The Thames Barrier
The Thames Barrier
High water level at London Bridge has risen about two and a half feet per century, due to the melting of the polar ice caps. However, the main cause of flooding in the London area is from surge tides. These come from the North Atlantic, and usually pass to the north of the British Isles. On occasion northerly winds will sometimes force the tides into the North Sea, sending millions of tons of extra water up the Thames. One and a quarter million people are at risk over an area of 45 square miles.
There are a number of defences in the form of moveable gate structures, the largest being the Thames Barrier.
The Thames barrier is a set of ten moveable gates that are positioned across the river at a length of 520m. The four main openings have a span each of 61m and a gate for one of these openings is over 20m high, weighing about 3700 tonnes. Each of these gates is capable of withstanding a load of over 9000 tonnes. Aswell as these four larger gates there are two smaller gates with 31m opening spans and four radial gates adjacent to the river banks.
When the barrier is in the closed position the upper Thames is sealed off from the sea meaning that tides cannot affect the surrounding land.
When not in use the six main gates lie in concrete recesses in the riverbed allowing boats through the openings between the piers. When high tidal surge threatens the gates move up 90 degrees from there resting position and the radial gates are lowered forming a continuous steel wall to stem the tidal flow.
Dangerous conditions can be forecast over 36 hours prior to an event. The decision to close the barrier is based on predicted heights of the incoming tide estimated by the Storm Tide Forecasting Service (STFS), part of the Meteorological Office, aswell as the barriers own computer analysis. Although the gates can be closed within minutes, more time is allowed to reduce the possibility of a reflective wave being created, which itself would have devastating effects.