Topic 17 Tides

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Topic 17Tides

• GEOL 2503
• Introduction to Oceanography

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Bay of Fundy, Nova Scotia, Canadalow tide
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Bay of Fundy, Nova Scotia, Canadahigh tide
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Tides

• Periodic waves caused by the gravitational pull
  of the moon and sun on the earth.
• Timing set by earth's rotation
• Long wave lengthsup to 1/2 the circumference of
  the earth

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The gravitational pull of the moon on the Earth
creates the Tide patterns in the World.
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Types of Tide Patterns

• Diurnal tideone high and one low daily
• Semidiurnaltwo cycles daily, with the two highs
  about the same height and the two lows drop to
  about the same level
• Semidiurnal mixedtwo cycles daily, but the two
  high tides reach different heights and the two
  low tides drop to different levels

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Tide LevelsDiurnal or Semidiurnal

• High watergreatest height to which the water
  rises on any day
• Low waterlowest point to which the tide drops on
  any day

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Tide LevelsSemidiurnal Mixed

• Higher high waterhigher of the two highs
• Lower high waterlower of the two highs
• Higher low waterhigher of the two lows
• Lower low waterlower of the two lows

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More Tide Terminology I

• Average (or mean) tidesaverage of all water
  levels taken over many years
• Mean high wateraverage high water level
• Mean low wateraverage low water level
• Tidal datumreference depth for reporting water
  depth (for navigation safety, mean low water is
  usually the tidal datum)

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More Tide Terminology II

• Minus tidewhen water level falls below the mean
  value
• Flood tidewhen water level is rising
• Ebb tidewhen water level is falling

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Tidal Currents

• Currents associated with rising or falling of the
  tides
• Important currents for nearshore navigation
• Flood tide currentwater level rising and current
  is toward the land
• Ebb tide currentwater level falling and current
  is toward the sea
• Slack waterturning tide, between ebb/flood or
  flood/ebb

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Two Ways to Study Tides

• Equilibrium Tidal Theory
• Mathematical idealized principles
• Dynamic Tidal Analysis
• Measure the real world

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Equilibrium Tidal Theory

• Mathematically ideal wave
• Assumes uniform layer of water covering Earth
• Used to simplify Earth, Moon, Sun relationships
• Good for illustration of principles, but not good
  for predicting actual tides

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Dynamic Tidal Analysis

• Study of tides as they occur naturally
• Modified by landmasses, shape of ocean basins,
  and Earths rotation

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Equilibrium Tidal Theory

• Equilibrium tide theory explains
• effects of the Suns and Moons gravity
• effects of rotation
• Consider Earth and Moon as a single unit, the
  Earth-Moon system orbiting the Sun

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Moon

• Orbits Earth
• Held by Earths gravitational force
• Force acting to pull Moon away from Earth is
  centrifugal force
• The two forces balance

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Earth-Moon System

• Earth and Moon rotate about a common center of
  mass
• Held in orbit about the Sun by the Suns
  gravitational attraction
• Suns gravitational attraction balanced by a
  centrifugal force acting to pull Earth-Moon
  system away from the sun

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The Moon Tide

• Water on Earths surface closer to the moon (side
  of Earth facing Moon) acted on by excess
  gravitational force
• Water moves along Earth surface toward a point
  directly under the Moon
• Produces a bulge in the water covering the Earth

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Moon and Centrifugal Force

• Centrifugal force is equal to the gravitational
  attraction of the Moon, but operates in the
  opposite direction
• Causes an opposing bulge on Earths surface away
  from the Moon

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The opposing gravitational and centrifugal forces
create two tidal bulges
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Earths rotation uder the tidal bulge gives the
observer two high tides and two low tides each day
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Lunar Complications to Equilibrium Tidal Theory

• Rotation of Earth and Moon
• Leads to tidal day differing from solar day
• Moons declination
• Moon above or below celestial plan
• Moons elliptical orbit
• Distance from Earth varies

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• Tidal Day 24 hours 50 minutes
• Earth rotates on its axis in 24 hrs
• Moon also revolving around Earth in same
  direction
• After 24 hrs, Moon has moved forward
• Earth must rotate 50 minutes more so point A
  can catch up to Moons new location to be back at
  high tide

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Moons Declination Changes
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This drawing is to scale. Note the variation in
the distance of the moon from the earth and the
distance of the moon above and below the orbital
plane.
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Complications of the Sun

• Like the Moon, the Sun also produces two
  equilibrium tidal bulges
• Even though the Sun is huge, it is so far away
  that it has only 46 of the tide-producing effect
  of the Moon
• Tides vary with phase of the Moon as Earth-Moon
  system orbits the Sun

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As the Moon revolves around the Earth, the
Earth-Moon system is revolving around the Sun.
In a lunar month, the moon passes through a
series of phases as seen from Earth.
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Spring Tides

• Occur at New Moon and Full Moon phases
• Earth, Moon, and Sun aligned
• Moon tidal bulge adds to Suns
• Higher high tides
• Lower low tides

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Neap Tides

• Occur at 1st quarter and 3rd quarter phases
• Earth, Moon, and Sun at right angles
• Moon tidal bulge subtracts from Suns
• Lower high tides
• Higher low tides

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Springfrom Old English word springere meaning
to rise or spring up Neapacronym for near even
as possible
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Note timing of Spring/Neap tides and Lunar phases
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Proxigean Spring Tide

• A rare, unusually high tide.
• Occurs when the moon is both unusually close to
  the Earth (at its closest perigee, called the
  proxigee) and in the New Moon phase (when the
  Moon is between the Sun and the Earth).
• The proxigean spring tide occurs at most once
  every 1.5 years.

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When Everything Lines Up
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Dynamic Tidal Analysis

• Needed to explain the real tides on Earth
• There are many complications with Equilibrium
  Tidal Theory

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Real Factors Controlling Tides

• Size of basin (lakes have no tides)
• Width of continental shelf (wider-higher)
• Shape of shoreline, embayments
• Water depth variations

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The Tidal Wave

• Wavelength is one-half Earths circumference
• Shallow-water wave

Courtesy of Dr. Tom Garrison
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Predicting Tides

• Cant use Equilibrium Tidal Theory alone
• Need to combine years of local observations with
  astronomical data
• Minimum 19-year record to allow for 18.6-year
  declinational period of the Moon
• Result is Tide Tables with local predictions of
  water level and timing of tides

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Some of the worlds highest tides are in the Bay
of Fundy because of the funneling effect of the
embayment.
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Tidal Properties Near Shore
A very small tide in deep water piles up in
shallow water to form a higher tide near shore.
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Cotidal Lines