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.