110
B. Kjerfve et al.
loses its characteristics as the bay rapidly becomes more shallow. Mud
covers the interior parts of the bay as a result of the high sediment load
during the past century, accelerated by anthropogenic activities in the
catchment. Because of the rapid mud sedimentation, the area-weighted bay
depth measures only 5.7 m. Although muddy sand, sandy mud, and mud
cover the bottom of most of the bay (Amador 1992), the central channel
from the continental shelf to the Rio-Niter6i bridge is filled with sand
(Kjerfve et al. 1997). Isolated areas of relict sands are also found northeast
and southwest of Ilha de Governador (Amador 1992), indicating strong
tidal scouring.
A submerged sandbank, consisting of medium quartzose sands, exists
12m below the surface seaward of the bay mouth. This asymmetrical bank
appears to be entering the bay but is kept from extending further inland by
tidal exchanges and the flux of freshwater. The sand shoal is similar to the
adjacent sub-aerial sand barrier formations, which stretch for more than
200 km along the coast of the state of Rio de Janeiro from the entrance to
Baia de Sepetiba to Arraial do Cabo. Were it not for the seaward pressure
gradient due to the freshwater runoff from the bay, it is likely that the sandbank at the entrance to Baia de Guanabara would also have been transformed into an emergent barrier system.
The benthic sediment distribution reflects the hydrodynamic forcing.
For example, sand waves are common along the eastern margin of the central channel at depths of 10-26 m. These sand waves have heights of
0.5-2.5 m, lengths of 18-98 m, and decrease in both height and wavelength
from the ocean into the bay in response to decreasing tidal energy. The
sand waves have steeper slopes facing the bay, indicating wave progression
and bottom sand transport into Baia de Guanabara. The sand waves and
their characteristics result from energetic ocean swells associated with
meteorological frontal passages and the tidal flood-dominance of bottom
currents.
8.4 Climate and Weather
The local marine climate is tropical-humid wet with wet, warm summers
and dry, cool winters. The mean air temperature is 23.7 oc, and the mean
relative humidity is 78%. The mean 30-year rainfall varies across the drainage basin from a maximum 291 mm month- 1 in the mountains to a minimum 30 mm month- 1 in the winter in low-lying areas. The mean rainfall
for Baia de Guanabara is 1,173 mm annually, and the mean evaporation is
1,198 mm annually. There is a well-defined wet (December-April) and dry
B. Kjerfve et al.
loses its characteristics as the bay rapidly becomes more shallow. Mud
covers the interior parts of the bay as a result of the high sediment load
during the past century, accelerated by anthropogenic activities in the
catchment. Because of the rapid mud sedimentation, the area-weighted bay
depth measures only 5.7 m. Although muddy sand, sandy mud, and mud
cover the bottom of most of the bay (Amador 1992), the central channel
from the continental shelf to the Rio-Niter6i bridge is filled with sand
(Kjerfve et al. 1997). Isolated areas of relict sands are also found northeast
and southwest of Ilha de Governador (Amador 1992), indicating strong
tidal scouring.
A submerged sandbank, consisting of medium quartzose sands, exists
12m below the surface seaward of the bay mouth. This asymmetrical bank
appears to be entering the bay but is kept from extending further inland by
tidal exchanges and the flux of freshwater. The sand shoal is similar to the
adjacent sub-aerial sand barrier formations, which stretch for more than
200 km along the coast of the state of Rio de Janeiro from the entrance to
Baia de Sepetiba to Arraial do Cabo. Were it not for the seaward pressure
gradient due to the freshwater runoff from the bay, it is likely that the sandbank at the entrance to Baia de Guanabara would also have been transformed into an emergent barrier system.
The benthic sediment distribution reflects the hydrodynamic forcing.
For example, sand waves are common along the eastern margin of the central channel at depths of 10-26 m. These sand waves have heights of
0.5-2.5 m, lengths of 18-98 m, and decrease in both height and wavelength
from the ocean into the bay in response to decreasing tidal energy. The
sand waves have steeper slopes facing the bay, indicating wave progression
and bottom sand transport into Baia de Guanabara. The sand waves and
their characteristics result from energetic ocean swells associated with
meteorological frontal passages and the tidal flood-dominance of bottom
currents.
8.4 Climate and Weather
The local marine climate is tropical-humid wet with wet, warm summers
and dry, cool winters. The mean air temperature is 23.7 oc, and the mean
relative humidity is 78%. The mean 30-year rainfall varies across the drainage basin from a maximum 291 mm month- 1 in the mountains to a minimum 30 mm month- 1 in the winter in low-lying areas. The mean rainfall
for Baia de Guanabara is 1,173 mm annually, and the mean evaporation is
1,198 mm annually. There is a well-defined wet (December-April) and dry