166
M.B. Framiftan . M.P. Etala . E.M. Acha . R. A. Guerrero· C.A. Lasta . O.B. Brown
to model tidal elevations and currents in the estuary; a description and validation of
the model can be found in CARP (1989). O'Connor (1991) adapted a model, that had been
extensively used elsewhere, to the estuary and part of the continental shelf, and typified situations based on historical information on storm surges. A similar model is presented by Guarga et al. (1992), in a work mainly focused on engineering applications.
As the mass of water in the estuary is small, direct astronomical forcing is considered negligible. For that reason all the models mentioned above are forced by some
previous knowledge of the tides at the open ocean. Also all of them are based on the
so-called shallow water equations, a vertically integrated approach to Navier-Stokes
equations, making use of the fact that tide and surge wave lengths are much greater
than the depth. Results from a model of this type (Etala 1996a) will be used in the following sections to illustrate some aspects of the tide and surge-induced water levels
and currents in the estuary. The constituents considered in this model are M 2 , 52' N2
and 01. Although the agreement with their counterparts, for instance CARp· (1989), is
good, it is considered desirable to include more constituents in the model to represent the tide more accurately.
The tidal regime in the estuary is mixed, dominantly semidiurnal. At the upper zone,
where the wave amplitude to depth ratio becomes sufficiently large, the wave starts
deforming by nonlinear effects during low tide. Ebb tide is then slower than flood tide,
resulting in an asymmetric wave shape. This effect is already noticeable in Buenos
Aires, and it increases upstream in the estuary, as noted by Balay (1961).
Rio de la Plata tides effect on the tidal regimes of some of the main tributaries has
been studied by several authors. For example, in the Parana de las Palmas, Junod (1996)
finds that the transition area, where the tidal effects are not observable, is usually located between Campana and San Pedro. Its position varies according to the Parana
River stage, the surge in the Rio de la Plata, and the tidal amplitude. The effects of
extraordinary floods in the Rio de la Plata, provided the Parana River stage is favourable, can be detected up to Rosario (approximately 250 km upstream of Parana de las
Palmas mouth).
8.3.2
Tidal Currents
Water levels are easier to measure than currents and consequently these data are more
widespread. Hydrodynamical models allow the specification of currents using knowledge of water level. The intention is not to substitute measured with modelled current values, but a properly verified model can be a useful tool to provide information
that otherwise would not be obtained for logistical reasons. Model limitations are
associated with model resolution, bottom topographic specification, limited number
of constituents, uncertainty in open boundary tides, and other assumptions made in
the governing equations, for example in relation to parameterizations of subgrid processes, such as bottom stress and horizontal diffusion.
The model run presented here (Etala 1996a) does not include flow from fresh water sources. Consequently, a better representation of the currents is expected in the
outer estuary rather than in the inner part, where the effect of this discharge on the
current pattern might be more pronounced. For the verification that follows, constant
values of discharge current have been added to model results, following the proce-
M.B. Framiftan . M.P. Etala . E.M. Acha . R. A. Guerrero· C.A. Lasta . O.B. Brown
to model tidal elevations and currents in the estuary; a description and validation of
the model can be found in CARP (1989). O'Connor (1991) adapted a model, that had been
extensively used elsewhere, to the estuary and part of the continental shelf, and typified situations based on historical information on storm surges. A similar model is presented by Guarga et al. (1992), in a work mainly focused on engineering applications.
As the mass of water in the estuary is small, direct astronomical forcing is considered negligible. For that reason all the models mentioned above are forced by some
previous knowledge of the tides at the open ocean. Also all of them are based on the
so-called shallow water equations, a vertically integrated approach to Navier-Stokes
equations, making use of the fact that tide and surge wave lengths are much greater
than the depth. Results from a model of this type (Etala 1996a) will be used in the following sections to illustrate some aspects of the tide and surge-induced water levels
and currents in the estuary. The constituents considered in this model are M 2 , 52' N2
and 01. Although the agreement with their counterparts, for instance CARp· (1989), is
good, it is considered desirable to include more constituents in the model to represent the tide more accurately.
The tidal regime in the estuary is mixed, dominantly semidiurnal. At the upper zone,
where the wave amplitude to depth ratio becomes sufficiently large, the wave starts
deforming by nonlinear effects during low tide. Ebb tide is then slower than flood tide,
resulting in an asymmetric wave shape. This effect is already noticeable in Buenos
Aires, and it increases upstream in the estuary, as noted by Balay (1961).
Rio de la Plata tides effect on the tidal regimes of some of the main tributaries has
been studied by several authors. For example, in the Parana de las Palmas, Junod (1996)
finds that the transition area, where the tidal effects are not observable, is usually located between Campana and San Pedro. Its position varies according to the Parana
River stage, the surge in the Rio de la Plata, and the tidal amplitude. The effects of
extraordinary floods in the Rio de la Plata, provided the Parana River stage is favourable, can be detected up to Rosario (approximately 250 km upstream of Parana de las
Palmas mouth).
8.3.2
Tidal Currents
Water levels are easier to measure than currents and consequently these data are more
widespread. Hydrodynamical models allow the specification of currents using knowledge of water level. The intention is not to substitute measured with modelled current values, but a properly verified model can be a useful tool to provide information
that otherwise would not be obtained for logistical reasons. Model limitations are
associated with model resolution, bottom topographic specification, limited number
of constituents, uncertainty in open boundary tides, and other assumptions made in
the governing equations, for example in relation to parameterizations of subgrid processes, such as bottom stress and horizontal diffusion.
The model run presented here (Etala 1996a) does not include flow from fresh water sources. Consequently, a better representation of the currents is expected in the
outer estuary rather than in the inner part, where the effect of this discharge on the
current pattern might be more pronounced. For the verification that follows, constant
values of discharge current have been added to model results, following the proce-