Bottom sediment is transported by flow if the velocity at the bed is large enough to bring the sediment particles into motion. Initially the particles start to rollover the seabed: this is bottom or bedload transport. For larger velocities, the sediment particles will also go into suspension. Sediment which is transported within the moving fluid is called suspended load transport.

One of the major mechanisms which causes net transport in the cross-shore direction (i.e. perpendicular to the coast) is the undertow. Undertow is a circulation current in the surf zone that is initiated by the process of wave breaking (see Fig. 51). Dune erosion during a storm surge (see Fig. 52) is an example of the effect of sediment transport in the cross-shore direction.

Incoming waves with their crests oblique to the coast induce a longshore current in the surf zone (see Fig. 53). This long shore current is the result of net longshore directed forces that emerge in the wave breaking process. The rather small long shore current transports relative large amounts of sediment. This can be understood since the breaking waves cause large flow velocities near the bed which bring large quantities of sediment into the flow to be transported by the (rather weak) current.