FEDERICI P.R. & MAZZANTI R.
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 of the Late Tertiary and the Quaternary. These sediments have filled in part of the Versilia graben and by their progradation the shoreline has been built forward farther and farther west and many kilometres away from the city of Pisa. Thus, besides the Holocene fluvial and palustrine sediments, we find that the surface of the plain consists of dune bar and coastal barrier beach sands. The plain is very low, lying slightly above sea level, and shows only a very slight gradient (0,0005%). Thus, for centuries vast lakes and lagoons covered the plain surface and the courses of the rivers changed several times. Man has intervened since the period of the Roman Empire, attempting to modify the plain by reclaiming the marsh areas, building a network of drainage canals, and rectifying the courses of rivers, so that today drainage takes place through an entirely canalized system. In spite of this, hydrological hazards are still present and they are represented mainly by backwater and river oveiflow. 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 ground-water level is low due to excessive exploitation. These facts indirectly bring about another hazard: the intmsion of salt water 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 river beds by man. A long list of overflows of the Arno and the Serchio rivers testifies to the oveiflow hazard affecting the countryside, cities and towns. A large artificial drainage canal, the Scolmatore, was built for the diversion and transport of the floodwater. From Pontedera, where the sluice-gates are located, the canal should transport the floodwater (900 m3/s) to the sea, following a route that is 28,300 km long. The structure has not yet been completed. Moreover, there is still no absolute certainty as to its efficiency in the event or a largescale overflow accompanied by sea-storm waves, which would obstruct the flow of the waters at the canal’s sea outlet. The latter, like the mouths of the Serchio and the Morto rivers, is almost always obstructed by deposits.