incipient motion was primarily influenced by wave shape, stone position, stone 

 exposure, and stone looseness. 



2^ Experimental Measurements 



Throughout the experiments, vertical and horizontal wave velocities were 

 measured in the vicinity of the armor layer. Figure 2.3 shows typical time series of the 

 horizontal and vertical velocities on the structure measured for one run of plan 3 with 

 the following characteristics: F, = 12 cm, /■= 1 s, ^ = 24 cm, and fif^ = 8.8 cm, where H, 

 is the average wave height measured at the toe, f is the average wave period, d, is the 

 toe water depth, and di^ is the depth of the laser measurement. The laser measurement 

 was made 1 cm outside the armor layer, measured perpendicular to the outer armor layer 

 profile line. The sign convention is such that the horizontal velocity is positive seaward 

 while the vertical velocity is positive upward. Typically for these measurements, the 

 horizontal velocity signal was considerably smoother than the vertical velocity signal, 

 due primarily to the small amplitude of the vertical velocity relative to the horizontal 

 velocity and the relatively large amount of turbulence near the armor layer. Figure 2.4 

 shows a velocity vector time series over one wave period, measured 1 cm outside the 

 armor layer profile line for one run of plan 3 with the following characteristics: H, = 8.4 

 cm, T= 2s,d, = 24 cm, and di=\l cm. Also shown is the wave profile at the point of 

 maximum vertical velocity. The plot shows a large vertical velocity just below the steep 

 wave front. Observed maximum stone movement for this wave profile position is also 

 shown. Figure 2.5 shows an example of vertical velocities outside and inside the armor 



18 



