VI. GEOMETRIC LIMIT DEPTH OF THE SHOREFACE 



Changes in the volume of sand on a beach are related, in part, to 

 changes in the volume of sand and the profile shape farther seaward. 

 Neglecting longshore transport, sand may move onto the beach from sources 

 farther seaward, or may be supplied to the seaward region from beach 

 sources. Wave action initiates most of the sand movement. Currents, 

 either wave-produced or resulting from other mechanisms, transport the 

 sand. There is presently an increasing interest in the seaward limit 

 beyond which sand will no longer move to or from the beach, or beyond 

 which changes in bathymetry will not affect processes on the beach. 

 This interest is primarily directed toward the use of offshore sand 

 sources for beach fill. For example, if sediment is removed from the 

 Inner Continental Shelf between the seaward limit and the foredune, i.e., 

 the region often defined as the active profile, the excavation may 

 subsequently fill, possibly with sand that originated near or on the 

 beach, thereby contributing to a decrease of sand on the beach. Con- 

 versely, sediment artificially placed on the active profile will likely 

 move in such a manner that the profile will tend toward an equilibrium 

 shape for the waves acting upon sand of that size, shape, and density. 

 Thus, sand may be placed seaward of the beach during certain times of the 

 year with the expectation it will ultimately move landward to nourish the 

 beach. If placed seaward of the seaward limit, or at a time when it moves 

 beyond the seaward limit, the sand will not fulfill the purpose of the 

 dumping. Additionally, in calculating the sediment budget of a coastal 

 area where sediment volume changes are occurring, the seaward limit of 

 sediment movement is a required parameter. One of its practical uses 

 comes in predicting the volume of sand needed to artificially extend the 

 shoreline while keeping the active profile in equilibrium. 



The data in this report may be used when two general geometric pro- 

 cedures are considered in establishing a seaward limit. Each procedure 

 is based wholly on geometric characteristics of the Inner Continental 

 Shelf, and not on direct evidence of sediment transport. No evidence is 

 available that indicates the results actually designate a seaward limit 

 of sediment movement. 



The geometric criteria that might be useful to establish a limit 

 depth of the shoreface, and possibly a seaward-limiting depth of signifi- 

 cant sediment transport, are (a) depth at the shoreface-ramp intersection, 

 coordinates (g, 3c) in Figure 5, and (b) depth at the transition from 

 shore-parallel, smooth bathymetric contours near the coast to irregular 

 contours farther offshore (U.S. Army, Corps of Engineers, Coastal Engineer- 

 ing Research Center, 1977). Examples are shown in Figures 2 and 3, and 

 both depths are given in Table 2. As illustrated in Figure 10, the depth 

 of the seawardmost shore-parallel bathymetric contour does not agree well 

 with the seaward-limit depth obtained using the shoreface-ramp criterion. 

 For the Atlantic coast and the gulf coast east of the Mississippi River 

 Delta, the shoreface-ramp criterion depth on 89 percent of the profiles 

 was greater than the depth obtained using the shore-parallel contour 

 criterion. West of the Mississippi River Delta, 85 percent of the shore- 



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