The presence of the sea floor in the depth range of moderate or strong water movement is responsible for frictional effects that cannot be ignored in the coastal ocean but are usually of no importance in the deep ocean basins. Water movement over the sea floor establishes a frictional boundary layer near the bottom, in the same way as wind blowing over the sea surface produces the Ekman layer, a frictional boundary layer at the sea surface through which momentum is transferred from the wind to the current. Bottom currents in the deep ocean are usually so sluggish that the associated frictional boundary layer can be disregarded in the dynamics. Chapter 3 investigates the role of frictional boundary layers in shallow water. The second effect of shallow water depth is an enhancement of tidal motion .
Tides are oceanic waves of very long wavelength and generally small amplitudes in deep water.
This is associated with a significant increase of the tidal current which in many shelf seas becomes the dominant water movement. These and related aspects are the topic of Chapter 5. The third difference between shelf seas and the deep ocean relates to the presence of coasts . Coastlines are an obstacle to the free flow of water. A current approaching a coast will develop flow convergence, which will result in acceleration of the flow.