The Pisa plain (Italy) and its hydrological hazards

Abstract

The Pisa plain, which is crossed by the lower sections of the Arno and the Serchio rivers, consists of a thick accumulation of marine and continental sediments from the Late Tertiary and the Quaternary. These sediments have filled part of the Versilia graben and, through their progradation, have extended the shoreline farther and farther west, now many kilometers away from the city of Pisa. Thus, besides the Holocene fluvial and palustrine sediments, the surface of the plain consists of dune-bar and coastal barrier beach sands. The plain is very low, lying just above sea level, and shows only a very slight gradient (0.0005%). As a result, for centuries, vast lakes and lagoons covered the plain's surface, and the courses of the rivers changed several times. Human intervention began during the Roman Empire, with efforts to modify the plain by reclaiming marsh areas, building a network of drainage canals, and straightening river courses, so that today drainage takes place through an entirely canalized system. In spite of this, hydrological hazards are still present, mainly represented by backwater and river overflow. The backwater hazard exists in less permeable areas after prolonged periods of rainfall. It should be kept in mind that the plain is undergoing subsidence and that the groundwater level is low due to excessive exploitation. These facts indirectly cause another hazard: the intrusion of saltwater inland, a hazard accentuated by major coastal erosion (a retreat of 1 km in 100 years). The coast is no longer replenished with the material that is removed from riverbeds by humans. A long list of overflows of the Arno and Serchio rivers testifies to the overflow hazard affecting the countryside, cities, and towns. A large artificial drainage canal, the Scolmatore, was built for the diversion and transport of floodwater. From Pontedera, where the sluice gates are located, the canal is meant to transport the floodwater (900 m³/s) to the sea, following a route that is 28.3 km long. The structure has not yet been completed. Moreover, there is still no absolute certainty as to its efficiency in the event of a large-scale overflow accompanied by sea-storm waves, which could obstruct the flow of the waters at the canal's sea outlet. The latter, like the mouths of the Serchio and Morto rivers, is almost always obstructed by deposits.