large rubble-mound breakwaters in the last 10 years (Stickland 1983). The 



Iribarren formula in its original form appears as follows (d'Angremond 1975): 



3 3 



W = ^ 3 (2) 



A (y cos 9 + sin 9) 



where 



N = empirical coefficient related to the armor material character- 

 istics (comparable to K^ in Equation 1) 



y - coefficient of static friction between individual armor units 

 (equivalent to the tangent of the angle 9 at which armor would 

 slide from gravity alone; values found by Graveson, Jensen, and 

 Sorensen (1980) are presented in Table 1) 



Table 1 

 Values for the Coefficient of Static Friction y 



Type of Armor Coefficient y Angle of Repose 9 



Round seastones 1.0 45 



Quarrystones 1.1 48 



Concrete cubes 1.2 50 



Concrete tetrapods -1.4 -55 



Concrete dolosse -2.7 \ -70 



19. The original Iribarren formula has only one additional parameter y 

 with M being essentially equivalent to K . . This additional explicit pa- 

 rameter appears to have little advantage to offer, except that static friction 

 has recently been investigated as a potentially critical factor in the overall 

 stability of complex artificial shapes such as dolosse (Price 1979). It al- 

 lows the Iribarren formula to account for the marginal stability of materials 

 placed at their natural angle of repose. This factor might be used also in 

 the future as a measure of seismic stability of rubble-mound structures. 



20. Engineers at the Danish Hydraulics Institute (DHI) have proposed a 

 modification to the Iribarren formula for application with scale model tests 

 using irregular waves (Graveson, Jensen, and Sorensen 1980). This DHI- 

 Iribarren formula is 



3 2 



P gy H^L 

 ^ s p 



3 3 



K A (y COS 9 - sin 9) 

 o 



(3) 



14 



