The Thames Barrier was destroyed by the Tenth Doctor in DW: The Runaway Bride in order to drown the children of the Racnoss, who were emerging from a hole drilled to the center of the earth by Torchwood using H.C. Clements, using the Thames to stop people from stumbling upon the secret research taking place.
In an alternate timeline, the encounter between the Doctor and the Empress ended differently. Although she was still killed, the Doctor also died, leading to a chain of events that drastically altered Earth history. The Thames remained closed for over a year, resulting in a great number of financial problems for Britain as a whole as well as the locals who had to cross the Thames on a regular basis. (DW: Turn Left)