The Thames Barrier, which protects London from tidal flooding, is to carry out its annual tide test closure on September 23.

You can see the iconic example of British design and engineering that protests 1.25 million people and a huge amount of important London infrastructure.

It is built across a 520-metre stretch of river and its annual test closure offers visitors the chance to see how it works and meet the teams who operate it.

Andy Batchelor, Thames Barrier operations manager, said: “The barrier is quite a spectacle and the annual test closure makes an interesting and unique day out for all the family.

"There’ll be an opportunity to meet our staff, listen to talks about the barrier and see information displays. This year our newly refurbished information centre will be open across the weekend and entry will be free of charge.”

During the test closure, the Thames Barrier gates rotate 90 degrees into the fully closed defence position, stopping the tide from flowing into London.

As the tide continues to rise, a higher level of water will build up downstream of the barrier creating different water levels on either side of the gates.

Shortly after high tide, the gates rotate further, to leave a two-metre gap underneath - this is called the underspill position, which is visually spectacular, as it creates a dramatic white-water effect behind each of the gates, which continues for approximately two hours.

Approximate event times and when to see the Thames Barrier in operation:

• 6.45am Barrier gates start to close

• 8am Barrier fully closed

• 9am Public outside event, displays, children's activities, food court and information centre open

• 12noon-1pm Underspill starts; Q & A session on the riverfront.

• 3pm-3.45pm Underspill ends

• 3.30pm Public outside event, displays, children's activities and food court close

• 4pm Barrier starts to re-open, information centre closes.

• 4.25pm Thames Barrier closure completed.