Water And Oceanography

All about Tides



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What causes the tides?

Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also  being pulled toward the moon Since the earth is rotating while this is happening, two tides occur each day. Isaac Newtonwas the first person to explain tides scientifically. Tides are most pronounced along the coastline of the oceans and in bays where tidal range (the difference in height between low tide and high tide) is increased due to the topography and other factors. 

 The Bay of Fundy between Nova Scotia and New Brunswick in Canada experiences the world's greatest tidal range of 50 feet (15.25 meters). This incredible range occurs two times ever 24 hours 52 minutes so every 12 hours and 26 minutes there's a single high tide and a low tide.

The Bay of Fundy tides are one of 30 locations worldwide where the power of tides can be harnessed to turn turbines to produce electricity. This requires tides greater than 16 feet (5 meters). In areas of higher than usual tides a tidal bore can often can be found. A tidal bore is a wall or wave of water that moves upstream (especially in a river) at the onset of high tide.

Predicting Tidal Currents by Machine

Since the tidal current velocities in any locality may be expressed by the sum of a series of harmonic terms involving the same periodic constituents that are found in the tides, the tide-predicting machine may be used for their prediction. The harmonic constants for the prediction of current velocities are derived from current observations by an analysis similar to that used in obtaining the harmonic constants from tide observations. For the currents, however, consideration must be given to the direction of flow, and in the use of the machine some particular direction must be assumed. The machine can be used for the prediction of reversing currents in which the direction of the flood current is taken as positive and the maximum velocity in this direction corresponds to the high water of the predicted tide. The ebb current is then considered as having a negative velocity with its maximum corresponding to the low water of the predicted tide. Rotary currents may be predicted by
taking the north and east components separately but the labor of obtaining the resultant velocities and directions from these components would be very great without a machine especially designed for the purpose. Predictions can, however, be made along the main axis of a rotary movement without serious difficulties.

The knowledge of the height of tides, both low and high, is vital for many functions, including navigation, fishing, and the construction of coastal facilities.



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