A subtle and hidden natural hazard: origin, development and evolution of sinkholes

Among the many geological hazards affecting the Italian territory (earthquakes, volcanic eruptions, slope movements, floods, wildfires, etc.), and causing severe losses and casualties, sinkholes are rarely considered in hazard and risk analysis. Nevertheless, their occurrence, with the consequent negative effects on the built-up environment and human infrastructures, affect large parts of Italy, covering the whole country with a variety of typologies. Sinkholes mostly occur in karst areas, where soluble rocks such as carbonates and evaporites crop out or are present at low depth (De Waele et al., 2011). Development of sinkholes, however, may also be registered in alluvial plains, involving thick sequences of alluvial and colluvial deposits (Fig. 1). Recently, an effort was produced by the Institute of Research for the Hydrogeological Protection with the National Research Council of Italy (CNR-IRPI) to build a chronological catalogue on sinkhole phenomena in Italy, addresses toward the evaluation of the sinkhole hazard (Parise & Vennari, 2013; Parise et al., 2013). The chronological catalogue of sinkholes is specifically focused on the time of occurrence of the events, that is considered a crucial element for the definition of the sinkhole hazard. Even though the catalogue takes into account both natural and anthropogenic sinkholes (these latter being those events related to presence of manmade underground caves), given the topic of this conference, the present contribution will deal exclusively with natural sinkholes. Knowledge of the dates of occurrence of sinkholes represents the necessary element for definition of the hazard; at the same time, discriminating between sinkholes related to presence of natural or man-made caves may determine very different scenarios to be faced, in terms of civil protection actions. A sinkhole at the surface appears as punctual (site-located), and typically of limited dimensions. This, however, does not mean that the affected area is limited to that specific point, since underground the caves responsible for the event may potentially develop for a much wider extension. With respect to natural sinkholes, these typically are found in areas with soluble rocks affected by karst processes, or alluvial sediments in peculiar geological situations. Thus, the zonation of the sinkhole-prone areas may be done for natural sinkholes based solely upon geology as a first approximation.