Tales of the Coast
Coastal Dynamics



Circulation is the movement of currents through bodies of water, often transporting sediments in the process. Sometimes this transport results only in a local rearrangement of sand into bars and troughs, while at other times there are extensive longshore displacements of sediment, possibly moving hundreds of thousands of cubic meters of sediment every year.

Currents associated with nearshore circulation generally produce local movement of beach sediments. The rip currents of the circulation can be important in the cross-shore transport of sand, but there is minimal net displacement of sand along the coast.

More important to the longshore movement of sediments are waves breaking obliquely to the coast and the longshore currents they generate, which may flow along an extended length of beach. The resulting movement of beach sediment along the coast is referred to as longshore sediment transport.

Longshore sediment transport can also result from the currents generated by alongshore gradients in breaking wave height, commonly called diffraction currents. This transport is manifest as a movement of beach sediments toward the structures which create these currents, such as jetties, long groins and headlands.



Longshore Sediment Transport: The longshore movement of beach sediments is important because the transport can be interrupted by structures, such as jetties and breakwaters, or captured by inlets and submarine canyons. In the case of a jetty, the result is a buildup of the beach along the updrift side of the structure and erosion of the beach downdrift of the structure. The impacts pose problems to the adjacent beach communities and threaten the usefulness of nearby waterways, such as channels and harbors.