Mr Red,
The Thames Barrier (as I understand it)serves the same purpose as the Cardiff Bay Barrage. The Cardiff version was built to prevent future flooding by managing the water levels in the three rivers flowing through Cardiff to the Bristol Channel (The Taff The Ely & The Rhymney)
Previous flooding (described as "once in a hundred years")happened in 1979 & 1960. In each case the cause was an unusual combination of heavy rainfall in South Wales (putting the three rivers in spate), a high tide in the Bristol Channel, and strong onshore winds, raising the tide level & holding back the water in the rivers.
By creating a barrage, the Cardiff Bay marina was formed where a suitable level of water can be maintained. The height of the barrage prevents the high tide from forcing its way into the three rivers when their flow is high, and allows the surplus water from the rivers to be released at low tide into the Bristol Channel.