QUESTION:

When swimming in the ocean, have you ever wondered why you tend to be carried down the beach, away from your towel in an undertow?

The answer is simple. This process is called longshore drift and is generated by wave and current action.

Water and sediment are transported in a zig-zag pattern as waves and currents approach the shoreline at an angle. This zig-zag pattern is the mechanism for sediment transport as the swash is immediately followed by the backwash to the ocean. This energy created from the breaking waves allows for weak currents to carry large amounts of coarse-grained sediment down the shoreline. These weak currents, known as longshore currents, carry water and sediment parallel to the shoreline toward the downdrift end of the beach. For an animated view of lonshore drift, click here.

Along South Carolina’s coast, longshore drift is generally north to south. The net effect of sediment moving southward causes the north end of barrier islands in the southeastern United States to erode and the south end to accrete (Keener-Chavis, P. and Sautter, L, 2000). Because of this movement, large volumes of sand are transported along our barrier islands each year. Direct evidence of this sand depostion is spit formation along the south end. Spits are hooked-shaped lobes of sand extending downdrift from the barrier island.

The action of southerly-directed longshore currents, wave refraction, and tidal currents, large reservoirs of sand in the form of sand bars tend to drift downward and inward toward the upper parts of islands that are south of coastal inlets. This gives the South Carolina barrier islands their characteristic drumstick shape, with the upper ends made thicker by the accumulation of sand (www.scccl.org, 2001). Tide-dominated environments tend to have short, wide "drumstick" shaped islands.

For more information on longshore drift, click here.