Portus was Rome’s maritime port during the Roman Empire. In AD 42, the harbour location was selected about 3 km north of Ostia, along the Tyrrhenian coast, on the margin of the Tiber River. Portus and its maritime façade are well known, however the fluvial aspects of Portus are poorly documented. How did Roman engineers preserve a continuous waterway from the basins of Portus to the Tiber River without accelerating siltation inside the harbour? Were their choices efficient? The present Canale di Comunicazione Traverso is the only canal attested to link the Tiber River and the harbour basins. The objective of this work is to analyse the Canale Traverso sediments infill in order to establish the role of this canal in relation to the infill deposits of the harbour basins and to define the functions of the canal. This study is based primarily on a reinterpretation of the available archaeological data, as well as mainly on sedimentological analysis and the interpretation of the Passega diagram. A Passega diagram is presented for understanding the deposit processes for the harbour environments at the entrance to the Trajanic basin (TR-XIV). This diagram is compared to the Passega diagram of the Canale Traverso (CT-1) which is characterised by a stronger influence of the Tiber River. This study concludes that the Canale Traverso was a canal that was well protected from the influence of the Tiber River, with the exception of occasional flooding. Also, this paper presents the patterns of sedimentation at Portus and allows us to define the use of the canal and the maintenance procedures.

Certains organismes marins vivent fixés sur les quais des ports actuels. La limite supérieure des peuplements médiolittoraux définit le niveau marin biologique. De telles faunes fixées viennent d’être découvertes sur les quais du port antique de Rome, le Portus, construit au ier et iie s. ap. J.-C. sur le littoral du delta du Tibre. Ce niveau marin biologique antique apporte quatre informations géoarchéologiques fondamentales. (1) Sa datation au radiocarbone, 2115 ± 30 BP soit 230 à 450 ap. J.-C., indique la phase ultime de connexion entre la mer et les bassins portuaires. (2) Sa position 80cm sous le zéro biologique actuel indique, avec précision, une hausse relative de 80 ±10cm du niveau marin depuis le iiie / ve s. ap. J.-C. (3) La différence altimétrique entre cet ancien niveau marin et les données stratigraphiques des carottages apporte des informations sur les paléoprofondeurs. Ainsi, à l’entrée du bassin hexagonal de Trajan, la profondeur était de 7m. (4) La présence ou l’absence de faunes fixées signifie un contact ou non des moles avec la mer et renseigne sur la configuration, encore mal connue, de Portus.