Les changements hydromorphologiques de l’estuaire de la Loire et l’évolution du port de Rezé/Ratiatum (Loire-Atlantique)

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En 2005, au démarrage d’un important programme de recherche sur le quartier antique de Saint-Lupien à Rezé (Loire-Atlantique) et de son port supposé, la connaissance des paysages ligériens et de la dynamique du fleuve durant l’Antiquité en était à ses balbutiements. Les études sur site, archéologiques et environnementales, et celles développées en parallèle sur la basse Loire (dans le cadre de deux Programmes collectifs de recherche – PCR – et d’une étude documentaire qui se sont succédé entre 2005 et 2019), ont permis de discerner les entités géomorphologiques présentes dans le paysage ligérien antique (îles, chenaux et plaines alluviales) et ont apporté une résolution assez fine à l’évolution hydrologique de la basse Loire depuis l’âge du Bronze jusqu’à nos jours. Ces études, focalisées ensuite sur l’Antiquité et circonscrites à Rezé, ont permis d’analyser sept siècles d’aménagements riverains et portuaires liés à la vie urbaine et contraints à évoluer pour s’adapter, dans un contexte eustatique relativement stable, aux crises et changements hydrologiques liés aux variations climatiques et/ou à la pression anthropique, d’une Loire de fond d’estuaire toujours plus fluviale.

In 2005, when a major research programme was initiated with the aim of investigating the ancient district of Saint-Lupien and its supposed port, little was known about the Loire river landscapes and the dynamics of the river during Antiquity was in its infancy. On-site archaeological and environmental studies, and those developed in parallel on the lower Loire, both as part of two collective research programmes (PCR) that succeeded each other between 2005 and 2015, and during a documentary study carried out in 2019, made it possible to describe the geomorphological entities present in the ancient Loire landscape (islands, channels and alluvial plains) and provided a fairly fine resolution on the hydrological evolution of the lower Loire from the Bronze Age to the present day. Many of the geomorphological features present in the Loire Valley, between the Roman port of Nantes/Condevicnum (Loire-Atlantique) on the right bank and that of Rezé/Ratiatum (Loire-Atlantique) on the left bank, appeared gradually between 5700 and 4500 BP, during the final Neolithic period. At that time, the marine influences which were dominating the Loire valley in Nantes, then a veritable ria, retreated after a slowing down of the sea level rise and gave way to a conquering river which incised the arms of Pirmil and the Madeleine. The Loire area structured in this way emerged subsequently. On the other hand, the active strip of these two arms was much more mobile and many islands appeared there, notably at the end of the hydrological crises linked to the climatic deterioration during the Early Iron Age and the beginning of the Late Iron Age. Among these were the Chevaliers island, located in front of the ancient harbour quarter of Saint-Lupien, and the Vertais island, whose location facilitated the crossing of the valley and later served as a foundation at the southern end of the alignment of the medieval bridges. In Saint-Lupien the initial riverbank developments, followed by the construction of the monumental quays, took place between the beginning of the 1st c. AD and the beginning of the 2nd c. AD, on the margins of the relatively shallow waters of a secondary arm of the Loire river subject to tidal dynamic. The latter facilities were adapted to suit this environment and could accommodate mixed navigation, both fluvial and estuary. Subsequent to the water crisis of the 2nd and 3rd c. AD caused by human activities, marine influences gradually diminished and these infrastructure became antiquated and were recycled. The quays were transformed into boat ramps, a redevelopment that restricted the activity of the port to mainly river transport. Nevertheless, the activity of this port continued up to the beginning of the 7th c. AD and subsequently massive sand sedimentation definitively obstructed the access channel. This umpteenth water crisis coincided with the beginning of a climatic deterioration which lasted until the 19th c. and frustrated the project to maintain a port in this sector at all costs, despite the digging of a channel which quickly became backfilled.