2.7 Conclusions from Incipient Motion Study 



Experiments on incipient motion of breakwater armor shiowed several 

 modes of displacement. One dominant mode was due to vertical wave forces which are 

 shown to occur at the point of maximum vertical velocity under the steep wave front. A 

 simple relation was derived assuming a Morison-like wave force balanced by the armor 

 unit submerged weight. The wave force model was composed of drag, due to the 

 maximum vertical velocities, and inertia, due to the vertical convective accelerations. 

 The maximum vertical convective acceleration is shown to be roughly linearly related to 

 the square of the velocity, which puts the inertial force term into the same form as the 

 drag term. The resulting incipient motion stability relation is similar in form to the 

 Shields sediment motion criteria. Further, when expressed as a traditional stability 

 number, incipient motion is shown to be a function of the Froude number, v/igH)'^\ The 

 incipient motion criterion shows promise in predicting the incipient motion of spheres 

 for the conditions tested, but further experiments are required. This study provided 

 some insight into how breaking waves can instigate armor motion and remove armor 

 units from an intact armor layer. Further experiments were conducted using a high- 

 precision force transducer to measure the forces on exposed spherical armor units. 

 These measurements have not been analyzed completely and are not included herein. 



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