Its purpose is to protect London from flooding due to surge tides or a general rise in sea level due to changes in weather and the melting of polar ice caps. The design of the barrier allows for ships to pass freely in and out of the docks in London, but in times of dangerously high water, the barrier can be closed with huge steel gates each weighing up to 1,500 tonnes. The piers are founded on solid chalk, over 15m below the level of the river.