0.15 u'. For LDV data, pressure records are used to define the reference 

 wave phase. Waves were divided into 10-deg (0.17-rad) slots for phase 

 averaging. Phase averages are similarly calculated for BASS data. 



Sediment concentration. The sediment concentration is obtained from the 

 number of sand grains crossing the sample volume of the LDV which produce 

 velocity realizations. The number of grains crossing the sample volume is 

 directly proportional to the sand grain volumetric concentration. However, in 

 order to obtain unbiased estimates of concentration as a function of height 

 above bed, it is important to recognize that the number of realizations itself 

 depends upon the concentration and the velocity. In other words, for a given 

 concentration, the volume of fluid swept past the sample volume is propor- 

 tional to the velocity of the fluid; therefore, the number of sand grains 

 crossing will be proportional to the velocity. The concentration is corrected 

 via normalization as: 



(11-4) 



222 



EN 



where 



n c = corrected number density 

 n = measured counts 

 v, = velocity realizations 



The magnitude is taken to ensure that the volume swept is always positive. 



Sediment Flux. The flux of sand grains is also obtainable as follows. 



^ |v f | 

 The quantity F « Yj — — represents the directional transit of particles; 



i v * 



i.e., each velocity realization is associated with the transport of a sand grain 

 in the direction represented by the sign of the velocity. Clearly, flux is 

 measurable in this manner and is obtained from the present data. Absolute 

 flux can be obtained by applying a multiplicative constant that includes sample 

 volume cross-sectional area, effects arising from grains grazing the sample 

 volume, and size of the grains. In SUPERTANK, sand grains were well 

 sorted, so the vertical distribution of F represents the profile shape faithfully. 



Sample time series 



Figure 11-4 shows an example of the near-bottom velocity measured by the 

 LDV under random waves (Run A2811A, Table 11-2). Under such strong 

 wave agitation, there are sufficient sediment particles suspended to serve as 

 the seeding for laser Doppler measurement. In this figure, the LDV-measured 

 longitudinal velocity u is shown as circles, and the solid curve is the surface 

 elevation. In comparison, the LDV velocity measurements under similar 



Chapter 1 1 LDV in the Bottom Boundary Layer 



