agitation (H = 0.6 and 0.8 m), the turbulence intensity increases. Similarly, 

 the profiles under 8-sec waves with wave heights of 0.4 (o, Runs S0410A), 

 0.5 (x, Runs S0609A, S0610A, S0612A, S0614A), and 0.7 m (+, Run 

 S0409B) are shown with dotted lines. There are some common features 

 between these two sets of data. In the lower region, typically 2 to 3 cm above 

 the bed, the rms turbulence intensity is approximately constant, suggesting the 

 presence of a constant flux layer. Above this constant flux layer, the turbu- 

 lence intensity decreases upward. Data at the highest elevation are sparse and 

 hence have poor statistics so that they may be disregarded. The dimensionless 

 turbulence intensity, normalized by the amplitude of the wave orbital velocity, 

 ranges from 0.14 to 0.25 for the 3-sec wave conditions and 0.08 to 0.16 for 

 the 8-sec wave conditions. 



Sediment concentration 



The number of sand grains passing through the sampling volume 

 (Figure 11-6, symbol x) is directly proportional to the sediment concentration 

 and velocity. The concentration level appears to display a periodic distribu- 

 tion with maxima near the points of flow reversal (corresponding to the zero- 

 crossing points of the surface elevation or longitudinal velocity profile). 

 Presumably, this is a consequence of the nature of turbulence generation in an 

 oscillatory flow where the growth of instabilities at reversals is known to 

 produce high turbulence intensity. 



The vertical distribution of the sediment suspension is derived from the 

 integration of sediment distribution over the complete wave cycle. 

 Figure 11 -7b shows profiles of sediment suspension under 3- and 8-sec wave 

 conditions (the symbol notation is the same as Figure 11 -7 a). It is interesting 

 to see that in the near-bed region (z < 3 cm, where z is the distance above 

 the bed), the vertical sediment distribution is nearly uniform; above this 

 region, the concentration can be modeled by a power-law decay. These fea- 

 tures were also observed in the distribution of the turbulence intensity, as 

 shown in Figure ll-7a. As noted, the profile of Run S0314B (T = 3 sec, H 

 = 0.2 m) is distinctively different from the other runs. The turbulence inten- 

 sity of this test is approximately 0.8 to 1.4 cm/sec with very weak vertical 

 distribution (Figure 1 l-7a). The settling velocity of sand particles used in this 

 experiment is 3.3 cm/sec (median diameter d x = 0.25 mm, data supplied by 

 Dr. Nicholas C. Kraus). Since a sufficiently large turbulence motion is essen- 

 tial to keep the sediment particles in suspension (e.g., Kennedy and Locher 

 1972; Raudkivi 1976; Yalin 1976), the low turbulence intensity of Run 

 S0314B suggests that the measured particle concentration is probably not due 

 to active wave agitation. For unidirectional steady flow, the sediment suspen- 

 sion profile is well represented by the Rouse equation 





' 



n 





— 



= exp 



n 





a 



V 



/-?* 



226 



Chapter 1 1 LDV in the Bottom Boundary Layer 



