estuarine basin and where evaporation leads to denser
saline surface water which sinks into the less dense water.
Horizontal mixing can take place where strongly
inflowing flood tidal waters enter a large estuarine basin
through a relatively narrow inlet to meet a stream of
strongly outflowing freshwater emanating from
a similarly narrow river tract. The interaction is a largescale circulation pattern. Some degree of horizontal
mixing is also effected in large estuarine basins by
Coriolis force, which is the flow induced on currents due
to the rotation of the Earth (Gross, 1972; Dyer, 1973).
Finally, smaller-scale mixing results from eddying
induced by bathymetry complexity and by plan geometry
of the estuarine basin. This is the result of river flows, tidal
flows, and wind-driven currents occurring in estuaries of
complex planar topography or with complex crosssectional and longitudinal bathymetry.
At finer scales, along the saltwater/freshwater interface,
mixing is effected by the following: entrainment (where
the interface is sheared and disrupted by the flowing freshwater and saline water is incorporated into the overlying
water by advection) progressively leading to turbulence
(where flow is rapid enough to change from laminar flow
along the saltwater/freshwater interface to turbulent flow)
(Wu, 1973; Kantha et al., 1977) and wave-driven vertical
mixing induced by wave orbitals (Martin and
McCutcheon, 1999; Qiao et al., 2004; Babanin, 2006).
The description of mixing in the text above has largely
concentrated on the interaction of saline and freshwater
wherein the density differences are mainly due to salt content and to some extent temperature differences. Further
complications in mixing occur where density of water is
determined by sediment in suspension or as bed load sediment transported along the river or estuary floor in traction. Mud suspended in freshwater surface flows and
sand-saturated basal traction loads have a higher density
than sediment-free freshwater. Instead of freshwater
flowing out into denser seawater in the estuarine basin,
many estuaries experience more unpredictable flows,
due to density-determined dynamics of sand-dominated
and mud-dominated current flows resulting in turbidity
along the estuary floor and in the upper water column,
respectively. Similarly, the dynamics of mixing become
complicated where turbid river flow interacts with tidal
flow that has been enriched with suspended mud or with
turbid basal entrained sand.
Bates (1953) and Orton and Reading (1993) describe
variability of turbidity and sediment loads and interactions
with contrasting density in the receiving waters in three
situations: (1) hypopycnal flows in which density of the
suspended sediment flow is less than that of the receiving
estuarine water body, (2) homopycnal flows in which density of the suspended sediment flow is equal to that of the
receiving estuarine water body, and (3) hyperpycnal flows
in which density of the suspended sediment flow is
more than that of the receiving estuarine water body.
The dynamics of sediment-laden river flow with its various amounts of suspension and/or traction load entering
and interacting with an estuarine receiving water body of
different density (ranging from fresh to brackish to marine)
determine the different types mixing and, for sediment
deposition, the different styles of sediment accumulation.
Thus, riverine freshwater flowing into an estuarine
basin of denser brackish water or marine salinity will
exhibit hypopycnal flow, with freshwater overlying the
denser estuarine water. Riverine freshwater flowing into
an estuarine basin of similar freshwater salinity, or turbid
freshwater flowing into brackish water, will exhibit
homopycnal flow, with the river water invading the estuarine water of similar density in a turbulent mixing front. At
the other extreme, sediment-laden turbid riverine freshwater flowing into an estuarine basin of freshwater or weakly
brackish salinity will exhibit hyperpycnal flow, with the
denser sediment-laden river (composed of sediment in
suspension and in traction) in a base flow under the less
dense estuarine water.
Various estuaries and models of estuarine settings have
been investigated empirically and mathematically to provide an understanding of the dynamics of mixing associated with bathymetrical variations and of circulation
patterns that result from interacting processes. For
instance, Lerczak and Geyer (2004) modeled the lateral
circulation in straight, narrow, stratified estuaries to contrast them with homogeneous or with weakly stratified
estuaries comparing flood-tide lateral hydrodynamics
with ebb-tide hydrodynamics, and addressing the issues
of along-channel and cross-channel differential advection
and Coriolis forcing. They determined that in straight narrow stratified estuaries, lateral circulation during flood
tides is about four times as strong as during ebb tides,
the flood–ebb-tide asymmetry being due to a feedback
between lateral circulation and the along-channel tidal
currents, as well as to time-varying stratification over
a tidal cycle. The modeling showed that as the stratification increased, the lateral circulation was reduced because
of the adverse pressure gradient setup by isopycnals being
tilted by the lateral flow itself.
Armi and Farmer (1986) explored the difference in
hydrologic exchange between an estuary with a submerged
barrier and one with a contraction (or constriction). They
showed that exchange over a submerged barrier or sill
(such as an inundated moraine) is fundamentally different
to that through a contraction. Sill crests primarily affect
the deeper layer into which the sill projects and only indirectly control the surface layer, consequently resulting in
asymmetrical flows. Where both a sill and a contraction
are present, the contraction affects flow at the sill crest only
if it occurs between the sill and the source of denser water.
Chen and Sanford (2009) explored the effects of axial
wind influences on salinity stratification and longitudinal
transport of saline water in partially mixed straight estuaries. They showed that wind straining along the channel
MARINE/FRESHWATER MIXING
409
saline surface water which sinks into the less dense water.
Horizontal mixing can take place where strongly
inflowing flood tidal waters enter a large estuarine basin
through a relatively narrow inlet to meet a stream of
strongly outflowing freshwater emanating from
a similarly narrow river tract. The interaction is a largescale circulation pattern. Some degree of horizontal
mixing is also effected in large estuarine basins by
Coriolis force, which is the flow induced on currents due
to the rotation of the Earth (Gross, 1972; Dyer, 1973).
Finally, smaller-scale mixing results from eddying
induced by bathymetry complexity and by plan geometry
of the estuarine basin. This is the result of river flows, tidal
flows, and wind-driven currents occurring in estuaries of
complex planar topography or with complex crosssectional and longitudinal bathymetry.
At finer scales, along the saltwater/freshwater interface,
mixing is effected by the following: entrainment (where
the interface is sheared and disrupted by the flowing freshwater and saline water is incorporated into the overlying
water by advection) progressively leading to turbulence
(where flow is rapid enough to change from laminar flow
along the saltwater/freshwater interface to turbulent flow)
(Wu, 1973; Kantha et al., 1977) and wave-driven vertical
mixing induced by wave orbitals (Martin and
McCutcheon, 1999; Qiao et al., 2004; Babanin, 2006).
The description of mixing in the text above has largely
concentrated on the interaction of saline and freshwater
wherein the density differences are mainly due to salt content and to some extent temperature differences. Further
complications in mixing occur where density of water is
determined by sediment in suspension or as bed load sediment transported along the river or estuary floor in traction. Mud suspended in freshwater surface flows and
sand-saturated basal traction loads have a higher density
than sediment-free freshwater. Instead of freshwater
flowing out into denser seawater in the estuarine basin,
many estuaries experience more unpredictable flows,
due to density-determined dynamics of sand-dominated
and mud-dominated current flows resulting in turbidity
along the estuary floor and in the upper water column,
respectively. Similarly, the dynamics of mixing become
complicated where turbid river flow interacts with tidal
flow that has been enriched with suspended mud or with
turbid basal entrained sand.
Bates (1953) and Orton and Reading (1993) describe
variability of turbidity and sediment loads and interactions
with contrasting density in the receiving waters in three
situations: (1) hypopycnal flows in which density of the
suspended sediment flow is less than that of the receiving
estuarine water body, (2) homopycnal flows in which density of the suspended sediment flow is equal to that of the
receiving estuarine water body, and (3) hyperpycnal flows
in which density of the suspended sediment flow is
more than that of the receiving estuarine water body.
The dynamics of sediment-laden river flow with its various amounts of suspension and/or traction load entering
and interacting with an estuarine receiving water body of
different density (ranging from fresh to brackish to marine)
determine the different types mixing and, for sediment
deposition, the different styles of sediment accumulation.
Thus, riverine freshwater flowing into an estuarine
basin of denser brackish water or marine salinity will
exhibit hypopycnal flow, with freshwater overlying the
denser estuarine water. Riverine freshwater flowing into
an estuarine basin of similar freshwater salinity, or turbid
freshwater flowing into brackish water, will exhibit
homopycnal flow, with the river water invading the estuarine water of similar density in a turbulent mixing front. At
the other extreme, sediment-laden turbid riverine freshwater flowing into an estuarine basin of freshwater or weakly
brackish salinity will exhibit hyperpycnal flow, with the
denser sediment-laden river (composed of sediment in
suspension and in traction) in a base flow under the less
dense estuarine water.
Various estuaries and models of estuarine settings have
been investigated empirically and mathematically to provide an understanding of the dynamics of mixing associated with bathymetrical variations and of circulation
patterns that result from interacting processes. For
instance, Lerczak and Geyer (2004) modeled the lateral
circulation in straight, narrow, stratified estuaries to contrast them with homogeneous or with weakly stratified
estuaries comparing flood-tide lateral hydrodynamics
with ebb-tide hydrodynamics, and addressing the issues
of along-channel and cross-channel differential advection
and Coriolis forcing. They determined that in straight narrow stratified estuaries, lateral circulation during flood
tides is about four times as strong as during ebb tides,
the flood–ebb-tide asymmetry being due to a feedback
between lateral circulation and the along-channel tidal
currents, as well as to time-varying stratification over
a tidal cycle. The modeling showed that as the stratification increased, the lateral circulation was reduced because
of the adverse pressure gradient setup by isopycnals being
tilted by the lateral flow itself.
Armi and Farmer (1986) explored the difference in
hydrologic exchange between an estuary with a submerged
barrier and one with a contraction (or constriction). They
showed that exchange over a submerged barrier or sill
(such as an inundated moraine) is fundamentally different
to that through a contraction. Sill crests primarily affect
the deeper layer into which the sill projects and only indirectly control the surface layer, consequently resulting in
asymmetrical flows. Where both a sill and a contraction
are present, the contraction affects flow at the sill crest only
if it occurs between the sill and the source of denser water.
Chen and Sanford (2009) explored the effects of axial
wind influences on salinity stratification and longitudinal
transport of saline water in partially mixed straight estuaries. They showed that wind straining along the channel
MARINE/FRESHWATER MIXING
409