The Effect of Air Bubbles in Ancient Long-Distance Pipelines

Abstract

Air–water interaction in certain types of gravity-flow water pipelines, so-called inverted siphons, has the potential to disturb the flow of water significantly. If such a pipeline is laid across high points in the landscape, air bubbles can, in extreme cases, render the pipeline useless. The mechanics of air–water interaction in pipes is very complex and represents an engineering challenge even today. In antiquity, the builders of a number of aqueducts chose, nonetheless, to lay the pipelines across such high points, running the risk of air bubbles disrupting the flow of water. This article illustrates the precise phenomenon by which large air bubbles become enclosed in pipelines with intermediate high points, quantifies the problem of air bubbles for the pipelines at Pergamon, Smyrna, Alatri, and Lyon, suggests a procedure by which the ancient technicians could have identified and solved the problems associated with air pockets in the line, and points to archaeological evidence that they may, indeed, have done so.