monochromatic waves (Run S0315A, Table 11-2) are shown in Figure 11-5. 

 In both cases, longitudinal velocity and surface elevation are generally in 

 phase with each other, as expected. Note that although the data acquisition 

 program samples the LDV output at a 25-Hz rate (the sample rate), the rate of 

 sediment particles crossing the sensing volume (the data rate) was consider- 

 ably lower, creating a significant amount of dropout; this is especially obvious 

 at higher elevations. High data rates at wave peaks and troughs are evident in 

 both Figures 11-4 and 11-5. In particular, near 10 sec in Figure 11-4, the 

 data rate near zero crossings becomes very small, which is indicative of the 

 rapid settling out of sand grains on scales smaller than the wave period. It is 

 for this reason that spectral methods are not suitable for analysis. Instead, 

 phase averaging is used as the major data analysis tool. 



Figure 1 1-4. Time series of water surface elevation (solid curve, cm) and LDV-measured 

 longitudinal velocity at 1 cm above the bed (circle, cm/sec); Run A281 1 A 

















50 



9» o 



o o 

 o 



• 9 * 1 ° °° ° 1 * °° i 1 % e 



? * ° i o I % §> 



O O 





0' 



L * 1 M 













-50 



i ° 



o 1 1 

 







* J o i f « * ° 



f « 





( 



) 



10 



20 30 40 50 



60 70 80 



9 











t (sec) 







Figure 1 1-5. Time series of water surface elevation (solid curve, cm) and LDV-measured 

 longitudinal velocity at 1 cm above the bed (circle, cm/sec); Run S0315A 



Chapter 1 1 LDV in the Bottom Boundary Layer 



223 



