What Is The Thames Tideway Tunnel?

Sarah Jayne Bell
By Sarah Jayne Bell Last edited 79 months ago

Last Updated 23 November 2017

What Is The Thames Tideway Tunnel?
Lee Tunnel under construction. Photo by Matt Brown in Londonist Flickr pool.

Whatever the problem in London, the engineers' solution is usually a tunnel. That's why the Thames Tideway Tunnel (aka 'super sewer') is London's proposed solution to sewage overflows into the River Thames. It will be a 7.2m diameter, 25km-long tunnel, up to 65m below ground. It will run from Acton to the Abbey Mills Pumping Station in Stratford, mostly under the river. It will collect dilute sewage that currently pollutes the Thames. The Thames Tideway Tunnel will link to the Lee Tunnel, which Londonist visited in 2015.

Proposed route of the Thames Tideway Tunnel.

Why do we need it?

In central London, we have a combined sewer system. Storm water runoff and waste water go into the same sewers. Most of the time this is fine, but during a rainstorm it's a problem. When it rains heavily the sewers fill up quickly, and the dirty water needs to go somewhere. Instead of flooding our streets and bathrooms, the sewers are designed to overflow into the Thames.

There are 57 'combined sewer overflow' (CSO) sites on the Thames. Back in the 19th century, engineers designed the sewers to overflow about 12 times a year. Now it happens about 60 times. CSOs pollute the Thames, and London is in breach of the EU Urban Waste Water Directive.

Combined Sewer Overflow outlet. Photo by Lee Jackson in Londonist Flickr pool.

Why do the sewers overflow so often?

There are two reasons why the sewers overflow more often now than when they were built. One is the increase in 'base flow'. This is the dirty water that flows more or less consistently each day from kitchens and bathrooms. The other is the increase in 'storm flow', which is water entering the sewers when it rains.

Increased population has contributed to increased in 'base flow'. This is the usual justification for the project. The project website says CSOs happen because London's population is double what the sewers were designed for. Londonist thinks this is a bit simplistic.

The sewers in central London do not serve the entire population inside the M25. London has expanded in size, not just population, since the sewers were built in the 1860s. Newer suburbs built in the 20th century have their own sewer systems. They don't all contribute to CSOs in central London.

In fact, much of the increase in 'storm flow' has happened because London has been paved over. In the 19th century, when it rained in London, most of the rainfall soaked into the ground. It had to be a very big storm before water would run into the sewers and overflow. Now, more of our rainfall flows directly into the sewers because we've built over green spaces. That means sewers fill up more quickly and overflow during smaller, more frequent rain events.

How will the super sewer work?

The Tideway Tunnel will intercept sewage from 34 CSOs before it overflows into the Thames. The CSO discharge points will be connected to the tunnel under the Thames. Instead of flowing into the river, the dirty water will be stored in the super sewer. It will then be pumped to the Beckton Sewerage Treatment Works. Once treated, the clean water will be finally released into the river.

Thames Tideway Tunnel intercepting sewer overflows.

Who will pay for it?

The total cost of the project is estimated to be £4.2 billion. Thames Water customers will pay for the tunnel through their water and sewerage bills, and Thames Water customers can expect their bills to increase by up to £80 per year by 2020. That includes customers who don't live in London.

The tunnel itself will be owned by a new company, Thames Tideway Tunnel Ltd. This in turn is owned by an 'infrastructure provider' called Bazalgette Tunnel Ltd. These companies have been formed specifically for the project. Bazalgette Tunnel Ltd will be paid for building, operating and maintaining the super sewer.

The government provides a financial guarantee for the project in case of unanticipated costs and risks. However, there is no base funding for the project from the Treasury.

Protest against super sewer construction site. Photo by Matt Brown in Londonist Flickr pool.

What are people protesting about?

There are two types of objection to the Thames Tideway Tunnel; opposition to the entire project, and specific local objections based on the impact of construction sites. The project received planning approval in September 2014, so it will go ahead regardless.

The project will require 24 construction sites, including 11 sites on the Thames. Any construction site in central London is likely to increase local noise, traffic and pollution. Hammersmith and Fulham council strongly objected to the construction site proposed for Carnwath Road. Locals in Southwark have also protested against the construction site at Chambers Wharf. Now the project has been approved, locals are trying to make sure Thames Tideway Ltd minimise disruption.

Green roofs help reduces sewer overflows. Photo by Andrew Smith from the Londonist Flick pool.

The wider objections to the project question its cost and highlight the benefits of alternative solutions. When the tunnel was first proposed in 2005 it was estimated to cost £1.7 billion. The current estimate is more than three times more. Professor Chris Binnie, who led the original study in 2005, now says the project is a waste of money.

Professor Richard Ashley and past President of the Institution of Civil Engineers Jean Venables are also opposed. They insist that alternative solutions could deliver wider benefits to the environment than the tunnel. Alternatives are based on green infrastructure, such as green roofs, rain gardens and rainwater harvesting. They stop or slow down storm water before it gets into the sewers in the first place. Such systems could also make London healthier for people and wildlife, and save water.

Londonist thinks green infrastructure is a good idea. It will still be a good idea when the tunnel is built. Any water that we stop getting into the sewers is less sewage to pump and treat. Pumping and treating sewage takes a lot of energy. Meeting London's targets for reducing carbon emissions will be easier if we use less energy.

However, getting decent green infrastructure in London requires serious effort; this means stronger policy from central Government and clearer strategy from the Mayor. Boroughs need to insist on more green infrastructure in their planning decisions. Developers need to deliver urban design that is good for London, not just their pockets.

When will it all kick off?

It's already underway. Planning approval was given in 2014 and finance secured in 2015. Construction started in 2016, and tunnelling began in 2017. It should take four years to build the tunnel and the whole project should be completed by 2023.

Want to know more?

Thames Tideway Tunnel Ltd have a slick website explaining the project and providing news on developments. Alternatively, Clean Thames Now and Always and Blue Green UK provide a critique of the tunnel and promote green infrastructure.