Chapter 5
Hydrographical Characteristics of the Estuarine Area
of Patos Lagoon (30 0 S, Brazil)
Osmar O. Moller Jr. . Patrice Castaing
5.1
Introduction
The Patos Lagoon (Fig. 5.1a) situated in Southern Brazil, between 30 and 32°S is a very
important water resource system for the Rio Grande do SuI State. It serves as nursery
ground for important commercial fish and shrimp species sustaining a fishery
production of 182 kg ha- I ye l in average (Castello 1985). Major man activities are
navigation and recreation. It also receives untreated domestic, agricultural and industrial sewage disposals produced by population living along the margins of the
lagoon. The construction of two 4 km long jetties fixed the entrance and allowed
navigation along the entrance channel. About 14 million m3 of sediments where transported to the coastal zone in the first two years after the construction of these structures (Motta 1969). In recent years the channel has been constantly dredged to keep
depth around 14 m.
Early studies (Rodrigues 1903; Malaval1922) indicated that the circulation of this
microtidal lagoon was driven by the NE-SW wind regime in absence of large river
discharge. These aspects have also been discussed by Motta (1969), Herz (1977), Castello
(1985), Costa et al. (1988) and Moller Jr. et al. (1991). Studies carried out through time
series analyses and mathematical modelling by Moller Jr. et al. (1996); Moller Jr. (1996)
demonstrated that:
a the wind drives the subtidal circulation of Patos Lagoon in the time interval of
3-16 days coincident with frontal system passages over this area;
b the lagoon responds to the local wind through the set up/set down mechanism of
oscillation;
c the long period oscillations generated offshore by remote winds are attenuated as
they propagate into the lagoon;
d subtidal exchanges between the lagoon and the coastal ocean are driven by the pressure gradient resulted from combined local and non local forcing effects with
SWINE winds driving landward/seaward flows;
e river flow dominates circulation during the seasonal strong flood period. The residual flow is directed to the ocean since it combines important contributions from
the fluvial basin with the dominance of the NE winds.
If the basic circulation processes are, in a certain way, well established, the same is
not true for mixing and salt transport mechanisms in spite of several studies conducted
in the area (Castello 1976a-d; 1977a,b; 1978a,b; Hartmann 1984; Paim and Moller Jr. 1986).
Hydrographical Characteristics of the Estuarine Area
of Patos Lagoon (30 0 S, Brazil)
Osmar O. Moller Jr. . Patrice Castaing
5.1
Introduction
The Patos Lagoon (Fig. 5.1a) situated in Southern Brazil, between 30 and 32°S is a very
important water resource system for the Rio Grande do SuI State. It serves as nursery
ground for important commercial fish and shrimp species sustaining a fishery
production of 182 kg ha- I ye l in average (Castello 1985). Major man activities are
navigation and recreation. It also receives untreated domestic, agricultural and industrial sewage disposals produced by population living along the margins of the
lagoon. The construction of two 4 km long jetties fixed the entrance and allowed
navigation along the entrance channel. About 14 million m3 of sediments where transported to the coastal zone in the first two years after the construction of these structures (Motta 1969). In recent years the channel has been constantly dredged to keep
depth around 14 m.
Early studies (Rodrigues 1903; Malaval1922) indicated that the circulation of this
microtidal lagoon was driven by the NE-SW wind regime in absence of large river
discharge. These aspects have also been discussed by Motta (1969), Herz (1977), Castello
(1985), Costa et al. (1988) and Moller Jr. et al. (1991). Studies carried out through time
series analyses and mathematical modelling by Moller Jr. et al. (1996); Moller Jr. (1996)
demonstrated that:
a the wind drives the subtidal circulation of Patos Lagoon in the time interval of
3-16 days coincident with frontal system passages over this area;
b the lagoon responds to the local wind through the set up/set down mechanism of
oscillation;
c the long period oscillations generated offshore by remote winds are attenuated as
they propagate into the lagoon;
d subtidal exchanges between the lagoon and the coastal ocean are driven by the pressure gradient resulted from combined local and non local forcing effects with
SWINE winds driving landward/seaward flows;
e river flow dominates circulation during the seasonal strong flood period. The residual flow is directed to the ocean since it combines important contributions from
the fluvial basin with the dominance of the NE winds.
If the basic circulation processes are, in a certain way, well established, the same is
not true for mixing and salt transport mechanisms in spite of several studies conducted
in the area (Castello 1976a-d; 1977a,b; 1978a,b; Hartmann 1984; Paim and Moller Jr. 1986).