4.2.4 Model-Prototype similitude 



Many authors have discussed similitude requirements for breakwater 

 stability including Dai and Kamel (1969), Hudson et al. (1979), Sakakiyama and Kajima 

 (1992), and Hughes (1993). As such, only a brief review will be provided herein. In 

 order for damage measurements in the physical model to be scalable, similitude must be 

 maintained, where the ratios of the dominant physical forces in the model are the same 

 as they are in the prototype. Because there is no mathematical relationship that 

 describes armor damage, dimensional and inspectional analysis and the resulting 

 empirical relations are relied on to establish similitude requirements (Hudson et al. 

 1979, Hughes 1993). 



The first requirement for simiUtude is that the model be geometrically and 

 kinematically undistorted. Therefore, the size and shape of the breakwater, armor and 

 underlayer stone, surface characteristics of the stone, and the height and length of the 

 waves must all be undistorted. As described in Chapter 2, the wave forces on the armor 

 units act to cause damage while the armor unit self-weight and inter-unit friction act to 

 prevent armor movement. Also shown in Chapter 2, wave forces are typically 

 decomposed into fluid drag, fluid inertia, and buoyancy. The two important force ratios 

 that include these forces are the Froude number and the Reynolds number (e.g. Hudson 

 et al. 1979). The Froude number is the square root of the ratio of the inertial and 

 gravitational forces or Fr = u/iglf" where u is the fluid velocity, g is the gravitational 

 acceleration, and / is a characteristic dimension of an armor unit or wave height. Froude 



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