Some of you may remember the Kingsway tram tunnel. The bit nearest the river was converted in 1964 to the Kingsway underpass but there remains, to this day, an unused section which comes to the surface just north of Holborn tube station.
During the 70s a friend was part of the team who would be called out in the event of London being threatened by a flood (the barrier opened in 82). His place of work (should it be needed) was a portacabin in that unused part of the tunnel. When I asked him whether that was a good idea he said "it's uphill from the river"!