Hudson also expressed this equation in a slightiy different form as 



iS^-l)D^, (2.2) 



where N^ is the dimensionless stability number and D^q = (^5(/Ya)^'^ is termed the 

 nominal stone diameter. Hudson did not use the nominal stone diameter variable. It 

 was used by both Thomson and Shuttler (1976) and Van der Meer (1988) who noted 

 that D„sQ is the length of a side of a cube with volume equivalent to that of the median in 

 the stone weight distribution. A glance at equations 2. 1 and 2.2 shows that stone 

 movement due to wave forcing, characterized by the wave height, is resisted primarily 

 by the stone weight. As shown by Hudson (1958), Ahrens and McCartney (1975), Van 

 der Meer (1988) and others, the wave shape, structure porosity, armor shape, and 

 structure cross-sectional shape all affect the wave force. Hudson also noted that the 

 resistance to movement is affected by the friction between armor units, stone shape, 

 upslope armor layer weight, and armor slope. Equation 2. 1 is preferable from a physical 

 perspective because it maintains the inertial form of the forces, which is appropriate for 

 armor stability. But equation 2.2 combines the stability coefficient with the structure 

 slope, which is desirable because the stability coefficient is a function of the structure 

 slope (e.g. Shore Protection Manual (SPM) 1984). This second form also presents the 

 stone stability equation in a form that is similar to sediment transport formulae. This 

 will be discussed further later in this chapter. 



