Geohazard map of cover-collapse sinkholes in the 'Tournaisis' area, southern Belgium

This paper reports the methodology developed to draw up a geohazard map of cover-collapse sinkhole occurrences in the 'Tournaisis' area. In this area, carboniferous limestones are overlain by a mesocenozoic cover mainly consisting of marls, sand and clay. The thickness of this cover ranges from a few meters to more than one hundred meters. The surficial morphology of the area does not show any karstic evidence except for the occurrence of these collapses. From a paleogeographical point of view, a developed quaternary karst is not conceivable in the area. Recent works suggested that the collapses are set off fi om reactivated paleokarsts. The paleokarsts studied in the area proved to be the result of a particular weathering of the limestone. The organization of these paleokarsts seems very low and mainly guided by the limestone fracturing. As for most induced sinkholes, the reactivation of these paleokarsts is linked to the lowering of piezometric heads. In most of the area a thick cover and intensive land use mask potential surface hints of the buried paleokarsts and of the fracturing of the bedrock. Aerial photographs and remote sensing techniques have therefore shown little results in delineating collapse hazard zones up to now. The study of the surficial morphology is also of little help. In order to draw up the geohazard map in such a difficult context, hydrogeological data and geological mapping information only could be used. These information are based on a limited number of boreholes and piezometers thus only valid on a regional scale. Records of former collapses were also available. These records were of great interest since sinkholes distribution is obviously clustered in the area. Bedrock roof and cover formations floor altitudes were digitized and adapted to produce digital thematic maps. Piezometric heads were imported from a calibrated groundwater model of the aquifer. These data and a digital elevation model of the area were integrated into a GIS to produce a coherent 3D description of the area on a regional scale. Parameters such as the dewatering of the limestone, thickness of cover formations where sinkholes occurred were then estimated. Density of former collapses were also computed. This showed that zones of high sinkholes occurrences coincide with zones of heavy lowering of piezometric heads. Combining the density of former collapses with the dewatering of the limestone enabled us to delineate zones of low, moderate and high collapse hazard.