Tides

• Tides are waves with very long wavelengths. In this lecture we will learn about:– Tidal characteristics– The origin of the tides– Tides in small and elongated basins– Tidal currents– Power from the tides

The Tides (low tide in Brittany)

High tide in Brittany

Tidal Characteristics

• Tides are single waves that stretch across ocean basins.

• They are also shallow-water waves because their wavelengths greatly exceed the depth of the ocean.

• They occur due to complex interactions of the sun and moon and Centrifugal force

Tidal Characteristics

• High tide : Wave crest

• Low tide : Wave trough

• Tidal range : Wave height

Tidal periods are either 12 hours, 25 minutesTidal periods are either 12 hours, 25 minutes or 24 hours, 50 minutes depending on location.or 24 hours, 50 minutes depending on location.

Key wave features of tides:Key wave features of tides:

Tidal Range – The difference in water level between the high and low tides

The Bay of Fundy, Nova Scotia has the greatest range in the world – 53 feet.

Tidal range is the difference between high and low tide volume.

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2. Deep water Waves

3. Interactive Waves

4. Tsunamis

Tides occur because of the interactions of the

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25% 25%25%25%1. Sun, Gravity and the Moon

2. Moon, Gravity and Centrifugal force

3. Sun, Moon, Centrifugal force

4. Sun and Moon only

Tidal Characteristics

• Tides can be classified based upon their monthly record. The tidal range varies slightly each day in a systematic pattern.– Spring tide: phase when tidal range is

maximal.– Neap tide: phase when tidal range is

minimal.

There are two spring and two neap tides each month.

Springs & Neaps

Origin of the Tides

• Unlike wind-driven surface waves, tides are cause by two principal factors:

–Gravitational attraction

–Centrifugal force

Origin of the TidesThe gravitational effect of the moon creates a tidal bulge.The gravitational effect of the moon creates a tidal bulge.

– Centrifugal forces arise as the Earth and Moon revolve around one another.

– Because of the unequal masses of the Earth and Moon, the center of rotation lies beneath the Earth’s surface.

– The water of the ocean shifts away from the center of rotation creating a second tidal bulge.

Centrifugal forces cause a second tidal bulge.Centrifugal forces cause a second tidal bulge.

Gravitational attraction and centrifugal force create two bulgesGravitational attraction and centrifugal force create two bulges

Tides are extremely long waves caused by the gravitational attraction of the Moon and the Sun on Earth. They are also affected by the:

NOAA

1. shape and depth of the ocean floor

2. latitude (greater pull at equator)

3. volume of waterNASA

• The sun exerts a gravitational attraction on the earth and its ocean – 46% of the Moon’s effect.

• The moon because of its proximity to the Earth has a greater gravitational effect on the Earth than the Sun, despite the fact that the Sun is much more massive than the Moon.

• If we include the effect of the Sun, we can account for neap and spring tides.

– The Sun creates two smaller tidal bulges.

– If we take into account the alignment of the Sun, Moon, and Earth we can explain neap and spring tides.

When the Earth, Moon and Sun are orbiting in a linear arrangement, the Moon is in either the new or full stage.

With the Sun and Moon pulling together on the water, tides are increased - called a spring tide.

• Spring tides occur when the Earth, Sun, and Moon are aligned.– New Moon and Full Moon phases– Now you have these large tidal ranges– They are called spring tides because they

seem to “spring up” with a surge of water.– They occur twice a month (full and new

moons)

At quarter phases, the Moon pulls opposite and equal to the Sun - resulting in lower tides called neap tide.

• Neap tides occur when the Sun and Moon are aligned at right angles to one another.– Quarter Moon phases– Small tidal ranges– Also occur twice a month when the moon is

in its first and third quarters (half moons)

Springs & Neaps

This shows the different views of the Moon from Earth during a lunar month’s revolution of 28 days.

SUN

Day 1

Day 7

Day 14

Day 21

The Earth and Moon revolve around each other and rotate on their axes.

Earth’s rotation = 24 hours (one day)

Moon’s rotation = 24 hours and 50 minutes (lunar day)

Tides occur 50 minutes later every day because of the rotation time difference.

At high tide (flow), the Moon pulls water out toward it. Centripetal force pulls water out on the opposite side - causing a tidal bulge.

As the Earth turns, locations move out of the center of gravity and low tides (ebb) result.

high tide

low tide

Moon

• The reason why tidal periods are not 24 hours or 12 hours in length is that the Moon moves forward in it’s orbit each day. In fact it takes 50 additional minutes for a spot on the Earth’s surface to regain it’s position relative to the Moon.

• For example- you see the moon at a certain spot in the night sky tonight at 9:00pm the moon will be at that same spot tomorrow night at 9:50pm

The Moon has an elliptical course, and the Earth’s distance from the sun also varies

Tides are also greater at times during the month when the moon and earth are closer together.

There are some exceptions but basically there are 3 types of Tidal Patterns:

1.Semi-Diurnal = 2 equal high and low tides per day each 6 hours and 12 1/2 minutes apart

ex: east coast of U.S.

2. Diurnal = 1 high and 1 low tide per day each 12 hours and 25 minutes apart. Usually Unequal but differences usually not great.ex: Gulf of Mexico, S.E. Asia

3. Mixed = 2 high and 2 lows of unequal range at 6 hrs. and 12 1/2 minutes apartex: west coast of U.S.

Tides in Small and Elongated Basins

• The shape of basins greatly influences the tidal pattern.

• In broad basins the pattern resembles the open ocean.

• In narrow basins the wave cannot rotate. Instead the wave (tide) moves in and out such that tidal currents reverse flow

Bay tides

• The dimensions of a bay can affect tidal range

– the tide can be enhanced (Bay of Fundy, 15 m tide)

And this is not eventhe lowest tide…...

Low Tide - Bay of Fundy

High Tide

The Qiantang River estuary in China

Tidal Bore

A tidal bore is a wall of water that moves up certain low-lying rivers due to an incoming tide.

Tidal bores form when an incoming tide rushes up a river, developing a steep forward slope due to resistance to the tide's advance by the river,

which is flowing in the opposite direction.

The height of the tidal bore increases with the range of the tide and may very in height from just a ripple to several feet.

Thus we have the phenomenon of the river changing its flow before your very eyes, flowing in OVER the outgoing river water.

The Qiantang River estuary in China

They can be quite powerful– churning up the mud and water

Fishing for eels isdone on the incomingtidal current, but onlyafter the tremendousturbulence of the bore has passed.

Power from the Tides

• The rise and fall of the tides can be used to generate electrical power.– There must be a substantial tidal range (>

5 m).– Water must flow through a restricted inlet

into a large basin. At the mouth a dam can be constructed such that water flow can be controlled and used to turn giant turbines.

The tidal powerstation on the La Rance estuaryin France

Special turbinesat the La Rancepower stationallow water to flow in both directions togenerate power

Summary• Tides are long wavelength shallow-

water waves

• They are generated by the gravitational attraction of the Moon and the Sun as well as Centrifugal Force of the spinning Earth.

• Tidal Periods are either 12hrs 25 Min or 24 Hrs 50 Min.

Summary•Three Types of Tidal Patterns are•Mixed - 2 high and 2 lows of unequal range at 6 hrs. and 12

1/2 minutes apart

•Diurnal - 1 high and 1 low tide per day each 12 hours and

25 minutes apart

•Semi-Diurnal- 2 equal high and low tides per day each 6 hours and 12 1/2 minutes apart

• Tidal bore - is a wall of water that moves up certain

low-lying rivers due to an incoming tide.

•Electrical power can be generated by having flood and ebb tide currents flow over specially designed turbines.