In equation 2.2, the stability number is a convenient scale for characterizing 

 the incident wave height relative to armor stone size. It is typically in the range of 1 to 4 

 for stable or marginally stable rubble mound breakwaters (Van der Meer 1988). The 

 SPM (1984) provides K^ values for various armor layers at different slopes on 

 breakwater trunks and heads exposed to breaking and nonbreaking waves. Zero-damage 

 Kj) values are typically used corresponding to less than 2 percent by count, or five 

 percent by volume, of the armor in a layer being displaced. Using this no-damage 

 guidance from the SPM (1984), the stabihty number is in the range of 1 to 1.6 for 

 angular stone armor layers at slopes of 1 V:2H or steeper exposed to breaking waves. 

 The SPM specifies Kj) values up to 2.2 for stone armor layers at slopes of 1 V:3H 

 exposed to nonbreaking waves. So for most stable coastal breakwaters, the stability 

 number covers a narrow range from 1 to 2.5. For deformable structures where the 

 armor stone is expected to be mobile, such as S-shaped breakwaters and berm 

 breakwaters. Van der Meer (1988) suggests N^ = 3 -6. 



Hundreds of studies have been conducted to quantify the single empirical 

 parameter Kp in equation 2. 1 for the wide variety of prototype conditions that might 

 exist. Originally Hudson only explicitly included the effect of regular wave height, 

 structure slope, and armor stone specific weight. Hudson found no clear effect of wave 

 period on_armor stability for the nonbreaking regular wave conditions he and his 

 colleagues tested. They simply determined Kp corresponding to the lowest stability 

 condition over a range of typical wave periods. The Hudson equation has been 



