Ocean Tides are the periodic rise and fall of the ocean waters due to the gravitational pull of the moon and the sun on the surface of the earth. This paper is all about oceanic tides in which we shall discuss what causes tides, what are high and low tides, the types of tides, the phenomenon of tidal currents and waves, the effect of tides on the coastlines, and on fishing, and how tides can be harnessed for energy. We shall also discuss other areas in which knowledge of tidal data can prove useful.
What Causes Tides?
Tides (the periodic rise and fall of the ocean levels) are caused by the gravitational pull of the moon and the sun. Apart from the oceans, tides also occur to a lesser extent in large lakes, the atmosphere and in the solid crust of the earth. Although gravity of the moon and the earth is the single most important factor in making tides happen, other non-astronomical factors such as configuration of the coastline, depth of the water, and ocean-floor topography also play a role in altering the range, interval between high and low water, and times of arrival of the tides. ("Our Restless Tides" -- Chapter 1: Introduction)
High and Low Tides
The gravitational force of the earth at its surface acts in an inward direction towards its center. This force keeps the ocean (and other) waters confined to the surface. At the same time, the gravitational force of the moon and the sun exert an outward pull on the surface of the earth. Since the earth solid surface is a rigid body, it remains relatively unaffected by this "pull." However, the ocean waters, being liquid and less rigid, are physically pulled outwards towards the moon and the sun by their gravitational force. This outward pull is not uniform over the earth's surface since the gravitational force of the moon (or the sun) acts on the side of the earth facing it. Hence 'high tide' is produced in the ocean waters by the "heaping" action resulting from the horizontal flow of water towards regions of the earth facing the moon or the sun. 'Low tide' occurs in the regions of the earth where the waters of the ocean have withdrawn to compensate for the water "heaped" towards the gravitational "pull" of the moon or the sun. ("Tides"-Encarta; "Our Restless Tides" -- Chapter 2) The highest tides in the world occur in the Bay of Fundy in Canada, where the difference between high and low tide is about 60 ft.
Lunar and Solar Tides
Since the moon is much nearer to the earth than the sun, its gravitational force at the surface of the earth is much greater; therefore the moon is the major cause of ocean tides and tides that occur due to the moon's gravitational pull are called 'lunar tides.' The solar tide is caused by the sun the gravitational force exerted by it is only about 46% of lunar force. The lunar and solar tide-causing forces may accentuate or cancel other depending on the relative positions of the moon and the sun with respect to the earth.
The tidal period or the cyclic pattern of the tides depend on the speed of Earth's rotation as well as that of the revolution of the moon around the Earth. If the moon were stationary in space, the tidal cycle would have been exactly 24 hours long. However, since the moon is not stationary and revolves around the Earth (once approximately every 27 days), it adds about 50 minutes to the tidal cycle. The tidal period is, therefore, 24 hours and 50 minutes in length. (Pidwirny)
Spring and Neap Tides
Spring Tide occurs when the gravitational forces of the moon and the sun complement each other, i.e., the direction of their gravitational forces are in a straight line. It produces the highest and lowest tides and occurs every 14~15 days coinciding with the new and full moon.
Neap Tide occurs when the gravitational pull of the moon and sun are at right angles to each other. It also occurs every 14 to 15 days, coinciding with the first and last quarter of the moon. The tidal variations during neap tides are the least. ("Tides"-Encarta; Pidwirny)
Diurnal, Semi-diurnal, & Mixed Tides
The positions of the moon and sun relative to specific locations on the Earth's surface result in three different types of tides. When tides have one high and one low water per tidal period, the tide is known as diurnal tide. This type occurs in parts of the northern Gulf of Mexico and Southeast Asia.
When tides have two high and two low waters per tidal period, they are known as Semi-diurnal tides. They commonly occur on the Atlantic coasts of the United States and Europe.
The third type of tides is known as mixed tide in which there is a higher high water and lower high water as well as higher low water and lower low water. This type of tide occurs around the west coast of Canada and the United States. (Pidwirny)
Effect of Tides on Coastlines
Various horizontal and lateral movements that are known as tidal currents or streams also accompany the vertical rise and fall of water levels during a tidal cycle. Rising tides produce flood current while falling tides produce ebb current. While the flood and ebb currents do not cause significant erosion by themselves, they can cause considerable damage to the coastlines when accompanied by tidal waves initiated by hurricanes, or under-sea earthquakes and volcanic eruptions. Fluctuating tides, however, are responsible for a unique environment along shorelines with the current continually circulating and replenishing a rich supply of nutrients along beaches. Frequent shifts between complete subversion and open air give rise to the development of unique life forms and marine organism on the coastlines that adapt to survive in contrasting environment. (Gore)
Effect of Tides on Fishing
The relationship between various tidal periods and their effect on fishing is a matter of great speculation. Opinions range from no relationship at all to very strong relationship. Most experienced outdoorsmen and fishermen believe that high or spring tide periods are particularly suited for fishing since strong tides produce more water movement. An experienced outdoorsman Captain Robert McCue says: "The strongest tides will happen during the time when the moon is either directly overhead - what's referred to as moon over, or directly behind your position on the earth - moon under." (Quoted by Davidson-"Moon Phases.."). "Overhead" occurs about 6 hours after moonrise and "directly behind" 18 hours after moonrise.
Apart from tide and moon phases the other phenomenon that affects fishing is the solunar period. McCue says: "There is no question that fish, especially harder to catch species like tarpon and snook, will feed more freely and strike a bait more readily during a major solunar period." (Quoted by Davidson-"Solunar Period"). The best time for fishing, according to McCue is when a major Solunar Period coincides with a strong (spring) tide. The "minor" solunar period is also considered to be a better than normal fishing period, while the time "in between" solunar periods is reputed to be the worst time for fishing. (Ibid.)
Tides in Gulf of Mexico and the Texas Gulf Coast Regions
Nowadays accurate and detailed tidal data is available for the current as well as day-ahead periods on various web sites. Web Site addresses on which tidal data at different points in the Gulf of Mexico and the Texas Gulf Coast Regions are available.
The energy of tides can be harnessed to produce electricity. The principle used is the same as that which drives conventional hydropower turbines. The incoming tide is made to flow through a dam, driving turbines, and then is trapped behind the dam. When the tide ebbs, the trapped water is released and flows back through the dam, again driving the turbines. Such tidal power plants are feasible only where the difference between high and low tides is great. The first such 'tidal' plant of 240,000 kW capacity was put in operation in 1966 on the Rance River, an estuary of the English Channel in northwestern France. ("Tides"-Encarta) The difference between high and low tides at the Rance estuary is about 28 ft.
Apart from its usefulness in 'fishing' and 'electricity generation' knowledge of the times, heights, and extent of inflow and outflow of tidal waters is of importance in several applications. These include navigation through intracoastal waterways, and within estuaries, bays, and harbors; work on construction of bridges, docks, breakwaters, and deep-water channels. It is also important for demarcation of legal coastlines for determining offshore territorial limits, and for activities such as underwater demolition work, military engineering uses; and for water sports activities like boating, and surfing. ("Our Restless Tides" -- Chapter 2)
Daily Tide Predictions." Nautical Software's Graphical Tide Charts. Tide Data for Gulf of Mexico. 2003. April 17, 2003. http://duckcentral.com/daily_tideC.htm