they noted that stone counts were more accurate when only a few stones moved, but the 

 volume method improved accuracy of the damage measurement for advanced damage. 

 They also noted that the depth of cover d^ was useful in describing the degree of 

 damage. They noted that, for an armor layer thickness of approximately 2D„^q, when 

 d^=D^Q, the underlayer was visible through a hole D„^q in size, and when d^=0, 

 significant damage to the underlayer had occurred. 



3.3 Damage Measurement Experiments 



Historically, breakwater design has been accomplished using an empirical 

 stabiUty equation, such as the Hudson equation (Hudson 1958, 1959) as shown in 

 Equation 2.1. As described earlier, for this equation, Kp is defined for a given level of 

 performance, typically the no-damage condition represented by D% less than 2 percent 

 by count or 5 percent by volume (SPM 1984). This technique assumed damage 

 approached an equilibrium level of Z)%, where further regular waves at the design 

 condition induced no further damage. This is based on regular wave experiments where 

 damage reaches an equilibrium level or failure occurs relatively quickly. Hudson (1958) 

 and Van der Meer (1988) expressed Equation 2.1 as a stability number as shown in 

 Equation 2.2 or 



(3.5) 





^s 



= (KpCotQ)'" - 



H 



where A = 5.-1. 









41 



