CRITICAL WAVE CONDITIONS FOR SAND MOTION INITIATION 



by 

 Robert J. Eatlevmeier> 



I . INTRODUCTION 



Various engineering activities require consideration of the seaward limit 

 for significant bed activity caused by surface waves. Sometimes the ultimate 

 seaward limit to bed activity by waves is of interest; examples include field 

 borrow of inactive sand for beach nourishment, and design of laboratory tests 

 to include the entire active nearshore region. The ultimate seaward limit to 

 wave-induced bed activity can be defined using a criterion for sediment motion 

 initiation by oscillatory flow. 



This report provides a procedure for calculating critical conditions for 

 wave-induced motion of sand grains (median sediment diameter, D, between 

 0.06 and 2.0 millimeters), based on the motion initiation criterion for a 

 level bed established by Hallermeier (1980) . That report reviews the available 

 experimental data and concludes that the criterion used here is simpler and 

 more reliable in predicting oscillatory-flow initiation of sand motion than the 

 well-known Shields criterion. The present criterion is also an improvement on 

 the somewhat qualitative guidance presented in Section 4.522 of the Shore Pro- 

 tection Manual (SPM) (U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, 1977). 



Section II presents this criterion by giving the threshold flow velocity for 

 sand motion in terms of sand and fluid characteristics and the flow period. 

 Relationships for linear waves are provided to permit conversion of this peak 

 near -bottom velocity into critical wave conditions for sand motion — either a 

 minimum wave height with a given water depth, or a maximum water depth with a 

 given wave height. Possible effects of physical factors ignored in this cal- 

 culation procedure are also discussed. 



Section III provides example calculations of critical conditions for sand 

 motion using the equations and graphical relationships presented in this report. 



II. THE CALCULATION PROCEDURE 



1. Threshold Velocity for Sand Motion Initiation . 



Using empirical results, two approximate asymptotic forms can be identified 

 for motion initiation on a level sand bed, and these forms permit a simple pre- 

 diction procedure for sand motion using physical values which are usually speci- 

 fiable. These asjnnptotic forms for motion initiation correspond to viscous 

 near-bed effects being either dominant or negligible. The five values to be 

 specified for a calculation are: 



D = median sand-grain diameter; 



0) = the oscillatory-flow frequency = (2Tr/T) , where T is flow period; 



y' = (Ps~P)/P> where pg is sediment mass density and p is fluid 

 mass density; 



