The Tides
tide - rhythmic oscillation of the ocean surface due to gravitational &
centrifugal forces between the Earth, Moon and Sun.
Tide-Generating Forces
Equilibrium Tidal Theory
Dynamic Tidal Theory
Analysis of Tidal Signal
tidal cycle – one low tide and one high tide consecutively
diurnal tide - one low tide, one high tide a day
semidiurnal tide – two tidal cycles daily; high and low tide levels are
approximately equal
semidiurnal mixed tide - same as semidiurnal except high and low tide
levels vary
diurnal tide
semidiurnal tide
semidiurnal mixed tide
MTL
=
Mean Tide Level
MHW
=
Mean High Water
MLW
=
Mean Low Water
MHHW
=
Mean Higher High Water
MLLW
=
Mean Lower Low Water
tidal range – difference between MHW and MLW
For navigation, the reference level is MLW or MLLW
Flood Tide: tide wave is propagating onshore
water level is rising
Ebb Tide:
tide is moving out to sea
water level is dropping
Slack tide:
period when tide wave is reversing
low current velocity
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Study of Tides
Equilibrium Tidal Theory - ideal approach to understand basic
principles
Assumptions:
1: entire Earth surface covered in water
2: infinitely deep basin (no shoaling)
3: tidal bulge fixed relative to the moon
universal law of gravitation
Dynamic Tidal Analysis - realistic approach, studying the tides as
they occur on earth, accounts for modification due to landmasses,
geometry of ocean basins, earth’s rotation.
F= G m1 m2 / R2
G = universal gravitational constant
m1, m2 = mass of bodies
R = distance between centers of
A, B & C = gravitational forces
A’, B’ & C’ = centrifugal forces
mass of bodies
A, B & C = gravitational forces
A’, B’ & C’ = centrifugal forces
The difference in force (for each unit mass of earth) between
gravitational force of moon (or sun) and centrifugal force at an earth
surface point is proportional to GM/R3
The Tidal Day = 24 hr 50 min
Diurnal = 24 hr 50 min
Lunar Orbit: the Earth – Moon system
has a period of 27.3 days
Semidiurnal = 12 hr 25 min
The Lunar Tide and how we get a ‘wave’
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Declination Tide
Moon’s declination cycle 28.5N – 28.5S = 29.5 days
Sun’s declination cycle 23.5N (summer solstice) – 23.5S (winter
solstice) = annual
The Sun Tide
Spring & Neap
Tides
Spring – Neap Cycle
Spring Tide: greatest tidal range,
highest high tides and lowest low
tides
Neap Tide: least tidal range, low
high tides and high low tides
Lunar Cycle: the Earth – Moon system
has a period of 29.5 days
Spring and Neap tides at two places on earth
Orbital Influence on Tides
Astronomical High Tide: Moon at perigee, Sun at perihelion, and EarthMoon-Sun system at syzygy (full or new moon)
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Dynamic Tidal Analysis
Amphidromic
Point
Amphidromic
System
Continents break up wave propagation
Tide wave moves continuously around the globe only in the
Southern Ocean (Antarctica).
shallow-water wave: speed is controlled by depth of ocean
standing wave: oscillates because it is contained in ocean basins
Cotidal
Lines
reflected by continents, refracted by changes in depth, and
diffracted (spread of energy sideways) as it passes through gaps
Coriolis affects the water movement because it is a large scale
phenomenon
Co-range Lines
Bay of Fundy
Tidal Resonance
British Columbia
10 meter range
17m Tidal Range
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Tidal Bores
Tidal Resonance
Length = 110 miles
Avg Depth = 78 feet
Shelf Width and Tidal Range
Shelf Width and Tidal Range
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