International Geologiical Congress - Oslo 2008


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PEM-01 Megacities: going deeper, building safer


Georesources and Geohazards at the northern periphery of a future megalopolis: a case study of Belo Horizonte, Brazil


Allan Büchi, Universidade Dederal De Minas Gerais (Brazil)
Monika Hofmann, TU-Darmstadt (Germany)
Joachim Karfunkel, UFMG (Brazil)
Andreas Hoppe, TU-Darmstadt (Germany)
Ricardo Pagung, UFMG (Brazil)


Belo Horizonte, capital of the state of Minas Gerais (Brazil), is growing rapidly and has been considered by the UNO as one of the cities that will become a megacity with more than ten million inhabitants by the middle of the century. The city already does not support its growth to the south due to accentuated relief. Thus, the future tendency will be to expand towards the north. As the analyses of geo-resources and their economical and ecological viability is essential for a sustainable urban growth, this project aims to evaluate these aspects as support for urban planning in the northern periphery of Belo Horizonte. As a case study, 6.68 km2 of the alluvial plain of the Riberão da Mata has been chosen. This area has been subject to sand, clay and gravel extraction during the last decades, and many of the pits have been left open. A visual analysis of aerial photos covering nearly half a century shows the land use change in this area. Frequent changes of the riverbed, erosion at its margins, in the alluvial plain could be observed. These analyses were accompanied by field observations where a special focus has been set on the sedimentological structures visible in sand and clay pits in the central part of the alluvium. Above the channel sediments that provide the sand, quite often 4-5 m of overburden in the form of floodplain sediments have to be cleared away. The peak of the activities was between 1960 and 1980, when the city was growing rapidly and environment legislation was less strict. Since the middle of the last century, more than 30% of the area have been subject to extraction of sand and clay (1.5 km2 in 1977 and only 0,5 km2 in 1990). This reduced activities in the last twenty years can be mainly assigned to the depletion of easily accessible sand above the water level, land use conflicts and a stricter environmental legislation that protects a buffer zone along rivers. Still, it is one of the few areas close to the city where sand and gravel for construction purposes can be found and extracted. In some abandoned areas, industrial plants e.g. breaking carbonate rocks for gravel substitution have been constructed a decade ago, as the alluvium is one of the few plain construction sites in the area. The gravel extraction has nearly stopped since, as the natural gravel resources are deep and few. At localities at which extraction sites have been left open, the natural recuperation is slow and erosion carries away fertile soil that increases the sediment load of the river. In 1977, 0,4 km2 of the area could be classified as exposed soil showing signs of erosion. Due to extraction of raw material, the area covered with water has strongly increased, thus reducing the available area for building sites.


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