4.1 Introduction

Dry sandy areas such as the berm and dunes represent only a small part of the active beach system.  Most sediment movement occurs underwater on the shoreface, a narrow, relatively steep surface seaward of the low tide line (Figure 1.2).  This energetic zone extends offshore to deeper water where waves and currents generally do not move sediment.  The width and depth of the shoreface vary along the coast, depending on wave climate, sediment supply, and the nature of the underlying geology.  The overall shape of the beach/shoreface system changes seasonally as eroded sediment is stored offshore during stormy weather and later returned to the beach in fair weather. 

Rates of shoreline retreat and the response of a beach to storms largely depend on the geologic framework of the shoreface that is being excavated by ocean processes.  Coastal engineers who design stabilization or renourishment projects typically assume that the shoreface is a pile of loose sediment with a uniform grain size.  In their forecasts of beach behavior, waves acting on this idealized shoreface produce a predictably smooth, concave-up profile of equilibrium that retains its general shape as the shoreline changes position (Dean, 1991; Thieler and others, 2000).  In South Carolina, however, framework components beneath the shoreface widely differ in their resistance to erosion.  Assuming that physical and biological processes are uniform across the region, we can reasonably expect more change to occur where less resistant materials (that is, loose sandy sediment) underlie the shoreface.  Conversely, a section of coast underlain by more resistant materials (this is, sedimentary rock) is likely to be relatively stable.

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