Análise da evolução da linha de costa entre as Praias do Futuro e Porto das Dunas, Região Metropolitana de Fortaleza (RMF), estado do Ceará, Brasi

Abstract

Coastal areas are included among the most dynamic environments existing on our planet and have been suffering with the advance and retreat of the coastline, constantly changing its position on various time scales. These areas have high tourism and urban potential, and the disorderly urban sprawl is a growing problem, with numerous examples of impacts of coastal erosion on cities and resorts, in addition to loss of important habitats. The systematic mapping of the coastline and monitoring its changes represents tools for generating of valuable information for planning and coastal management because they provide subsidies for the establishment of temporal variations of the shoreline. This study is about a shoreline variation analysis at inter decadal and inter annual scales involving 38, 31, 7 and 5 years time span evaluation in four sections of the east coast of the Metropolitan Region of Fortaleza, from the analysis of multitemporal remote sensing images and aerial photographs associated with geoprocessing techniques. The rates of advance and retreat of the shoreline were obtained using the Digital Shoreline Analysis System, software for ArcGIS 10.0, integrating georeferenced data, between 1972 and 2010.The results show that the dynamics of this coastal area has become quite unstable in its entirety. From Porto das Dunas beach to the left margin of Pacoti river it was set up an erosion stage on an average of – 3,53m/year and -17.65m in 5 years. It was possible to set the erosive stage of Porto das Dunas beach to the left edge of the Pacoti river as uncritical in the face of changing trends retreat. Through on-site observations, it can be seen that fixed structures built on the dunes, on the beach strip or offshore represent obstacles to the continuous mobility of the sands, interfering in the dynamic replacement and sediment transport. This fact could be confirmed when the sediment balance of the monitored points had considerable losses over the deposition. Important to mention that the dunes migration reaches the lower estuary of the Pacoti River, providing a source of longshore supplier through estuarine dynamics. Between the mouth of the Pacoti river and Abreulândia beach it was observed erosion rates, average of -0,31m/year and -2.18m in 7 years. This erosion process is associated with the increase of buildings installed in unsuitable areas, already suffering from the occupation of fixed and mobile dunes and praial erosion track in this sector of the coast. However, this retrogradation shows to be not very expressive with respect to time and factors, which may be associated with the presence of cord discontinuous sandstones beach (beachrocks) parallel to the coastline, which protects praial range of the power erosional of waves .From Abreulândia beach from to the mouth of the Cocó river showed erosional average of -0.07m/year, -2,59m in 38 years, roughly speaking, the process of erosion can be linked together with a reflection of the reduction in the flow of the Pacoti river basin for estuarine region, as a result of artificial dams along its present route, plus the reduction of aeolian sediment delivery deposits in the estuarine zone, caused by reduce of sands mobility, a consequence of accelerated and disorderly installation of numerous anthropogenic structures in the high beach and dune fields in sectors 1 and 2 in the barlamar zone. Featuring stability in this sector, probably due to discontinuous cord beachrocks parallel to the coast. Praia do Futuro beach has a strong progradational average of +1.43m/year, +54.33m in 38 years. The standard deviation of this cell is shown in two scales fairly representative expressing significant mobility of the shoreline along the profiles. This process is possibly associated with the major dynamic players operating in this edge exposed, morphodynamic character ranging from intermediate to dissipative, in some points reflective, in the E-W direction, and normal orientation to the direction of incidence of the dominant action of the winds and waves. It is worth mentioning the presence of the Titanzinho spike located in the terminal portion of the Praia do Futuro Beach, a structure that prevents the continuity of the sediment transport by longshore drift and leads to the beach nourishment. The coastal dynamics balance of the entire surveyed area has been heavily influenced by urban growth and it´s exponentially impact, where it is necessary for the deployment of anthropogenic structures the morphologic deconfiguration and burial of some ecosystems, for example, stretch of the beach, dunes field and fluvial-marine plain. The impact in this environment involves the sediment removal that would be transported to the beach, to the estuarine zone and the littoral drift, feeding also other surrounding areas. Developing a multitemporal base of satellite images and aerial photographs in an environment of Geographic Information Systems facilitated the identification and understanding of the dynamics of natural and environmental analyzed areas, being essential as input to decision making about the use and sustainable management of these sectors. It is an important tool in planning and environmental intervention, particularly easily indicating trends easily retreat and /or progradation of a region over the years.