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Tales of the Coast
Coastal Dynamics

Circulation and Sediment Transport

There are two modes of sediment transport: suspended sediment transport, in which sediment is carried above the bottom by turbulent eddies of water, and bed-load sediment transport, in which the grains remain close to the bed and move by rolling and saltating (bouncing along a surface).

Longshore sediment transport can remove significant amounts of sediment from beaches, and it is responsible for accretion and deposition in other locations. For example, this form of sediment transport is one of the principal processes for forming barriers.

When structures such as groins, jetties, and breakwaters interrupt longshore sediment transport, they can cause severe downdrift erosion. Likewise, inlets modify currents and sediment transport patterns, resulting in the funneling of sediment into the inlet channel where some is deposited as shoals. Mitigation measures, such as sand bypassing, can be implemented to counteract erosion from these interruptions to natural longshore sediment transport.

Assateague Island
Source: IAN Image Library

Circulation and Sediment Transport

Assateague Island: The stabilization of Ocean City Inlet by jetties blocked longshore sediment transport immediately after their construction in 1935. Assateague Island, downdrift from the jetties, underwent severe erosion and a massive ebb-tidal delta formed seaward of the inlet because of changes to the sediment transport pathways.