Roman arch bridges were usually semicircular. A number were also segmental arch bridges (such as Alconétar Bridge), meaning it has a curved arch that is less than a semicircle. Such a ssegmental arch bridge allowed great amounts of flood water to pass under it. This would prevent the bridge from being swept away during floods. Also the bridge itself could be more lightweight. Generally, Roman bridges had wedge-shaped primary arch stones (called voussoirs) of the same in size and shape. The Romans built both single spans and lengthy multiple arch aqueducts. Their bridges featured flood openings in the piers.