How do Tides Occur?


What are tides?” They are simply the rise and fall of the sea levels mainly affected at the shores. Before we look into how tides occur, we must understand “why do tides occur?”, and the answer is “Gravity”.  Any object which has mass possesses a gravitational field and the larger the mass the stronger the gravitational field is. The gravitational force between two bodies also depends on the mass of the two bodies and the distance that separates their centres of gravity. As the bodies go further apart the gravitational force grows weaker. So, if we look at the gravitational fields that are at a significant value and distance are the ones of the Moon and the Sun. Although the sun is far heavier than the moon and hence has a stronger gravitational field, the moon’s gravitational field is the major factor contributing partner to the rise and fall of the sea levels. This is due to the moon being closer to the earth (3, 86,240 kilo meters) than the sun (approx. 150 million kilo meters).

Why do tides occur?

Why do tides occur? (Image source: sxc.hu)

Interaction of the Moon with the Earth

The area where the moon’s gravity interacts directly with the water bodies is the place where the high tides occur (when the moon is either directly above that area). The area whose face makes a 90 degree with the gravitational field is the location of the occurrence of the low tides. The figure given below illustrates the same.

 How do Tides occur

The figure above only shows the tidal formations at a particular instant. The earth and moon are not still; they have a relative motion in between. The earth rotates constantly and the moon revolves around the earth. Because of this, one particular shore experiences an alternate high tide – low tide for two times in a time period known as the “Tidal Cycle”. The tidal cycle lasts for 24 hours and 52 minutes. In this period, there will be two each of high tides and low tides.

 

Effect of the Relative Motion between Moon and Earth

The oceanic bulge that forms because of the moon’s gravitational field follows the path of the revolution of the moon. This moving tide is also influenced by the earth which rotates from west to east. Therefore the eastward rotation passes through the tidal bulge once in every tidal cycle. Because of this, the opposite side of the earth which is directly away from the moon’s influence also faces a high tide because of “inertia”. While two sides have a high tide, the areas in between these high tides experience a low tide.

 

To understand the effect of inertia, take a bowl of water and slide it across the table. One end of the water rises up and the water level becomes inclined opposite to the direction of motion. Once you stop the bowl, a high wave is generated on the exactly opposite end of the bowl. This is because of inertia (Newton’s first law).

The Tidal Cycle

Let us suppose that the tidal cycle begins with the high tide. After 1/4th of the tidal cycle (6 hours and 13 minutes), the high tide begins to recede back into the sea. This receding tide is known as an “Ebb Tide”. The ebb tide is a transition phase from the high tide to low tide. This causes a low tide. Again after another 1/4th of the tidal cycle, the water starts to flood back in bringing in the high tide. This tide is known as the “Flood tide”.

 

Effect of Topography on the Tides

When we are out at sea, we cannot observe a low magnitude tide up to a noticeable level. It just passes us like a big size wave. But, when there is an inconsistency in the topography, such as a deep trench or areas along the coastline, the tide can be noticed more prominently.

 

These irregular geometries fuel tides with greater velocity and power. Sometimes they can be so powerful that some can capsize boats. The heat generated by the tidal currents can corrode the propellers and metal sheets of ships too.

 

The world’s highest tidal range occurs in Canada in the Bay of Fundy which is between the cities of Nova Scotia and New Brunswick. The height of the tides can climb up to 16 meters (50 feet). The Bay of Fundy experiences such tides not once, but twice in a tidal cycle (24 hours 50 minutes). So, there is a tide of 15 meters followed by a low tide every 12 hours and 25 minutes.

Bay of Fundy

Left: High tide at the Bay of Fundy. Right: Low tide at the same harbour at the Bay of Fundy (Image source: Wikipedia)

Harvesting Tidal Power

Tides of significant altitudes can be made to rotate turbines to generate power. There are thirty locations worldwide which are capable of harvesting energy from tides (including the Bay of Fundy). The coastline of Australia is also well known for high altitude tides. Turbines are placed in the path of the tidal currents or power generating dams (impoundment dams) can be built across the path of the tides.

 

There are many problems which occur when we need to harvest energy from tides. The first and foremost concern is the timing. The energy can only be harvested when the tides are in movement. Next is the installation of turbines can disrupt the ships and building of dams can obstruct the natural river cycle. The tidal currents can produce frictional heat on the turbine materials causing the metals to corrode lowering the efficiency of production.

 

Common Misconceptions about Tides
  • Giant waves caused by hurricanes (known as ‘hurricane tides’ or ‘rip tides’) are also confused with the actual ‘tide’ caused by the moon’s gravitational field  due to the use of the word ‘tide’ to describe them.
  • Most people think that all tides are killer waves and wipe out people but reality is that most tides are even too small to notice unless they are measured by scales.
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