Pilkey et al. (1993) state that one of the most essential assumptions that 

 must hold true for the concept of the equilibrium beach profile to be valid 

 is: "There must exist a closure depth beyond which there is no net off- 

 shore or onshore transportation of sediment - a depth of no net sediment 

 movement to and from the inner shelf even during storm-induced down- 

 welling events." Pilkey et al. (1993) also defined the depth of closure as 

 the depth where no vertical changes to the bed take place and where grain 

 size distribution remains constant. Pilkey et al. (1993) state that the depth 

 of closure does not exist, as field evidence shows that large volumes of 

 sand may be moved beyond the closure depth. Such movement occurs 

 mostly during offshore-directed storm flows. Studies in the Gulf of 

 Mexico measured offshore bottom currents of up to 200 cm/sec and sedi- j 

 ment transport to the edge of the continental shelf (Hayes 1967a,c; Mor- 

 ton 1981; Snedden, Nummedal, and Amos 1988). The amount of sediment 

 moved offshore was large, but it was spread over such a large area that the 

 change in seabed elevation could not be detected by standard profiling 

 methods (Hayes 1967a, c) (±10 cm). 



Several studies have found closure depths ranging from -5 m to -30 m 

 for the U.S. Atlantic coast. Birkemeier (1985) stated that the measured 

 depth of closure at Duck, North Carolina, fluctuates between -3.9 m and 

 -6.4 m. However, the first conspicuous inner shelf configuration change 

 at Duck occurs at -15 m, where sediments change from well-sorted fine 

 sand to muddy fine sand with the fines bound in fecal pellets (Wright, in 

 press). Perhaps this depth is more likely to be the maximum depth of nor- 

 mally occurring, shore-normal sediment exchange. This compares to Hal- 

 lermeier's (1981b) seaward limit of sediment motion initiation (d) of 

 -22.1 m for the Atlantic coast. 



Depth of closure estimates using Hallermeier's (1977) method and the 

 hindcast data of Jensen (1983) include Brevard County, Florida (-7.1 m); 

 Walton County, Florida (-6.4 m); and Virginia Beach, Virginia (-5.5 m) 

 (Hansen and Lillycrop 1988). Pearson and Riggs (1981) state that the 

 depth of closure at Wrightsville Beach, North Carolina, was at least -16 m 

 based on the presence of beach sediments at this depth. Wright (1987), in 

 inner shelf studies including the use of bed elevation changes and sedi- 

 ment and profile data, shows that the depth of closure was located 

 between depths of -10 and -30 m depending on regional energy regimes. 



An additional estimate of depth of closure for the U.S. Atlantic coast is 

 -9 m as presently used in engineering project design. This is the esti- 

 mated depth where waves first affect the bottom as they move onshore, 

 and there is no measurable (within the error bars of the profiling method) 

 change in pre- and post-storm inner shelf profiles. In addition, sand 

 ridges and irregular topography are typically located onshore of this clo- 

 sure depth while a uniform sloping shelf is located seaward. 



Where Equation 4 predicts closure during the annual extreme 12-hr 

 event, there exists a deficit of knowledge in predicting the depth of clo- 

 sure as a function of time. In order to develop a predictive method to 



Chapter 2 Inner Shelf Concepts 



13 



