KFY 

 l^ffl^ PERIOD 



H-0.6 1=7 



Maximum depth 

 for onset of 

 3heet *low 



Transition zone 

 between symniatnc 

 and asymmetric 

 bedforms 



Zone of sheet flow. 



Maximum depth 

 initiotion 

 sediment motion 



50 60-— Depth(m) 



Zone of Symmetric 

 bedfomis 



H=3 T=7 



H=3 T=10 



Figure 4. Fluctuations of inner siieif bed form zones and initiation of sediment motion with 

 respect to significant wave Height (H) and period (T) (after Boyd (1981)) 



longer produce a measurable change in depth. He calls this latter limit, 

 the minimum water depth at which no change occurs (as measured by engi- 

 neering means) the depth of closure. The depth of closure is not the loca- 

 tion where sediment ceases to move, but that location of minimum depth 

 where profile surveys before and after a period of wave action, a storm 

 perhaps, lie on top of one another. 



Kraus (1992) also stated that the depth of closure is time-dependent, 

 that is, dependent upon the transporting capacity of the particular incident 

 waves. For example, we expect the average depth of closure for the sum- 

 mer to be less than that in winter. Similarly, the "storm of the decade" 

 will alter the profile elevation to a much greater depth than occurs during 

 a typical storm season. In engineering projects, the depth of closure is 

 best determined through repeated accurate profile surveys, such as per- 

 formed with a sled. 



Chapter 2 Inner Shelf Concepts 



11 



