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. Alternatively, convergence of surface currents can force surface water to sink to greater depth. Where a wind produces water movement away from the coast, the resulting surface divergence causes a rise of subsurface water to the surface, a process known as upwelling. Chapter 6 addresses these processes in detail. A fourth, important aspect is again a consequence of the presence of coasts and related to variations in sea level produced by flow convergence and divergence.