Since the processes in the coastal zone depend strongly on the local sediment, the tide and the effects of wind and waves, these topics will be illustrated briefly in this chapter.

Tides
The tide affects the coastal area in two ways. Firstly, it induces the rise and fall of the water level. Secondly,. it produces currents in the coastal zone.

The tide is produced by the interactive forces of the moon, sun and earth. The forces are closely related to the relative motion of these bodies. In order to give some insight to the process, it is schematized in Fig. 32. On the left side of the figure, the forces on the earth's surface that result from the earth-moon interaction only, have been presented. Whereas the gravitational force and the centrifugal force balance for the earth as a whole, there is a resulting force at each individual point of the earth's surface: the tide producing force. On the right side of the figure, the tide produced by the moon and sun has been shown under the assumption that the earth is completely covered with deep water: the equilibrium tide. The position of the moon and sun have been shown both for spring tide and for neap tide. Due to the larger distance to the earth of the sun, its effect is about half of that of the moon. In Europe the tides are mostly semi-diurnal (i.e. two high waters and two low waters each day).

An impression of the effects of the tide on European ports is given by the tidal ranges (i.e. the water level variation due to the tide) listed in Table 1.