The coastal zone is extremely dynamic and its morphology depends on climatic, geologic
and physical factors. The Algodoal Island, which is focused in the present study, is located at
the northeast of Pará State, being part of the Amazon coastal zone. With the aim to
understand the local beach morphodynamics, two different beaches (Praia da Princesa e
Praia da Caixa d’Água) were evaluated using topographic profiling and sediment sampling
along a year, within the main climatic/hydrodynamic periods: december/08 (end of the dry
season), march/09 (high rain season and equinoctial tides), may/09 (maximum fluvial
discharge), september/09 (high dry season, strong winds and equinoctial tides) and
december/09, enclosing the annual cycle. The results have shown a very dynamic area, but
indicating a long-term stability. The results showed a general erosional tendency for both
beaches. Between dez/08 and mar/09 the sediment balance was negative, being approx. -77
m³/m for Profile 1 (Praia da Princesa) and -48 m³/m for Profile 2 (Praia da Caixa d’Água).
This erosion was attributed to the equinoctial tides, as well as water table flow at the beach
face, improving erosion by backwash, affecting both beaches. From mar/09 to may/09 the
sedimentary balance was positive for the two beaches: 15 m³/m for Praia da Princesa and 40
m³/m at the Praia da Caixa d’Água. In this case the deposition was correlated to the gradual
reduction in tidal range after the equinoctial tides of march and the maximum fluvial
discharge period of may, which also affects both beaches, especially at the Praia da Caixa
d’Água, due to its location in the Marapanim estuary. Between may/09 and sep/09 the
sedimentary balance was again negative in both beaches, although the erosion was much
more conspicuously at the Praia da Caixa d’Água (-52 m³/m) than in the Praia da Princesa (-
11 m³/m). Considering the strong easterly winds of september, they result in erosion in the
exposed Praia da Princesa region and aeolian transport toward the Praia da Caixa d’Água
area. Furthermore, the equinoctial tides of september would also imply in erosion of both
beaches. From sep/09 to dez/09 slight deposition was observed in both beaches (9 and 4
m³/m for Princesa and Caixa d’Água beaches, respectively). Comparing dez/08 and dez/09
one could observe strong erosion in both beaches: -63 m³/m (Praia da Princesa) and -55
m³/m (Praia da Caixa d’Água). This erosion seems to be the result of episodic migration of
subtidal sediment banks. The observed variations in each profile place the Praia da Princesa
as a dissipative oceanic beach, where the main morphological changes are the result of
bar/trough migration. Furthermore, the Praia da Caixa d’Água is classified as an estuarine
beach of the reflective with low tide terrace type. Therefore, waves are affecting especially
the Princesa beach, whereas fluvial discharge is affecting especially the Caixa d’Água
beach, as expected. During the dry season winds are resulting in erosion in Princesa beach
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and deposition in Caixa d’Água beach, because of wind direction and beach orientation. The
results on granulometric analyses corroborate to the interpretation of topographic changes,
where grain size, asymmetry and sorting indicate erosion or deposition of aeolian sediments,
as well as muddy sediments from the fluvial discharge. The results represent a good
characterization of the evaluated profiles and beaches, and of the acting processes in
oceanic and estuarine beaches of the Amazon littoral. Though it is helpful for further studies
in any beach of the Amazon littoral, where rain/fluvial discharge seasonality, as well as
macrotidal regime imply in a complex beach morphodynamics.
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