CHAPTER 7 . Hydrography and Sediment Transport Characteristics of the Rio de la Plata
155
ing from the N to W quadrant produce a rapid and simultaneous effect over the whole
Rio de la Plata adding its effect to the discharge flow and reducing water level. For winds
blowing from the S to E quadrant, wind and river runoff effects are opposed and circulation patterns are strongly influenced by the presence of Arquimedes and Ingles banks.
Meteorological forces have great influence on the general Rio de la Plata hydrodynamic patterns. In the upper and middle regions, winds are responsible for extreme
water levels, while at the outer region wind influence is smaller. The main effect is in
producing residual currents and controlling the degree of vertical mixing.
The salt intrusion limit has been located between Punta Tigre and Punta Brava
transverse sections, shifting to the southeast along the Argentinean coast. The turbidity
maximum has been located between Punta Yeguas and Punta Espinillo transverse sections (Nagy et al. 1987). This observations agrees with satellite data (Gagliardini
et al. 1984; Karszenbaun et al. 1983; Jackson 1984; Ayup 1987; Nagy 1989; Framifian and
Brown 1993, 1996) and its also reflected by bottom sedimentology (onlap marine sands
and offlap clayey silts) and morphology (Barra del Indio).
Eulerian studies conducted at anchored stations near the upper salt intrusion limit
(Lopez Laborde et al. 1991; Lopez Laborde and Perdomo 1991; Lopez Laborde, unpublished data) allowed to observe simple dilution processes of the fluvial load and the
development of the turbidity maximum associated with entrapment in the estuarine
circulation and with tidal scour (even with salinity greater than lO psu).
A general conceptual model for salinity and suspended sediment behaviour at the salt
intrusion limit could be sketched. During high-tide, maximum velocities at one metre above
the bottom or depth averaged coincides with maximum depth averaged salinity and maximum bottom to surface salinity difference, this means vertical stratification. During ebbtide, mixing is an important phenomenon. During low-tide, bottom induced turbulence
may create intense mixing and bottom sediment resuspension. During flood-tide, this
turbulence together with water advection tend to destroy the previous mixing conditions.
In fresh water, suspended sediment concentration is clearly related to the axial velocity component. The greater concentrations develops during ebb-tide. When velocity decreases, concentrations fall and continue decreasing even during flood-tide, after current inversion, until axial component velocity (at one metre above the bottom)
reaches 15 cm S-I, atthis point concentrations begin to increase. When salty wateris present, salinity (flocculation and aggregation processes) affects suspended sediment behaviour. The spatial and temporal distribution of the turbidity front shows a high degree
of variability, at the northern coast varies between 57°00' and 54°12'W and at the southern coast coincides with 5.0 m isobath (Framifian and Brown 1996). River discharge
and meteorological history are important in determining how far salinity penetrates
and fresh water extends, and consequently, in affecting stratification patterns.
Franja Costera Sur and Samborombon Bay, due to their location windward of dominant winds with low wave energy, and due to their relationship with the Gran Hoya
del Canal Intermedio, appear as areas with exceptional conditions for sediment deposition by mechanical sedimentation processes. Physico-chemical processes, related to
the salt intrusion limit, are mainly responsible for silty clays along the Uruguayan coast
and middle Rio de la Plata (Barra del Indio), although relict sediments masking modern ones can be observed.
Lusquifios and Valdez (1971), Hubold (1980), and many others, reported the NNE
Rio de la Plata influence along the continental shelf. Depending on the fluvial discharge,
155
ing from the N to W quadrant produce a rapid and simultaneous effect over the whole
Rio de la Plata adding its effect to the discharge flow and reducing water level. For winds
blowing from the S to E quadrant, wind and river runoff effects are opposed and circulation patterns are strongly influenced by the presence of Arquimedes and Ingles banks.
Meteorological forces have great influence on the general Rio de la Plata hydrodynamic patterns. In the upper and middle regions, winds are responsible for extreme
water levels, while at the outer region wind influence is smaller. The main effect is in
producing residual currents and controlling the degree of vertical mixing.
The salt intrusion limit has been located between Punta Tigre and Punta Brava
transverse sections, shifting to the southeast along the Argentinean coast. The turbidity
maximum has been located between Punta Yeguas and Punta Espinillo transverse sections (Nagy et al. 1987). This observations agrees with satellite data (Gagliardini
et al. 1984; Karszenbaun et al. 1983; Jackson 1984; Ayup 1987; Nagy 1989; Framifian and
Brown 1993, 1996) and its also reflected by bottom sedimentology (onlap marine sands
and offlap clayey silts) and morphology (Barra del Indio).
Eulerian studies conducted at anchored stations near the upper salt intrusion limit
(Lopez Laborde et al. 1991; Lopez Laborde and Perdomo 1991; Lopez Laborde, unpublished data) allowed to observe simple dilution processes of the fluvial load and the
development of the turbidity maximum associated with entrapment in the estuarine
circulation and with tidal scour (even with salinity greater than lO psu).
A general conceptual model for salinity and suspended sediment behaviour at the salt
intrusion limit could be sketched. During high-tide, maximum velocities at one metre above
the bottom or depth averaged coincides with maximum depth averaged salinity and maximum bottom to surface salinity difference, this means vertical stratification. During ebbtide, mixing is an important phenomenon. During low-tide, bottom induced turbulence
may create intense mixing and bottom sediment resuspension. During flood-tide, this
turbulence together with water advection tend to destroy the previous mixing conditions.
In fresh water, suspended sediment concentration is clearly related to the axial velocity component. The greater concentrations develops during ebb-tide. When velocity decreases, concentrations fall and continue decreasing even during flood-tide, after current inversion, until axial component velocity (at one metre above the bottom)
reaches 15 cm S-I, atthis point concentrations begin to increase. When salty wateris present, salinity (flocculation and aggregation processes) affects suspended sediment behaviour. The spatial and temporal distribution of the turbidity front shows a high degree
of variability, at the northern coast varies between 57°00' and 54°12'W and at the southern coast coincides with 5.0 m isobath (Framifian and Brown 1996). River discharge
and meteorological history are important in determining how far salinity penetrates
and fresh water extends, and consequently, in affecting stratification patterns.
Franja Costera Sur and Samborombon Bay, due to their location windward of dominant winds with low wave energy, and due to their relationship with the Gran Hoya
del Canal Intermedio, appear as areas with exceptional conditions for sediment deposition by mechanical sedimentation processes. Physico-chemical processes, related to
the salt intrusion limit, are mainly responsible for silty clays along the Uruguayan coast
and middle Rio de la Plata (Barra del Indio), although relict sediments masking modern ones can be observed.
Lusquifios and Valdez (1971), Hubold (1980), and many others, reported the NNE
Rio de la Plata influence along the continental shelf. Depending on the fluvial discharge,