Transcript Slide 1

TIDES
Periodic short term changes in the
height of the ocean surface at a
particular place
TIDAL MARSH
Moon's Gravity Pulls
Oceans - Near-side Bulge
is Easy to Understand
Moon and Earth actually
orbit around the EarthMoon Center of Mass
(about 1500 km beneath
the surface of the Earth)
Motion of Earth Around
Center of Mass Creates a
Bulge on the Far Side of
the Earth
Both Moon and Sun Cause Tides
DEFINITIONS
• Tidal day
– 24 hr 50 min
– Time between successive phases of moon over a
given point on the earth
• Tidal Period
– Time between two successive high or low tides
• Tidal Range
– Difference between highest and lowest tide levels
• Daily inequality
– Difference in height between successive high or
low tides
The Tidal Cycle
• In general, a complete tidal cycle takes 24 hours
and 50 minutes.
• This is the time it takes for the Earth to rotate on
its axis back to its original position with respect
to the moon, the primary tide-causing force.
• Because it takes the moon about 27.3 days to
complete one orbit around the Earth, the moon
moves a little bit further around the Earth each
day.
• Thus, the time of the tides advances about 50
minutes each day.
TIDES
• Periodic changes in sea level relative to land
along a coast
• Daily or Diurnal Tides
– One high and one low tide each day
• Semi-daily or Semidiurnal Tide
– Two high and two low tides of approximate equal
heights occur each day
• Mixed Tide
– Two high and two low tides of unequal heights
(HHW, LHW, HLW, LLW)
TIDES
• Many other factors influence the nature
and intensity of the tides, including the
shape of the ocean basin and the
Coriolis effect.
• These factors create high and low tides.
Depending on the position of the Earth
with respect to the moon and the sun,
differences in the height of sea level
during the high and low tides may be great
or small
AMPHIDROMIC POINT
• As the tidal bulge moves across the Atlantic it
encounters the
American Continents
• Because the Moon keeps on moving overhead,
the tidal bulge gets left behind and the tidal
wave is reflected back into the Atlantic
• The lagging bulge and the reflection of the tidal
bulge give rise to different types of tides
depending on the dimensions and shapes of
the basins.
AMPHIDROMIC POINT
• As the tidal bulge moves across an ocean and is
reflected back from the opposite side, the
Coriolis Effect causes the moving water to be
deflected.
• The peak of the tidal bulge moves around the
basin rather than just straight back and forth
across it.
• In an open ocean the crests and troughs of the
wave actually rotate around a point near the
center of the ocean.
• This point is called the amphidromic point.
SPRING AND NEAP TIDES
• Spring Tides
– Occur at Full and New Moon Sun,
– Moon and earth in a line
– Greatest tidal range
• Neap Tides
– Occur at the first and third quarter of moon
– Least tidal range
TIDAL RANGE
The Bay of Fundy
Nova Scotia, Canada
In the Bay of Fundy
the tidal range can
be up to 16m
TIDAL CURRENTS
• Horizontal water movement caused by
tides
• Tides are like Shallow water waves
• Orbital motion of water is highly elliptical:
can be assumed to be to and from motion
• Flood tides when water moves in
• Ebb tide when water moves back
TIDAL BORE
Tidal Friction
• Tides stretch the oceans, and to a small extent,
the solid mass of a planet or satellite.
• In one complete rotation, the planet material
keeps deforming and relaxing.
• This takes energy away from the rotation,
transforming it into heat.
• In effect, this is a frictional loss, like a giant
brake on the planet.
• Over the centuries, the moon's rotation on its
own axis has slowed until it presents essentially
the same face to the earth.
• Each century, the day increases by about 3
milliseconds.
• Over 100 million years, the day will increase by
about an hour.
Tidal Friction
Rotation and Friction Causes Tides to
Lead Moon
Bulge Pulls Moon, Throws into Larger
Orbit
Friction Slows Earth
Precambrian (900 m.y.):
Year = 500 Days,
Day = 18 Hr.,
Month = 23.4 Days
Cambrian (500 m.y.):
Year = 400 Days,
Day = 22 Hr.
Predicting Tides
• Predicted tidal heights are those expected under
average weather conditions.
• When weather conditions differ from what is
considered average, corresponding differences
between predicted levels and those actually
observed will occur.
• Generally, prolonged onshore winds (wind
towards the land) or a low barometric pressure
can produce higher sea levels than predicted,
• While offshore winds (wind away from the land)
and high barometric pressure can result in lower
sea levels than predicted.
Tidal Power
• The potential energy contained in a volume of
water is
– E = hMg
• where h is the height of the tide, M is the mass
of water and g is the acceleration due to gravity.
• Therefore, a tidal energy generator must be
placed in a location with very high-amplitude
tides.
• Suitable locations are found in the former USSR,
USA, Canada, Australia, Korea, the UK and
other countries
Tidal Power
• In 1966, France built the World’s first tidal
power station on the river Rance, in the
process constructing 24 earth dams which
generate approximately 502 millions KW/H
of electrical power/year.
Severn Barrage, UK
• John Hutton, Secretary of State for Business,
Enterprise and Regulatory Reform, announced a
further feasibility study on 25 September 2007.
• The proposal for a hydro-electric barrier to
generate 8.6 GW and meet five percent of
Britain's power needs, is being opposed by
environmental groups
• Power would be equivalent to about 18 million
tons of coal or 3 nuclear reactors.
• This decreases the output of greenhouse gases
into the atmosphere.
Operating Stations
• SITE
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Cook Inlet
Knik-Arm
Turnagain-Arm
Rio Gallegos
Golfo de San Jorge
Bahia Sao-Jose
Golfo-Nuevo
Belem
Bay of Fundy
Cumberland Bay
Cobequid Bay
Shepody Bay
Tidal Range (m)
Power (MW)
8,4
8,4
7,6
4,2
5,6
3,6
5,9
1440
9002
7003
9,9
11,8
9,6
• Annapolis-Royal (1984) 6,4
7000
110005
306
1080
4030
15507
208
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SITE
Ungava Bay 9 dam sites
Severn River
Strangford-Lough
Solway-Firth
River Rance* (1966)
Chausse de Sein
Cotentin Peninsula
Kislaya Guba (1968)
Lumbovskiy Bay
Mezenskiy Guba
Guba Penzhinskaya:
south range
north range
Tugurskiy Bay
Inchon 6,050021
Tsien-tien (1980)
Gulf of Cambay
Gulf of Kachchh
Walcott Inlet
Security Bay
Tidal Range (m)
7,7
8,3
3,0
5,1
8,5
8,5
8,0
2,3
4,2
6,0
Power (MW)
92609
720010
32011
683012
24013
1200014
5000015
0,416
67017
1520018
6,2
6,2
4,7
87400
2140019
1030020
5,0
6,8
5,3
5,0
5,6
322
736023
160024
125025
570