ESTUARINE CIRCULATION
Roberto F. C. Fontes
1 , Luiz B. Miranda
2 and
Fernando Andutta
3
1
Littoral Campus/UNESP – Univ Estadual Paulista,
São Vicente, SP, Brazil
2
Oceanographic Institute of the University of São Paulo,
São Paulo, SP, Brazil
3
School of Physical, Environmental and Mathematical
Sciences, University of New South Wales, Canberra,
Australia
Definition
An estuary is a semi-enclosed coastal body of water which
has a free connection with the open sea and within which
salt water is measurably diluted with fresh water derived
from land drainage.
Introduction
Estuaries are unique aquatic environments that share
terrestrial and marine contributions, creating a set of
interesting physical characteristics including bimodal
circulation patterns (fresh water flowing along the surface
and saline water entering along the bottom) and mixing
processes induced by tides. To a lesser extent, continental
shelf processes such as storm surges and wind-driven
circulation can also influence circulation in estuaries.
At first glance, most estuaries are governed by tidal
oscillation and influenced by river discharges, so
mixing processes account for most of the hydrodynamic
characterization of estuaries. The resulting velocity
and salinity fields have strong time variability, and
the non-steady-state current patterns may be altered by
sediment erosion, transport, and sedimentation, which
contribute to bathymetric and margin changes.
Formation and geological age
Estuaries are transitional environments that were formed
in the narrow coastal boundary zone between the ocean
and the continents during the Holocene rise in sea level
which began about 15,000 years ago, when the sea was
about 125 m below its present level. They were formed
due to the Flandrian Transgression, the secular increase
of global sea level that ended about 7,000 years ago. Local
changes in sea level as well as tectonic activity may have
also contributed to the formation of estuaries. At the end
of that period, the continental coastal plains and the river
valleys were slowly flooded forming the bays, coastal
lagoons, inlets, and estuaries.
The geomorphology of estuaries depends on coastal
topography, geomorphology, continental shelf characteristics, and river discharges, which change due to natural
processes of erosion and sediment deposition. These processes have often been modified due to the anthropogenic
activities in estuaries and in their drainage basins.
Recent studies conclude that during the late Holocene
sea-level rise amounted to 5–7 m. This was corroborated
by the earlier work of Fairbridge (1961). Villwock
(1972), Suguio and Martin (1978), and Kowsmann
et al. (1977) also confirmed this phenomenon in the south
and southeastern coastal plain of Brazil.
Transitional environments associated with estuaries,
such as salt marshes and mangroves, are colonized by
the salt-tolerant species and distributed in regions of temperate and tropical climates. They are environments ultimately dependent on the dynamics of estuarine systems,
but have been strongly affected by human activities
around the world Herz (1992) . As such, their areas have
declined significantly. Some have been protected by
reserve status since the nineteenth century. Human
impacts and their interference on estuaries and coasts are
described in French (1997).
Herz (1992) used digital mapping, based on processing
of multispectral images, to describe the mangrove colonization area along the 8,000 km of the Brazilian coast. The
mangrove area decreased from 85.0 % in the northern
coast of Brazil to 9.9 % and 5.1 % in the eastern and southern coastal regions.
Pioneer investigations
In 1875, the Swedish researcher F. L. Ekman performed
experiments in the Götaelf River Estuary (Sweden) to
investigate estuarine processes (quoted in Defant (1961),
539). From the analysis of the salinity distribution on
August 1875, Ekman observed that a strong outflowing
current could not be compensated only by river discharges, but instead by a compensating upward motion
(upwelling) of saline water from the bottom.
Upwelling is a vertical advective motion that breaks the
continuity of the bidirectional motion field observed by
Ekman. The vertical motion was calculated theoretically
under steady-state conditions for the first time by Pritchard
(1954), solving the salt balance equation using extensive
experimental data from the James River Estuary
(Virginia, USA) during the summer of 1950.
Ekman investigated the occurrence of currents
up-estuary in the lower layers of the estuary in a paper
published in 1899, quoted in Defant (1961, 539).
Although this paper was only descriptive, Ekman was
aware that the river discharge input and the salinity stratification were the main forcing mechanisms of the observational results.
It was only in 1952 – almost 60 years later – that
D. V. Pritchard became the first researcher to link estuarine
circulation to the forcing by the horizontal density
gradient. He used observations from the James River
estuary to demonstrate this mechanism, quoted in Geyer
(2010, 13).
Definitions and variability
Ketchum (1951) focused on the exchanges of fresh and
salt water in tidal estuaries and defined an estuary as “a
body of water in which the river water mixes and measurably dilutes sea water.” The most classical definition is
ESTUARINE CIRCULATION
247
Roberto F. C. Fontes
1 , Luiz B. Miranda
2 and
Fernando Andutta
3
1
Littoral Campus/UNESP – Univ Estadual Paulista,
São Vicente, SP, Brazil
2
Oceanographic Institute of the University of São Paulo,
São Paulo, SP, Brazil
3
School of Physical, Environmental and Mathematical
Sciences, University of New South Wales, Canberra,
Australia
Definition
An estuary is a semi-enclosed coastal body of water which
has a free connection with the open sea and within which
salt water is measurably diluted with fresh water derived
from land drainage.
Introduction
Estuaries are unique aquatic environments that share
terrestrial and marine contributions, creating a set of
interesting physical characteristics including bimodal
circulation patterns (fresh water flowing along the surface
and saline water entering along the bottom) and mixing
processes induced by tides. To a lesser extent, continental
shelf processes such as storm surges and wind-driven
circulation can also influence circulation in estuaries.
At first glance, most estuaries are governed by tidal
oscillation and influenced by river discharges, so
mixing processes account for most of the hydrodynamic
characterization of estuaries. The resulting velocity
and salinity fields have strong time variability, and
the non-steady-state current patterns may be altered by
sediment erosion, transport, and sedimentation, which
contribute to bathymetric and margin changes.
Formation and geological age
Estuaries are transitional environments that were formed
in the narrow coastal boundary zone between the ocean
and the continents during the Holocene rise in sea level
which began about 15,000 years ago, when the sea was
about 125 m below its present level. They were formed
due to the Flandrian Transgression, the secular increase
of global sea level that ended about 7,000 years ago. Local
changes in sea level as well as tectonic activity may have
also contributed to the formation of estuaries. At the end
of that period, the continental coastal plains and the river
valleys were slowly flooded forming the bays, coastal
lagoons, inlets, and estuaries.
The geomorphology of estuaries depends on coastal
topography, geomorphology, continental shelf characteristics, and river discharges, which change due to natural
processes of erosion and sediment deposition. These processes have often been modified due to the anthropogenic
activities in estuaries and in their drainage basins.
Recent studies conclude that during the late Holocene
sea-level rise amounted to 5–7 m. This was corroborated
by the earlier work of Fairbridge (1961). Villwock
(1972), Suguio and Martin (1978), and Kowsmann
et al. (1977) also confirmed this phenomenon in the south
and southeastern coastal plain of Brazil.
Transitional environments associated with estuaries,
such as salt marshes and mangroves, are colonized by
the salt-tolerant species and distributed in regions of temperate and tropical climates. They are environments ultimately dependent on the dynamics of estuarine systems,
but have been strongly affected by human activities
around the world Herz (1992) . As such, their areas have
declined significantly. Some have been protected by
reserve status since the nineteenth century. Human
impacts and their interference on estuaries and coasts are
described in French (1997).
Herz (1992) used digital mapping, based on processing
of multispectral images, to describe the mangrove colonization area along the 8,000 km of the Brazilian coast. The
mangrove area decreased from 85.0 % in the northern
coast of Brazil to 9.9 % and 5.1 % in the eastern and southern coastal regions.
Pioneer investigations
In 1875, the Swedish researcher F. L. Ekman performed
experiments in the Götaelf River Estuary (Sweden) to
investigate estuarine processes (quoted in Defant (1961),
539). From the analysis of the salinity distribution on
August 1875, Ekman observed that a strong outflowing
current could not be compensated only by river discharges, but instead by a compensating upward motion
(upwelling) of saline water from the bottom.
Upwelling is a vertical advective motion that breaks the
continuity of the bidirectional motion field observed by
Ekman. The vertical motion was calculated theoretically
under steady-state conditions for the first time by Pritchard
(1954), solving the salt balance equation using extensive
experimental data from the James River Estuary
(Virginia, USA) during the summer of 1950.
Ekman investigated the occurrence of currents
up-estuary in the lower layers of the estuary in a paper
published in 1899, quoted in Defant (1961, 539).
Although this paper was only descriptive, Ekman was
aware that the river discharge input and the salinity stratification were the main forcing mechanisms of the observational results.
It was only in 1952 – almost 60 years later – that
D. V. Pritchard became the first researcher to link estuarine
circulation to the forcing by the horizontal density
gradient. He used observations from the James River
estuary to demonstrate this mechanism, quoted in Geyer
(2010, 13).
Definitions and variability
Ketchum (1951) focused on the exchanges of fresh and
salt water in tidal estuaries and defined an estuary as “a
body of water in which the river water mixes and measurably dilutes sea water.” The most classical definition is
ESTUARINE CIRCULATION
247