History : Nov-Dec 2016
62 HISTORY NATIONAL GEOGRAPHIC FROM SOURCE TO CITY Roman aqueducts were an engineering marvel, designed to circumvent any number of natural challenges—from hills and valleys to purification and storage. Roman engineers used gravity to power the water’s flow on its journey. The water traveled down along a gentle, uniform slope moving through hills in underground tunnels and over valleys via soaring arches. Pools to decant impurities and water distribution tanks were built along the route. Without the aqueduct, Rome could not have grown to one million people. Local rivers, wells, and springs would have become polluted or exhausted in such a large city. An aqueduct’s underground channel. This cross section shows the vaulted construction and the coating of the cavity up to the water’s normal level with opus signinum, a mixture of lime mortar, sand, and silica. Trap system. The water flowed from a cistern into a U-shaped pipe. The pressure from the water backed up behind could push it up to a slightly lower point on the other side of the valley. The arches reduced the slope that had to be negotiated. The lead pipes were buried some three feet deep into the hillside. WATER SOURCES The ideal source produced clean water, free of vegetation and silt. In order for the water to be conveyed to a population center, the source had to be located on higher ground. POOLS In the aqueduct’s initial stretch, the water went through a decanting tank. This slowed the flow, so that silt and other impurities in the water would settle at the bottom. MULTISTORY ARCHES One option for crossing a river was to build an arched bridge, usually with two tiers of arches. The channel, or specus, flowed along the top and was usually covered. CASTELLUM AQUAE When the water reached the city, it was collected in a tank, the castellum aquae. Often exquisitely decorated, this tank fed different channels that distributed water through the city. DIAGRAMS: SANTI PÉREZ CHANNELS AND SHAFTS The underground channels were built using shafts (putei) placed at regular intervals. On completion of the channels, they were used as access points for maintenance.