Flood barriers are employed all over the world to prevent a sudden deluge of water from damaging property and potentially taking lives.
A few sandbags lining the floor of a porch will be an effective defence against a small flash flood, but something a little more robust is required to hold back the mighty swell of a river such as London’s Thames.
In 1953, London and the surrounding area was hit by a flood that claimed 307 lives and caused millions of pounds’ worth of damage. It prompted the creation of the Thames Barrier, an impressive feat of engineering that took eight years to build and cost GBP 534 million (USD 839 million) by the time it was completed in 1982. It stretches across 520 metres (1,706 feet) of the Thames near Woolwich and comprises ten separate gates on pivots supported by concrete piers that house the machinery for operation of the gates.
As tends to be the case with all the best engineering designs, the Thames Barrier is based on a very simple concept. Each gate is radial, using hollow cylinders that rotate the barrier through 90 degrees according to whether they’re in open, flood control or maintenance positions. Four central gates over 20 metres (65 feet) high and weighing 3,300 tons each span the middle section of the barrier, with two smaller gates next to them and four non-navigable radial gates that sit above the river during normal flow.
When a potential storm is forecast to hit the Thames Estuary, the barrier flood defence can be erected in around 90 minutes. Two hydraulic cylinders move rocking beams that shift the gate into the closed position. Here, the Thames Barrier can hold back the downstream tide until it’s level with upstream again and the decision is made that it is safe to be opened once more.
The 1953 North Sea Flood was one of the worst natural disasters to happen in the UK in recorded history. It occurred when a high spring tide combined with a severe storm over the North Sea, sending a swell that was magnified in the narrow estuarine channels along the coast. In places it reached up to 3.2 kilometres (two miles) inland, destroying 24,000 properties, 1,600 kilometres (994 miles) of coastline and flooding 1,000 square kilometres (386 square miles).
The Netherlands – parts of which lie below sea level – was even worse hit: the country’s sea defences buckled under the pressure and killed 1,835 people, 30,000 animals and caused over a billion Dutch guilders (about GBP 360 million/USD 570 million) of damage.
The biggest barrier
The Oosterscheldekering is the largest movable flood barrier in the world. Like the Thames Barrier, this Dutch feat of engineering was built in response to the 1953 North Sea Flood. It’s actually part of 13 dams known as Delta Works and is nine kilometres (5.6 miles) long, four kilometres (2.5 miles) of which include huge sluice-gate doors that are normally open, but can be closed in the event of a tidal surge. It comprises 65 concrete pillars weighing 18,000 tons each, with 62 steel doors, each 42 metres (138 feet) wide. Dutch engineers created an artificial island in the centre of the estuary to facilitate the construction of the structure and, unlike the Thames Barrier, which is estimated to be effective until around 2030, Oosterscheldekering has a predicted lifetime of 200 years.