separate water outlet 404 is provided at the base of the structure 300 in various embodiments lateral passages not shown are provided at least between adjoining cells in the bottom rear row so that a single outlet can drain all of the cells 302 in the barrier 300 with reference to fig5 in some embodiments 500 the cells 302 include passive automatic valves 500 that seal the passages 400 after the cells 302 are filled with water so that deflation of one cell due to a puncture or some other cause will not cause the cells beneath it to deflate in the embodiment 500 of fig5 the valves 502 are flaps of elastic material joined to the upper surfaces of the cells 302 by living hinges 504 a small air bladder 506 is included in the region of the valve 502 that is positioned to cover the passage 400 when the cell 302 is empty gravity causes the valve 502 to fall away from the passage 400 so that the cell 302 can fill with water however once the cell 302 is full of water the air bladder 506 lifts the valve 502 into place and closes the passage 400 once the valves 502 are closed if a cell should develop a leak and deflate only the cells directly above it.