CHAPTER 6 . The Argentina Estuaries: A Review
liS
homogeneous. The boundary between both reaches is transitional depending on the
river discharge. Fully turbulent conditions resulting in Estuarine Richardson Numbers less than 2 characterize the mixing regime. According to Hansey and Rattray
(1966), the estuary is classified as Type 1a. However, when stations located across the
different sections of the estuary are considered, the southern flank presents well defined Type 1a characteristics, meanwhile the northern flank, Type 2a one.
The residual circulation shows a marked difference in the direction of the mass
transport. On the deeper parts of the sections (northern flank) the flow reverses with
depth, being headward near the bottom. The net transport is completely landward on
the shallower parts. This behaviour, added to the evaporation processes, produce a
concentration of salt in the inner portion of the estuary and over the tidal flats, resulting in salinities larger than those observed in the inner continental shelf when these
areas are flooded. In the southern flank of the Principal Channel the asymmetry of
the tidal current is mainly due to the extensive tidal flats bordering it. Perillo and
Sequeira (1989) have also shown the same pattern for the middle reach of the Bahia
Blanca Estuary which can be explained by the fact that the volume of water crossing a
point in the channel adjacent to the tidal flat during flood does not necessarily returns through the same point at ebbing tide, thus mass conservation is not attained
for specific cross-sections.
A quasi-stationary tidal wave provides the main energy input to the Bahia Blanca
Estuary. However, the complex geometry of the estuary and the wind produce deviations from the estimated values from theoretical estimations. The calculated damping coefficient is small thus resulting in an hypersynchronous estuary where convergence is the dominant factor producing a marked increase in the tidal amplitude.
Unfortunately the number of tidal stations with long and continuous records is small
and all of them located on the northern shore of the Main Channel. Therefore with
these data it is very difficult to assess the actual influence of the tidal flats on the tidal
wave. The approach followed here to estimate the convergence and friction effects only
used mean tide conditions. One can imagine that the convergence effect ought to disappear as soon as the tide overcomes the channel banks and runs freely over the extensive tidal flats. To the authors' knowledge no theory has been developed yet that
takes into consideration large tidal flats.
In a region where the wind blows on average at 16 km h- 1 with average maximum
of 58 km h- 1 and gusts of over 100 km h- 1 (Piccolo 1987) its influence over the tide is
very important. The deviations of the real tide in relation to the predicted reaches up
to 4 m. This type of differences seriously affect the general circulation of the estuary
specially when it is compared with the tidal range that varies from 2-4 m and the
amplitude-to-depth ratio is relatively large (Perillo and Piccolo 1991).
Heat exchange across the sediments of the tidal flats of the estuary were analysed
by Piccolo and Davila (1993) during one year. Thermistors were installed at 0.05 and
0.15 m below the sediment surface, and at 1 and 10 m above the mud flat. Assuming
that soil temperature varied as a sinusoidal function of time, thermal diffusivity (k)
was determined from a wave amplitude-depth relationship resulting in a mean value
of 0.7 x 10- 6 m 2 S-1. The magnitude and rate of heat exchange during tidal inundation
is dependent upon the relative temperature of the mud and the incoming water layer.
The net result of tidal inundation during daylight hours, specially after noon, is a very
sharp fall in temperature, the magnitude of which is closely related to the timing of
liS
homogeneous. The boundary between both reaches is transitional depending on the
river discharge. Fully turbulent conditions resulting in Estuarine Richardson Numbers less than 2 characterize the mixing regime. According to Hansey and Rattray
(1966), the estuary is classified as Type 1a. However, when stations located across the
different sections of the estuary are considered, the southern flank presents well defined Type 1a characteristics, meanwhile the northern flank, Type 2a one.
The residual circulation shows a marked difference in the direction of the mass
transport. On the deeper parts of the sections (northern flank) the flow reverses with
depth, being headward near the bottom. The net transport is completely landward on
the shallower parts. This behaviour, added to the evaporation processes, produce a
concentration of salt in the inner portion of the estuary and over the tidal flats, resulting in salinities larger than those observed in the inner continental shelf when these
areas are flooded. In the southern flank of the Principal Channel the asymmetry of
the tidal current is mainly due to the extensive tidal flats bordering it. Perillo and
Sequeira (1989) have also shown the same pattern for the middle reach of the Bahia
Blanca Estuary which can be explained by the fact that the volume of water crossing a
point in the channel adjacent to the tidal flat during flood does not necessarily returns through the same point at ebbing tide, thus mass conservation is not attained
for specific cross-sections.
A quasi-stationary tidal wave provides the main energy input to the Bahia Blanca
Estuary. However, the complex geometry of the estuary and the wind produce deviations from the estimated values from theoretical estimations. The calculated damping coefficient is small thus resulting in an hypersynchronous estuary where convergence is the dominant factor producing a marked increase in the tidal amplitude.
Unfortunately the number of tidal stations with long and continuous records is small
and all of them located on the northern shore of the Main Channel. Therefore with
these data it is very difficult to assess the actual influence of the tidal flats on the tidal
wave. The approach followed here to estimate the convergence and friction effects only
used mean tide conditions. One can imagine that the convergence effect ought to disappear as soon as the tide overcomes the channel banks and runs freely over the extensive tidal flats. To the authors' knowledge no theory has been developed yet that
takes into consideration large tidal flats.
In a region where the wind blows on average at 16 km h- 1 with average maximum
of 58 km h- 1 and gusts of over 100 km h- 1 (Piccolo 1987) its influence over the tide is
very important. The deviations of the real tide in relation to the predicted reaches up
to 4 m. This type of differences seriously affect the general circulation of the estuary
specially when it is compared with the tidal range that varies from 2-4 m and the
amplitude-to-depth ratio is relatively large (Perillo and Piccolo 1991).
Heat exchange across the sediments of the tidal flats of the estuary were analysed
by Piccolo and Davila (1993) during one year. Thermistors were installed at 0.05 and
0.15 m below the sediment surface, and at 1 and 10 m above the mud flat. Assuming
that soil temperature varied as a sinusoidal function of time, thermal diffusivity (k)
was determined from a wave amplitude-depth relationship resulting in a mean value
of 0.7 x 10- 6 m 2 S-1. The magnitude and rate of heat exchange during tidal inundation
is dependent upon the relative temperature of the mud and the incoming water layer.
The net result of tidal inundation during daylight hours, specially after noon, is a very
sharp fall in temperature, the magnitude of which is closely related to the timing of