2.4 Incipient Motion Observations 



Several dominant incipient motion modes were identified during the stone 

 stability experiment in Plan 3, Table 2.1. The following descriptions pertain to initial 

 annor motion on an intact, as-built structure. Rolling was the only mode of motion for 

 stones on the toe. Although both onshore and offshore motion was observed, the toe 

 units always moved out of the layer in the onshore direction. Armor near the still water 

 level was more likely to displace than armor in other areas. This appeared to be due to 

 the fact that the armor was loosened in this area due to high velocities in the breaking 

 wave jet. Once loosened, the motion would depend on the armor shape and its position. 

 If the armor shape was flat, then the armor unit would flop back and forth until it rolled 

 out of the armor layer, generally rolling upslope during uprush. If the armor shape was 

 rounded, which was normally the case, the armor units would jump vertically under the 

 steep wave face if the wave was severely plunging or collapsing. If the wave was 

 surging, then loose units would only be displaced if they were exposed. There did not 

 appear to be sufficient lift in dowimash or uprush flows along the armor layer to displace 

 the stones unless they were odd shaped (flat). The only displacement mechanism 

 observed for rounded stones sufficiently hidden in the armor layer was uplift under the 

 steep wave face. 



These observations indicated that a fluid velocity or acceleration component 

 in the vertical direction is normally required to initiate armor motion for hidden armor 

 units. Additionally, this early qualitative study indicated that, for a given wave height. 



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