Prof. B.S. Thandaveswara. The periodic rise and fall of the planetary ocean level in response to the gravitational

Transcription

1 37.5 Tides The periodic rise and fall of the planetary ocean level in response to the gravitational interaction of the earth, moon, and sun. The periodic rise and fall of sea level that results from the gravitational attraction exerted on the earth by the moon, and to a lesser extent by the gravitational pull on the sun, is the tide. Tides are complex because of: (1) the movements of the moon in relation to the earth's equator, () changes in positions of the moon and sun with respect to the earth, (3) uneven distribution of the water on the earth's surface, and () irregularities in the configuration of ocean basins. Because of these variables, tides vary. For example, in the Bay of Fundy, between New Brunswich and Nova scotia, they are over 1 meter high, at Gulf of Camby (Gujarat, India, it is of the order of 8 m and in chennai it is above 1 m) but along the coast of the Mediterranean Sea, they are virtually unnoticeable. Measurements of tides can be made by using an ordinary graduated pole anchored to the ocean bottom. Periodically at, say, one-hour intervals, reading of the water level on the pole are recorded. The height of the water at given interval will determine the construction of the tidal curve.

2 N A E N P S September E New York S MLW MLWS Port Adelaide Seattle S Total datum MHHW MLW MLLW* Los Angeles MHHW MLW MLLW* Tidal curves for selected coastal cities (Source: Nathaniel Bowditch, American Practical Navigation, rev. ed., H.O. Publ. 9, Washington, D.C.) Tides are at their maximum when the moon and the sun are in the same plane as the earth. These spring tides occur every 1 days, at new and full moons. When the moon and the sun are at right angles to each other, low tides occur. These neap tides occur every 1 days, always at half moon. See figure

3 Last quarter (neap tide) New (Spring tide) Sun Full (Spring tide) First quarter (neap tide) Spring tides occur with greatest amplitude when the Sun and are lined up at the same time of new and full moon. Neap tides occur with lowest amplitude when the moon is at first and last quarter Diurnal and semidiurnal tides occur because of the gravitational attractions of the earth, the moon, and the sun. The moon, because of its closeness to earth, is the strongest influence on tides. The tide producing force of the moon is twice as strong as that of the sun. The moon takes 9.53 earth days to complete one revolution around the earth. During the course of this revolution, the earth and the moon are gravitationally attracted. But this gravitational attraction is balanced by centrifugal force, which orginates from their orbital motion about each another (figure). Interaction between gravitational and centrifugal forces is the primary source of tides.

4 N Water surface (tidal bulge) F c (Centrifugal force) F g (gravitational force) S The primary cause of tides is the gravitational pull of the moon. Although its pull is only 1/3, of the earth's gravitational force, it is sufficient to raise the sea level on the moon-facing portion of the earth; Centrifugal force balances the opposite side Tidal Bores: A steep-nosed tide crest rushing (along with a high tide) upstream. Often under favourable conditons in some tidal rivers, incoming high tides reverse their flows as they move upstream in breaking waves called tidal bores. Tidal bores have been reported to rush between 5 and 3 kilometers per hour and have heights up to 1 metres. Notable examples of tidal bores include the Tsientang Kiang River at Hangschou Bay in China (which has a 3.5 meter height and moves at about 1 meter per second) and the Amazon river (which has a 5-meter height and moves at more than.5 meter per second). Summary 1. Waves are formed in response to prevailing winds. Waves are described in terms of height, wavelength and period.. The Mechanism of wave formation in complex and is not yet fully understood. 3. Major wind generated waves include seas, swells, and surfs.. Wave refraction is a phenomenon that waves display as they approach shore.

5 5. Longshore currents and rip currents are important types of wave currents.. Catastrophic waves include Tsunamis, which are produced either by volcanic or by earthquake activity. 7. Storm waves or suges produced by abnormal meteorlogical and oceanic conditons. 8. Internal waves are found below surface waters. When they emerge at the surface, they are identifiable as slicks and carry fine sediments and plankton. 9. Tides are a daily phenomenon produced by the gravitational force of the moon, and to lesser extent, by the sun. 1. Tides are harnessedto manufacture electricity, for example, in France. There are 1 locations throughout the world under consideration to obtain energy from tides. World's Potential Tidal Power Plant Sites (Selected Sites only) 1. Cook inlet (Alaska);. Baja Calfornia; 3. Passamaquoddy (Maine);. Bay of Fundy; 5. Frobisher Bay (Canada);. Maranho (Venezuela); 7. San Jose Gulf (Argentina); 8. Severn River (Great Britian); 9. Rance River (France); 1. Kislaya (USSR); 11. Cambay River (India); 1. Seoul River (South Korea); 13. Abidjan (Africa); 1. Darwin (Australia); 15. Kimberleys (Australia);

6 Most tidal curves record two high tides and two low tides per tidal day, occuring every hours and 5 minutes (Figure 8-1). The time 1 hours and 5 minutes, between high or low tides is called tidal period. The combination of two high and low tides is called semidaily or semidiurnal tides. When there is only one high tide and one low tide daily, they are called daily or diurnal tides. Often the tidal curves record two high tides and two low tides per tidal day, but if there are differences between two high tides and between two low tides, mixed tides result. Mixed tides are abbreviated HHW (higher high water) and LHW (lower high water). Similarly, low and lower marks are expressed as LLW (lower low water) and (higher low water). Tide predicting machines are used at many locations.