Tides

Mont St. Michel

What is a tide? Tides are huge shallow water waves that happen at given periods throughout the day. They cause short-term changes in the height of the ocean surface in a particular place, due to the gravitational force of the sun and the moon and the Earth’s rotation.

Earth-Moon System The Earth and moon form a system around which they both rotate. Because the Earth’s mass is 81 times larger, the common center of mass is located 1,023 miles below the Earth’s surface. Each revolution takes 27.3 days.

Earth-Moon System

What causes the tides? The sun and moon both play a part in the tides. Even though the sun is so much larger than the moon, it plays less of a role than the moon does. Due to the fact that the moon is much closer to Earth than the sun, the moon has about twice as much influence on the tides as does the sun.

Tidal Bulges

The Moon’s Influence The moon’s gravity pushes the ocean’ s surface. As the Earth spins on its axis and the moon pushes at the oceans, a bulge occurs under the side nearest the moon. Another bulge occurs on the side opposite the moon due to Earth’s rotation and inertia.

Tidal Bulges As the Earth turns eastward, an island on the Equator moves in and out of the tidal bulges. When the island is in the bulge, the tide is high and when the island passes through the trough (the area between the bulges), it is low tide. Every 12 hours and 26 minutes, the place will experience a high tide. Six hours and 13 minutes after a high tide, it will experience a low tide.

Due to the fact that the moon’s orbit does not stay directly above the Equator, but moves north 28.5 degrees and south 28.5 degrees, the tidal bulge will follow the moon. This means that an island at a latitude above or below the Equator will experience a very high tide, a low tide, a lower high tide and a low tide in the tidal cycle.

More of the Moon’s Influence Because the moon rises 50 minutes later each day, the highest tide will also arrive 50 minutes later. This is known as a complete tidal cycle which is 24 hours and 50 minutes long.

The Sun and the Tides The sun also has an influence over the tides. Even though the sun is 27 million times more massive than the moon, it is also 387 times farther away. Because of this, the sun’s influence on the tides is 54% less than that of the moon.

The Sun and Moon Together The sun and moon both influence the Earth’s tides. They can either work to increase the height of the tides or to decrease the height of the tides. The position of the Earth, moon, and sun in relation to each other is what will cause the changes.

Spring and Neap Tides

Spring Tides When the sun and moon are in line with Earth, the tides are higher and lower than normal. This is because the pull of the moon and the sun is added together. These higher and lower tides are known as spring tides, even though the term has nothing to do with the season of the year. Spring tides occur every two weeks in conjunction with the full and new moons.

Neap Tides Neap tides are when the Earth, moon and sun form a right angle. The sun’s pull will decrease the height of the tides, so that the high tides will not be very high and low tides will not be very low. These tides occur every two weeks also (arriving one week after the spring tide), in conjunction with the first and third quarters of the moon.

Astronomical Tides The orbits of the moon around the Earth and the Earth around the sun are not perfect circles. They are ellipses or ovals. When the Earth is closer to the sun or moon or both, they increase the pull on the oceans, causing extremely high and low tides. These unusual extremes are referred to as astronomical tides.

Tide Patterns Due to the shape of the coasts and whether or not they are somewhat enclosed, the tides have different patterns. For example, Cape Cod has a high tide and a low tide twice per day of equal heights, while Mobile, AL has only one high and one low tide per day of equal heights. On the west coast, the tides are mixed, having two high and low tides daily, but they are of different height, as seen in the next slide.

Tidal Datum The reference point to which tide height is compared is called tidal datum. It is the zero point seen in tide graphs. It may either be set at mean sea level, the height of the ocean surface averaged over 2 years or by the average level of the low tides, mean low water.

Tidal Range Tidal range is the difference between high and low water mark. It varies around the world depending on the location, the basin and it’s shape. In places like the Bay of Fundy, the range can be up to 50 feet. Along the coast of Maine, in our area the range is under 12 feet.

Tidal Range

Tidal Bores A tidal bore forms in some inlets due to the narrow configuration of the inlet. The tidal bore is a steep wall of water moving toward the end of the inlet at speeds up to 25 mph. The bores could be extremely dangerous if it weren’t for the predictability of their occurrences. In China, there is a tidal bore on the Qiantang River that is up to 26 feet high.

Bay of Fundy

Tidal Currents Tidal current refers to the rise and fall of sea level as the tide crest approaches and passes in a bay or harbor. The incoming tide is referred to as the flood current or flood tide and the outgoing current is the ebb current or ebb tide. As the tides are turning (when it is changing direction), it is known as slack water. The terms flood and ebb tide have no technical meaning but are often heard in reference to the direction the tide is moving. Tidal currents can move as fast as 7 mph.

Tidal Currents

Tidal Friction The tidal friction is gradually slowing the Earth’s rotation by a few hundredths of a second every century. As the Earth slow, the days become longer and the years shorter. 350 million years ago, the year contained a little over 400 days, each day being 22 hours long. It has also effected the moon, locking in the moon’s rotation with that of the Earth. This is why the same side of the moon is always facing Earth.

Amphidromic points are points where the amplitude of the tides is zero, in other words the height doesn’t change. They occur because of the Coriolis effect and interference within ocean basins, seas and bays create a wave pattern — called an amphidromic system— which rotates around the amphidromic point. Amphidromic Systems

Amphidromic Points

Tidal Predictions When trying to predict tides, there are at least 140 factors and forces that affect the tides. To mathematically determine high and low tides, 7 of the most important ones must be considered. Also, the study of past records allows for construction of future tide tables that are accurate to within 1.2 inches.

Tide Table

Other Factors Affecting Tide Height Meteorological tides are caused by weather- related factors such as hurricanes. The storm surge that accompanies a hurricane can cause severe flooding if it arrives at the time of high tide. Other types of storm systems can also cause storm surges drastically affecting the height of the tides.

Non-weather Related Factors Seiches are gently moving waves that slosh back and forth in a basin. If the seiche and high tide arrive at the same time, there is an increase in the height of the tide. Tsunamis also can cause massive amounts of damage from flooding whether at the time of high tide or not, as seen by the one that hit the Bay of Bengal in December of 2004.

Intertidal Zones Between the areas of high and low tide is the intertidal zone. Animals and plants have sorted themselves into at least 3 bands or subzones, depending on their ability to survive in that zone. Each zone has a specific amount of time it is exposed to the air and organisms adapt to that environment.

Intertidal Zones

Tidal Power The power of the tides has been harnessed in France by construction of a 2,800 foot long dam which can produce 544 million kilowatt- hours in a year. The seawater flows through turbine generators in both directions depending on the tide. A similar project has been discussed for years for Passamaquoddy Bay between New Brunswick and Downeast Maine and it being tested and implimented.

Rance Estuary in Western France

Advantages of Tidal Power -low operating costs -power is free, endless and constant -no carbon dioxide or pollutants released -not as dangerous as other sources -not as unsightly as a fossil fuel or nuclear plant

Disadvantages of Tidal Power -equipment damage from storms -corrosion of metal parts -disruption of plankton and benthic marine life -an increase in tidal friction causing a decrease in Earth’s rotation -changes in height of the tides

The End