approach was unique and yielded insight into the average frictional force on sections of 

 the amaor layer. They found that the frictional force tending to dislodge armor units was 

 greatest below the still water level and that the character of the force depended strongly 

 on the type of wave breaking. 



This chapter discusses a series of physical model experiments to identify 

 and develop predictive models for breakwater armor incipient motion and to relate this 

 motion to existing empirical stability relationships. The experiments were conducted in 

 wave flumes at the Waterways Experiment Station in Vicksburg, Mississippi. The first 

 experiment consisted of measuring wave-induced fluid velocities on and within the 

 armor layer and runup/down. In addition, free surface oscillations were measured while 

 observing armor motion on stone and Core-Loc armor. The observations from this early 

 study led to an incipient motion experiment using a fixed-sphere armor layer with 

 several loose spheres placed at various depths within the armor layer. A dominant 

 incipient armor motion mode and predictive stability equation were verified. 



2.3 Experimental Setup 



The initial experiments were conducted to determine the nature of armor 

 incipient motion and surrounding flow. The instrumentation included a laser Doppler 

 velocimeter (LDV), high-resolution video, and runup and vertical free-surface-piercing 

 gages near and within the armor layer. The experiments discussed in this chapter for 

 incipient motion were all carried out using regular monochromatic waves. Data analysis 



12 



