where S^ = armor specific gravity, g = acceleration of gravity, and v^ = critical vertical 

 velocity at which armor just begins to lift. In terms ofN^ from Equation 2.2, Equation 

 2.11 becomes 





where H^ = critical wave height at toe. It is interesting to note that the stability number 

 is primarily a function of the Froude number, v^ / {gHj^\ This formula ties the 

 traditional stability relations to local vertical velocity measurements. 



Based on results of detailed velocity measurements in the interior and just 

 outside the armor layer, the vertical velocity gradient was foimd to be proportional to the 

 ratio of the vertical velocity and the armor diameter, as assumed in Equation 2.8. The 

 empirical convection coefficient is K^ = 0.90 for this experiment. This is shown in 

 Figure 2.8 for a group of experiments summarized in Table 2.3. All experiments listed 

 in Table 2.3 were conducted with a seaward slope of 1V:2H, D„ = 4.6 cm, and dj = 24 

 cm. The velocity values are positive peaks from the aligned inner and outer vertical 

 velocity time series. In Table 2.3, the velocity gradient Av/Ay = | (v^ - v^yiy^ - j,) | , where 

 Vg = outer peak velocity, v, = inner peak velocity, >', = inner velocity measurement 

 elevation, y^ = outer velocity measurement elevation. 



25 



