TIDAL AND NONTIDAL OSCILLATIONS
Bruce Parker
Center for Maritime Systems/Davidson Laboratory,
Stevens Institute of Technology, Hoboken, NJ, USA
Definition
Tidal oscillation is the oscillation in the vertical and horizontal movement of water (and the properties of water)
caused by the tidal forces of the moon and sun.
Nontidal oscillation is the oscillation in the vertical and
horizontal movement of water (and the properties of
water) caused by nontidal phenomena that affect the estuary and ocean, such as wind, atmospheric pressure, air
temperature, river runoff, and earthquakes.
Introduction
The movement of water into, out of, and inside an estuary
is critically important for understanding all processes that
take place in that estuary, whether they be physical, geological, chemical, or biological. There are numerous
examples of water movement effects on such processes,
only a few of which include transporting nutrients, flushing pollutants, moving sediments, moving floating ecosystems, changing the characteristics of water in which
an ecosystem occurs, eroding shorelines, mixing water
masses, and affecting stratification.
Oscillations
Many of the most important water motions involve some
type of oscillation, where there is movement around some
mean value (an equilibrium), that movement fluctuating
between positive and then negative values around that
mean. This can take many forms, such as the oscillation
of the water surface up and down (around a mean sea
level), water repeatedly flowing up and then down an estuary, water temperature increasing and then decreasing,
salinity increasing and decreasing, as well as fluctuations
in numerous chemical, biological, and geological characteristics affected by these physical oscillations. Properties
in an estuary undergo oscillations caused by a number of
the physical phenomena mentioned below.
Many of the most important of these oscillations are the
result of some type of wave motion. A wave is essentially
an oscillation that travels through space (in this case
through water or along the water surface), transferring
energy from one location to another and often transporting
mass (there are mass oscillations, but with the nonlinear
systems inherent in shallow estuaries, there is usually
overall transport of mass and other properties after each
complete oscillation). These waves can be of different frequencies (periods) and wavelengths, with different driving/restoring forces. Some of these waves (e.g., the long
tide wave generated by the moon and sun in the open
ocean) are forced waves, where their generation is continuous. Most other waves are free waves, where an initial
generating force sets them in motion, to either travel as
far as they can or cause oscillations in an estuary as long
as they can, using that initial input of energy.
Long waves
The most powerful of these waves tend to be long waves,
that is, waves whose wavelength (the distance from one
peak to the next peak) is much greater than the water depth
of the estuary (or for some long waves, even much greater
than the depth of the ocean to which the estuary is
connected). Most important are the tides, storm surges,
nearshore wind waves, and tsunamis (although the wavelengths of nearshore wind waves are not nearly as long
as for the other three types). Each of these long waves
has a different generating force.
In their largest form, each of these long waves can be
the cause of deadly natural disasters along the coast and
inside estuaries. Although the easy and accurate prediction
of tides makes tidal extremes less dangerous today, in the
past large tides, tidal bores, and tidal whirlpools killed
many people. Storm surges have killed many millions
over the centuries, because they were not predictable and
people living in low-lying coastal areas drown in the
floods caused by the storm surges. That did not change
until late in the twentieth century when satellites and
hydrodynamic computer models allowed us to accurately
predict where and when a storm surge would strike
and then provide warnings to those in the danger area.
Tsunamis are still not predictable (because the submarine
earthquakes that cause them are not predictable), which
is why we lost 300,000 people to a tsunami as recently
as 2004 in countries around the Indian Ocean or 25,000
people even in a tsunami-knowledgeable country like
Japan in 2011. Because of much better weather prediction,
we can now predict when large wind waves are likely to
strike the coast. For a history of marine disasters
and a history of how scientists slowly developed
a marine prediction capability, see Parker (2012).
In their less extreme form, tides, storm surges, and nearshore wind waves have very significant day-to-day effects
on an estuary. They account for most of the water movement in the estuary (and its effect on biological and geological systems) along with river discharge (which could
be thought of as having an annual asymmetric oscillation).
Many of these long wave phenomena can happen at the
same time, combining their effects. They also interact with
each other because the shallow water causes nonlinearity
in the motion. They do not just add to each other, they
change each other, by transferring energy from one to
another through various nonlinear mechanisms. (see Tidal
Hydrodynamics, and also Parker, 2007, for explanations
of mechanisms that cause the tide to interact with storm
surges, wind waves, and river discharge).
Types of oscillations
In most estuaries, the tides provide the dominant forces
that cause the water to move. The gravitational effects of
the moon and sun on the oceans cause the water to move
TIDAL AND NONTIDAL OSCILLATIONS
701
Bruce Parker
Center for Maritime Systems/Davidson Laboratory,
Stevens Institute of Technology, Hoboken, NJ, USA
Definition
Tidal oscillation is the oscillation in the vertical and horizontal movement of water (and the properties of water)
caused by the tidal forces of the moon and sun.
Nontidal oscillation is the oscillation in the vertical and
horizontal movement of water (and the properties of
water) caused by nontidal phenomena that affect the estuary and ocean, such as wind, atmospheric pressure, air
temperature, river runoff, and earthquakes.
Introduction
The movement of water into, out of, and inside an estuary
is critically important for understanding all processes that
take place in that estuary, whether they be physical, geological, chemical, or biological. There are numerous
examples of water movement effects on such processes,
only a few of which include transporting nutrients, flushing pollutants, moving sediments, moving floating ecosystems, changing the characteristics of water in which
an ecosystem occurs, eroding shorelines, mixing water
masses, and affecting stratification.
Oscillations
Many of the most important water motions involve some
type of oscillation, where there is movement around some
mean value (an equilibrium), that movement fluctuating
between positive and then negative values around that
mean. This can take many forms, such as the oscillation
of the water surface up and down (around a mean sea
level), water repeatedly flowing up and then down an estuary, water temperature increasing and then decreasing,
salinity increasing and decreasing, as well as fluctuations
in numerous chemical, biological, and geological characteristics affected by these physical oscillations. Properties
in an estuary undergo oscillations caused by a number of
the physical phenomena mentioned below.
Many of the most important of these oscillations are the
result of some type of wave motion. A wave is essentially
an oscillation that travels through space (in this case
through water or along the water surface), transferring
energy from one location to another and often transporting
mass (there are mass oscillations, but with the nonlinear
systems inherent in shallow estuaries, there is usually
overall transport of mass and other properties after each
complete oscillation). These waves can be of different frequencies (periods) and wavelengths, with different driving/restoring forces. Some of these waves (e.g., the long
tide wave generated by the moon and sun in the open
ocean) are forced waves, where their generation is continuous. Most other waves are free waves, where an initial
generating force sets them in motion, to either travel as
far as they can or cause oscillations in an estuary as long
as they can, using that initial input of energy.
Long waves
The most powerful of these waves tend to be long waves,
that is, waves whose wavelength (the distance from one
peak to the next peak) is much greater than the water depth
of the estuary (or for some long waves, even much greater
than the depth of the ocean to which the estuary is
connected). Most important are the tides, storm surges,
nearshore wind waves, and tsunamis (although the wavelengths of nearshore wind waves are not nearly as long
as for the other three types). Each of these long waves
has a different generating force.
In their largest form, each of these long waves can be
the cause of deadly natural disasters along the coast and
inside estuaries. Although the easy and accurate prediction
of tides makes tidal extremes less dangerous today, in the
past large tides, tidal bores, and tidal whirlpools killed
many people. Storm surges have killed many millions
over the centuries, because they were not predictable and
people living in low-lying coastal areas drown in the
floods caused by the storm surges. That did not change
until late in the twentieth century when satellites and
hydrodynamic computer models allowed us to accurately
predict where and when a storm surge would strike
and then provide warnings to those in the danger area.
Tsunamis are still not predictable (because the submarine
earthquakes that cause them are not predictable), which
is why we lost 300,000 people to a tsunami as recently
as 2004 in countries around the Indian Ocean or 25,000
people even in a tsunami-knowledgeable country like
Japan in 2011. Because of much better weather prediction,
we can now predict when large wind waves are likely to
strike the coast. For a history of marine disasters
and a history of how scientists slowly developed
a marine prediction capability, see Parker (2012).
In their less extreme form, tides, storm surges, and nearshore wind waves have very significant day-to-day effects
on an estuary. They account for most of the water movement in the estuary (and its effect on biological and geological systems) along with river discharge (which could
be thought of as having an annual asymmetric oscillation).
Many of these long wave phenomena can happen at the
same time, combining their effects. They also interact with
each other because the shallow water causes nonlinearity
in the motion. They do not just add to each other, they
change each other, by transferring energy from one to
another through various nonlinear mechanisms. (see Tidal
Hydrodynamics, and also Parker, 2007, for explanations
of mechanisms that cause the tide to interact with storm
surges, wind waves, and river discharge).
Types of oscillations
In most estuaries, the tides provide the dominant forces
that cause the water to move. The gravitational effects of
the moon and sun on the oceans cause the water to move
TIDAL AND NONTIDAL OSCILLATIONS
701