that stated by Pritchard (1952) and Cameron and Pritchard
(1963), “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.” Several other definitions are found in the literature. Perillo (1995) defined an
estuary as “a semi-enclosed coastal body of water that
extends to the effective limit of tidal influence, within
which sea water entering from one or more free connections with the open sea, or any other saline coastal body
of water, is significantly diluted with fresh water derived
from land drainage, and can sustain euryhaline biological
species from either part or the whole of their life cycle.”
These definitions along with others by Dionne (1963),
Kjerfve (1987), and Dalrymple et al. (1992) have the ability to include all basic marine disciplines, which are necessary for studying this transitional water body as an
ecosystem. There are interchanges between three estuarine domains: the tidal river (TR), the mixing zone (MZ),
and the estuarine plume (EP), which must be investigated
together as a unique transitional system Okubo (1970).
In these definitions, salt-water dilution is due to mixing
processes, which involve advection (large-scale) and diffusion (small-scale). These occur internally in the estuary
and tend to produce uniformities in the concentration of
dissolved properties (Bowden, 1963; Okubo, 1970). The
small-scale component of mixing is generated by surface
(due to the wind stress) and bottom shears and the vertical
internal shears.
The estuarine drainage basin and the fresh water discharge (Q f ) into the estuary may change as a result of natural processes and human activity, such as inadequately
planned land use and pollutants from homes, farms, and
factories. According to Ji (2008), every surface water system is unique and many face similar environmental problems such as eutrophication, pathogen contamination,
toxic chemicals, loss of habitat, and declines in fish and
wildlife. Estuaries also have problems that cause declines
in water quality, living resources, and overall ecosystem
health. Rivers, lakes, and estuaries contain a very small
fraction of the total earth’s total water budget driven by
the hydrologic cycle.
Circulation variability in estuaries is classified as intertidal when it occurs at semi-diurnal or diurnal tidal frequencies (>1 cycle/day), or subtidal at lower frequencies
(<1 cycle/day). The beat period of the main semi-diurnal
tidal lunar and solar components (M 2 and S 2 ) results in
fortnightly subtidal frequency (modulation between successive spring tides, %15 days).
Classification of estuaries
Advection and mixing processes can be used to compare
and classify different estuaries. The major classification
schemes are based on salinity stratification, circulation,
and mixing. Figure 1 shows a simple steady-state dynamic
balance analysis of the interaction of these processes based
on a longitudinal section presented by Geyer (2010).
The first estuary classification was suggested by
Stommel (1951) taking into account the main forces as
tides, fresh water discharge, and wind. Raritan, Pamlico
Sound, and Mississippi river estuaries of New Jersey,
North Carolina, and Louisiana (USA) have as primary
forcings the tide, wind, and river, respectively. The important characteristic of these estuaries is the vertical salinity
stratification. The Raritan River estuary, which is the
shallowest one, is nearly vertically homogeneous. The
Mississippi River estuary is the deepest, being forced by
micro-tides.
Pritchard (1952) introduced a classification scheme
based on the main geomorphologic estuarine features.
Four types of estuaries were differentiated: (1) coastal
plain estuaries or drowned river valleys; (2) bar built estuaries; (3) fjords, typically in higher latitude regions; and
(4) tectonic estuaries.
Taking into account vertical salinity stratification,
Stommel (1953) suggested the following classification:
(1) salt wedge estuaries, river discharge dominates with
vertical mixing absent; (2) fjords, deep estuaries (several
hundred meters) characterized by a highly stratified upper
layer; and (3) moderately and highly stratified estuaries,
dependent on the intensity of the vertical mixing and
establishment of a steady-state bidirectional circulation.
This classification was described by Pritchard (1955),
Cameron and Pritchard (1963), and Schubel and Pritchard
(1972), taking into account improvements related
to dynamic conditions due to estuarine circulation and
salinity stratification.
During the 1960s, estuarine classifications were based
on quantitative criteria. Ippen and Harleman (1961)
introduced the stratification number, based on laboratory
experimental results, and Hansen and Rattray (1966)
introduced the stratification and circulation parameters.
Hansen and Rattray (1965) were the first investigators
to link estuarine classification with its physical properties.
They used salinity stratification and circulation in
a theoretical development based on the analytical solutions of a bi-dimensional system of equations (motion,
mass and salt conservation, and a linear state equation)
under steady-state conditions, applied to a laterally homogeneous estuary. The results were vertical profiles of the
longitudinal velocity and salinity used for a theoretical
deduction of the stratification-circulation diagram
(Hansen and Rattray, 1966). The coordinates (axis) of this
diagram are the stratification (p e ) and the circulation (p c )
parameters defined by p e ¼
S f ÀS s
S
¼
dS
S
and p c ¼
u s
u f
, respectively, where S f and S s are the time mean salinities on the
bottom and surface, respectively, and S its mean-depth
value. The velocity u s is the time mean value on the
estuary surface and u f is the one generated by the river
discharge (Q f ) defined by u f ¼
Q f
A
, where A is the
cross-sectional area.
The stratification (p e ) and circulation (p c ) parameters
are linked to a third parameter u defined by u ¼
f D
f D þf A
,
248
ESTUARINE CIRCULATION
(1963), “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.” Several other definitions are found in the literature. Perillo (1995) defined an
estuary as “a semi-enclosed coastal body of water that
extends to the effective limit of tidal influence, within
which sea water entering from one or more free connections with the open sea, or any other saline coastal body
of water, is significantly diluted with fresh water derived
from land drainage, and can sustain euryhaline biological
species from either part or the whole of their life cycle.”
These definitions along with others by Dionne (1963),
Kjerfve (1987), and Dalrymple et al. (1992) have the ability to include all basic marine disciplines, which are necessary for studying this transitional water body as an
ecosystem. There are interchanges between three estuarine domains: the tidal river (TR), the mixing zone (MZ),
and the estuarine plume (EP), which must be investigated
together as a unique transitional system Okubo (1970).
In these definitions, salt-water dilution is due to mixing
processes, which involve advection (large-scale) and diffusion (small-scale). These occur internally in the estuary
and tend to produce uniformities in the concentration of
dissolved properties (Bowden, 1963; Okubo, 1970). The
small-scale component of mixing is generated by surface
(due to the wind stress) and bottom shears and the vertical
internal shears.
The estuarine drainage basin and the fresh water discharge (Q f ) into the estuary may change as a result of natural processes and human activity, such as inadequately
planned land use and pollutants from homes, farms, and
factories. According to Ji (2008), every surface water system is unique and many face similar environmental problems such as eutrophication, pathogen contamination,
toxic chemicals, loss of habitat, and declines in fish and
wildlife. Estuaries also have problems that cause declines
in water quality, living resources, and overall ecosystem
health. Rivers, lakes, and estuaries contain a very small
fraction of the total earth’s total water budget driven by
the hydrologic cycle.
Circulation variability in estuaries is classified as intertidal when it occurs at semi-diurnal or diurnal tidal frequencies (>1 cycle/day), or subtidal at lower frequencies
(<1 cycle/day). The beat period of the main semi-diurnal
tidal lunar and solar components (M 2 and S 2 ) results in
fortnightly subtidal frequency (modulation between successive spring tides, %15 days).
Classification of estuaries
Advection and mixing processes can be used to compare
and classify different estuaries. The major classification
schemes are based on salinity stratification, circulation,
and mixing. Figure 1 shows a simple steady-state dynamic
balance analysis of the interaction of these processes based
on a longitudinal section presented by Geyer (2010).
The first estuary classification was suggested by
Stommel (1951) taking into account the main forces as
tides, fresh water discharge, and wind. Raritan, Pamlico
Sound, and Mississippi river estuaries of New Jersey,
North Carolina, and Louisiana (USA) have as primary
forcings the tide, wind, and river, respectively. The important characteristic of these estuaries is the vertical salinity
stratification. The Raritan River estuary, which is the
shallowest one, is nearly vertically homogeneous. The
Mississippi River estuary is the deepest, being forced by
micro-tides.
Pritchard (1952) introduced a classification scheme
based on the main geomorphologic estuarine features.
Four types of estuaries were differentiated: (1) coastal
plain estuaries or drowned river valleys; (2) bar built estuaries; (3) fjords, typically in higher latitude regions; and
(4) tectonic estuaries.
Taking into account vertical salinity stratification,
Stommel (1953) suggested the following classification:
(1) salt wedge estuaries, river discharge dominates with
vertical mixing absent; (2) fjords, deep estuaries (several
hundred meters) characterized by a highly stratified upper
layer; and (3) moderately and highly stratified estuaries,
dependent on the intensity of the vertical mixing and
establishment of a steady-state bidirectional circulation.
This classification was described by Pritchard (1955),
Cameron and Pritchard (1963), and Schubel and Pritchard
(1972), taking into account improvements related
to dynamic conditions due to estuarine circulation and
salinity stratification.
During the 1960s, estuarine classifications were based
on quantitative criteria. Ippen and Harleman (1961)
introduced the stratification number, based on laboratory
experimental results, and Hansen and Rattray (1966)
introduced the stratification and circulation parameters.
Hansen and Rattray (1965) were the first investigators
to link estuarine classification with its physical properties.
They used salinity stratification and circulation in
a theoretical development based on the analytical solutions of a bi-dimensional system of equations (motion,
mass and salt conservation, and a linear state equation)
under steady-state conditions, applied to a laterally homogeneous estuary. The results were vertical profiles of the
longitudinal velocity and salinity used for a theoretical
deduction of the stratification-circulation diagram
(Hansen and Rattray, 1966). The coordinates (axis) of this
diagram are the stratification (p e ) and the circulation (p c )
parameters defined by p e ¼
S f ÀS s
S
¼
dS
S
and p c ¼
u s
u f
, respectively, where S f and S s are the time mean salinities on the
bottom and surface, respectively, and S its mean-depth
value. The velocity u s is the time mean value on the
estuary surface and u f is the one generated by the river
discharge (Q f ) defined by u f ¼
Q f
A
, where A is the
cross-sectional area.
The stratification (p e ) and circulation (p c ) parameters
are linked to a third parameter u defined by u ¼
f D
f D þf A
,
248
ESTUARINE CIRCULATION