54 



approaching the shore. In addition, data from many areas suggest that under 

 certain circumstances, there is an interchange of sediments between the upper 

 shoreface and the beach. Although the interchange occurs on a seasonal basis, 

 it may be unidirectional at times. 



Because of the cost and difficulty of obtaining data and sediment samples 

 from offshore areas, less is known about the shoreface zone than the adjacent 

 shore and coast. However, repetitive profiles indicate that shoreface 

 morphology is variable over the long term (Moody 1964), and that episodic 

 sediment transport, with suspension of sediments at least 1 m above the sea 

 floor, occurs in response to changes in waves and currents (Young et al. 

 1982; Vincent, Young, and Swift 1983). 



Everts (1978) analyzed 49 composite shoreface profiles based on 441 mea- 

 sured profiles from the Atlantic and gulf coasts of the United States. Of the 

 49 composite profiles, only three, all from northwestern Florida, did not 

 contain a definable shoreface slope. In general, the profiles show the shore- 

 face to have a concave-upward slope and to be significantly steeper than the 

 usually planar shelf floor, perhaps representing a cutoff point of significant 

 active modification of the profile by waves and currents. 



Shoreface variability is more pronounced on the upper shoreface because of 

 the decrease in wave and current forces with increasing depth. The most 

 pronounced short-term effects appear to be seasonal, with the more energetic 

 winter regimen tending to move material from the beach to the shoreface and 

 summer fair weather conditions tending to move material onshore. Lower 

 shoreface deposits are disturbed less often and probably become active only 

 during major storms. 



On many transgressing barrier coasts, the barriers are overriding their own 

 back-barrier deposits, which may crop out on the beach or the shoreface. 

 Evidence of outcropping back-barrier deposits on transgressive coasts include 

 blocks of salt marsh peat and shells of back-barrier fauna such as Crassostrea 

 virginica (Gmelin). Most back-barrier sediment is fine-grained and tends to 

 move rapidly offshore to deeper water where it is more stable. Coarser sand 

 may also occur in back-barrier deposits in the form of storm washover 

 deposits and flood-tidal shoal complexes adjacent to relict inlets. In most 

 cases this material was originally derived from beach sediments moving in the 

 alongshore drift or from relict river channel deposits. Thus, these deposits 

 are more stable and are not likely to increase shoreface recession rates. 



Usually, shoreface areas are primarily composed of unconsolidated sands. 

 However, many shoreface zones are underlain by consolidated material such 

 as rock or reefs, glacial till, or clay. These shorefaces may be more stable 

 and may not follow the equilibrium profiles of primarily depositional shore- 

 faces or of those underlain by ancient unconsolidated material. Admixtures of 

 silt and clay are not uncommon, especially on the lower shoreface, where 

 outcrops of fine-grained back-barrier deposits often occur on transgressive 

 coastlines. 



. Chapter 3 Variable Coastal Features 



