Riscos e gestÃo ambiental no estuÃrio do Rio malcozinhado, Cascavel-CE

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

The Rio Malcozinhado lagoon-estuarine system is located in the Cascavel municipality, in the Cearà State Southeastern littoral 75km away from Fortaleza, Northeastern Brazil. Its 380 km2 hydrographic basin lies on basement rocks, Plio- Pleistocene Barreiras Formation sediments and Quaternary deposits. The urban expansion of the area in the last two decades was responsible for a series of impacts in the estuarine dynamics and adjoining beaches. There is a growing tendency of this process once a reservoir has been already implanted with a capacity of 3.8 million of square meters upstream the estuarine system. Aware of these problems and considering the hypothesis of minimizing and/or to eliminate them, this work considered the structural, dynamic and controlling elements of the state of current evolution in the area of direct influence of the estuary, aiming at the delimitation of the risks and impacts for its adequate use and respective environmental management. The present morphodynamics is subordinated to a seasonal regimen of precipitations, together with the longshore drift, remobilization processes by aeolian deflation and tide oscillations. The sediment accumulation verified at the right edge of the river provided to a canal migration of approximately 110 meters causing erosion in Aguas Belas, over a period of 9 years. It is a shallow estuarine system with average depth of 1.50 m, and predominantly sandy mud an sandy clay substratum, as shown by the bathymetry and collected sediments and analysis which have been carried out. The longshore drift sediment transport ranges from 1,4 gs-1 and 3,34 in the dry and wet season, respectively. The euhaline regimen predominates as the dry period lasts while that in the rainy period was polyhaline with salinity varying from zero to 23,95. The high evaporation in September caused the inversion of the longitudinal distribution of the salinity. Concerning to circulation, currents varied seasonally in speed from 0 to 0,892 ms-1, with the highest mean values in the rainy period. The current axis keeps with the estuary geomorphology. The estuary system, according to classification of Hansen &Rattray (1966), is of the subtype 1a showing at some points gravitational circulation. As shown in the results, the process of turbulent diffusion is the principal source to mixing the system and to carry on salt upstream and downstream the estuary. The salt exported volumes in the wet and dry periods were 1,12 and 0,426 tons, indicating the importance of the fluvial action in making the water less saline. The stretch of the beach under direct influence of the River Malcozinhado is a strong unstable environment with the backshore zone being totally occuppied and undergoing the implantion of protecting structures.The erosion is recurrent independently of the reduction or increment of sediments stemmed from the Malcozinhado River and other associates processes. In order to handling and protecting this estuarine system a matrix has been mounted to assess the responses arisen from the relationships between the natural and anthropic elements and work out what they represent to the vulnerability, sustainability, and potential use of this area at medium range

ASSUNTO(S)

oceanografia gestÃo ambiental estuÃrios environmental management estuaries

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