Tides are another aspect of the restless ocean visible to a keen observer on a beach. Forced by the gravitational attraction of the Moon and the Sun, tides are periodic (Fig. 19) and highly predictable. Tide tables are prepared for major ports a year in advance. Tides are a special class of surface gravity waves: they are shallow-water waves, so called because they occur in water that is shallower than half their wavelength. The speed of shallow-water waves is given by √gH , where g is the Earth’s gravitational acceleration and H is the ocean depth. In the open ocean, which is about 3700 m deep, these waves have a speed of 700 km/hr, which is the speed of a typical jet airplane. In shallow-water waves, momentum does not decay with depth and there is motion even at the bottom (Fig. 24).
Tides have wavelengths ranging from 100-10000 km, and periods of 12.5 (semidiurnal tides) and 24 hours (diurnal tides). Along the Indian coast, the tides observed are a combination of both semi-diurnal and diurnal tides. The tides and tidal currents are weaker along the southern part of the Indian coast (current speeds of the order of a few tens of cm/s). The magnitude of tidal currents increases northward, and it reaches very high values in certain areas like the Gulf of Kutch and the Gulf of Khambhat, where the speeds can exceed 2 m/s.