Lagan Weir on the River Lagan in Belfast was completed in 1994 and looks like a smaller version of the Thames Barrier. Its purpose, however is the opposite of the Thames Barrier. Whereas the latter is intended to keep rising tides out of central London, in the case of the Lagan the aim is to keep upstream river levels high to improve conditions for wildlife and, to a lesser extend, to improve the view of the river from riparian developments.
Like the Thames example, there is an underwater passage that runs the full width of the river and is used to access the central piers and to route power and other services. There is a small visitor centre known as the Lagan Lookout but the tunnel itself is normally off-limits. Sub Brit was fortunate in being given permission to visit the tunnel during a trip arranged by Chris Rayner in 2016.