Hedar (1986), Losada and Gimemenz-Curto (1979), and Van der Meer (1988). Pfeiffer 

 found that the Hudson equation and the Losada and Gimemenz-Curto equations 

 performed the best when compared to field data, but none of the equations matched the 

 prototype records all that well. Pfeiffer also noted that none of the stability models 

 mentioned above are specifically suited for predicting extended damage. 



2.2 Armor Incipient Motion Studies 



The empirical stability models of Iribarren (1938), Hudson (1958), and 

 others are based on a free body analysis of an armor unit undergoing forcing due to 

 shallow- water waves. Early stability models assumed 1) the principal wave force was 

 due to down- or up-rush on an unsheltered and unrestrained unit, 2) the wave force was 

 drag dominant, and 3) the drag force would be critical if the maximum horizontal fluid 

 velocity was used, which was considered to be proportional to the shallow-water 

 incident wave celerity. But for an intact structure and prior to initiation of incipient 

 motion, the armor units are typically partially hidden and restrained from up or down 

 slope movement; so lift, inertia, and convection across the armor layer must be 

 considered. Moreover, Sawaragi et al. (1982) showed that the maximum fluid velocity 

 on a rubble mound was not necessarily proportional to the wave celerity. Sigurdsson 

 (1962) made force measurements on armor composed of spheres set on extremely steep 

 slopes with an impermeable underlayer and derived incipient equations of motion; but 

 concluded by stating that the dominant mechanism of initiation of armor motion was 

 still unknown and required further investigation. Although many authors have 



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