Thames Tideway Tunnel: Digging deep to avoid the next big stink

Francesca Washtell
Follow Francesca
The Thames Tideway Tunnel project is due to be completed in 2023 (Source: Thames Tideway)

When asked to think of major infrastructure projects taking place in London, the first to come to mind for many will be Crossrail.

But there is another transformational infrastructure development taking place beneath our feet that will arguably change London over the next century more than any single train line ever could and which, for most of us, is going unnoticed. The £4.2bn Thames Tideway Tunnel, dubbed the “super sewer”.

It is a large-scale solution designed to last for 100 years or more to address the problem of London’s continued, and potentially disastrous, sewage overflow problem into the Thames.

The current sewer network was designed in the 1860s after the Great Stink of 1858 by engineer Sir Joseph Bazalgette for a city with a population of two million.

By 2015, London’s population was at eight million and counting, and although the sewage network is in excellent condition, it has been feeling the strain on its maximum capacity.

Read more: We flush out the myths about London's sewers

On average, 39m tonnes of untreated sewage currently enters the tidal River Thames each year as a result of overflows from rainwater.

When London’s current low-level interceptor sewers fill up, the overflow is diverted straight into the river. This is exacerbated by intense rain after a period of dry weather, when more concentrated sewage builds up and is released en masse into the river, destroying wildlife and fish populations in the process.

On the northern bank of the Thames, just west of Blackfriars Bridge, one of these combined sewage overflow (CSO) points discharges untreated sewage into the river around 21 times a year, reaching a total volume of about 520,000 tonnes.

The Tideway project will divert this overflow into the tunnel instead of going into the river, and will work alongside upgrades to five major sewage works and the recently-constructed Lee Tunnel in east London (undertaken by Thames Water).

Read more: EU row sparked after EIB backs London's "super sewer" with £700m loan

Without the Thames Tideway Tunnel, the Lee Tunnel and upgrades to sewage treatment works in London, the amount discharged could reach 70m by 2020.

By the time the tunnel is in place in 2023, the three projects combined will ensure that a maximum of 2.4m tonnes will be released into the river each year, and will be in more dilute concentrations.

The Thames Tideway Tunnel will run for more than 15 miles from Acton in west London to Abbey Mills in the east, while the tunnel itself will be 7.2 metres in diameter.

The project hasn’t been short of investors, attracting BAM Nuttall, Morgan Sindall and Balfour Beatty Group in a joint venture for the west section alone, while Vinci Construction Grands Projets, Costain and Laing O’Rourke Construction are among the groups involved in the central and east sections.

Read more: London super sewer is a "real disaster" says ex-Ofwat boss

Debbie Leach, chief executive of waterway charity Thames 21, told City A.M.: “The Thames Tunnel project is the essential response to an extremely serious environmental problem here in London. Almost weekly, sewage is overflowing from the system and into the Thames, polluting the river and causing serious damage to wildlife, including the wide range of freshwater and marine fish.

“At this time of year, the Thames is an important nursery area for millions of juvenile fish, which are particularly vulnerable to sewage in the river. Every time there is a major overflow of sewage, tens of thousands of fish die, damaging the fragile ecosystem,” Leach added.

However, the tunnel alone is not the end of the sewage saga. As it is rainwater rather than sewage that is the key to the overflow problem, another crucial focus will be to implement sustainable land drainage systems. The risk of flooding in the capital will increase as the population edges towards 10m and more and more green areas are paved over, leaving less permeable land to soak up water.

Former mayor Boris Johnson announced in April that he had set an ambitious target to achieve a 25 per cent reduction in surface water flows into the sewer network by 2040. The most ambitious of these is now set be constructed at the Nine Elms development in Battersea.

Initiatives that remove the burden of surface water from the sewer system will also help to prolong the life of the Thames Tideway Tunnel itself. Otherwise, within decades, London could be forced to launch another “super sewer” project all over again.

Related articles