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Figure 4.7.  Schematic diagrams showing seasonal changes in a sediment-limited beach typical of the central Grand Strand. (TOP) During times of fair weather. (BOTTOM) During times of stormy weather. More »

4.5 Exposure and Burial of Hardbottom Areas

Sediments that compose the shoreface of the Grand Strand vary in thickness and constantly shift across an irregular, rocky surface.  In the short term, fair-weather conditions promote landward transport of sediment from the lower shoreface to the upper shoreface and beach system.  The wide, elevated berm formed by these mobile sediments remains dry at mean high tide and is a favorite spot for beachgoers.  Just offshore, however, sediment is scarce, and rock is exposed on the shallow seafloor (Figure 4.7).  Erosion of these exposed hardbottoms provides relatively small, but significant quantities of new sediment that are incorporated in the active beach (Riggs and others, 1998).  Over the long term, repeated cycles of exposure wear down the erosion-resistant rocks of the shoreface and effectively increase water depths just offshore of the beach.

During storms, waves and currents remove sediment stored in the dunes and beach and transport it offshore (Figure 4.7).  The seasonal movement of sediment away from the beach temporarily buries the hardbottom areas on the shoreface.  Stripped of sediment cover, the rocks on the upper shoreface are eroded and contribute some new sediment to the active beach.  When fair weather returns, sediment on the shoreface is driven back onto the upper shoreface and beach, reestablishing the berm and dunes.  This onshore-offshore shifting of sediment alternately buries and exposes rocks on the beach and shoreface.  The balance of this process, along with gains and losses from alongshore transport, determines whether there is erosion or accretion of a particular stretch of beach.  As a result of this constant shifting, the beach and its underlying rocky foundation wear away, and the beach and shoreface migrate landward as a system.

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