CHAPTER 3 . Coastal Lagoons of Southeastern Brazil
43
water input have a profound impact on the ecological viability of coastal lagoons by
controlling the salinity, increasing the lagoon water level, and maintaining the entrance
channel open. Some smaller coastal lagoons in Brazil, in particular along the coasts
of Rio de Janeiro near Macae and Rio Grande do SuI, intermittently close, isolating
the lagoon from the sea as a result of active wave-infilling of the channel during periods of low runoff.
Although gravitational currents due to longitudinal and vertical density differences are very small in coastal lagoons, usually less than 0.01 m s-', they can become
important in the lagoon entrance channel. Where the channel is relatively deep,
which is the case in the canal leading into Lagoa dos Patos, gravitational circulation
is important in maintaining the water balance. On the average, lagoon outflow as a
result of river runoff is balanced by landward transport in the bottom layer of lagoon inlets. This gravitational circulation is often an order of magnitude greater
than the river discharge and is an effective mechanism in ensuring lagoon water exchange.
Rainfall and freshwater runoff have profound impacts on coastal lagoons. During
freshets, the water level can increase rapidly and cause extensive flooding of adjacent
lands. This effect is most pronounced in choked lagoons, where the water level can
rise one or more metres seasonally or in response to storms (Kjerfve et al. 1990). Flooding also causes large shifts in salinity in choked lagoon systems. As the runoff from
the drainage basin crests in the lagoon, the resident water mass is rapidly advected
out of the system, but when the runoff crest has receded, the salinity in the lagoon
returns gradually to the former salinity as a result of lagoonward dispersion of coastal
waters (Sikora and Kjerfve 1985).
In arid coastal regions, choked lagoons sometimes become hypersaline, at least
seasonally, because of greater evaporation than runoff. A good example of a hypersaline
coastal lagoon is Lagoa de Araruama, which remains permanently hypersaline with
an average salinity of 52 and also the smaller sized Vermelha L. with a salinity of 180
(Kjerfve et al. 1996).
3.6
Wind Effects
Wind forcing affects water level, seiches, currents, and circulation in choked lagoons.
The wind effect consists of both local wind stress on the lagoon and far-field wind
forcing on the adjacent coastal ocean. Both contribute to the filling and emptying of
the lagoon waters, depending on strength and direction of winds. The local winds
generate currents, setup and setdown, and seiches and short-period wind waves. Farfield wind effects are manifested by oscillations in coastal sea level with a period of
2-20 days and result in either landward or seaward pressure gradients to aid in filling
and emptying of the lagoons.
Wind stress causes current flow, but because of fluctuations in both wind speed
and direction, the resultant currents are usually variable and intermittent. If the wind
persists for long periods, however, a surface current will develop and cause a setup at
the downwind end of the lagoon. Locally, the setup can cause extensive flooding of
adjacent low-lying lands. The surface slope due to wind setup can be anywhere from
10- 6 -10-4 (Kjerfve 1975). Rapidly moving meteorological fronts often cause seiches
43
water input have a profound impact on the ecological viability of coastal lagoons by
controlling the salinity, increasing the lagoon water level, and maintaining the entrance
channel open. Some smaller coastal lagoons in Brazil, in particular along the coasts
of Rio de Janeiro near Macae and Rio Grande do SuI, intermittently close, isolating
the lagoon from the sea as a result of active wave-infilling of the channel during periods of low runoff.
Although gravitational currents due to longitudinal and vertical density differences are very small in coastal lagoons, usually less than 0.01 m s-', they can become
important in the lagoon entrance channel. Where the channel is relatively deep,
which is the case in the canal leading into Lagoa dos Patos, gravitational circulation
is important in maintaining the water balance. On the average, lagoon outflow as a
result of river runoff is balanced by landward transport in the bottom layer of lagoon inlets. This gravitational circulation is often an order of magnitude greater
than the river discharge and is an effective mechanism in ensuring lagoon water exchange.
Rainfall and freshwater runoff have profound impacts on coastal lagoons. During
freshets, the water level can increase rapidly and cause extensive flooding of adjacent
lands. This effect is most pronounced in choked lagoons, where the water level can
rise one or more metres seasonally or in response to storms (Kjerfve et al. 1990). Flooding also causes large shifts in salinity in choked lagoon systems. As the runoff from
the drainage basin crests in the lagoon, the resident water mass is rapidly advected
out of the system, but when the runoff crest has receded, the salinity in the lagoon
returns gradually to the former salinity as a result of lagoonward dispersion of coastal
waters (Sikora and Kjerfve 1985).
In arid coastal regions, choked lagoons sometimes become hypersaline, at least
seasonally, because of greater evaporation than runoff. A good example of a hypersaline
coastal lagoon is Lagoa de Araruama, which remains permanently hypersaline with
an average salinity of 52 and also the smaller sized Vermelha L. with a salinity of 180
(Kjerfve et al. 1996).
3.6
Wind Effects
Wind forcing affects water level, seiches, currents, and circulation in choked lagoons.
The wind effect consists of both local wind stress on the lagoon and far-field wind
forcing on the adjacent coastal ocean. Both contribute to the filling and emptying of
the lagoon waters, depending on strength and direction of winds. The local winds
generate currents, setup and setdown, and seiches and short-period wind waves. Farfield wind effects are manifested by oscillations in coastal sea level with a period of
2-20 days and result in either landward or seaward pressure gradients to aid in filling
and emptying of the lagoons.
Wind stress causes current flow, but because of fluctuations in both wind speed
and direction, the resultant currents are usually variable and intermittent. If the wind
persists for long periods, however, a surface current will develop and cause a setup at
the downwind end of the lagoon. Locally, the setup can cause extensive flooding of
adjacent low-lying lands. The surface slope due to wind setup can be anywhere from
10- 6 -10-4 (Kjerfve 1975). Rapidly moving meteorological fronts often cause seiches