The Tides Chapter 11. Tidal Range Tide Patterns Diurnal tide T = 1 day One high and one low per day.
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Transcript of The Tides Chapter 11. Tidal Range Tide Patterns Diurnal tide T = 1 day One high and one low per day.
The Tides
Chapter 11
Tidal Range
Tide Patterns Diurnal tide
T = 1 day One high and one low per day
Tide Patterns Semidiurnal tide
T = ½day Two highs and two lows per day
Tide Patterns Mixed semidiurnal tide
Worldwide Tidal Patterns
Tide Terms
Average tide
Tidal datum
Minus tide
Flood & Ebb
Slack water
Tidal Analyses Equilibrium Tidal Theory This model is a simplification of the real
world and makes several assumptions There are no landmasses or effects from
the sea floor The ocean is assumed to be of a deep,
uniform depth Water is assumed to be in equilibrium with
tide generating forces = gravity and centrifugal effect
Origin of Tides
Tides are caused by two factors:1. Gravitational attraction2. Centrifugal force
Gravitational attraction Sun-Moon-Earth system Strength varies with the mass of an
object
Gravity The strength of gravity also varies with
the distance separating any two masses Tide raising forces varies inversely as the
cube of the distance between them As you double the distance between the
objects the tide raising force decreases by a factor of 8 (23)F = G(m1m2/r2)
T = G(m1m2/r3)
Distance is more important than mass in tide generating forces
Newton’s Universal Law of Gravitation:
Gravitational Pull Gravitational force pulls on the oceans
causing the water to be drawn toward the side of the Earth facing the moon This creates a tidal bulge
Centrifugal Force
This force arises as the Earth and moon revolve around each other Centrifugal force in everyday situations
The water of the oceans shifts away from the center of rotation creating the second tidal bulge away from the side facing the moon
Centrifugal Force
Two Tidal Bulges
Tide Generating Forces
Tidal Day Diurnal tides 24 hours and 50
minutes Semidiurnal tides 12 hours and 25
minutes
What about the Sun? Large but very far away Tide generating force only 46% as
large as that of the Moon Solar tide wave
Diurnal 24 hours Semidiurnal 12 hours
Spring & Neap Tides
Spring & Neap Tides New & full
Earth-Moon-Sun aligned Constructive interference Highest tide range
1st & 3rd Earth-Moon-Sun perpendicular Destructive interference Lowest tide range
Spring
Neap
Declinational Tides The latitude at which the Moon and
Sun are directly overhead varies with time in a regular fashion
Diurnal tide
Elliptical Orbits Due to elliptical orbits, the
distances from the Moon and Sun to Earth change
Therefore, tide generating forces also change
Elliptical Orbits Earth is closest to the Sun during
the Northern Hemisphere winter
Thus, the solar tide is largest during the Northern Hemisphere winter.
ReviewThe equilibrium model is an excellent
start to understanding tides, but we must remember the assumptions:• There are no landmasses or effects of the
sea floor • The ocean is assumed to be of a deep
uniform depth• Water is assumed to be in equilibrium
with tide generating forces = gravity and centrifugal effect
Dynamic Tidal Analysis Generating Forces
Gravity & inertia
The Tide Wave
The Tide Wave Free wave
~200 m/sec Forced wave at the equator
Balance between friction & gravity
Less in higher latitudes
Progressive Wave Tides Tide wave that
moves, or progresses, in a nearly constant direction
Western North Pacific
Eastern South Pacific
South Atlantic Ocean
Progressive Wave Tides Cotidal lines
Marks location of crest at certain time intervals
1 hour Shallow water
wave
Standing Wave Tides The reflection
of the tide wave can create a rotary standing wave
The bulge on the western edge of the basin creates a pressure gradient (to the east) as the earth continues to rotate
At some point the water will flow down the pressure gradient and be deflected to the right in the Northern Hemisphere.
Due to the Coriolis effect the water forms a mound in the South
This bulge creates another pressure gradient (to the north)
When the water flows it is deflected once again to the right and piles up in the eastern margin
Once this balance is reached the tidal bulge that forms is called a rotary wave This wave is similar to the wave that can be
produced by swirling a cup A rotary wave creates both high (crests)
and low (troughs) tides each day
The node is seen half-way along the basin, where the color is always greenish-yellow regardless of the phase of the wave.
Rotary Wave Movement
Tide crest rotates counterclockwise around the basin
Tidal current rotates clockwise because the current is deflected to the right in the Northern Hemisphere
Amphidromic Point Node for a
rotary wave
Tidal range is zero
Tidal range increases away from node
Corange Lines Lines of equal
tidal range
Rose Diagram
Shows direction of tidal current at a specific hour
Speed of current correlated to length of arrow
Progressive-Vector Diagram
Diurnal One complete
circle
Semidiurnal Two circles
Mixed Two unequal circles
Tides in Small & Narrow Basins Tides can be quite different due to
the shallowness, smallness and shapes of many bays and estuaries
In the nearby Bay of Fundy it is much narrower and more elongated (restrictive basin) the tidal wave cannot rotate as it does in the open ocean Instead the tide moves in and out of the
estuary and does not rotate around a node
The Bay of Fundy
Two reasons:
Gradual tapering & shallowing that constricts tidal flow into the bay
Dimension of the bay Tidal resonance This creates a seiche causing the water to
slosh back and forth like a standing wave
Tidal Bores High tide crest that
advances rapidly up an estuary or river as a breaking wave
3 conditions contribute to tidal bores
Large tidal range, greater than 17 feet
A tapering basin geometry
Water depths that systematically decrease upriver
Tidal Bores Qiantang River
9m 40 km/hr (25 miles/hr)
Amazon River Pororoca
Tide Predictions Astronomical data and local measurements
Measurements made for at least 19 years, to allow for the 18.6-year declinational period of the Moon
Harmonic analysis Used to separate the tide record into
components or partial tides that combine to form the actual tide
Can then isolate the effect of local geography Local Effect
Tide Tables
Tide Tables
Tide Current Tables
Tide Current Tables
Ripple Rock
Tidal Energy Two systems to extract energy
from tides: Single-action power cycle Ebb only
Annapolis River, NS
Tidal Energy Two systems to extract energy
from tides: Double-action power cycle Ebb & flood
Rance River Estuary, France
The Future of Tidal Power