CHAPTER 5 . Hydrographical Characteristics of the Estuarine Area of Patos Lagoon
93
1983) and the Delaware (Garvine et al. 1992) estuaries. The seaward increase of Kx is
normally expected (Hamilton and Rattray 1988) however, in this case, it is mainly due
the role played by the morphology of the lagoon.
In the same longitudinal section the estuarine zone may pass from a highly stratified to a partially stratified estuary (Fig. 5.7a) or it can behave as partially mixed estuary (cases 7b, 7C and 7d) with large fluctuations of the inner limit of the transition
zone. In some of these cases, instantaneous two-layered reversed flows were observed
with surface waters running seaward.
5.4.2
Temporal Variations of Water Level, Currents and Salinity
The series of four cruises carried out in the Praticagem area had the purpose of establishing the main mechanisms involved in the exchange processes between the lagoon and the coastal zone. This includes dynamical behaviour, mixing processes and
salt flux parameters in the channel area.
In all examined cases the lagoon was subject to the influence of, at least, one meteorological front. During these events the wind rotates anticlockwise from NE to SW
directions, returning to the NE position as the front migrates northwards (Stech and
Lorenzzetti 1992). The way this system responds to this effect is shown in Fig. 5.8 that
reproduces the data collected during PLATES I (6-11/04/92). A positive sea level elevation (Fig. 5.8b) reaches its maximum some 40 h after the establishment of the SW
wind regime (Fig. 5.8a). This long period oscillation behaves like a progressive wave
in the sense that the generated current velocity peaks (Fig. 5.8c) some hours before
water level has attained its maximum. As the wind velocity decreases it no longer sustains the pressure gradient it generated at the coast and the flow reverses even before
the establishment of the NE wind regime. The seaward velocities are stronger than
the landward ones. The vertically stratified water column (Fig. 5.8d) observed in the
first 15 h associated with a two-layered flow forced by tidal action, becomes well mixed
when the current reverses throughout the entire column. This situation is also presented in Fig. 5.9 for the same period of time (6-11/04/92). It can be denoted that coastal
waters exert the control on the maximum salinity values found within the lagoon.
Figure 5.9 presents the temporal variation of vertical longitudinal velocity and salinity profiles for the three series of measurements (PLATES I, II and IV) where mixing takes place within the lagoon. This wind forcing mechanism discussed above for
PLATES I can be generalized to explain the situations found during PLATES II
(18-23/05/92) and part of PLATES IV (5-10/10/92) cruises. The PLATES III (22-27/06/92)
situation is not shown because the lagoon was subject to a river flow of 8000 m3 S-l
and just behaved like a riverine system with mixing taking part in the adjacent coastal
zone. Unfiltered data present maximum values around 2.5 m S-l for seaward flows observed during PLATES II and 3 m S-l in PLATES III.
During PLATES II (18-23/05/92) the situation is similar to that of the first cruise
except that the SW wind regime with wind speeds between 6 and 10 m S-l was already
established when measurements started. The freshwater discharged by Guaiba and
Camaqua Rivers was 3 000 m3 S-l. vertically well mixed column has been formed and
salinity decreased as the flow turned towards the coast. A second meteorological front
reverses it at 100 h, forcing a return of mixed water into the lagoon.
93
1983) and the Delaware (Garvine et al. 1992) estuaries. The seaward increase of Kx is
normally expected (Hamilton and Rattray 1988) however, in this case, it is mainly due
the role played by the morphology of the lagoon.
In the same longitudinal section the estuarine zone may pass from a highly stratified to a partially stratified estuary (Fig. 5.7a) or it can behave as partially mixed estuary (cases 7b, 7C and 7d) with large fluctuations of the inner limit of the transition
zone. In some of these cases, instantaneous two-layered reversed flows were observed
with surface waters running seaward.
5.4.2
Temporal Variations of Water Level, Currents and Salinity
The series of four cruises carried out in the Praticagem area had the purpose of establishing the main mechanisms involved in the exchange processes between the lagoon and the coastal zone. This includes dynamical behaviour, mixing processes and
salt flux parameters in the channel area.
In all examined cases the lagoon was subject to the influence of, at least, one meteorological front. During these events the wind rotates anticlockwise from NE to SW
directions, returning to the NE position as the front migrates northwards (Stech and
Lorenzzetti 1992). The way this system responds to this effect is shown in Fig. 5.8 that
reproduces the data collected during PLATES I (6-11/04/92). A positive sea level elevation (Fig. 5.8b) reaches its maximum some 40 h after the establishment of the SW
wind regime (Fig. 5.8a). This long period oscillation behaves like a progressive wave
in the sense that the generated current velocity peaks (Fig. 5.8c) some hours before
water level has attained its maximum. As the wind velocity decreases it no longer sustains the pressure gradient it generated at the coast and the flow reverses even before
the establishment of the NE wind regime. The seaward velocities are stronger than
the landward ones. The vertically stratified water column (Fig. 5.8d) observed in the
first 15 h associated with a two-layered flow forced by tidal action, becomes well mixed
when the current reverses throughout the entire column. This situation is also presented in Fig. 5.9 for the same period of time (6-11/04/92). It can be denoted that coastal
waters exert the control on the maximum salinity values found within the lagoon.
Figure 5.9 presents the temporal variation of vertical longitudinal velocity and salinity profiles for the three series of measurements (PLATES I, II and IV) where mixing takes place within the lagoon. This wind forcing mechanism discussed above for
PLATES I can be generalized to explain the situations found during PLATES II
(18-23/05/92) and part of PLATES IV (5-10/10/92) cruises. The PLATES III (22-27/06/92)
situation is not shown because the lagoon was subject to a river flow of 8000 m3 S-l
and just behaved like a riverine system with mixing taking part in the adjacent coastal
zone. Unfiltered data present maximum values around 2.5 m S-l for seaward flows observed during PLATES II and 3 m S-l in PLATES III.
During PLATES II (18-23/05/92) the situation is similar to that of the first cruise
except that the SW wind regime with wind speeds between 6 and 10 m S-l was already
established when measurements started. The freshwater discharged by Guaiba and
Camaqua Rivers was 3 000 m3 S-l. vertically well mixed column has been formed and
salinity decreased as the flow turned towards the coast. A second meteorological front
reverses it at 100 h, forcing a return of mixed water into the lagoon.