surrounding the loose sphere, providing a path for water motion under the sphere. The 

 only motion observed for the raised loose spheres was vertical motion under the steep 

 wave front, following a slightly elliptical path, and landing back in their hole after the 

 wave front passed. For tests with vertical velocities corresponding to the critical value, 

 the spheres were just lifting off. For the larger vertical velocities, the spheres were 

 lifting entirely out of their initial holes, but settling back into their holes. This sphere 

 motion under the breaking wave is shown in the sequence of photographs in Figure 2.9. 

 The sphere in motion is just left of the black rectangle on the right side of the 

 photograph. 



In the incipient motion experiment, spheres at a depth of one-third the toe 

 depth were the most mobile while spheres at the still water level were somewhat less 

 mobile. This movement corresponded to the variation of the vertical velocities in the 

 water column as shown in Figure 2.6. Figure 2.10 shows the incipient motion criterion 

 of Equation 2.16 versus wave steepness for Plan 4 using a few representative points 

 from each motion category as summarized in Table 2.4. For this figure v^ = 61.8 cm/s 

 computed using Equation 2. 16 with £)„ = 3.03 cm, S, = 2.083, and g = 980.6 cm/s. The 

 dark horizontal line represents the theoretical incipient motion criteria while the velocity 

 measurements are represented by the dark dots. Observed movement is noted for each 

 data point. The vertical gaps between the lifting group of points and the stationary and 

 rolling groups occurred because the vertical velocity increased dramatically under the 

 steep breaking wave face. Therefore, it was difficult to get a continuous set of points 



29 



