On and offshore sand movement

Figure 1. The coastal sand-sharing system, fair weather and foul. In fair weather, collapsing breakers and tides move sand onto the beach from offshore bars (A), sometimes creating a narrow tidepool on the beach (B). In foul weather, spilling and plunging breakers erode sand from the beach and carry it offshore to form sandbars (C). In major storms, high waves and tides move sand over and between sand dunes to form overwash fans behind the dunes (D). Overwash events often carry sand onto roads and buildings built close to the beach. (Illustration by the author. More about the illustration)

Figure 1 is a diagram from the author’s book entitled The Nature of North Carolina’s Southern Coast, published by UNC press in 1997. The figure illustrates how sand is moved on and off oceanfront beaches by waves and storms.

As you can see, when waves are low and gently sloped from crest to trough, sand moves onto the beach from offshore sand bars. When storms occur and waves become high and steeply sloped, sand is eroded off of the beach and carried back to the sandbar. This on- and offshore sand movement characterizes the seacoast, and goes on almost constantly. As a result, the only constant in beach geology is constant change. When storms come to the coast, the sand sharing system responds by extending both landward and seaward of the intertidal beach. Routine storms erode the beach and carry sand offshore, where subsequent periods of small waves gradually carry it back to the beach.

Hurricanes and other major storms, however, carry sand both further offshore and onto the land where it can be permanently lost to the beach. Such large storms can also take beach sand along the shore to places where it will not return. As a result, big storms like hurricanes have the capacity to reduce and move our sandy barrier islands toward land. This is particularly noticeable in a period of rising sea level like the one we are in now.

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