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Tue 03 Mar 2007Holding back the tideBarrage benefits:The aim of a tidal barrier or barrage is to protect low-lying areas from sea-based floods. In cities they are normally located at river estuaries or deltas. Massive engineering structures, they act by allowing fresh water to flow into an estuary, but can block any tidal surge, the normal cause of flooding in low-lying coastal-city areas.Many of the newer versions are based on hydraulic systems, the best known being the Thames flood barrier, where its gates revolve around an axis, allowing water craft to pass through them. There are also versions that can be heightened using hydraulic mechanisms at high-risk periods.