I've always associated barrage schemes as being ways of generating electricity. The problem is that they need the tide to flow through the barrages. The barrage in Cardiff Bay was built to create a freshwater lagoon - a permanent artificial lake - and to stop it being tidal. So the barrage is not an electricity generator although it must let water out from the river Taff or else the water levels would rise and rise. However someone has proposed building a proper tidal lagoon outside the barrage and into the Severn Estuary in the area shown below
See
http://www.telegraph.co.uk/news/earth/energy/11445579/Cardiff-tidal-energy-lagoon-could-power-every-home-in-Wales.html The article explains well how such a tidal lagoon generates electricity. There are turbines in holes in the dam wall. Start with the lagoon empty and close the holes whilst the tide is coming in. The water level outside is higher than inside. Just as the water level peaks, open the holes and the water flows into the empty lagoon, generating electricity in the turbines as it goes. When the water levels are the same inside and out, shut the holes again and wait for the tide drop the water level outside the lagoon. Then open the holes again and let the lagoon empty, with the water generating electricity as it flows out.
