42
Bastiaan Knoppers . Bjorn Kjerfve
resents the penultimate transgressive-regressive period during the Late Pleistocene
(Martin et al. 1988). Only the inlet of L. Patos breaches Barrier IV.
3.4
Tidal Choking
Whatever tidal variability occurs in a coastal lagoon is a result of ocean tides (Kjerfve
et al. 1990). However, the Brazilian lagoons are largely choked; they extend parallel to
the coast, with a single connection to the sea, and consist of two or more elliptical
lagoon cells. The exception to this rule is the restricted Canam!ia-Igmipe L. system
(Fig. 3.4), with two ocean connections. In choked coastal lagoons, the entrance channel serves as a dynamic filter that effectively reduces or eliminates tidal water fluctuations and tidal currents (Keulegan 1967; Kjerfve 1986). In Guarapina L., which is
characterized by a 1.5 km long and 40 m wide tidal channel, water level oscillations
are usually reduced to 1% or less as compared to the adjacent coastal tide (Kjerfve
and Knoppers 1991). Similar conditions of tidal choking are encountered in Fora L.
and Urussanga L. (of the Saquarema L. system), and Piratininga L. (Knoppers et al.
1991), Concei
Kjerfve et al. 1996).
Residual tidal circulation is dominant only where the entrance channel does not
sufficiently reduce tidal forcing, as in Itaipu L., which exhibits tidal water level oscillations of more than 30% as compared to the coastal tide (Knoppers et al. 1991). In
Cananeia-Iguape L., tidal and wind forcing may attain equal importance in driving
circulation and mixing (Miyao and Harari 1989). The lower estuarine portion of
Patos L. is also affected by tides, but it is governed mainly by wind forcing because of
the small range of the tides (Herz 1977; Costa et al. 1988).
A useful indicator of the dominant transport mode in coastal systems is the nondimensional Peelet number. The Peelet Number is defined by the ratio of diffusive to
advective transport times. When it is greater than 1, advective transport takes less time
than dispersive mixing; when it is less than 1, diffusive processes dominate (Zimmerman 1981). In most choked coastal lagoons, the Peelet number is on the order of
0.01-0.10, and mixing is more rapid than advective transport (Zimmerman 1981). The
Peelet Number, however, often measures 1-100 within entrance channels to choked
lagoons, indicating that advective processes dominate in the channel (Kjerfve et al.
1990; Kjerfve and Knoppers 1991), as in the case of Araruama L. (Kjerfve et al. 1996)
and most other systems. Ebbing currents in the entrance channel to Guarapina L., for
example, are largely uniform in salinity, but flooding currents often exhibit sharp temporal and horizontal salt gradients as stratified water masses from the adjacent ocean
invade the lagoon before becoming mixed (Kjerfve et al. 1990).
3.5
Hydrology and Water Balance
Fresh water enters lagoons via river discharge, groundwater seepage, and rainfall, and
is lost via evaporation and exchange with the coastal ocean. Seasonal pulses of fresh-
Bastiaan Knoppers . Bjorn Kjerfve
resents the penultimate transgressive-regressive period during the Late Pleistocene
(Martin et al. 1988). Only the inlet of L. Patos breaches Barrier IV.
3.4
Tidal Choking
Whatever tidal variability occurs in a coastal lagoon is a result of ocean tides (Kjerfve
et al. 1990). However, the Brazilian lagoons are largely choked; they extend parallel to
the coast, with a single connection to the sea, and consist of two or more elliptical
lagoon cells. The exception to this rule is the restricted Canam!ia-Igmipe L. system
(Fig. 3.4), with two ocean connections. In choked coastal lagoons, the entrance channel serves as a dynamic filter that effectively reduces or eliminates tidal water fluctuations and tidal currents (Keulegan 1967; Kjerfve 1986). In Guarapina L., which is
characterized by a 1.5 km long and 40 m wide tidal channel, water level oscillations
are usually reduced to 1% or less as compared to the adjacent coastal tide (Kjerfve
and Knoppers 1991). Similar conditions of tidal choking are encountered in Fora L.
and Urussanga L. (of the Saquarema L. system), and Piratininga L. (Knoppers et al.
1991), Concei
Kjerfve et al. 1996).
Residual tidal circulation is dominant only where the entrance channel does not
sufficiently reduce tidal forcing, as in Itaipu L., which exhibits tidal water level oscillations of more than 30% as compared to the coastal tide (Knoppers et al. 1991). In
Cananeia-Iguape L., tidal and wind forcing may attain equal importance in driving
circulation and mixing (Miyao and Harari 1989). The lower estuarine portion of
Patos L. is also affected by tides, but it is governed mainly by wind forcing because of
the small range of the tides (Herz 1977; Costa et al. 1988).
A useful indicator of the dominant transport mode in coastal systems is the nondimensional Peelet number. The Peelet Number is defined by the ratio of diffusive to
advective transport times. When it is greater than 1, advective transport takes less time
than dispersive mixing; when it is less than 1, diffusive processes dominate (Zimmerman 1981). In most choked coastal lagoons, the Peelet number is on the order of
0.01-0.10, and mixing is more rapid than advective transport (Zimmerman 1981). The
Peelet Number, however, often measures 1-100 within entrance channels to choked
lagoons, indicating that advective processes dominate in the channel (Kjerfve et al.
1990; Kjerfve and Knoppers 1991), as in the case of Araruama L. (Kjerfve et al. 1996)
and most other systems. Ebbing currents in the entrance channel to Guarapina L., for
example, are largely uniform in salinity, but flooding currents often exhibit sharp temporal and horizontal salt gradients as stratified water masses from the adjacent ocean
invade the lagoon before becoming mixed (Kjerfve et al. 1990).
3.5
Hydrology and Water Balance
Fresh water enters lagoons via river discharge, groundwater seepage, and rainfall, and
is lost via evaporation and exchange with the coastal ocean. Seasonal pulses of fresh-