CHAPTER 5 . Hydrographical Characteristics of the Estuarine Area of Patos Lagoon
91
a between days 30 and 60 due to the influence of river discharge and an intense period of NE winds;
b in spring (after day 270), when the salt water was almost totally expulsed from the
lagoon during the late winter/early spring flood period, a salt wedge type of estuary was then formed in the near entrance area.
This is a very common situation in this period of the year (Castello and Moller Jr.
1978; Castello 1985).
The comparison between the surface (Fig. 5.5a) and bottom (Fig. 5.5b) situations
illustrates that, for the cases already discussed, stratified conditions have prevailed which
indicates the importance of river discharge in the circulation of this system. This general
trend for stratified conditions can be better observed in Fig. 5.6a. The curves correspond
to an exponential decrease towards the inner parts with larger differences between
surface and bottom occurring in the first 80 km from the mouth. Farther northwards,
the vertical gradients decrease and the water column becomes weakly stratified. It may
be argued also that this trend in stratification can be due to the fact that sampling was
carried out along the deeper areas of the navigation channel. However the same trend
stands even for shallow zones where salinity variations up to 16 psu were observed in
areas with maximum depths between 2 and 2.5 m (Castello 1976b; 1977a; 1978a).
The longitudinal distribution of the depth averaged salinity (s) calculated for data obtained during the first ten cruises, i.e., excluding spring data (when salty water was flushed
out of the lagoon) is shown in Fig. 5.6b. If this distribution is assumed to be the steady
state response to the mean river discharge (936 m3 S-I) the effective longitudinal dispersion coefficient Kx can be calculated from the balance between advection and diffusion as:
K = us
x
ds
dx
where u is given by the ratio of the river flow to the mean cross sectional area and
ds/ dx the longitudinal salinity gradient. This one dimensional approach was initially
3 0 , - - - - - - - - - - - - - - - - , 3 0 , - - - - - - - - - - - - - - - - , -6000
a
b
25
25
5000 ,
'"
20
4000 1
3000
2000
1000
0
50
100
150
200
Distance from Praticagem (km)
Distance from Praticagem (km)
Fig. 5.6. a Mean surface (55) and bottom (5b) salinity (psu) distribution along the longitudinal profile of the lagoon. b Mean depth salinities (psu) along the lagoon (dots); the exponential decrease in
salinity calculated through regression analysis represented by the dotted line (r2= 0.94) and the variation of the longitudinal dispersion coefficient (Kx)
~
tt:: <1J
0
u
g.
'0
til
<::
.3
91
a between days 30 and 60 due to the influence of river discharge and an intense period of NE winds;
b in spring (after day 270), when the salt water was almost totally expulsed from the
lagoon during the late winter/early spring flood period, a salt wedge type of estuary was then formed in the near entrance area.
This is a very common situation in this period of the year (Castello and Moller Jr.
1978; Castello 1985).
The comparison between the surface (Fig. 5.5a) and bottom (Fig. 5.5b) situations
illustrates that, for the cases already discussed, stratified conditions have prevailed which
indicates the importance of river discharge in the circulation of this system. This general
trend for stratified conditions can be better observed in Fig. 5.6a. The curves correspond
to an exponential decrease towards the inner parts with larger differences between
surface and bottom occurring in the first 80 km from the mouth. Farther northwards,
the vertical gradients decrease and the water column becomes weakly stratified. It may
be argued also that this trend in stratification can be due to the fact that sampling was
carried out along the deeper areas of the navigation channel. However the same trend
stands even for shallow zones where salinity variations up to 16 psu were observed in
areas with maximum depths between 2 and 2.5 m (Castello 1976b; 1977a; 1978a).
The longitudinal distribution of the depth averaged salinity (s) calculated for data obtained during the first ten cruises, i.e., excluding spring data (when salty water was flushed
out of the lagoon) is shown in Fig. 5.6b. If this distribution is assumed to be the steady
state response to the mean river discharge (936 m3 S-I) the effective longitudinal dispersion coefficient Kx can be calculated from the balance between advection and diffusion as:
K = us
x
ds
dx
where u is given by the ratio of the river flow to the mean cross sectional area and
ds/ dx the longitudinal salinity gradient. This one dimensional approach was initially
3 0 , - - - - - - - - - - - - - - - - , 3 0 , - - - - - - - - - - - - - - - - , -6000
a
b
25
25
5000 ,
'"
20
4000 1
3000
2000
1000
0
50
100
150
200
Distance from Praticagem (km)
Distance from Praticagem (km)
Fig. 5.6. a Mean surface (55) and bottom (5b) salinity (psu) distribution along the longitudinal profile of the lagoon. b Mean depth salinities (psu) along the lagoon (dots); the exponential decrease in
salinity calculated through regression analysis represented by the dotted line (r2= 0.94) and the variation of the longitudinal dispersion coefficient (Kx)
~
tt:: <1J
0
u
g.
'0
til
<::
.3