Tides are regular fluctuations in water level due to the gravitational attraction of the Sun and the Moon on the Earth’s hydrosphere, but their effects are hardly noticed in deep water. This is not the case in the near-shore areas and tidal currents have an important role to play in sediment movement. By way of introducing their importance we will outline the fundamentals of their generation. Tides are in fact waves with extremely long wavelength, but unlike sea waves are unable to progress without astronomical forcing due to the shallowness of the oceans. Since gravitational force is a function of a body’s mass and declines with distance, the Moon is of greater importance than the Sun to the Earth’s tides. Although the Sun is much bigger than the Moon, its pull is less than half that of the Moon because it is so much further away. The basic tidal pattern is a semi-diurnal one, with two high tides in 24 hours. However, the relative magnitude of the two high tides may vary from location to location, giving a more complex tidal signature. In fact, in some cases, the second high tide is so small that the tidal pattern is effectively a diurnal one. In other areas, the tidal signature changes from a diurnal to a semi-diurnal pattern during a lunar month, which is the time taken for the Moon to rotate around the Earth’s axis (29 days). The timing of a high tide progresses by 50 minutes every 24 hours, so high tides do not occur at a constant time each day. The magnitude of high tides varies over a lunar month and over a year. High tides are often much greater close to the shore, in shallow water, than offshore in the ocean.
Tidal range in some estuaries or bays is extremely high relative to that in adjacent sea or ocean basins. This adds to the scale of regional and local variations in tidal range. Tides form when ocean water on the surface of the Earth passing under the Moon or the Sun is pulled towards it, forming a bulge. A bulge also forms on the opposite side of the Earth as a result of centrifugal force. This can be explained as follows: the Earth and the Moon rotate around a common axis and do not collide as a result of their mutual gravitational attraction due to the centrifugal forces generated by their rotation; the gravitational forces are equal and opposite to the centrifugal force, which gives stability. This balance is only a planetary average, however All objects on the Earth experience the same centrifugal force, but those closest to the Moon at a particular point in time experience a slight excess of gravitational attraction, leading to the first water bulge on the surface of the ocean. At the opposite side of the globe, where the gravitational attraction of the Moon is at its least, there is a slight excess in centrifugal force over gravitational attraction, which gives the second and complementary bulge in the water surface. The consequence of this double bulge is to give a semi-diurnal periodicity to the tide.