Chem 1103: Lecture 8: Tides: Chap. 8
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Why are tides important
o Mixing in the ocean
o Transport sediments
o Erosion
o Navigation
Tides are waves
o Caused by gravitational pull of Moon and Sun
o Have frequencies related to rotation of Earth
o Tide tables
Tidal Characteristics
o Tides have a wave form but differ from other waves because they caused by
gravitational attraction between the ocean and the Sun and Moon
 Crest of the wave form is high tide and trough is low tide
 The vertical difference between high tide and low tide is the tidal
range
 Tidal period is the time between consecutive high or low tides and
varies between 12 hrs 25 min to 24 hrs 50 min
 There are three basic types of daily tides defined by their period and
regularity
 Diurnal tides have one high and one low tide each day and tidal
periods of 24 hrs or 24 hrs 50 min
 Semidiurnal tides have two high and two log tides of similar
tidal range each day and a tidal period of 12 hrs 25 min or 12
hrs
 Mixed tides have two high and two low tides of dissimilar tidal
range each day
 Show how diurnal and semidiurnal add to get these three
 Over a month the daily tidal ranges vary systematically with the cycle
of the moon
 Spring tides are the two tides each month with the largest tidal
range
 Neap tides are the two tides each month with the smallest tidal
range
 Tidal range is also altered by the shape of a basin and sea floor
configuration
Origin of the tides
o The tides result from gravitational attraction and centrifugal effect
 Gravity varies directly with mass, but inversely with distance
 Although much smaller, the moon exerts twice the gravitational
attraction and tide-generating force as the Sun because it is closer
 Gravitational attraction pulls the ocean towards the Moon and Sun,
crating two gravitational tidal bulges in the ocean (high tides)
 Centrifugal effect is the push outward from the center of rotation
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As the Earth orbits the Sun, centrifugal effect raises a tidal
bulge on the side of Earth facing away from the Sun
 Earth and Moon revolve around a point between them.
Centrifugal effect raises a tidal bulge on the side of the Earth
facing away from the Moon
 Latitude of the tidal bulges is determined by the declination, the angle
between Earth’s axis and the lunar and solar orbital plane
 As Earth rotates on its axis a point on Earth’s surface
alternately passes through and between tidal bulges,
experiencing high and low tides respectively
 In two weeks, the tidal range varies from maximum to minimum to
maximum
 Spring tides occur when the Earth, Sun an Moon are aligned in
a straight line and the tidal bulges display constructive
interference, producing very high high tides and very low low
tides
o Spring tides coincide with full and new moons
 Neap tides occur when the Earth Moon and Sun are aligned
forming a right angle and tidal bulges display destructive
interference, producing low high tides and high low tides
o Neap tides coincide with the first and last quarter moon
o Equilibrium model of tides
 Earth’s surface is covered by seawater of infinite depth
 Waves progressive
 Water in equilibrium with tide generating force
 Two bulges
 Gravity and Coriolis
BREAK
o Dynamic model of tides
 Movement of tides across the ocean basins is deflected by Coriolis,
block by continental landmasses and forms a rotary wave, which each
day completes two cycles around the basin if the tide is semidiurnal or
one cycle if it is diurnal
 High tide at the ocean basin’s western edge creates a pressure gradient
sloping downward towards the east
 As water flows down the gradient, Coriolis deflects water towards the
equator, where it accumulates and establishes a pressure gradient
sloping downward towards the pole
 Water flowing down this gradient is deflected eastward, forming a
pressure gradient sloping downward to the west
 Westward flow along this gradient is diverted poleward forming a
pressure gradient sloping downward toward the equator
 Finally, the flow toward the equator is deflected westward completing
the cycle
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o A rotary wave is part of an amphidromic system (rotary standing wave) in
which the wave progresses about a node (no vertical displacement) with the
antinode (maximum vertical displacement) rotating about the basin’s edges
 Cotidal lines connect points on the rotary wave that experience high
tide at the same time
 Cotidal lines are not evenly spaced because tides are shallow
water waves and their speed depends upon water depth
 Corange circles are lines connecting points which experience the same
tidal range
 The lines form irregular circles which are concentric about the
node
 Tidal range increases outward from the noted
 Amphidromic systems rotate clockwise in the southern hemisphere
and counterclockwise in the northern hemisphere because of the
difference in the direction of Coriolis deflection
 Irregular coastlines distort the rotary motion
 Actual tide expressed at any location is a composite of 65 tidal
components – book wrong – 180 tidal constituents
Tides in small and elongated basins
o In long and narrow basins tides cannot rotate
 Currents in these basins simply reverse direction between high and low
tide, flowing in with the high tide and out with the low tide
 Cotidal and corange lines are nearly parallel to each other
 Tidal ranges increase if a bay tapers landward because water is
funneled towards the basin’s narrow end
 Tidal resonance occurs if the period of the basin is similar to the tidal
period
 Resonance = period of wave matches natural period of basin
 Resonance can greatly enhance the tidal range
 A tidal bore is a wall of water that surges upriver with the advancing
high tide
 A tidal bore occurs if:
o Tidal range is larger than 5 m
o The river valley tapers headward
o Water depth systematically decreases upstream
 As the tide flows in, it is constricted on the bottom and sides,
forcing to flow faster and the front to steepen into a collapsing
wave
Tidal currents
o The movement of water towards and away from land with the high and low
tides, respectively, generates tidal currents
 Flood current is the flow of water towards the land with the
approaching high tide
 Ebb current is the flow of water away from the land with the
approaching low tide
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Far off shore the tidal currents inscribe a circular path over a complete
tidal cycle
Near shore the tidal currents produce simple landward and then
seaward currents
Antarctic tides
o Ice shelves
o Amphidromic points
o Tidal currents
o Internal tides
o Mixing and why it is important
END
If extra time– then discuss homework and/or word problems
Lab
1. Types of tides
2. Tsunamis
3. Storm surge
Bring
1.
2.
3.
4.
Tide tables
Tide calendar
Tanks
Sugar water
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