Thames Barrier
When the river is in flood upstream, if the gates are closed shortly after low tide, a huge empty volume holds behind the barrier which can act as a reservoir to hold the floodwater coming over Teddington weir. Most river floods will not fill this volume in the few high tide hours when the barrier needs to be closed. If the barrier was not there, the high tide would fill up this volume instead, and the floodwater could then spill over the river banks in London. About a third of the closures up to 2009 were to prevent flooding caused by heavy rain to the west of London.