Osmar O. Moller Jr .. Patrice Castaing
5.5
Discussion and Conclusions
Coastal lagoons are normally considered as well mixed type of estuaries. The wind
action over shallow water bodies combined with low river discharge are the main factors that cause this situation. The longitudinal salinity distribution presented here
indicate however that this is not the case of Patos Lagoon where stratification is, probably, the dominant feature. The considerable river runoff combined with a downestuary
dominant wind field are the main reasons for this system to be stratified. The morphology of Patos Lagoon may also play its role because of the funneling in the lagoon
section that tends to enhance the effect of river flow. The deep (14 m) inlet may contribute, sometimes, to flow separation in a two-layered vertical profile. In this case a
lens of salt water can propagate near the bottom into the estuary, as described by
Largier and Taljaard (1991) for microtidal estuaries, forming a highly stratified water
column. Well mixed conditions were only found along the entrance channel after strong
meteorological fronts or after a river freshet.
Stratification and destratification cycles in estuaries have been reported to be due:
to the relative strength of river and wind action (Schroeder et al. 1990); to strong wind
events (Goodrich et al. 1987) and to the river flow action (Smith 1985). In the case of Patos
Lagoon it depends on the relative strength of the barotropic circulation induced by
the wind field and river flow. Even during periods of high river flows (>4000 m3 S-I) well
mixed conditions can be attained in the inlet during the passage of a strong cold front
that force large amounts of coastal waters to enter into the lagoon. However, velocity
of winds in excess of 10 m S-1 represents less than 1% of occurrences (Tomazelli 1993).
The salt balance is dominated by two terms: the residual seaward salt transport
forced by river discharge and the advection of sea water caused by the passage of
meteorological fronts. Diffusion is important only in the inner parts of the estuarine
area where the salinity gradient is strong enough to balance river flow. Values of the
longitudinal dispersion coefficient (KJ between 100 and 500 m 2 s-" that are comparable with those obtained for many estuarine systems, were found only for the region
near Ponta da Feitoria (some 50-60 km from the entrance) as indicated in Fig. 5.6b.
Near the entrance, the salinity gradient is very weak and the requirement of balance
between advection and diffusion would lead to unrealistically high values of Kx. This
suggests that in fact the salinity distribution is dominated by advection in this area.
A basic mechanism for the formation of the estuarine zone is given by the pressure gradient generated between the coastal ocean and the lagoon by the action of
SW winds. As the salty waters enter into the lagoon the mixing zone (denoted by the
strongest part of the salinity gradient) is formed and displaced towards the inner parts
of the lagoon as this process persists. Behind this area, the advection of unmixed coastal
waters form a region with a very weak salinity gradient.
The time series indicate that for river flow exceeding 4500 m3 S-1 only strong winds
can force salt water into the lagoon. This intrusion is rapidly flushed out of the channel.
The data presented here can only give an insight into the main processes related
with mixing and salt transport processes in the Patos Lagoon. 3D mathematical modelling and longer time series obtained at different sites are needed to understand these
processes and their consequences within the lagoon.
5.5
Discussion and Conclusions
Coastal lagoons are normally considered as well mixed type of estuaries. The wind
action over shallow water bodies combined with low river discharge are the main factors that cause this situation. The longitudinal salinity distribution presented here
indicate however that this is not the case of Patos Lagoon where stratification is, probably, the dominant feature. The considerable river runoff combined with a downestuary
dominant wind field are the main reasons for this system to be stratified. The morphology of Patos Lagoon may also play its role because of the funneling in the lagoon
section that tends to enhance the effect of river flow. The deep (14 m) inlet may contribute, sometimes, to flow separation in a two-layered vertical profile. In this case a
lens of salt water can propagate near the bottom into the estuary, as described by
Largier and Taljaard (1991) for microtidal estuaries, forming a highly stratified water
column. Well mixed conditions were only found along the entrance channel after strong
meteorological fronts or after a river freshet.
Stratification and destratification cycles in estuaries have been reported to be due:
to the relative strength of river and wind action (Schroeder et al. 1990); to strong wind
events (Goodrich et al. 1987) and to the river flow action (Smith 1985). In the case of Patos
Lagoon it depends on the relative strength of the barotropic circulation induced by
the wind field and river flow. Even during periods of high river flows (>4000 m3 S-I) well
mixed conditions can be attained in the inlet during the passage of a strong cold front
that force large amounts of coastal waters to enter into the lagoon. However, velocity
of winds in excess of 10 m S-1 represents less than 1% of occurrences (Tomazelli 1993).
The salt balance is dominated by two terms: the residual seaward salt transport
forced by river discharge and the advection of sea water caused by the passage of
meteorological fronts. Diffusion is important only in the inner parts of the estuarine
area where the salinity gradient is strong enough to balance river flow. Values of the
longitudinal dispersion coefficient (KJ between 100 and 500 m 2 s-" that are comparable with those obtained for many estuarine systems, were found only for the region
near Ponta da Feitoria (some 50-60 km from the entrance) as indicated in Fig. 5.6b.
Near the entrance, the salinity gradient is very weak and the requirement of balance
between advection and diffusion would lead to unrealistically high values of Kx. This
suggests that in fact the salinity distribution is dominated by advection in this area.
A basic mechanism for the formation of the estuarine zone is given by the pressure gradient generated between the coastal ocean and the lagoon by the action of
SW winds. As the salty waters enter into the lagoon the mixing zone (denoted by the
strongest part of the salinity gradient) is formed and displaced towards the inner parts
of the lagoon as this process persists. Behind this area, the advection of unmixed coastal
waters form a region with a very weak salinity gradient.
The time series indicate that for river flow exceeding 4500 m3 S-1 only strong winds
can force salt water into the lagoon. This intrusion is rapidly flushed out of the channel.
The data presented here can only give an insight into the main processes related
with mixing and salt transport processes in the Patos Lagoon. 3D mathematical modelling and longer time series obtained at different sites are needed to understand these
processes and their consequences within the lagoon.