tides document

7/29/2019 Tides Document

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The regular rise and fall of the oceans waters are known as tides. Along coasts, the water

slowly rises up over the shore and then slowly falls back again. When the water has risen

to its highest level, covering much of the shore, it is at high tide. When the water falls toits lowest level, it is at low tide. Some lakes and rivers can also have tides.

Causes of Tides

Forces that contribute to tides are called tidal constituents. The Earths rotation is a tidal

constituent. The major tidal constituent is the moons gravitational pull on the Earth. Thecloser objects are, the greater the gravitational force is between them. Although the sun

and moon both exert gravitational force on the Earth, the moons pull is stronger because

the moon is much closer to the Earth than the sun is.

The moons ability to raise tides on the Earth is an example of a tidal force. The moon

exerts a tidal force on the whole planet. This has little effect on Earths land surfaces,

because they are less flexible. Land surfaces do move, however, up to 55 centimeters (22

inches) a day. These movements are called terrestrial tides. Terrestrial tides can changean objects precise location. Terrestrial tides are important for radio astronomy and

calculating coordinates on a global positioning system (GPS). Volcanologists studyterrestrial tides because this movement in the Earths crust can sometimes trigger a

volcanic eruption.

The moons tidal force has a much greater effect on the surface of the ocean, of course.

Water is liquid and can respond to gravity more dramatically.

High TidesThe tidal force exerted by the moon is strongest on the side of the Earth facing the moon.

It is weakest on the side of the Earth facing the opposite direction. These differences ingravitational force allow the ocean to bulge outward in two places at the same time. Onebulge occurs on the side of the Earth facing the moon. This is the moons direct tidal

force pulling the ocean toward it. The other bulge occurs on the opposite side of the

Earth. Here, the ocean bulges in the opposite direction of the moon, not toward it. Thebulge may be understood as the moons tidal force pulling theplanet(not the ocean)

toward it.

These bulges in the ocean waters are known as high tides. The high tide on the side of theEarth facing the moon is called the high high tide. The high tide caused by the bulge on

the opposite side of the Earth is called the low high tide. In the open ocean, the water

bulges out toward the moon. Along the seashore, the water rises and spreads onto theland.

Low Tides and Ebb TidesOne high tide always faces the moon, while the other faces away from it. Between these

high tides are areas of lower water levelslow tides. The flow of water from high tide to

low tide is called an ebb tide.


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Most tides are semidiurnal, which means they take place twice a day. For example, when

an area covered by the ocean faces the moon, the moons gravitational force on the water

causes a high high tide. As the Earth rotates, that area moves away from the moonsinfluence and the tide ebbs. Now it is low tide in that area. As the Earth keeps rotating,

another high tide occurs in the same area when it is on the side of the Earth opposite the

moon (low high tide). The Earth continues spinning, the tide ebbs, another low tideoccurs, and the cycle (24 hours long) begins again.

The vertical difference between high and low tide is called the tidal range. Each month,the range changes in a regular pattern as a result of the suns gravitational force on the

Earth. Although the sun is almost 390 times farther away from the Earth than is the

moon, its high mass still affects the tides.

Because the Earths surface is not uniform, tides do not follow the same patterns in all

places. The shape of a seacoast and the shape of the ocean floor both make a difference in

the range and frequency of the tides. Along a smooth, wide beach, the water can spread

over a large area. The tidal range may be a few centimeters. In a confined area, such as anarrow, rocky inlet or bay, the tidal range could be many meters. The lowest tides are

found in enclosed seas like the Mediterranean or the Baltic. They rise about 30centimeters (about a foot). The largest tidal range is found in the Bay of Fundy, Canada.

There, the tides rise and fall almost 17 meters (56 feet).

Twice each month, the moon lines up with the Earth and sun. These are called the new

moon and the full moon. When the moon is between the Earth and the sun, it is in the

suns shadow and appears dark. This is the new moon. When the Earth is between the sun

and moon, the moon reflects sunlight. This is the full moon.

When the sun, moon and Earth are all lined up, the suns tidal force works with the

moons tidal force. The combined pull can cause the highest and lowest tides, calledspring tides. Spring tides happen whenever there is a new moon or a full moon and have

nothing to do with the season of spring. (The term comes from the German word

springen, which means to jump.)

In the period between the two spring tides, the moon faces the Earth at a right angle to the

sun. When this happens, the pull of the sun and the moon are weak. This causes tides that

are lower than usual. These tides are known as neap tides.

Tidal Features

Tides produce some interesting features in the ocean. Tides are also associated with

features that have nothing to do with them.

A tidal bore occurs along a coast where a river empties into the ocean or sea. The tidal

bore is a strong tide that pushes up the river, against the river's current. This is a true tidal

wave. The huge tidal bore of the Amazon River is called thepororoca. Thepororoca is a


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wave up to 4 meters (13 feet) tall, traveling at speeds of 15 kilometers (9 miles) per hour.

Thepororoca travels 10 kilometers (6 miles) up the Amazon.

While a tidal bore is a tidal wave, a tsunami is not. Tsunami is taken from the Japanesewords for harbor wave. Tsunamis are caused not by tides, but by underwater

earthquakes and volcanoes. Tsunamis are associated with tides because their reach

surpasses the tidal range of an area.

So-called red tides also have nothing to do with actual tides. A red tide is another term

for an algal bloom. Algae are microscopic sea creatures. When billions of red algae form,or bloom, in the ocean, the waves and tides appear red.

Finally, rip tides are not a tidal feature. Rip tides are strong ocean currents running along

the surface of the water. A rip tide runs from the shore back to the open ocean. Rip tidescan be helpful to surfers, who use them to avoid having to paddle out to sea. Rip tides can

also be very dangerous to swimmers, who can be swept out to sea.

Intertidal Life

The land in the tidal range is called the intertidal zone. The intertidal zone is oftenmarked by tide pools. Tide pools are areas that are completely underwater at high tide but

remain as pockets of seawater when the tide ebbs. Tide pools are home to some of the

oceans richest biodiversity.

The intertidal zone can be hard-bottomed or soft-bottomed. A zone with a hard bottom is

rocky. A zone with a soft bottom has silt or sand. Wetlands and marshes are often soft-

bottomed intertidal zones. Different creatures have adapted to different types of intertidalzones. Hard-bottom zones often have barnacles and seaweeds, while soft-bottom zones

have more sea plants and slow-moving creatures like rays.

Intertidal zones are marked by vertical zonation. Different organisms live in different

zones in the tidal range, depending on how much water reaches them. This zonation can

often be seen vertically, with dry plants near the top of the tidal zone and seaweeds nearthe bottom.

The intertidal zone can be broken into four major mini-zones. The highest is called the

splash zone (1). This area is splashed by water and mist during high tide, but is neverfully underwater. Barnacles live on rocks in the splash zone. Many marine mammals,

such as seals and sea otters, can live in the splash zone.

The high-tide zone (2) is pounded by strong waves. Animals that live in the high-tide

zone often have strong shells and are able to cling tightly to rocks to avoid being swept

out to sea. These animals include mussels and barnacles. Crabs, which have toughexoskeletons and can hide under rocks, also live in the high-tide zone.

The mid-tide zone (3) is usually the busiest part of the intertidal zone. This is where tide

pools usually form. Animals from the high- and low-tide zones come here to feed.


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Animals that live in the mid-tide zone are still tough, but can have softer bodies than their

neighbors in the high-tide zone. Brightly colored sea anemones, which are soft-bodied

but strongly anchored to rocks, live in tide pools. Snails and hermit crabs use shells toprotect their soft bodies.

Sea stars (sometimes called starfish, although they are not related to fish at all) areperfectly adapted to life in tide pools. They have a tough, leathery body that can

withstand strong tides and waves. They have thousands of tiny, tube-like legs that help

them stick to rocks or put them on the move for prey. Sea stars are carnivores, and willeat anything, such as fish, snails, or crabs. They especially love mussels. The way sea

stars eat is unusual. Sea stars move over a mussel and use their arms to pry open the

mussels shell. Then, the sea star ejects its own stomach to surround the mussel. The sea

stars stomach contains powerful acids that dissolve the mussel and make it easy to digestwhen the sea star pulls its stomach back into its body.

The low-tide zone (4) is only dry at the lowest tide. Nudibranchs, a type of sea slug, live

in tide pools in the low-tide zone. Like the sea star, this animal is a carnivore.Nudibranchs eat sponges, barnacles and other nudibranchs. Nudibranchs can also eat sea

anemones, because they are immune to its poisonous tentacles.

People can be very active in the low-tide zone. Simple nets can catch fish here, and

fishers can collect animals like crabs, mussels, and clams. The tide is out, our table isset, is a traditional saying among the Tlingit nation (tribe), who live along the Pacific

Northwest coast in Alaska and Canada.

In the low-tide zone of the Puget Sound in the U.S. state of Washington, people practicetidal aquaculture. Aquaculture is the breeding, raising, and harvesting of plants and

animals that live in the water. One of the most harvested animals is a giant clam called a

geoduck. Geoduck farms have been set up in the Puget Sound tidelands, which are areascovered by the intertidal zone. On the farms, geoducks live in plastic pipes.

Environmental groups worry about the impact of these pipes on the environment. Tools

of aquaculture, such as unsecured pipes, nets, and rubber bands, can be washed away bytides. This debris can pollute the ocean, beach, and natural tide pools.

Tides and People

Tidal energy is a renewable resource that many engineers and consumers hope will be

developed on a large scale. Now, small programs in Northern Ireland, South Korea, and

the U.S. state of Maine are experimenting with harnessing the power of tides.

There are three different types of tidal power. All of these use tidal energy generators to

convert that power into electricity for use in homes and industry.

In most tidal energy generators, turbines are put in tidal streams (1). A turbine is a

machine that takes energy from a flow of fluid. That fluid can be air (wind) or liquid

(water). Because water is more dense than air, tidal energy is more powerful than wind


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energy. Placing turbines in tidal streams can be difficult, because the machine disrupts

the tide it is trying to harness. However, once the turbines are in place, tidal energy is

predictable and stable.

Another tidal energy generator uses a type of dam called a barrage (2). A barrage is a low

dam where water can spill over the top or through turbines in the dam. Barrages can beconstructed across tidal rivers and estuaries. Turbines inside the barrage can harness the

power of tides the same way a dam can harness the power of a river. Barrages are more

complex designs than single turbines.

The final type of tidal energy generator is a tidal lagoon (3). The lagoons function much

like barrages, but are usually constructed out of more natural materials, like rocks. Tidal

lagoons can sit along coasts and do not prevent the natural migration of wildlife.

Geographic imaging systems (GIS) rely on tidal calculations. GIS must account for tides

when mapping, especially when mapping the ocean floor. Tides affect the report on an

areas depth.

Predicting tides is very important for shipping and travel across oceans. Ships decidewhich channels they may navigate by calculating their own weight, the depth of the

ocean and an areas tidal range. Errors in navigation can strand ships along shores or on

sand banks. Cargo can sit and spoil while waiting for a tide. This was not a significantproblem after the 2004 tsunami in Southeast Asia. Even though the tsunami destroyed

kilometers of coastline, GIS technology helped disaster-relief agencies get aid to victims

in Indonesia, Thailand, and Sri Lanka.

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