High resolution positioning â suitability and aplication to coastal morphology / Posicionamento de alta resoluÃÃo â adequaÃÃo e aplicaÃÃo à morfologia costeira

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

The coastal zone concentrates a large economic potential and due to its importance to the sustainable development it was subject of discussion during the United Nations Conference on the Environment, Rio 92. The Agenda 21, the basic document generated during this conference, includes a specific chapter (Chap. 17) that deals with the protection of the oceans and coastal zones. In it, it is recommended that all countries conduct systematic observations and studies to identify critical areas and attempt to anticipate and monitor anthropic interventions along it. The present work focus on the development and test of a methodology employing high resolution spatial positioning to determine and monitor the shoreline position. The later defined as the feature at the horizontal plane corresponding to the interface between the dry area of the continent, or of an island, with the area under effective action of the water. A pair of geodesic GPS receptors was utilized, maintaining one at a fixed known location while the antenna of the other was displaced over the shoreline. The measurements where processed in a way to generate a 2-D databank of the position of the shoreline for the time of the measurement. Two areas where monitored: a coast stretch between the mouth of the Timbà river and the southern entrance of the Santa Cruz channel and the isle Coroa do AviÃo. At the first, the alterations occurred from 1969 and 2004 (last 35 years) where quantified. It was verified that during the first 19 years (1969-1988) a landfill of an area of 66.50 hectares in the continent and the retrocession of the shoreline in front of the fort Orange. Later, between 1988 and 2004, it was verified the progress of the erosion and the wear down of the fort Orange walls, simultaneously to the appearance of a sand spit immediately to the North of the fort and the retrocession of the shoreline in the continental portion. The isle Coroa do AviÃo, located by the southern entrance of the Santa Cruz channel, was monitored for 42 months (nov/00-may/04) with nineteen measurements of its emerged area. The data analyses considered two distinct phases, before and after the construction of a pier in the isle, in sept/2002, that destabilized the local dynamics. The isle, as a whole, has migrated and rotated. The position of its centroid had moved 11.62m to the north and 7.75m to the west. This displacement resulted from the combined effect of erosion along the southern isle shore and deposition along its northern shore face. The eroded area was of 5741.89 m2, at a rate of 136.71 m2.month-1 and the accretion area was of 6890.46 m2, at a rate of 164.04 m2.month-1. While by the end of the monitoring period the total variation of the emerged area was less than 5%, the displacements of the shoreline forced, initially, the relocation and later the removal of the installations of the museum and of the migratory bird research facility of the UFRPE. It was also verified that the installation of a pier in September/2002, cause a 28.3m retreat of the shoreline on the western extremity of the isle and the area still remains unstable. A prognostic is presented for the isle Coroa do AviÃo, considering besides the shoreline measurements, a 3-D survey, grain size analyses of its sediments and information on wind speed and direction. The precision and accuracy of the proposed methodology for shoreline positioning using GPS receptors was equally investigated taking into account the possibility of identification and degree of access to the local to be monitored. In real situation the average discrepancy between distinct observers in positioning shoreline was of 12-16 cm when it was easily identifiable and accessible and of 51-78 cm under adversity. In a field test, the average precision of the method was of 1.7 cm, varying punctually from 0.4 to 3.6 cm among distinct observers while the average accuracy of the method was estimated in 0.5 cm (min.=0.0; max.=24 cm). Considering the overall deviation, discrepancy in position the shoreline varied from less than 10 cm when shore line was easily identified and access to the area was free to up to 1m at location where shoreline identification and/or accesses was not ideal

ASSUNTO(S)

monitoramento oceanografia desenvolvimento sustentÃvel morfologia costeira

ACESSO AO ARTIGO

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