The tide phenomenon is the periodic motion of the waters of the sea—and is observed upstream of several rivers—caused by celestial bodies, mainly the moon and the sun. The tide results from the gravitational pull and the earth’s rotation. Tide and tidal currents must be differentiated, for the relation between them is not simple, nor is it everywhere the same. In its rise and fall the tide is accompanied by a periodic movement of the water, the tidal current; the two movements are intimately related.
The current experienced at any time is usually a combination of tidal and nontidal currents (cf. p. 67). Offshore, the direction of flow of the tidal current is usually not restricted by any barrier and the tidal current is rotary.
The tidal current is the rotary current that accompanies the turning tide crest in the open ocean and becomes a reversing current, near shore, moving in and out, respectively as flood and ebb currents. There is an instant or a short period—the slack period—when there is little or no current, at each reversal of current direction. During the flow in each direction current speed varies from naught to a maximum— strength of flood or ebb—about midway between the slacks. The shorewards, and upstream, movement is the flood and the seawards, and downstream, movement is the ebb.
Both the rise and fall of the tide, and the flood and ebb of the reversing current can be harnessed to produce mechanical and/or electrical power. Tidal currents are alternating and their maximum velocity occurs at high and low water. The motion is uniform from surface to bottom, except for wave interference at the surface and increases with distance. Because of superimposition by other currents, observation of tidal currents is difficult and requires extensive complex data.
Tidal currents may be semi-diurnal, diurnal or of mixed type, corresponding largely to the type of tide at the site, but often with a stronger semi-diurnal trend. The most common type is a to a greater or lesser degree, mixed one.
Treating tides as waves, a progressive tide wave will have a shallow wave horizontal orbital velocity (Us) given by (8.1) wherein A is the wave amplitude, a the angular velocity of a particle undergoing a circular motion as the tide wave passes by, k the wave number, h the water depth in which the wave is progressing, x the distance from a point of origin and t the time from a particular instant:
Us = Aa/khcos(kx + at) (8.1)
Tidal currents are an appreciable energy resource in relatively shallow water, near continents. When particular geometry comes into play, bottom and sides may impede the flow and speeds from 9 to 19 km/h have been registered.
