Barrage 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.

Other systems are effectively similar to dams or solid walls, such as the marina barrage in Singapore, which is currently under construction. This barrage will also create a new reservoir for the city. This option also has the side benefit of creating a new large, solid structure, the surface of which can be used for property development.

The Dublin feasibility study will look at the possibility of three separate barrages in the bay area. These include one between Bull Island and Sutton, and a second between Howth and Portmarnock (see map). A third, between North Bull and South Bull close to Dublin Port, will also be examined as part of the feasibility study.

In Dublin, the likeliest options to be considered are hydraulic barrages for the bay area near Sutton and Howth. The option expected to receive closest examination for Dublin Port may well be the solid version, because of the opportunity to create an entirely new quarter for the city.