Flood, war and economy. Leonardo da Vinci and the plan to divert the Arno River

This chapter explores the connections between Leonardo da Vinci and hydraulic works on the Arno, in particular the failed war scheme to divert the river in 1503–1504. Through observations of this river, the artist developed a number of studies on flooding, examining bridge projects, dams, channels, canals and levees connected with the Arno. Leonardo's activity for Cesare Borgia (1502), and then for Pier Soderini (1503–1504) in the context of plans to divert the river near Pisa, points up the relationship among hydraulic engineering, observations on nature and the measurement of terrain. Drawings and writings in Leonardo's corpus, such as the Windsor drawings (ca. 1503), which describe Leonardo's project for the ‘gran canale’, can be related to his studies on the Arno, to protect Florence from flooding. The essay reconstructed the entire episode (both from the point of view of the succession of events and the organisation of the building site), re-examining the published sources and finding new documents. In the light of this new research, Leonardo's contribution to the enterprise is circumscribed to the initial stages, while the figure of Biagio Rossetti emerges as a consultant called in by Machiavelli in the final stages of the work to try to avoid the enterprise's failure. This chapter thus points up the relationship among control, transport, regulation and measurement in peacetime and in wartime, to prevent flooding or, inversely, to cause flooding against an enemy or deny that enemy access to the sea. Leonardo's ideas may not have been realised, but his thinking echoed into the sixteenth century, influencing the extensive river works in Tuscany undertaken by Grand Duke Cosimo I de’ Medici (1519–1574) and his successors. All the many sources - of different types and content - recompose a picture of extreme complexity, revealing the dense web of relationships and contrasts that marked the genesis, development and conclusion of the project. In this undertaking, therefore, knowledge gained in the field of flood defence and the practice of using hydraulic engineering for military purposes converge. Machiavelli also highlights the prospects of using it for economic purposes. A great dream, to which Leonardo may indirectly have contributed in conceptual terms (but of which no specific documentary evidence can be found), and which is shattered in the face of objective technical-organizational difficulties.