natural setting. This relation is expressed as 



(10) 



2\ / 2 



V \ h 



where 



V = flow velocity 



g = acceleration of gravity 



L = a linear dimension associated with the flow 

 These scaling criteria provide that the linear dimensions of the model are all 

 geometrically similar to those of the prototype. Typical rubble-mound break- 

 water model scales range from 1:5 to 1:70. The Froude number theoretically 

 represents the ratio of inertial to gravitational forces, an appropriate mea- 

 sure in situations where gravity is the predominant force. It is widely 

 accepted that this is usually the case for rubble-mound breakwaters (Hudson 

 et al. 1979). 



43. Another scaling law sometimes applies, however, which requires that 

 the Reynolds numbers of the model and prototype be equal, or 



LV\ /LV\ (^^) 



m \ /p 



where v is the kinematic viscosity of the fluid. The Reynolds number the- 

 oretically represents the ratio of inertial forces to viscous forces. Viscous 

 forces in the primary armor layer, underlayers, and core are now thought to 

 have greater importance than they did in the pioneering days of rubble-mound 

 breakwater design. The Reynolds criterion conflicts in many instances with 

 the Froude criterion in sizing structural materials for models (particularly 

 in smaller, more economical models), and compromising measures are usually ne- 

 cessary. Other scale effects can come into play when model waves are so short 

 that surface tension has a significant effect (seldom a real problem in prac- 

 tice) or when the mechanical strength of armor units is critical. Dealing 

 with these conflicting criteria makes physical modeling of rubble-mound break- 

 waters a highly specialized practice. Proper execution of a rubble-mound 

 breakwater scale model study requires both specialized equipment and extensive 

 experience available only at a handful of hydraulic laboratories around the 

 world. 



28 



