High- and low-frequency morphodynamic events and natural risks in mountain catchments of the semi-arid Brazilian “Nordeste”: Crato (Ceará)
Résumé
Usually known as prone to severe droughts, the inner Northeast Brazil (“sertão”) is still hardly considered as an area of hazards and risks related to violent geodynamic events. However, such past or present phenomena are now recognized in a few narrow humid mountains exposed to the trade-winds, such as the eastern “Chapada do Araripe”, a high tabular sandstone plateau located at the border between the Ceará and Pernambuco states. Here, a 400-500 m high cuesta-like scarp covered by a dense forest overlooks drier dissected lowlands. The city of Crato (110,000 inhabitants) is built along one of its short rivers, the Rio Grangeiro, 4 km from the rim. Whereas existing works only bear on current and historically observed dynamics, flash-floods, gullying, and small-scale mass movements, we consider a wider range of geodynamic phenomena. So far, only the violent floods that periodically occur along the Grangeiro River have been taken into account in urban management, with the construction of a 2 km long concrete channel through the city. According to our analysis of climate-hydrological data over the last decade, big floods aggravated by this inadequate canalization have devastated the centre almost every year. Our study of landforms and deposits in the city and upstream also evidences largely distributed marks of past large-scale mass movements, debris flows, landslides, and even Sturzstrom. We show the importance of recurrent gravitational dynamics as usual processes in scarp evolution. Since such phenomena induced by favourable climatic and geological factors still might occur in a now densely populated area, and since some of them were catastrophic, they must be included in natural hazards and risks, in spite of the current lack of data on their chronology. Our first results justify further integrated studies of the past and current conditions of occurrence of both high frequency-low magnitude, and low frequency-extreme events identified through short-and long-terms records.