The growth and decline of Pisa (Tuscany, Italy) up to the Middle Ages: Correlations with landscape and geology

The history of Pisa up to the Middle Ages was influenced by the local landscape. Pisa is located at the confluence of two rivers, has easy access to the sea, is surrounded by wetlands, and overlies a clay-sand substrate characterized by highly plastic clay near the ground surface. All of these characteristics have played a significant role in the history of Pisa along with socio-economic and sociopolitical conditions. Pisa is located in western Tuscany, about 15 km from the Tyrrhenian Sea, in a very flat coastal plain (Pisa plain) crossed by the meandering Arno River. The origin of Pisa dates from the 5thcentury BC when Pisa was an Etruscan settlement. In the 2ndcentury BC the Romans built Portus Pisanus harbour. Pisa was known as an important commercial and political centre in the Middle Ages and obtained the status of Maritime Republic, similar to Venice, Genoa and Amalfi. In 1284, its defeat in the battle of the Meloria against Genoa caused the beginning of the city's decline. In 1509, Pisa passed definitively under the domination of Florence. The history of Pisa was influenced by many factors related to the local landscape. It started at the confluence of two large rivers, Arno and Auser, an old branch of the Serchio River. These rivers represented a resource for transport of goods; however, they also necessitated the continuous management of hydraulic works to protect the town from floods and to avoid the formation of the wetlands. The Pisa plain was characterized by wide and numerous wetlands. Unlike other areas of Italy, the wetlands were more an economic resource than a cause of health problems like malaria or developmental complications for transport and agriculture. In any case, a network of artificial canals was built to partially drain these wetlands and impede their expansion. Its sea- and river-harbours made Pisa an important Marine Republic during the Late Middle Ages. Pisa had commercial influence on the coast of the Tyrrhenian Sea, Corsica, Sardinia and the Balearic Islands until progressive siltation and catastrophic events caused the abandonment of the harbours. Foundation problems vexed Pisa, as evidenced by the famous Leaning Tower. The problem originates from the nature of the subsurface, which is characterized by highly plastic clay layers at shallow depth; the thickest (7-8 m called pancone) occurs at about 10 m below the surface.