Take a deep breath and lift open the flap to smell the River Thames in the hot summer of 1858.” Such invitations are irresistible and the Year 7 girls from Beth Jacob Grammar School in Hendon, duly sniffed and gasped.

They were on a visit to the Thames Barrier at Woolwich, south-east London.

The smell was just one item in the Information and Learning Centre which outlines the visit - explaining how the confluence of sea and river could result in disastrous flooding if the barrier had not been built.

The centrepiece of the exhibition is a working model of the barrier which shows how it works. Between each of the silver piers hang 10 huge metal gates, which normally lie flat on the river bed. But when a flood threatens, the quarter-circle gates pivot round to stop the tide flooding London.

It is important to see the model before starting on the next stage of the visit because pupils may not see the gates being tested as they walk along the river bank.

As we go, Sonia Gill, assistant manager at the learning centre, explains why the 20-year-old barrier is needed more than ever. London is sinking into its soft clay, and the whole southern half of Britain is tilting downwards, and sea-levels are rising as a result of global warming. Moving along the path the children pass a 100m-long map in concrete on the wall of the walkway, showing every bridge and town along the 180 mile-length of the Thames.

After 15 minutes we reached the London side of the barrier. The huge silver bonnets above the piers gleamed in the early summer sunshine. On this balmy day the gates didn’t move, but one was out of the water in its maintenance position and this helped clarify how the system works.

The second half of the visit included participation in the “River of Life” game. A huge plastic floor-map of the river was unrolled on the floor and pupils are divided into groups whose names explain the nature of the game.

Nature Builders populated the river and its banks with ready-made fish and birds and trees. Then along came the Town Builders who wanted to build houses and embank the river to protect their homes. Next were the Industry Builders creating pollution.

The Great Stink of 1858 was caused by the sewage in the Thames. At this time the population of London was 2.5million but there was no proper sewerage system. The invention of the flushing toilet by Thomas Crapper only made the situation worse as the extra water rapidly overflowed cesspits. Cholera outbreaks caused by dirty water also killed tens of thousands.

The solution was provided by the civil engineer Sir Joseph Bazalgette who between 1856 and 1859 designed and built a system of 82 miles of sewers, pumping stations, holding tanks and embankments that is still in use today.

Jo Draper said: “It’s good for pupils to get the same information from a different person to their teacher and in a different environment.”

Schools can book sessions to match their needs but packages include water-sampling (key stage 2 science), rivers and flooding (KS3 geography), and school parties can arrive by boat.

Thames Barrier Information and Learning Centre

1 Unity Way, London SE18 5NJ

Tel: 020 8305 4188

www.environment-agency.gov.ukregionsthames323150335688341764