dinâmica do sistema estuarino timonha / ubatuba (ceará – brasil)

ABSTRACT
The Timonha / Ubatuba estuarine system is located between Ceará and Piauí States,
500 km northwest of Fortaleza City, in Northeast Brazil. The hydrographic tributary basin of
this estuarine system has an area of 2 165 km 2 , and lies in Paleozoic lands of the Ibiapaba
Plateau, Embasamento Cristalino, Formação Barreiras and Quaternary deposits. It is
important to stand out that the south portion of this estuarine basin is inserted in Pre-Cambrian
lands where may be seen Granitóide Chaval exhumed inside of the system. The search for
areas suitable to shrimp farming and urban/agroindustrial development may represent a
potential risk to this ecosystem, which is considered the largest of the Ceará State. These
impacts tend to increase with the construction and operation of the Itaúna reservoir, with a
capacity of 77.5 million m 3 and located 17 km from the estuarine basin. The aim of this study
was considering the structural, dynamic and controlling elements of the current state of the
estuarine system, to contribute with useful information to the suitable and sustainable use of
this ecosystem. The hydrodynamic and sedimentologic processes are dominated by the rainy
seasons plus the sedimentation processes on the mouth by longshore current and tide
oscillation. The reservoir resulted in the reduction of 91% of the Timonha river discharge
presenting an outflow of 5.6 X 10 8 m 3 in a year. The regularized outflow is not enough to
promote the mixing and salt dilution processes, contributing to the inversion of salinity in the
drought season. In the rainy season, the salinity of Timonha River grows gradually seaward
and varied from limnetic (< 0.5) to euhaline (30.0 at 40.0). In the dry season, predominated
the euhaline pattern with strong signals of hipersalinization. In Ubatuba river, the salinity
varied from polihaline (18.0 at 30.0) to euhaline between the rainy and drought periods. The
estuaries are type 2a in the rainy period and 1a in the drought period. The salt balance of
Timonha river in the rainy period was + 0.3 ton, while in the dry period was 0.0 ton. In
Ubatuba river the salt balance was - 0.1 ton and in the dry period was - 0.2 ton. The average
depth of the system is 5 m and the deeper depths were seen in Ubatuba river. The bottom is
made up predominantly by medium sand and biogeneous sediments. The total of sediments
exported to the longshore current, in a year, was 250 tons and the load imported was 150 tons.
The volume of the tide prism in Timonha river was nearly 3.6 x 10 7 m 3 and in Ubatuba river
4.3 x 10 7 m 3 . Considering the simplified model, the period of water residence is 1 day in the
rainy period and 5 days in dry period. This period, in Ubatuba river, is 0.5 day in the rainy
period and 2 days in the dry period. Considering the water residence time calculated to each
segment of the channels of Timonha and Ubatuba Estuaries, it was observed that the linear
model is not applicable to the system dilution capacity analysis. Based on the segmented
model, it was verified, in Timonha river, that the area located between 3,7 km and 7,0 km
from the sea, has a strong tendency to water retention and reaches a residence time equivalent
to almost a week, been inadequate to the direct emission of effluent. Comparing to other
tropical estuaries, the system presents a good dilution capacity and a low residence time. The
lack of monitoring to correct the outflows released by Itaúna Dam in long dry periods and the
deposition processes on the mouth river will represent the main elements to jeopardize the
support capacity of the estuary.
Key-words: Estuaries, Timonha river, Salt and Water Balance.
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