The unique hydraulics in the Barbegal water mills, the world's first industrial plant

A mill complex consisting of a total of 16 water wheels in two parallel rows

Watermills were one of the first sources of energy that did not depend on the muscle strength of humans or animals. In the Roman Empire they were used to make flour and sawing stone and wood. As one of the first industrial complexes in European history, the Barbegal watermills are an outstanding example of the development at that time. The mill complex consisted of 16 water wheels in a parallel arrangement of eight wheels each, separated by central buildings and fed by an aqueduct. The upper parts of the complex were destroyed and no traces of the wooden structures have been preserved, which is why the type of mill wheels and how they worked remained a mystery for a long time.

However, carbonate deposits that had formed from the flowing water on the wooden components remained. These were stored in the archaeological museum in Arles and only recently examined in detail. The researchers found an imprint of an unusual, elbow-shaped flume that must have been part of the mill construction. "We combined measurements of the water basins with hydraulic calculations and were able to show that the flume to which this elbow-shaped piece belonged very likely supplied the mill wheels in the lower basins of the complex with water," said Professor Cees Passchier. "The shape of this flume was unknown from other watermills, either from Roman or more recent times. We were therefore puzzled as to why the flume was designed this way and what it was used for."

An elbow-shaped flume as a unique adaptation for the Barbegal mills

At first glance, the team found such a flume unnecessary and even disadvantageous, because it shortens the height from which the water falls onto the mill wheel. "However, our calculations show that the oddly shaped flume is a unique adaptation for the Barbegal mills," explained Passchier. The distribution of the carbonate deposits in the elbow-shaped flume shows that it was inclined slightly backwards against the direction of the current. This created a maximum flow rate in the first, steep leg of the flume, and at the same time the water jet to the mill wheel obtained the correct angle and speed. In the complicated mill system, with small water basins, this unique solution was more efficient than using a traditional, straight water channel. "That shows us the ingenuity of the Roman engineers who built the complex," emphasized Passchier.

"Another discovery was that the wood of the flume was probably cut with a mechanical, water-powered saw, which is possibly the first documented mechanical wood saw -- again evidence of industrial activity in ancient times." The research was carried out by a multidisciplinary team of experts in geology, geochemistry, hydraulics, dendrochronology, and archaeology.

The carbonate deposits that formed on the ancient hydraulic structures are an important tool for the researchers for archaeological reconstructions. In an earlier project, the team led by Professor Cees Passchier was able to show that the flour from the Barbegal mills was probably used to make ship biscuits. "The carbonate deposits give us extremely exciting insights into the skills of Roman technicians at a time that can be seen as the direct predecessor of our civilization," added Passchier, Professor of Tectonic Physics and Structural Geology at the JGU Institute of Geosciences from 1993 to 2019, now Senior Research Professor in Geoarchaeology.