Precisely curved trenches had been gouged into the banks to give the barrier arcs a place to abide during the long spans of time—on the order of decades—during which they were not actually needed. Like dry docks, these trenches lay below the level of the water, but most of the time they were kept empty, leaving the barrier arcs high and dry so that they could be inspected and maintained. The arcs themselves were hollow boxes, normally full of air. Once floated into position across the waterway, they could be flooded so that they would sink and form a seal against the floor of the channel.