c. Dimensionless Fall Time . Prediction of accreted (berm) or erod- 

 ed (bar) profiles is possible using a dimensionless fall-time parameter 



Ho 



F^ = , (4-20) 



O VrT 



where H^ is deepwater wave height, T is wave period, and Vj? is the 

 fall velocity of the beach sediment. (Dean, 1973.) 



The fall-time parameter Vq, is plotted against deepwater steep- 

 ness, Ho/Lo, in Figure 4-29 using the profile data of Rector (1954, Table 

 1, Column 25) and Saville (1957, unpublished). These data include wave 

 heights ranging from about 0.05 foot to 5.0 feet, a range compatible with 

 field conditions. They also include a range of initial slopes. 



In Figure 4-29, the line of demarcation between deposition offshore 

 and deposition onshore is approximately at the value F^ = 1. More com- 

 plete separation is possible when F^ is plotted against H^/Mj. (See 

 Figure 4-30.) 



Values of F^ > 1 indicate that the time required by the particle 

 to fall a distance about equal to the maximum depth in the surf zone is 

 greater than the time available between arriving wave crests. Thus, 

 values of F,^ significantly greater than 1 suggest significant concen- 

 trations of suspended sediment, which are expected to diffuse seaward 

 and deposit offshore. 



Since values of V^ range from about 0.066 foot per second (2 centi- 

 meters per second) for fine sand to 0.49 foot per second (15 centimeters 

 per second) for coarse sand (Fig. 4-31), ¥q ranges from about 0.25 for 

 low swell on coarse sand beaches to 10 or more for storm waves on fine 

 sand beaches. Such values suggest that typical field and laboratory con- 

 ditions define a range of conditions within which the importance of sus- 

 pended sediment in the surf zone may vary from significant to negligible. 



The effect of temperature on fall velocity (Fig. 4-31) is important 

 enough to be critical under some conditions. It appears that temperature 

 by itself, in its effect on fall velocity, can change a profile from erod- 

 ing to accreting. 



To summarize results on berm-bar criteria, the dimensionless fall 

 time, Vq = H^/(Vy?T), Equation 4-20 provides an estimate of the separa- 

 tion between berm and bar-type profiles. A value of V^ between 1 and 2 

 appears to be the critical value, with F^ = 2 being more appropriate for 

 prototype-size waves. For ¥q less than the critical value, the beach 

 accretes above MLW. The effect of fall velocity is important. Deepwater 

 wave steepness by itself is an unreliable criterion for prototype condi- 

 tions. 



4-81 



