Taming the river

The Loire communities who live alongside the river have always been concerned about how to control its waters. To settle, grow crops or sail there, it was essential to understand, anticipate and guide the Loire’s changing nature. Until the 20^th century, the methods deployed to adapt to it and restrict it were the result of an accumulation of developments linked to evolving practices and doctrines. In his paper on defensive structures against flooding, Guillaume Emmanuel

Comoy, a unique figure among 19^th-century engineers, mentions the long-standing opposition between those in favour of total control over the river and advocates of opening up levees and accommodating the water. And yet, neither the raising of the old levees in the 18^th century nor the opening or maintaining of overflow channels were enough to prevent disasters the following century. In 1846, 1856 and 1866, three flood events swept away convictions and the valley’s farmland: the ever-higher levees, formed from disparate materials and poorly looked after - if at all - had the difficult task of containing the water pressure, but gave away in countless places.

A doctrine still under development

Learning of the scale of the disaster, Napoleon III appointed Comoy to lead the studies from 1856. Unable to reach the appropriate height, which would have required a sea change of the valley, he called on expert engineering knowledge from before the French Revolution to orchestrate a new technical doctrine, combining a scientific approach to flows and in-depth field knowledge. He demonstrated the link between levees, the dimension of the major bed and the increase in the river’s maximum flow. He sought to strengthen some sections of the old dykes, whilst accepting “the idea of an occasional influx of water into the dyked valleys, [...] to mitigate [...] the effects thereof”. This protection doctrine, which is still applied today, halted major development works in the Loire upstream that were at odds with Comoy’s proposals.