Why is the Thames Tideway Tunnel needed?

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London’s Victorian sewerage system is still in good working order but was built to serve the needs of a much smaller city. The Thames Tideway Tunnel will boost the capacity of the network and stop it overflowing into the River Thames.

Victorian sewer system.

The long history of London’s sewerage network began with local people simply throwing their waste into the nearest stream or small river, which would usually wash it away into the Thames.

As time went on, these waterways were gradually covered over, but by the time of the famous ‘Great Stink’ in 1858, the Thames was so polluted that a completely new solution had to be found.

Sir Joseph Bazalgette designed a system of interceptor sewers under embankments in the middle of London, which would capture sewage from the Fleet, Tyburn and other ‘lost rivers’ before it could reach the Thames. But Bazalgette knew that even the largest sewers he could build would not cope after very heavy rain, so he designed the system with overflow points, known as Combined Sewer Overflows (CSOs), to allow any extra flows to go into the Thames, instead of backing up into streets and homes. 

London’s Victorian sewerage system has been very well maintained, and it’s still in perfect working order. But it was built to serve the needs of a much smaller city, with more green spaces for rainfall to soak away into the ground, and many fewer inhabitants, who bathed, showered and flushed their toilets less.

Since then, London’s population has increased from two million to eight million, and by 2031 it’s expected that there will be 10 million people living in the capital. Whereas Sir Joseph’s sewers overflowed into the Thames only once or twice a year when they were built, for a maximum population of four million people, it’s now happening almost weekly. Quite simply, the system is overloaded.

Today, millions of tonnes of sewage are discharged into the river each year, and regular pollution incidents cause problems for people and the environment. Rowers and other recreational users of the Thames can become ill with stomach infections, and summer storms can lead to the deaths of thousands of fish when sewage causes oxygen levels in the water to fall.

The Thames Tideway Tunnel has been chosen by the UK government, with cross-party support, as the best and most cost-effective way to boost the capacity of a sewerage system which was originally built in the 1850s. All of the suggested alternatives would take longer, cost more, cause more disruption, and fail to clean up the river properly.

When the tunnel is finished, the UK will be complying with international law – and the Thames will be safe for Londoners and wildlife again in the 21st century.