Vienne, contraintes hydrologiques et aménagements des rives du Rhône. De la komè allobroge à la ville du Haut-Empire

Since the end of the 5th century BC, a town has been established on a hill overlooking the eastern bank of the Rhône and its confluence with the Gère. Main town of the Allobroges, it became a large Roman civitas capital and long continuity and development are due to its position in the Rhône valley at the junction of several valleys. Study of geology carried out systematically since fifteen years, during excavations, reveals that the native settlement, originally perched on a promontory surrounded by streams, became one of the largest towns in Gaul, after progressive modification of the river system that started as soon as the 2nd century BC, and noticeable up to the turn of Christian era. At that time, the settlement was lying on two flat-lands on both banks of the Rhône out of reach of floods. The first buildings noticed on the low-lands show that even being near the natural ground level, some precautions were taken against dampness (place Saint-Pierre : under floor space made of amphorae ; place Camille-Jouffray : spreads of pottery sherds ; Saint-Romain-en-Gal : wooden and earth internal divisions on masoned "murs bahuts"). Between 20 and 50 AD, extensive measures were taken : the lower districts on both banks were quickly raised, from 1,50 m high up to 3 m, varying with the areas. These huge equipments in relation to the areas concerned are not really the achievement of cautious town-planners but could be an answer to a temporary return of floods of which evidence dated of the years 30-40, was yielded south of town.