They can fill buckets with silt or chipped-out calcium deposits left behind from hard water and then haul the buckets out. When faced with a deep valley, Roman engineers should use pressurized pipes that are inverted siphons. Roman water engineers build these rather than arcades because tall arcades are too unstable when built too tall. With siphons, water travels down one side of the valley in watertight pipes. Water pressure forces water up the other side.
Water exits the pipes at nearly the same height as it entered.
The pipes are usually built of lead, which is costly, but the material can handle strong water pressure. When aqueduct engineers have to cross shallow depressions in the landscape, they should build the aqueduct on a wall. Simple to construct, walls are easier to build than arcades, although walls can impede the natural flow of water and people. When engineers need to raise the aqueduct's channel more than approximately five feet above the ground, they should resort to arcades, which allow people and water to move freely beneath them. In a valley, water engineers should use arcades rather than aqueduct trenches.