SUBSIDENCE

Subsidence is the final result of a complex system of natural and anthropogenic processes that cause a vertical lowering of the soils, with annual variations ranging from 1 millimetre to few centimetres.

Natural subsidence has its main causes in the geological and geomorphological structure of the territory, with very slow millimetre evolutions, measurable in historical times (sometimes geological), and distributed mostly on a regional scale. The processes that cause natural subsidence are:

- deep tectonic phenomena (orogenesis, volcanic or seismic activity);

- physics-chemical transformations of sediments;

- natural compaction of soils;

- fluctuations in the groundwater level.

According to a recent study conducted by the University of Padua and by the National Research Council Department for Hydrogeological Protection (CNR-IRPI) and Department for geosciences and geo-resources (CNR-IGG), the Italian territory is strongly affected by the phenomenon of subsidence, and the risk reach its most relevant level in Veneto, Emilia Romagna, Tuscany, Campania, Apulia and Calabria. The study also highlighted that, due to the phenomenon of subsidence, about 200 km of the northern Adriatic coast are currently below sea level and this make these territories also more susceptible to flooding.

Human action amplifies the negative consequences of natural subsidence. The anthropo-induced subsidence (often call anthropic subsidence) is mainly due to the extraction of fluids from the subsoil. Following the extraction activities, the original pressure of the underground fluid decreases, causing a compaction of the subsoil, with consequent lowering of upper levels. Human-induced subsidence generally show negative effects in shorter times then natural subsidence (in the order of few tens of years) and more locally limited.

Due to time and extensions, the anthropo-induced subsidence causes much more devastating effects on the environment and requires targeted control and management actions. In the Emilia-Romagna Region, the extraction of fluids from the soils has increased the natural subsidence since the 1950s, reaching maximum values ​​between the 1960s and 1980s.

Linked to specific properties of the territory, natural subsidence cannot be stopped, just monitored: for this purpose, at the end of the 1990s, the Emilia-Romagna Region, together with ARPAE, activated a dedicated monitoring network (Regional subsidence monitoring network - see below).

On the contrary, local, regional and national Authorities are paying a lot of attention to anthropo-induced subsidence. Law no. 845/80 (Protection of the territory of the Municipality of Ravenna from the phenomenon of subsidence) was one of the first national regulation that dealt with the problem of anthropic subsidence, trying to mitigate the phenomenon: the legislation in fact regulated interventions for protect the coasts and inhabited areas and introduced specific prohibitions for extraction activities of groundwater. Few years later, Law ho.183/89 specifically indicated anthropo-induced subsidence among the causes of the hydrogeological instability. More recently, Legislative Decree 152/2006, containing specific environmental protection regulations, includes subsidence containment works among the phenomena for which is required to Municipalities and Regional Authorities a specific policy for planning and implementation of mitigation activities. At European level, the European IPPC Directive for integrated pollution prevention and reduction (96/61 / EC) provides dedicated reports for subsidence phenomena and integrated authorization and measures to limit its negative effects.