Tidal Energy - Theory

Tides and The Earth-Moon System

The Combined Roles of the Moon and the Sun

The Dynamic Theory of Tides

The previous animations illustrated the equilibrium theory. However, tides experienced on Earth do not exactly behave as equilibrium tides. The main reasons are:

• The wavelength of tidal waves depends on the depth of oceans,
• The Earth spins on itself too fast, resulting in a time-lag in the response of oceans to the tractive forces,
• Landmasses interfere and constrain tidal flows, so the tidal bulges cannot directly circumnavigate around the Earth,
• The Coriolis force affects the water movements.

These factors are included in the dynamic theory of tides and resulted in the conception of amphidromic systems. There are several amphidromic points on Earth around which the tidal waves circulate. The figure below illustrates the worldwide amphidromic systems, with the tidal range on a colour scale (red for the greatest range, blue for the weakest).

The Influence of Tides on Currents

A low tidal range does not necessarily imply slow tidal currents, and vice-versa. Tidal currents velocity greatly depends on local topography.
However, on a selected site, knowing the tidal regime and its range allow the prediction of tidal currents relative velocity and direction.

Tidal currents can have 3 states:

• flood current: currents flowing from the ocean to the shore,
• ebb current: currents flowing from the shore to the ocean,
• slack water: current having little or no horizontal motion

If slack waters happen at the time of maximum and minimum tidal height, the profile of the currents is called a standing wave. Instead, if the maximum current velocity happens at the same time as the maximum tidal height (and minimum velocity at the same time as minimum tidal height), the profile of the currents is called a progressive wave.