Intro to Tides

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Intro to Tides

• Regular rise and fall of ocean levels along the
  coasts of the world
• Produces currents which move nutrients and
  sometimes sediments landward and seaward
• Mainly due to the Moon and Sun gravitational
  pulls
• F Gm1m2/R2 where,
• m1 gravitational pull of the 1st planet
• m2 gravitational pull of the 2nd planet
• R distance between the planets
• G gravitational constant
• Though the Sun is 27 million times greater in
  mass than the moon, it also average distance of
  93.5 million miles away from the Earth.

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Tidal Bulge
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Earth-Moon-Sun Relationships
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• The pull of the Moon varies across the Earth
  since the Earth is rotating on its axis, and the
  Moon is revolving around the Earth
• Centrifugal force causes the water envelope of
  the Earth to bulge at the Equator
• -the bulge is greater on the side facing the
  Moon
• The offset of the axis of the Moons orbit around
  the Earth influences the bulge
• -two tides high and low
• Tidal wave ½ the circumference of the Earth

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Tidal Cycle

• Terms high tide, low tide, mean tide, slackwater
• The Moon revolves around the Earth every 27 days
  8 hrs in the same direction as the Earths
  rotation
• -because the Earth is also rotating on its axis,
  it needs 50 min every day to arrive at the same
  place relative the Moon the length of lunar day
  is thus 24 hr 50 min
• -this would determine the timing of tides but
  coastal configuration enters in
• Spring Tide alignment of Earth, Moon, and Sun
• Neap Tide Moon is aligned at 90 degrees to the
  plane of the Sun
• Perigean spring tide cause coastal flooding

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Tidal Curves
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Project

• Compare Hurricane Isabel to 1933 August
  Hurricane storm surge and propensity for
  flooding
• What would sea level rise do to coastal
  vulnerabilty from the same storm in year 2100
• Changing wind fields during Isabel and wave
  propagation

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Syzygy and Quadrature

• The Moon, Sun and Earth in line
• -the tidal forces of Sun and Moon are additive
• -produces Spring tides New Moon and Full Moon
• Quadrature when the moon is aligned at right
  angles to Sun
• -first quarter and ¾ (Last Quarter) Neap tides
• The 5 produces a long term tidal variation (18.6
  years), declination varying, because of the
  Earths axial tilt from 28.5 to 18.5

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The Most Important Tide-Generating Components
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Amphidromic System

• Tidal Wave advances from east to west
• Reflected and moves in opposite direction to the
  east
• Combine with other tidal waves and produce a
  standing wave
• The point where on wave crosses another is a
  called a node
• -point of oscillation is Tn 2Lb/(gh)1/2
• Coriolis Force acts to modify standing wave,
  causing oscillation to veer slightly to right
• -water moving west piles up on northern
  boundary, while moving east piles on southern
  boundary
• thr high water of the tidal now moves around a
  nodal point in counterclockwise fashion know as
  a Kelvin wave
• -can be complex around complex basins like the
  English Channel

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• Rotation leads to
• Amphidrome, with tidal Kelvin wave entering the
  gulf on the right (N. hemisphere) and rotating
  around the amphidromic point anti-clockwise.
  Thus, HW propagates round the gulf.
• A large gulf/semi-enclosed sea will have a series
  of amphidromic systems, at ? / 4, 3? / 4, 5? /
  4etc. from the head of the gulf.

.and friction

• Friction leads to
• The outgoing tidal wave has less energy than the
  incoming wave (dissipated by friction), and so a
  smaller amplitude.
• The amphidromic point is shifted to the left
  (looking into the gulf, N. hemisphere).
• Tidal range on the right of the gulf is greater
  than on the left.
• In strongly dissipative systems, the amphidromic
  point can be shifted onto the land, becoming a
  degenerate amphidromic point.

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See also http//www.es.flinders.edu.au/mattom/In
troOc/notes/figures/fig11a5.html
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Tidal co-range lines (in feet!) off eastern
USA. Co-range contour line of constant tidal
range. After Redfield, 1958. Journal of Marine
Research, 17, 432-448.
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Massaachusetts
North Carolina
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Florida
Georgia
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Tidal Range

• Microtidal 0-2 m Example Chesapeake Bay
• Mesotidal 2-4 m Example Georgia Coast
• Macrotidal gt4 m Example Bay of Fundy
• MHW mean high water MLW mean low range
• MHHW mean higher high water MLLW mean lower
  low water
• -apply to coasts with semidiurnal tides of
  unequal amplitude
• MHWS mean high spring tide MLWS mean low water
  spring
• MHWN mean high water neap MLWN mean low water
  neap

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Time Velocity Asymmetry

• Tides are unequal in terms of velocity
• -ebb-dominated stronger velocities at ebb
• -flood-dominated stronger velocities at flood
• Sediment always moves in the direction of the net
  highest velocities
• Postma model

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Wind (Storm) Driven or Meteorological Tides

• Where winds drive water offshore or onshore
  overwhelming the astronomical tides
• Tends to occur in coasts with weak tidal signals
  e. g., microtidal coasts like Chesapeake Bay
• Can have no high or low tide for over a number of
  normal tidal cycles

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Tide Tables