Tides
8-1
Tidal Characteristics
Tides have a wave form, but differ from other waves because
they are caused by the interactions between the ocean, 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 , Semidiurnal tides,
and Mixed tides.
8-1
• Over a month the
daily tidal ranges
vary systematically
with the cycle of the
Moon.
• Tidal range is also
altered by the
shape of a basin
and sea floor
configuration.
Tidal Characteristics
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 the Moon is
closer.
• Gravitational attraction pulls
the ocean towards the Moon
and Sun, creating two
gravitational tidal bulges in
the ocean (high tides).
• Centrifugal effect is the push
outward from the center of
rotation.
• Latitude of the tidal bulges
is
8-2
Origin
determined by the declination, the
angle between Earth’s axis and the
lunar and solar orbital plane.
• Spring tides occur when Earth,
Moon and Sun are aligned in a
straight line and the tidal bulges
display constructive interference,
producing very high, high tides and
very low, low tides.
• Spring tides coincide with the new and
full moon.
• Neap tides occur when the Earth,
Moon and Sun are aligned forming
a right angle and tidal bulges
displaying destructive interference,
producing low high tides and high
low tides.
• Neap tides coincide with the first and
last quarter moon.
• Earth on its axis and the Moon in
its orbit both revolve eastward and
this causes the tides to occur 50
minutes later each day.
of the Tides
Movement of tides across ocean basins is deflected
by Coriolis, blocked 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.
8-2 Origin of the Tides
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
celerity
depends
upon water
depth.
8-2
Origin of the Tides
• 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 node.
• 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
different tidal components.
8-2
Origin of the Tides
• 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.
Water movement around an amphidromic point. The only
place where the water level never changes (where the colour
is always yellow) is in the centre of the basin; this is the
amphidromic point. You can see the wave rotate around this
point by following its highest elevation (the red region).
If the basin is of comparable dimensions in all directions the
wave travels along the basin perimeter, in circular fashion
around an amphidromic point.
Wave movement around a node. The node is seen half-way
along the basin, where the color is always greenish-yellow
regardless of the phase of the wave.
This is usually the case in long narrow basins such as
channels or narrow lakes, where the water can move along
the axis of the basin but not much across it.
8-4
Tidal Currents
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.
• 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.
Bay of Fundy
15 m tides in New Brunswick
Tidal effects on organisms:
Fiddler crabs, Grunion spawns during sroing tides in summer in
Cailfornia
In long and narrow basins tides can not 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 can greatly
enhance the tidal range.
• A tidal bore is a wall of
water that surges upriver
with the advancing high
tide.
8-5
Power from Tides
Electricity can be generated from tidal currents if
the tidal range is greater than 5 m in a large bay
connected to the ocean by a narrow opening.
• A dam is
constructed
across the
opening and
water is allowed
to flow into and
out of the bay
when sufficient
hydraulic head
exist to drive
turbines and
generate power.