London Tideway Improvements

Much of London's sewerage system is combined, meaning that foul water mixes with rainwater.

When London's modern-day sewer network was built by Sir Joseph Bazalgette in the mid-19th century, he needed to ensure that his new interceptor sewers could cope with heavy storms. His solution was to install a series of overflow points, known as combined sewer overflows (CSOs), which allowed the excess sewage to escape into the river, rather than backing up and flooding streets and buildings.

Bazalgette's brick sewers are an engineering masterpiece. They have served London well and remain the backbone of the capital's sewer network, but now need new capacity to help the capital cope with modern-day challenges.

The population of the capital is now nearly 8 million and rising, far more than Bazalegette ever imagined his system serving.

In recent years, large parts of the capital have also been concreted over, reducing the availability of green spaces to soak up rainfall before it enters the sewers.

The impact of climate change adds to the urgent need for action to tackle the unacceptable levels of sewage entering the River Thames.

Today, as little as two millimetres of rainfall can lead to sewage entering the River Thames from one or more of the 57 CSOs integrated into Sir Joseph's original network

In an average year, 39m tonnes of sewage is discharged into the river via the CSOs. In wetter years, the figure can increase threefold.

In March 2007, the Government tasked us with developing a tunnel-based solution to this unacceptable and growing problem.

Our solution

Our London Tideway Improvements include two tunnels – the Lee Tunnel and the Thames Tunnel – and a programme to extend the capacity of our five main sewage treatment works (Beckton, Crossness, Mogden, Long Reach and Riverside).

All three elements are key to ensuring that the River Thames complies with the EU Urban Wastewater Treatment Directive.

The Lee and Thames Tunnels will collect the discharges from the most polluting CSOs and transfer them to Beckton Sewage Treatment Works, in east London.

The Lee Tunnel, which will run for approximately seven kilometres (four miles), will capture discharges from the largest CSO at Abbey Mills in Stratford. Initial construction work began in June 2010, and the project is due for completion in 2014.

The proposed Thames Tunnel will run for about 32 kilometres (20 miles) through central London.

Our preferred route, announced in September 2010, is the result of two years of detailed design and planning, aiming to ensure the tunnel meets its objectives while keeping the cost as low as possible and minimising disruption.

It needs to capture untreated sewage that currently discharges into the river from 34 CSOs that the Environment Agency has identified as the most polluting.

We anticipate submitting a planning application in 2012. Construction is provisionally due to start in 2013 and finish in 2020.

A drilling rig was used during our investigations of ground conditions beneath the Thames.

Consulting on our plans

We are committed to a genuine open consultation on our preferred route for the Thames Tunnel and the construction sites we think we need for its construction and operation.

Our 14-week 'phase one' public consultation (September to December 2010) has been designed to give all interested parties a chance to have their say.

We will be seeking feedback on our preferred route and sites, plus the other options we have considered. We will also invite comment on the need for the scheme, the tunnel as the means of meeting the need and our after-use proposals.

We have developed a bespoke website, enabling people to go online to scrutinise our plans and send us feedback. A programme of localised exhibitions (September/October 2010) also gives people a chance to question the project team face to face.

Liz Sale chats to local councillors at one of our stakeholder events.

Rachael Grenz (left) explains our proposed improvements.

We will be prepared to amend our plans in the light of the comments we receive during the consultation. Prior to submitting a planning application, we will undertake a second round of public consultation in 2011, so people can review our revised plans.

Since June 2008 we have staged over 200 briefings and events on the tunnels for interested groups.

In July 2010, 40 senior representatives from the potentially affected local authorities and other pan-London organisations accepted our invitation for a briefing on the River Thames, giving them an opportunity to see some of the CSOs for themselves.

The Thames Tunnel Forum, which we established in March 2009 to review strategic issues with key planning consultees, has continued to meet quarterly, and has seen positive feedback from those attending.