CHAPTER 5 . Hydrographical Characteristics of the Estuarine Area of Patos Lagoon
97
In this case the stratification parameter was 4.23, but it was not represented on the
diagram because, as explained before, it is not related to the mean measured flow.
5.4.4
Longitudinal Salt Transport
Methods for estimating the principal factors affecting the salt balance of estuaries have
been proposed by several authors (Dyer 1974; Officer 1976; Lewis and Lewis 1983). They
are based on cross-correlations between flow components and salinity that may include longitudinal and transversal terms (Murray and Siripong 1988; Hughes and
Rattray 1988) averaged over one or several tidal cycles. The same type of approach
has been used for the determination of residual fluxes of suspended sediments and
dissolved inorganic substances (Uncles et al. 1985; Dyer 1988; Pi no et al. 1994). In spite
of their short duration the data collected during cruises I, II and IV were used to establish the main factors involved with the longitudinal salt transport in the entrance
channel of Patos Lagoon. Following Smith (1994) the instantaneous longitudinal velocity and salinity values can be given by:
u = + U + u' + u" ,
5 = <5> + S + 5' + 5" ,
h = + H + h' ,
where: , <5> and represent the time averaged velocity, salinity and depth;
U, Sand H are the tidal deviations; u', 5' and h' are the low frequency deviations that
are associated with wind forcing; u" and 5" are the vertical shear effect which is a measure of the gravitational circulation. Deviations related to turbulence were disregarded.
The transport of salt (F) through the vertical water column of time averaged depth
can be given by:
F= + + +
2
3
4
where the bar indicates depth average.
The calculated values for each one of these components are presented in Table 5.l.
Two factors dominate this process: the advective term related to freshwater discharge
(term 1) and the cross products associated between offshore forced oscillations and
salinity (term 2). The tidal cross products and the shear stress are of minor importance. These terms do not balance each other resulting in a residual seaward salt transport for all the analysed cases which can be related to a decrease in the salt content in
the inner part of the estuary (Dyer 1974).
It is clear that the difficulties to separate different signals in short time series may
induce errors. This is evident in PLATES IV when the unbalanced salt flux may be tied
with incomplete filtering and in the case of PLATES II where the series of measurements have started when the landward flow was already at its maximum. Nevertheless these results show the mean advective seaward flow and the deviations associated with the passage of meteorological fronts are the most important factors for salt
transport in this part of the lagoon.
97
In this case the stratification parameter was 4.23, but it was not represented on the
diagram because, as explained before, it is not related to the mean measured flow.
5.4.4
Longitudinal Salt Transport
Methods for estimating the principal factors affecting the salt balance of estuaries have
been proposed by several authors (Dyer 1974; Officer 1976; Lewis and Lewis 1983). They
are based on cross-correlations between flow components and salinity that may include longitudinal and transversal terms (Murray and Siripong 1988; Hughes and
Rattray 1988) averaged over one or several tidal cycles. The same type of approach
has been used for the determination of residual fluxes of suspended sediments and
dissolved inorganic substances (Uncles et al. 1985; Dyer 1988; Pi no et al. 1994). In spite
of their short duration the data collected during cruises I, II and IV were used to establish the main factors involved with the longitudinal salt transport in the entrance
channel of Patos Lagoon. Following Smith (1994) the instantaneous longitudinal velocity and salinity values can be given by:
u = + U + u' + u" ,
5 = <5> + S + 5' + 5" ,
h =
where: , <5> and
U, Sand H are the tidal deviations; u', 5' and h' are the low frequency deviations that
are associated with wind forcing; u" and 5" are the vertical shear effect which is a measure of the gravitational circulation. Deviations related to turbulence were disregarded.
The transport of salt (F) through the vertical water column of time averaged depth
F=
2
3
4
where the bar indicates depth average.
The calculated values for each one of these components are presented in Table 5.l.
Two factors dominate this process: the advective term related to freshwater discharge
(term 1) and the cross products associated between offshore forced oscillations and
salinity (term 2). The tidal cross products and the shear stress are of minor importance. These terms do not balance each other resulting in a residual seaward salt transport for all the analysed cases which can be related to a decrease in the salt content in
the inner part of the estuary (Dyer 1974).
It is clear that the difficulties to separate different signals in short time series may
induce errors. This is evident in PLATES IV when the unbalanced salt flux may be tied
with incomplete filtering and in the case of PLATES II where the series of measurements have started when the landward flow was already at its maximum. Nevertheless these results show the mean advective seaward flow and the deviations associated with the passage of meteorological fronts are the most important factors for salt
transport in this part of the lagoon.