This is probably a stupid question, but how does the Thames Flood Barrier work? Not mechanically speaking, but the principle of it. As I understand it, they close the gates when the tide is forecast to be higher than a certain level. OK, so that stops water coming up the estuary towards London, but what happens to all the water pouring down the river in the meantime? Is it just that the volume of water coming downriver is not enough to appreciably raise the river level during the time that the gates are closed, or are there some sort of fancy one-way valves involved?
From Wikipedia: A Thames Barrier flood defence closure is triggered when a combination of high tides forecast in the North Sea and high river flows at the tidal limit at Teddington indicate that water levels would exceed 4.87 m in central London. Forecast sea levels at the mouth of the Thames Estuary are generated by Met Office computers and also by models run on the Thames Barrier’s own forecasting and telemetry computer systems. About 9 hours before the high tide reaches the barrier a flood defence closure begins with messages to stop river traffic, close subsidiary gates and alert other river users. As well as the Thames Barrier, the smaller gates along the Thames Tideway include Barking Barrier, King George V Lock gate, Dartford Barrier and gates at Tilbury Docks and Canvey Island. Once river navigation has been stopped and all subsidiary gates closed, then the Thames Barrier itself can be closed. The smaller gates are closed first, then the main navigable spans in succession. The gates remain closed until the tide downstream of the barrier falls to the same level as the water level upstream.
The volume of water further downstream may be huge, but the river is also much wider and deeper. Enough water beyond the barrier to cause a swell of many feet would only raise estuary levels by a much smaller amount. Google Maps helps give some idea of how the river widens - visualise how the cross-section increases exponentially.
Further point: the barrier is also essentially a cold calculation that while not everywhere can be saved from a 1953-size tidal surge, London can and should be protected. So yes, Westminster would be safe while property and land downstream would revert to its earlier flood plain existence.