Publications

Survey of Energy Resources 2007

The Tides

The tides are cyclic variations in the level of the seas and oceans. Water currents accompany these variations in sea level which in some locations, such as the Pentland Firth to the north of the Scottish mainland, can be extreme.

Small tidal 'mills' were used in Southern England and Northern France in the Middle Ages. Tidal flows in bays and estuaries offered the potential to drive cereal-grinding apparatus in areas that were too low-lying to allow the use of conventional water wheels. In the 20th century, tides were seriously re-examined as potential sources of energy to power industry and commerce.
The explanation of the existence of tides represented one of the greatest challenges to early oceanographers, mathematicians and physicists. It was not until Newton developed his theories of gravitation and the mechanics of motion that a satisfying theory emerged to explain at least some of the properties of the tides. The physics of the 'Newtonian Tidal Theory', which is sometimes referred to as 'Equilibrium Tidal Theory', gives a partial description of tidal behaviour for an abstract planet Earth entirely covered by water, and is outlined in most introductory texts on oceanography (Bearman, 1997).

This theory suggests the establishment of 'bulges' in the fluid surrounding the Earth as shown in Fig. 13-1 .

As the Earth rotates, the two tidal 'bulges' appear to travel round the Earth at the same rate as the Earth's rotation. The Moon rotates around the Earth (actually about the centre of mass of the Earth-Moon system) every 27.3 days, in the same direction as the Earth rotates every 24 hours. Because the rotations are in the same direction, the net effect is that the period of the Earth's rotation, with respect to the Earth-Moon system, is 24 hours and 50 minutes. This explains why the tides are approximately an hour later each day.

The equilibrium theory can be extended to include the influence of the Sun. It is possible to consider the establishment of solar 'bulges' in the Earth's oceans as well as the lunar 'bulges'. When these approximately superimpose at the full moon and the new moon, large spring tides occur. At the half-moon stage of the lunar cycle, the solar and lunar bulges are 90o out of phase and small neap tides occur

In effect, the tides represent the terrestrial manifestation of the potential and kinetic energy fluxes present in the Earth-Moon-Sun system. These fluxes are complicated by the presence of continents and other landmasses, which modify the form and phase of the tidal wave. This results in some regions of the world possessing substantially higher local fluxes than others. The Bay of Fundy in Canada and the Bristol Channel between England and Wales are two particularly noteworthy examples.