In the shallow waters of the continental shelves the tidal currents of different types are caused by the tides generated in the open oceans. In fact, tides and tidal currents are two manifestations of the same process.
There is a very close connection between them. Tidal current observations taken over longer time periods clearly show that tidal currents no more repeat themselves at one place than do tides. Tidal currents differ from place to place.
Tidal currents generally represent two dimensional processes, whereas tides are one dimensional. However, in exceptional cases namely, in rivers and in the immediate vicinity of the coast, where the bottom topography permits only alternating motion, the tidal currents become one dimensional.
It is to be kept in mind that in a tidal wave the water particles move only to and fro, but when under favourable conditions tide-producing forces produce the horizontal flow in water, we call it a tidal current.
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At the time of the high tide, on-shore tidal currents are produced, whereas at the time of low tide (ebb tide), the tidal currents move in the opposite direction i.e., towards the sea.
The tides have a dominant control over the tidal currents. For example, if there are semi-diurnal tides in a particular ocean, then on its continental shelf there would be semi-diurnal tidal currents.
Of course, the tidal currents are influenced by a number of factors, namely, the bottom relief, depth of water, the, width of the shelf, friction and the rotation of the earth.
If on the opposite ends of a strait the timing of the tides is different, then in such cases the tidal currents originate due to difference in the water level on opposite sides. In fact, such currents are compensatory currents. Such a current flows through the strait with great velocity.
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Wherever these currents flow between various small islands, a large number of eddies and whirlpools are produced in water. The tidal currents produced in estuaries and rivers are dependant on the tides of the sea and the nature of the coast line.
As stated earlier, the tidal currents are considerably affected by friction as well as density stratification. Friction affects the tidal current at the bottom. However, owing to turbulence in the water which depends on the density stratification, the friction influences the tidal current in the whole water column.
It may be mentioned that Sverdrup and Fieldstad have discussed the influences of friction on tidal currents on the basis of the Maud observations made in the north Siberian shelf seas.
Other oceanographers have also investigated the frictional influences on currents. Other technical aspects of the tidal currents need not be discussed here due to limit scope of the book.