Roman aqueducts carried water using gravity and gentle slopes. Their famous arched bridges formed only the sections crossing low ground; much of the route ran at ground level or underground.
Water supplied urban baths, fountains, and other facilities. Maintaining channels, settling impurities, and distributing water were all important parts of the system.
Conduits and gradients
Most Roman aqueducts were flat-bottomed, arch-section conduits, approximately 0.7 m (2.3 ft) wide and 1.5 m (5 ft) high internally, running 0.5 to 1 m beneath the ground surface, with inspection-and-access covers at regular intervals. Conduits above ground level were usually slab-topped. Early conduits were ashlar-built but from around the late Republican era, brick-faced concrete was often used instead. The concrete used for conduit linings was usually waterproof, with a very smooth finish. The flow of water depended on gravity alone. The volume of water transported within the conduit depended on the catchment hydrology – rainfall, absorption, and runoff – the cross section of the conduit, and its gradient; most conduits ran about two-thirds full. The conduit's cross section was also determined by maintenance requirements; workmen must be able to enter and access the whole, with minimal disruption to its fabric.
Bridgework, siphons and tunnels
Some aqueduct conduits were supported across valleys or hollows on multiple piered arches of masonry, brick or concrete, also known as arcades. The Pont du Gard, one of the most impressive surviving examples of a massive masonry multiple-piered conduit, spanned the Gardon river-valley some 48.8 m (160 ft) above the Gardon itself. Where particularly deep or lengthy depressions had to be crossed, inverted siphons could be used, instead of arcades; the conduit fed water into a header tank, which fed it into pipes. The pipes crossed the valley at lower level, supported by a low "venter" bridge, then rose to a receiving tank at a slightly lower elevation. This discharged into another conduit; the overall gradient was maintained. Siphon pipes were usually made of soldered lead, sometimes reinforced by concrete encasements or stone sleeves. Less often, the pipes were stone or ceramic, jointed as male-female and sealed with lead.
Vitruvius describes the construction of siphons and the problems of blockage, blow-outs and venting at their lowest levels, where the pressures were greatest. Nonetheless, siphons were versatile and effective if well-built and well-maintained. A horizontal section of high-pressure siphon tubing in the Aqueduct of the Gier was ramped up on bridgework to clear a navigable river, using nine lead pipes in parallel, cased in concrete. Modern hydraulic engineers use similar techniques to enable sewers and water pipes to cross depressions. At Romano-Gallic Arles, a minor branch of the main aqueduct supplied a local suburb via a lead siphon whose "belly" was laid across a riverbed, eliminating any need for supporting bridgework.
Sources and surveying
Springs were by far the most common sources for aqueduct water; most of Rome's supply came from various springs in the Anio valley and its uplands. Spring water was fed into a stone or concrete springhouse, then entered the aqueduct conduit. Scattered springs would require several branch conduits feeding into a main channel. Some systems drew water from open, purpose-built, dammed reservoirs, such as the two (still in use) that supplied the aqueduct at the provincial city of Emerita Augusta.
The territory over which the aqueduct ran had to be carefully surveyed to ensure the water would flow at a consistent and acceptable rate for the entire distance. Roman engineers used various surveying tools to plot the course of aqueducts across the landscape. They checked horizontal levels with a chorobates, a flatbedded wooden frame some 20 feet long, fitted with both a water level and plumblines. Horizontal courses and angles could be plotted using a groma, a relatively simple apparatus that was eventually displaced by the more sophisticated dioptra, a precursor of the modern theodolite. In Book 8 of his De architectura, Vitruvius describes the need to ensure a constant supply, methods of prospecting, and tests for potable water.