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
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.
The Battle of Tarawa
• The first amphibious assault in the Pacific during
World War II.
• Sketchy tide data for the island suggested a tidal
range of about seven feet.
• There were puzzling rumors of periods when the tides
on Tarawa almost ceased
• The invasion was set for November 20, 1943 when
tide conditions were expected to be favorable.
• At low tide in the early morning, the bombardment
would begin.
• As the tide rose and water levels in the lagoon
reached 1.5 meters (five feet), landing craft would
head ashore and by noon, at high tide, heavier craft
could come ashore bringing tanks and supplies.
The Battle of Tarawa
• November 20 was near last-quarter moon, resulting in
a neap tide. Military planners did not realize the moon
was unusually far from earth. Also, the Earth was only
seven weeks from perihelion, meaning solar tides were
unusually strong
• Landing craft hit bottom hundreds of meters offshore and
the Marines had to wade ashore under heavy fire.
• Once ashore, they had to fight without assistance,
because supply ships could not come in.
• For 48 hours, the tidal range was only 60 centimeters
(two feet), and it was four days before the tidal range
increased to normal.
• 1027 Marines were killed and 2292 wounded in the
battle.
The Grunion
• Grunion have adapted to tidal cycles in a precise
manner
• Along the Pacific coast of North America the two
daily high tides vary in height, and the higher of
the two occurs at night during spring and
summer months.
• Spawning must take place soon after the highest
tide in a series if the eggs are to have adequate
time to develop before the next series of high
tides.
• Looking at the tidal cycle, it becomes apparent
that there are only 3 to 4 nights following the
highest tide that spawning conditions are right
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
Severn Barrage, UK
• The Severn Barrage, when built, is projected to
produce 8640 MW during flow, or 2000 MW
average power.
• This would provide 17 TWh of power per year
(about 6% of UK consumption), equivalent to
about 18 million tons of coal or 3 nuclear
reactors.
• The cost in 1989 was calculated to be about £8
billion (£12 billion in 2006 money), and running
costs would be £70 million per year.
• This decreases the output of greenhouse gases
into the atmosphere.
Operating Stations
• The first tidal power station was built over a period of 6
years from 1960 to 1966 at La Rance, France
– It has 240MW installed capacity.
• The first (and only) tidal power site in North America is
the Annapolis Royal Generating Station, which opened in
1984 on an inlet of the Bay of Fundy.
– It has 20MW installed capacity.
• A small project was built by the Soviet Union at Kislaya
Guba on the White Sea.
– It has 0.5MW installed capacity.
• China has developed several small tidal power projects
and one large facility in Jiangxia.
• Scotland has committed to having 18% of its power from
green sources by 2010, including 10% from a tidal
generator.