observation of the bottom 

 boundary layer: a two-axis backscatter fiber-optic LDV, a pressure sensor, a 

 four-axis acoustic current meter BASS, and a light-scattering sensor OFS. 

 They measured, respectively, velocity and sediment concentration in the near- 

 bed region, wave height, velocity in the irrotational part of the wave motion 

 1.4 m above the bed, and sand-grain and suspended mass. The LDV was 

 placed on a profiler to obtain vertical flow structure. The BASS was mounted 

 at a fixed height above the bed. Of the two OFS used, one was mounted to 

 the LDV sensor head and profiled with the LDV and the other was fixed at a 

 height of 1.2 m above the bed. 



Using conditional sampling techniques, the phase-averaged and turbulence 

 properties such as the wave-induced velocity component and the longitudinal 

 distributions of the rms turbulent intensities were obtained. These results 

 allow the study of the distribution of turbulence in the water column. 



In addition to the velocity measurement, the data rate of the LDV was pro- 

 portional to the concentration of sediment in the water column as a result of 

 the high threshold setting. This information is used to construct the concen- 

 tration profile very close to the bed. The concentration and turbulence inten- 

 sity profiles in the lowest few centimeters were found to be nearly uniform, 

 indicating the presence of a constant-flux layer. 



The data set is especially unusual in the sense that velocity and concentra- 

 tion profiles are truly "simultaneous" and measured by a single instrument — 

 the LDV. These data are the first measurements of sand grain flux in the 

 boundary layer under full-scale conditions. 



Recommendations 



Results from the LDV measurements, as summarized above, represent the 

 most promising database for the study of turbulence properties in the wave 

 boundary layer and the detailed distribution of suspended sediment very close 

 to the sediment-fluid interface. In the next stage of data analysis, it would be 

 useful to compare turbulence results with the prevailing theoretical models of 

 turbulence generation in the wave-induced bottom boundary layer, and to 

 compare sediment concentration and flux results with sediment suspension 

 models. The specification of absolute value of a reference concentration at the 

 bed is thus within reach. Future work will require application of suitable 

 factors and corrections to the concentration data presented here to reduce the 

 present results to mass concentration. 



Simultaneous measurement of concentration and particle motion has permit- 

 ted deducing the flux of sediment in the boundary layer. These data are 



Chapter 1 1 LDV in the Bottom Boundary Layer 



231 



