The densimetric Froude number at which a grain will just move was 

 defined as : 



F., » U.^ g-l/^.■'/\„ -1/2 , (22) 



where, 



U^ = critical shear velocity; and 



D = median grain-size diameter. 



Assuming U* as proportional to U^ j,,^^ at the point of inception of 

 beach material movement offshore of the breaker, and substituting 

 equation (21) into equation (22) yield: 



F^ ^ H (y' Dd^)'^/^ . (23) 



The profiles were examined to determine dc, defined as the limiting 

 depth at which the profile changed initially, and the function H(Y'Dd(,)' * 

 was computed for each profile where d^, was determined to be less than the 

 water depth in the flume. If equation (23) is valid, it was expected that 

 this function would be a constant. Figure 31 shows the results obtained 

 when d(, is plotted versus H(y'D)^, indicating that the function is not a 

 constant. Most data are for the ground glass or glass beads since the 

 lighter weight materials generally moved to the bottom. 



None of the several theoretical and empirical relationships developed 

 to predict profile shape as a function of wave conditions and sediment 

 characteristics have been found to be universally applicable for modeling 

 purposes . 



VI. CONCLUSIONS AND RECOMMENDATIONS 



1 . Conclusions . 



(a) Effect of Model Particle Shape and Size Distribution . 



(1) Strongly bimodal grain-size distributions under small wave 

 steepnesses produce profiles with multiple bars. These multiple bars do 

 not appear when the sediment has the same median diameter but a unimodal 

 distribution. 



(2) Very narrow grain-size distributions and smooth spherical 

 shapes produce profiles having unstable bars which intermittently grow 

 and disappear. 



63 



