Engineering with water and gravity
The aqueduct that supplied the Roman city of Glanum was constructed in the 1st century BC. It carried water from a dam in the Alpilles mountains, channeling it over a distance of roughly 10 kilometers to the city's fountains and public baths. The system was built with meticulous planning, designed to use gravity alone to maintain a constant flow. Roman engineers aimed for an average gradient between 0.15% and 0.30%, a drop of 1.5 to 3 meters for every kilometer of distance. The gentle slope was fast enough to prevent stagnation but slow enough to avoid eroding the channel.
To achieve this, surveyors used a specialized instrument called a chorobates. This was a 6-meter (about 20-foot) wooden beam with legs, equipped with plumb lines for leveling. For windy conditions, it also had a carved channel on top; if water poured into the channel sat level, the instrument was horizontal. By taking repeated measurements with the chorobates and the groma, a tool for surveying straight lines, engineers could plot a course that maintained the necessary gradient with remarkable accuracy. Much of the aqueduct was not an elevated arcade, but a covered channel, or specus, set about 0.5 to 1.0 meter below the ground or carved directly into the limestone hills.
A channel built to last
The main channel of the Glanum aqueduct was sized for maintenance, large enough for a person to enter for cleaning and repairs. The builders used a mixture of local stone, brick, and a form of volcanic cement known as pozzolana. To ensure the channel was waterproof, it was lined with a special hydraulic mortar called opus signinum. This material, a mix of lime, sand, and crushed terracotta, gave the interior a distinctive pinkish hue and prevented leaks.
The water source was the Glanum Dam, also known as the Vallon de Baume dam, a Roman arch dam built in the 1st century BC. This structure, the earliest known of its kind, collected water that fed directly into the aqueduct system. The city of Glanum flourished with this reliable water supply for centuries. After the city was destroyed around 260 AD, the aqueduct fell into disuse. The remains of the Roman dam were eventually destroyed in 1891 during the construction of a modern replacement, the Lac de Peirou. Parts of the aqueduct's path can still be traced today, integrated into the landscape surrounding the main archaeological site.