9
2.4 Estuarine Fronts
Estuarine fronts are produced by the meeting of continental freshwaters and salty
marine waters. The later frequently form a salt-wedge below the former, leading to
the most frequently observed structure (Fig. 2.2d). These fronts develop usually in
bays, part of a bay, or inlets in which freshwater flows from land. These fronts are
controlled by salinity variations, and are frequently the most contrasting in terms
of water density. Estuarine fronts differ in stratification and dynamics mostly due
to diverse patterns in river discharge, and to the variable importance of external
forcing such as tides or wind. These frontal mechanisms lead to the formation of
plume, tidal intrusion and shear fronts at estuaries, some of which might develop
along-channel fronts, particularly during flood tide (O’Donnell 1993). Except for a
few very large estuaries (e.g. Río de la Plata; St. Lawrence) most estuarine fronts
have much smaller spatial scales than other types of marine fronts.
In some estuarine systems, a well-developed turbidity front characterizes the
innermost part of the estuary. This maximum gradient in turbidity is due to the
flocculation of suspended matter at the edge of the salt intrusion, and re-suspension of sediment due to tidal stirring. Turbidity fronts are clearly visible in satellite
images and frequently from the deck of ships (Fig. 2.2d).
2.5 Plume Fronts
In some situations, waters from either a river or an estuary pouring onto a continental shelf predominate over any tidal effects and flow into the neighboring
ocean creating a river plume, which may have a strong impact on the distribution
of water properties, sediments and biota. When the surface outflow onto the continental shelf is mainly of freshwater from the river itself, these plumes are referred
to as river plumes (e.g. those of the Mississippi or Amazon rivers); if the outflow
is of river waters mixed with salt water the flow constitutes an estuarine plume
(e.g. the Chesapeake; Río de la Plata or St. Lawrence estuaries). The Coriolis force
affects plumes turning them to the left (Southern Hemisphere) or right (Northern
Hemisphere), and the buoyant plume continues on its way as a coastal current parallel to the coast. Under favorable (downwelling) wind conditions buoyant river
plumes can extend hundreds of km away from the river mouth. Recent numerical
simulations have shown that in the absence of wind bottom-trapped plumes can
also propagate in the opposite direction (e.g. upstream). This is associated with
a baroclinic adjustment of the river discharge, while the downstream spreading
is generated by the cross-shelf barotropic pressure gradient (Matano and Palma
2010). Coastal currents; tidal currents and winds can modify plume dynamics in
a complex manner (Garvine 1975). At the boundary between the plume and the
marine coastal waters, the low salinity buoyant waters ride on the top of denser
saline waters, forming plume fronts where surface convergence and downwelling
occur (Mann and Lazier 2006) (Fig. 2.3a).
2.4 Estuarine Fronts
2.4 Estuarine Fronts
Estuarine fronts are produced by the meeting of continental freshwaters and salty
marine waters. The later frequently form a salt-wedge below the former, leading to
the most frequently observed structure (Fig. 2.2d). These fronts develop usually in
bays, part of a bay, or inlets in which freshwater flows from land. These fronts are
controlled by salinity variations, and are frequently the most contrasting in terms
of water density. Estuarine fronts differ in stratification and dynamics mostly due
to diverse patterns in river discharge, and to the variable importance of external
forcing such as tides or wind. These frontal mechanisms lead to the formation of
plume, tidal intrusion and shear fronts at estuaries, some of which might develop
along-channel fronts, particularly during flood tide (O’Donnell 1993). Except for a
few very large estuaries (e.g. Río de la Plata; St. Lawrence) most estuarine fronts
have much smaller spatial scales than other types of marine fronts.
In some estuarine systems, a well-developed turbidity front characterizes the
innermost part of the estuary. This maximum gradient in turbidity is due to the
flocculation of suspended matter at the edge of the salt intrusion, and re-suspension of sediment due to tidal stirring. Turbidity fronts are clearly visible in satellite
images and frequently from the deck of ships (Fig. 2.2d).
2.5 Plume Fronts
In some situations, waters from either a river or an estuary pouring onto a continental shelf predominate over any tidal effects and flow into the neighboring
ocean creating a river plume, which may have a strong impact on the distribution
of water properties, sediments and biota. When the surface outflow onto the continental shelf is mainly of freshwater from the river itself, these plumes are referred
to as river plumes (e.g. those of the Mississippi or Amazon rivers); if the outflow
is of river waters mixed with salt water the flow constitutes an estuarine plume
(e.g. the Chesapeake; Río de la Plata or St. Lawrence estuaries). The Coriolis force
affects plumes turning them to the left (Southern Hemisphere) or right (Northern
Hemisphere), and the buoyant plume continues on its way as a coastal current parallel to the coast. Under favorable (downwelling) wind conditions buoyant river
plumes can extend hundreds of km away from the river mouth. Recent numerical
simulations have shown that in the absence of wind bottom-trapped plumes can
also propagate in the opposite direction (e.g. upstream). This is associated with
a baroclinic adjustment of the river discharge, while the downstream spreading
is generated by the cross-shelf barotropic pressure gradient (Matano and Palma
2010). Coastal currents; tidal currents and winds can modify plume dynamics in
a complex manner (Garvine 1975). At the boundary between the plume and the
marine coastal waters, the low salinity buoyant waters ride on the top of denser
saline waters, forming plume fronts where surface convergence and downwelling
occur (Mann and Lazier 2006) (Fig. 2.3a).
2.4 Estuarine Fronts