Portus: The Harbour of Imperial Rome Built by Claudius and Trajan

Portus was the great artificial harbour of imperial Rome, built on the coast just north of the Tiber mouth near modern Fiumicino. Begun by the emperor Claudius around AD 42 and enlarged by Trajan in the early 2nd century AD with a famous hexagonal basin, it handled much of the grain and goods that fed a city of perhaps a million people. Today its remains lie some 3 km inland, beside Rome’s main airport.

Why Rome needed a harbour

By the 1st century AD Rome depended on imported food. Grain came above all from Egypt and North Africa, delivered under the state-run supply system known as the annona. Feeding the capital was a political necessity: shortages led to riots and could threaten an emperor.

Rome’s old port, Ostia, lay at the mouth of the Tiber, but the river mouth was shallow and silted up. Large merchant ships had to anchor offshore in the open sea or unload at Puteoli on the Bay of Naples, more than 200 km away, with goods transferred to smaller boats. Julius Caesar had considered building a proper harbour, but the project was not carried out.

Claudius’s harbour

According to Cassius Dio, Claudius decided to build a harbour in AD 42 after a grain shortage, despite engineers warning about the cost. The site chosen was about 3 km north of Ostia. Workers dug a large basin out of the land and extended two curving breakwaters, or moles, into the sea.

Suetonius and Pliny the Elder describe how the huge ship that had brought the obelisk now in St Peter’s Square from Egypt under Caligula was filled with concrete and sunk to form the foundation of a lighthouse at the harbour entrance. Underwater concrete of this kind was a Roman speciality, described in more detail in the article on Roman concrete. The new basin covered roughly 150–200 hectares, making it one of the largest harbours of the ancient world.

Canals linked the harbour to the Tiber, and an inscription of AD 46 records that Claudius dug them also to protect Rome from floods. The harbour was completed under Nero, whose coins of about AD 64 show it crowded with ships around a lighthouse.

Trajan’s hexagon

Claudius’s basin was too large and exposed. Tacitus reports that in AD 62 a storm destroyed about 200 ships inside the harbour itself. The solution came under Trajan, who in the early 2nd century AD had a second, sheltered basin dug further inland.

Trajan’s harbour was a precise hexagon, each side about 357 metres long, enclosing roughly 32 hectares of water with a depth of around 4–5 metres. Ships could moor along all six sides, with long warehouses, the horrea, behind the quays. A new canal, the Fossa Traiana, connected the harbour to the Tiber; it survives as the canal of Fiumicino. Trajan’s other great building projects, from his forum to Trajan’s Column, were funded partly by the wealth this trade carried.

A port city

Portus quickly grew into a town. It had warehouses, temples, baths, and an imperial residence often called the “Imperial Palace”. Between Portus and Ostia lay the Isola Sacra cemetery, whose tombs, many decorated with scenes of everyday work, reveal the lives of merchants and craftsmen.

Goods arriving at Portus included Egyptian and African grain, Spanish olive oil carried in amphorae, wine, marble and exotic luxuries. Most of it then went up the Tiber to Rome on barges hauled by oxen or men. Similar large harbour projects, such as Herod’s Caesarea Maritima, show how much the empire invested in seaborne trade.

Under Constantine I in the early 4th century, Portus received the status of a separate town, with a name honouring the emperor’s family. By then it had overtaken Ostia, which was in decline.

Working the port

Running Portus required a large workforce, much of it organised in professional associations, the corpora. Inscriptions from Portus and Ostia name guilds of boatmen, lightermen who ferried cargo between ships and quays, ballast handlers, divers who recovered goods from the water, grain measurers and shipowners. Many of these men were freedmen, former slaves who had acquired property and status through trade.

The state supervised the grain supply through an official called the prefect of the annona, whose staff checked cargoes and recorded deliveries. Grain was stored in the warehouses around the basins before being loaded onto river barges. A single large grain ship could carry several hundred tonnes, so the arrival of the Alexandrian fleet each year was a major event.

The harbour itself needed constant maintenance. The canals and basins had to be dredged to stop them silting up, and the moles and quays repaired after storms. Archaeologists have found evidence of this work in the form of rebuilt quay walls and changing water levels recorded in the sediments.

Excavations in the 2010s also identified a huge building near the hexagon that may have served as a shipshed or naval arsenal, large enough to house or build sizeable vessels. Its exact function is still debated, but it shows that Portus was far more than a set of quays.

Decline

Portus remained vital in late antiquity, and that made it a target. The Visigothic king Alaric seized it in 408–409 to put pressure on Rome by cutting the grain supply. During the Gothic War of the 6th century, the historian Procopius describes it as Rome’s harbour and records fighting there during the siege of Rome in 537–538.

As the city’s population shrank and long-distance trade fell away, the port was gradually abandoned. Sediment carried by the Tiber pushed the coastline steadily westward, leaving the basins stranded inland.

Rediscovery and excavation

Antiquarians and artists of the Renaissance studied the ruins and imagined their former grandeur, as in the 16th-century engraving shown with this article. Much of Trajan’s harbour later belonged to the Torlonia family, and the hexagon survives as a lake.

Building Fiumicino airport in the late 1950s and 1960s uncovered parts of Claudius’s harbour, including the remains of ancient ships. Since the late 1990s the Portus Project, led by Simon Keay of the University of Southampton with the British School at Rome and Italian heritage authorities, has used geophysical survey and excavation to map the whole port complex.

By the numbers

Timeline

• AD 42: Claudius begins the harbour, according to Cassius Dio.
• AD 46: Inscription records the canals linking the harbour to the Tiber.
• AD 62: A storm wrecks about 200 ships in the harbour.
• c. AD 64: Nero’s coins celebrate the completed harbour.
• Early 2nd century AD: Trajan builds the hexagonal basin and the Fossa Traiana.
• Early 4th century: Constantine grants Portus town status.
• 408–409: Alaric seizes Portus during his blockade of Rome.
• 2000s: The Portus Project begins large-scale survey and excavation.

Why it matters

Portus was the hinge between Rome and its empire: without it, the capital could not have been fed. Its hexagonal basin is one of the most striking pieces of Roman engineering to survive, and its excavation has transformed understanding of how the Mediterranean economy worked.