vertically (the tide) and horizontally (the tidal current), the
entire phenomenon usually referred to as the tides. Tidal
motion is actually the motion of extremely long waves
generated in the open ocean, which propagate over the
continental shelf and up into estuaries where their size is
usually amplified. Only the oceans are large enough for
the tide-generating forces to directly produce a tide of
significant size. The tides in an estuary are forced at the
estuary entrance by the tide wave from the ocean
(having been modified to some degree by propagating
over the shallower continental shelf). In estuaries we are
essentially speaking of shallow-water tides, which are significantly different than open-ocean tides, having not only
much larger tidal ranges and faster tidal currents but also
distorted and asymmetric oscillations caused by the
nonlinear effects of the shallow water, which can have significant effects on transport within the estuary (see Tidal
Hydrodynamics and also Parker, 2007).
Storm surges are very long waves, generated by the fast
winds of hurricanes and gales, which can push huge quantities of water onshore causing extensive flooding. Such
wind-generated changes in water level along the shore
also occur under more modest wind conditions (under
conditions not considered as “storm” conditions), but the
term storm surge is often used to define in general any
change in water level caused by wind. Atmospheric
pressure also plays a small role in storm-induced changes
in water level. The crest of a storm surge does the most
damage when arriving at the coast at high tide. Because
of the shallow water, storm surges can modify the tide,
sometimes decreasing the tide range when the storm surge
has made the water depth greater (Parker, 2007).
Nearshore wind waves are also generated by wind, sometimes local winds, and sometimes by distant storms whose
waves travel across the ocean, reaching distant shores as
swell, which are deep-water waves until they reach the shallow waters near the coast and inside the estuary. At this point
the bathymetry can focus wave energy via wave refraction,
increasing their size at shallow points along the shore, creating longshore currents and rip currents, eroding shorelines,
and mixing the water column. During storms, they do the
most damage when arriving at the shore at high tide. Wind
waves can also affect the tide by modifying the bottom friction that affects the propagation of the tide wave.
Tsunamis are very long waves generated most frequently by submarine earthquakes (ones that have significant vertical movement of the sea bottom) but also
occasionally by volcanic eruptions or submarine landslides. Their wavelengths are very long in the deep ocean
(hundreds of kilometers), where they also travel at great
speeds (e.g., 600 km/h). When tsunamis reach the
shallower continental shelf, their wavelength shortens,
their speed decreases, their height increases, and they send
large waves against the shore with periods usually on the
order of 20–40 min. They do the most damage when
arriving at the shore at high tide (Parker, 2012).
Beyond these four primary long-wave phenomena,
there are other phenomena that can cause oscillations
in an estuary. Examples are seiches, land breeze-sea
breeze, and river flow.
Seiches are oscillations in harbors or small bays caused
by wind waves at the harbor entrance. These oscillations
are largest if the period of the waves entering the harbor
matches the natural period of the harbor basin
(as determined by its depth and width).
Land breeze-sea breeze oscillations in the wind are
found in tropical regions and in temperate regions during
the summer. During the day, the land heats up more than
the sea and the air rises above the land to be replaced by
cooler air blowing toward the land from the sea. The opposite breeze occurs at night when the sea is warmer than the
land. A land breeze-sea breeze, which has roughly a 24-h
period, can produce an oscillating wind current in the
upper waters of the estuary or along the coast or even an
oscillation in the water level (although these oscillations
can be hidden by diurnal tidal oscillations). There are also
changes in wind speed and direction as weather systems
move over an estuary that can be quasiperiodic and can
cause oscillations in the estuary.
The daily change in air temperature also produces
a 24-h oscillation in the heating of the water surface,
which affects the estuary as does the 24-h cycle in the
effect of light on photosynthesis in phytoplankton. There
is, of course, an annual (seasonal) oscillation in temperature (and other meteorological effects).
River flow can be thought of as having a very asymmetric oscillation, with most high flow values occurring after
high spring runoff from melting snow or from rainfall produced by large storms or hurricanes, with the remaining
time being characterized by much less flow. River currents
augment the tidal current, making the ebb flow duration
much longer and the flood duration much shorter.
The river flow can also interact with the tide nonlinearly,
reducing the tide range and distorting the shape of the tide
curve (see Tidal Hydrodynamics and Parker, 2012).
Summary
Tidal oscillations are oscillations in the vertical and horizontal movement of water (and the properties of water)
caused by the tidal forces of the moon and sun. Nontidal
oscillations are horizontal and vertical oscillations in
water and its properties caused by nontidal phenomena
that affect the estuary and ocean, such as wind, atmospheric pressure, air temperature, river runoff, and earthquakes. This entry summarizes the effects of tides, storm
surge, long wind waves, and tsunamis (all of which have
long wavelengths), as well as other phenomena that can
cause oscillations in an estuary, including seiches, land
breeze-sea breeze, daily and annual changes in air temperature and light, and river discharge. Various aspects of the
biological, chemical, geological, and physical environments of an estuary have equilibria around which there
are oscillations caused by the above phenomena. Examples of these oscillations can be found elsewhere in this
encyclopedia.
702
TIDAL AND NONTIDAL OSCILLATIONS
entire phenomenon usually referred to as the tides. Tidal
motion is actually the motion of extremely long waves
generated in the open ocean, which propagate over the
continental shelf and up into estuaries where their size is
usually amplified. Only the oceans are large enough for
the tide-generating forces to directly produce a tide of
significant size. The tides in an estuary are forced at the
estuary entrance by the tide wave from the ocean
(having been modified to some degree by propagating
over the shallower continental shelf). In estuaries we are
essentially speaking of shallow-water tides, which are significantly different than open-ocean tides, having not only
much larger tidal ranges and faster tidal currents but also
distorted and asymmetric oscillations caused by the
nonlinear effects of the shallow water, which can have significant effects on transport within the estuary (see Tidal
Hydrodynamics and also Parker, 2007).
Storm surges are very long waves, generated by the fast
winds of hurricanes and gales, which can push huge quantities of water onshore causing extensive flooding. Such
wind-generated changes in water level along the shore
also occur under more modest wind conditions (under
conditions not considered as “storm” conditions), but the
term storm surge is often used to define in general any
change in water level caused by wind. Atmospheric
pressure also plays a small role in storm-induced changes
in water level. The crest of a storm surge does the most
damage when arriving at the coast at high tide. Because
of the shallow water, storm surges can modify the tide,
sometimes decreasing the tide range when the storm surge
has made the water depth greater (Parker, 2007).
Nearshore wind waves are also generated by wind, sometimes local winds, and sometimes by distant storms whose
waves travel across the ocean, reaching distant shores as
swell, which are deep-water waves until they reach the shallow waters near the coast and inside the estuary. At this point
the bathymetry can focus wave energy via wave refraction,
increasing their size at shallow points along the shore, creating longshore currents and rip currents, eroding shorelines,
and mixing the water column. During storms, they do the
most damage when arriving at the shore at high tide. Wind
waves can also affect the tide by modifying the bottom friction that affects the propagation of the tide wave.
Tsunamis are very long waves generated most frequently by submarine earthquakes (ones that have significant vertical movement of the sea bottom) but also
occasionally by volcanic eruptions or submarine landslides. Their wavelengths are very long in the deep ocean
(hundreds of kilometers), where they also travel at great
speeds (e.g., 600 km/h). When tsunamis reach the
shallower continental shelf, their wavelength shortens,
their speed decreases, their height increases, and they send
large waves against the shore with periods usually on the
order of 20–40 min. They do the most damage when
arriving at the shore at high tide (Parker, 2012).
Beyond these four primary long-wave phenomena,
there are other phenomena that can cause oscillations
in an estuary. Examples are seiches, land breeze-sea
breeze, and river flow.
Seiches are oscillations in harbors or small bays caused
by wind waves at the harbor entrance. These oscillations
are largest if the period of the waves entering the harbor
matches the natural period of the harbor basin
(as determined by its depth and width).
Land breeze-sea breeze oscillations in the wind are
found in tropical regions and in temperate regions during
the summer. During the day, the land heats up more than
the sea and the air rises above the land to be replaced by
cooler air blowing toward the land from the sea. The opposite breeze occurs at night when the sea is warmer than the
land. A land breeze-sea breeze, which has roughly a 24-h
period, can produce an oscillating wind current in the
upper waters of the estuary or along the coast or even an
oscillation in the water level (although these oscillations
can be hidden by diurnal tidal oscillations). There are also
changes in wind speed and direction as weather systems
move over an estuary that can be quasiperiodic and can
cause oscillations in the estuary.
The daily change in air temperature also produces
a 24-h oscillation in the heating of the water surface,
which affects the estuary as does the 24-h cycle in the
effect of light on photosynthesis in phytoplankton. There
is, of course, an annual (seasonal) oscillation in temperature (and other meteorological effects).
River flow can be thought of as having a very asymmetric oscillation, with most high flow values occurring after
high spring runoff from melting snow or from rainfall produced by large storms or hurricanes, with the remaining
time being characterized by much less flow. River currents
augment the tidal current, making the ebb flow duration
much longer and the flood duration much shorter.
The river flow can also interact with the tide nonlinearly,
reducing the tide range and distorting the shape of the tide
curve (see Tidal Hydrodynamics and Parker, 2012).
Summary
Tidal oscillations are oscillations in the vertical and horizontal movement of water (and the properties of water)
caused by the tidal forces of the moon and sun. Nontidal
oscillations are horizontal and vertical oscillations in
water and its properties caused by nontidal phenomena
that affect the estuary and ocean, such as wind, atmospheric pressure, air temperature, river runoff, and earthquakes. This entry summarizes the effects of tides, storm
surge, long wind waves, and tsunamis (all of which have
long wavelengths), as well as other phenomena that can
cause oscillations in an estuary, including seiches, land
breeze-sea breeze, daily and annual changes in air temperature and light, and river discharge. Various aspects of the
biological, chemical, geological, and physical environments of an estuary have equilibria around which there
are oscillations caused by the above phenomena. Examples of these oscillations can be found elsewhere in this
encyclopedia.
702
TIDAL AND NONTIDAL OSCILLATIONS