= 1.2 (2.15) 



where the critical vertical velocity, v^, depends on the nominal diameter, D„, and the 

 specific gravity, 5^ only, for a loose armor unit. So, if the vertical velocity exceeds this 

 critical velocity, motion of the sphere should occur. At the point of incipient motion, 

 this critical condition can be expressed as 



V = v^ = ^1.2Z)„g(5,-l) (2.16) 



Plan 4 in Table 2. 1 was designed to test the above criterion. For Plan 4, the 

 armor layer was constructed using silicon rubber spheres which were glued together and 

 attached to an inflexible yet porous metal mat. The metal mat was placed directly on the 

 underlayer and fixed to the flume walls. Several loose concrete spheres were placed in 

 the armor layer along a line from above the still water level down to the toe. Each two 

 loose spheres were separated by two glued spheres so that there was no interaction 

 between loose spheres. The sphere layer of Plan 4 was constructed to have the 

 mini mum porosity of a sphere layer of 0.33. 



For Plan 4, the loose spheres would not move under any conditions unless 

 they were slightly raised in the armor layer. This was accomplished by placing a 0.5- 

 cm-thick spacer under each sphere. The primary effect of this was to raise the porosity 



28 



