Longshore Transport

Sediment transport is the method by which dynamic coastline features, such as beaches, spits, dunes and offshore bars, are built and maintained.

Littoral transport is nearshore sediment transport driven by waves and currents. As shown in figure 1, this transport occurs both parallel to the shoreline (alongshore or longshore) and perpendicular to the shoreline (cross shore or on-off shore).

Storm waves carve beaches, ridges and banks, transporting large volumes of sand to nearshore bars. Where the rate of offshore sand transport exceeds the rate of supply from updrift sources, the beach erodes. During calmer periods, waves transport sand from offshore bars and deposit it on the beach face. Through these cycles, there is a movement of sand and gravel along shore in response to the shifting directions of waves. In many places this is a net movement in one direction. The transport direction depends on such factors as wave climate, bathymetry, shoreline orientation, and the presence of natural or artificial features that deflect waves and currents. Cross-shore transport is affected by changes in lake levels.

The "littoral zone," where littoral transport occurs, extends roughly from where the waves begin to break offshore to the limit of wave uprush on the beach (Figure 2). Wave conditions and current speed determine the size of material that can be transported. The rate of transport within the littoral zone is relatively small along erosion-resistant rocky shorelines but may reach several hundred thousand cubic yards (a hundred thousand cubic meters) per year along some sandy coastlines.