The concept of real tides
The concept of real tides
In a traditional sense, tides refer to the periodical rise and fall of water levels in the oceans and seas. Geologists, oceanographers and other scientists within earth sciences studied this phenomenon based on a simplified model compared to the actual model. Their studies revealed that tides resulted from the gravitational forces of three main celestial bodies; the sun, earth and moon. The moon’s gravity differential field acting upon the earth’s surface was discovered to be the principal tide generating force. Denny and Paine say it is responsible for the two equipotent bulges at opposite ends of the earth surface, which represent high tides due to centrifugal forces and the moon’s gravitational attraction (2). The areas remaining opposite areas represent low tides. This simplified model is however based on the equilibrium theory of tides.
Real tides are however much more complicated. The equilibrium theory of tides assumes that the sea surface that would be in equilibrium with the tide generating forces is elevated, and as such the earth surface is covered with water. The water would then respond instantaneously to these forces. Real tides on the other hand form on the real earth surface which is uneven in many respects. The ocean basins in reality have a very complex topographical bottom and coastal borderlines (Wright, Colling, and Parks 56). Real tides are the outcome of the interference of multiple tidal components by constructive and destructive waves resulting in a mixed wave. Real tides occur with the interference of semidiurnal tides and diurnal tides making them more complex. They are a representation of what actually takes place unlike the equilibrium theory of tides.
The main factors of real tides
In their research, Pham and Martin concluded that there are a numerous factors that directly influence real tides (67). The first factor that directly impacts the movement of water is the position of the sun and the moon. The moon is much closer to the earth compared to the sun. Its proximity and gravity differential field aggregated with that of the sun determine the timing and amplitude of real tides. The second factor is the topographical nature and features of the particular earth surface. In this case mainly land masses, ocean bed and coastal features. The third is the earth’s declination. The earth is tilted at 23°27’ vertically off, this coupled with the earth’s position relative to the position of the sun and moon as the earth revolves around the sun, the moon around the earth and both earth and moon rotating on their own axis (Pham and Martin 73).
Another important factor is the basin oscillation of the particular water body. Pham and Martin add that all water bodies, based on their size and shape, have a natural oscillation period. In any large water body such as oceans, there exists a number of different oscillating basins which affect real tide movement and wave formation, which are based on the degree of resonance with the tidal curvature (81). Cairns for example, is the most northerly part of Queensland, Austria. It is renowned for its frequent tropical cyclones. Most recently, Cyclone Yasi in 2011 resulted in the formation of waves up to 12 meters high at the coast of Cairns. These are as a result of the earth’s rotation and the subsequent blowing wind in the southern hemisphere (Tang, Yong Ming, et al. 18). The moon’s position relative to that of Cairns at the time played a big part. The Coriolis Effect of the earth’s rotation coupled with its inclination and the tide generating forces relative to the position of Cairns are also major determinant factors in the tidal formation in Cairns.
Denny, Mark W., and Robert T. Paine. “Celestial mechanics, sea-level changes, and intertidal ecology.” The Biological Bulletin 194.2 (1998): 108-115.
Pham, C., and Vanessa A. Martin. “Tidal current turbine demonstration farm in Paimpol-Brehat (Brittany): tidal characterization and energy yield evaluation with Telemac.” Proceedings of the 8th European Wave and Tidal Energy Conference, Uppsala, Sweden. 2009.
Tang, Yong Ming, et al. “A numerical study of storm surges and tides, with application to the North Queensland coast.” Journal of physical oceanography 26.12 (1996): 2700-2711.
Wright, John, Angela Colling, and Dave Park, eds. Waves, tides and shallow-water processes. Vol. 4. Butterworth-Heinemann, 1999.
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