London’s Super Sewer in Numbers: The Tideway Tunnel

London's super sewer captured 18.4 million tonnes of sewage in its first full year of operation, against a pre-tunnel baseline of roughly 39 million tonnes reaching the tidal Thames in an average year. The Thames Tideway Tunnel runs 25 kilometres from Acton to Abbey Mills, intercepts 34 of the capital's most polluting combined sewer overflows, and cost around £5 billion against a 2017 estimate of £4.2 billion.

Those are the headline figures. Several of the others in circulation are either wrong or describe something other than what they appear to describe. This page sets out what is published and sourced, reconciles the numbers that conflict, and separates what has been measured from what was forecast in 2011 and has never been reported on since.

Tideway's own first-year figure carries a caveat the company stated plainly: 2025 brought long periods of unusually dry weather. The tunnel has not yet met the condition it was sized for.

What is London's super sewer?

London's super sewer is the Thames Tideway Tunnel, a storage and transfer tunnel running 25 kilometres beneath the tidal Thames from Acton Storm Tanks in the west to Abbey Mills Pumping Station in the east. It intercepts 34 combined sewer overflows, holds the flow they would otherwise have spilled into the river, and passes it forward to Beckton Sewage Treatment Works for treatment.

The problem it addresses is a Victorian one. Joseph Bazalgette's interceptor sewers, built through the 1860s after the Great Stink, were sized for a city of roughly four million people and deliberately designed to overflow to the river rather than back up into streets and houses. London now holds about nine million. By 2011 as little as two millimetres of rain could trigger a discharge, and the overflows were operating around 60 times a year. Victorian engineers were not the first to build a city drain and discover the city had outgrown it, as Rome found with the Cloaca Maxima.

• What it is: a 25 km storage and transfer tunnel, 7.2 metres in internal diameter, running by gravity beneath the tidal Thames.
• What it intercepts: 34 combined sewer overflows. Twenty-one are taken directly, twelve indirectly, and one through a separate local solution.
• Where it goes: Acton Storm Tanks to Abbey Mills Pumping Station, then on through the Lee Tunnel to Beckton Sewage Treatment Works.
• What it does not do: it does not separate surface water from foul flow, and it does not add capacity at any individual connection point, which is why a scheme in the catchment still needs a foul drainage strategy that evidences capacity.

That last point is the one that matters to anyone building in the capital. A combined sewer carries foul flow and surface water in the same pipe, and it is still the network a new London development connects into — the distinction between foul, surface water and combined sewers decides what a scheme may discharge and where. A tunnel built downstream does not create headroom at a connection upstream of it, so a local planning authority will still expect a drainage strategy for the scheme that evidences where both flows go, at application stage rather than after it.

How much can the Tideway tunnel hold?

The published storage figure is 1.6 million cubic metres, and it is the most widely misquoted number attached to the project. It covers the whole London Tideway Tunnels system, meaning the Thames Tideway Tunnel together with the earlier Lee Tunnel. It is not the Tideway tunnel's capacity on its own.

The Lee Tunnel is a separate 6.9 kilometre tunnel running up to 75 metres deep from Abbey Mills to Beckton. It has been operating since 2016, four years before Tideway's first flows, and was forecast in its own right to capture around 16 million tonnes a year. The frequently repeated comparison with 600 Olympic swimming pools is derived from the 1.6 million cubic metre combined figure, which means it too describes the system rather than the tunnel.

1.6 million cubic metres is the combined Tideway and Lee Tunnel capacity. Any source attributing it to the Thames Tideway Tunnel alone overstates that tunnel's storage.

How long, how deep and how wide is the super sewer?

The tunnel's dimensions are settled on length and diameter and genuinely disputed on depth, where published maxima range from 62 to 70 metres. The variation is not an error in any one source; it reflects whether the figure measures the tunnel invert, the tunnel axis or the base of a shaft.

The gradient is the detail that explains the depth. A tunnel that runs on gravity for 25 kilometres has to keep falling, which is why the eastern end sits more than twice as deep as the western end. Nothing along the tunnel is pumped. The lift happens once, at Beckton. The shaft sites along the route are where that fall was engineered.

What did the super sewer cost, and who pays for it?

The National Audit Office put the project cost at £4.2 billion in its review of March 2017. The out-turn figure now generally quoted is around £5 billion. Both are accurate, because they sit at different points on an estimate that has risen steadily since the scheme was first costed.

• 2004: £1.7 billion.
• 2007: £2.2 billion.
• 2011: £3.6 billion for the Thames Tunnel, plus around £1 billion for the Lee Tunnel and treatment works upgrades.
• March 2017: £4.2 billion, the National Audit Office's stated estimate.
• Out-turn: around £5 billion, the figure generally reported on completion.

Thames Water customers carry the cost through their bills. The National Audit Office recorded £13 a year on the average Thames Water household bill in 2016-17, forecast to peak at between £20 and £25 a year by the early 2020s. Separately, the government provided a contingent support package transferring some risk to the taxpayer, which the National Audit Office assessed as reaching £6.6 billion in its reasonable worst case. Those bills sit inside a wider picture set out in Unda's review of the Thames Water crisis and what it means by area.

On value for money the National Audit Office reported in 2017 that it was “still too early to form a value for money conclusion”, and that the Department forecast benefits of between 1.8 and 3.1 times costs, with those estimates uncertain. Nine years on, no updated assessment has been published.

The bill impact is settled and the benefit case is not. The cost per household was fixed years ago; the environmental return it was supposed to buy has still never been measured against the forecast.

How much sewage has the super sewer captured?

Tideway reported on 27th February 2026 that the tunnel had captured 18.4 million tonnes of sewage in its first year of full operation, the twelve months following the final connection on 14th February 2025. That is the only published operational figure for the system. It measures captured volume, not spills prevented.

The same release recorded long periods of unusually dry weather during 2025, and that context changes how the figure should be read. A storage tunnel captures what the network sends it, and in a dry year the network sends less. Set against a pre-tunnel baseline of about 39 million tonnes a year, 18.4 million tonnes is not a shortfall against design. It is what a dry year looks like.

The corollary is the part nobody has written down: in nineteen months of operation, the tunnel has not yet been tested at the storm condition it was sized for.

Is London's super sewer working?

On the one measure that has been published, yes. On the measures the tunnel was actually designed against, the data has not been reported. The 2011 design case set out four performance targets, and as at September 2026 none of them has a published outcome to test it against.

The Environment Agency publishes Event Duration Monitoring returns for storm overflows annually, and its 2025 data showed 291,492 spills across England, a 35 per cent fall on 2024, with monitored spill duration down 48 per cent. Those are national figures. They reflect a dry year as much as investment, they are not a measure of this tunnel's performance, and the national total does not isolate the 34 overflows Tideway intercepts. Thames Water publishes storm discharge data site by site, which is the closest a reader can currently get. No published analysis pulls the Tideway overflows out of it and sets them against the 2011 forecast. What a spill record does reveal about a network's headroom is covered separately in Unda's explainer on dry weather flow and dry day spills.

Nineteen months of operation have produced one published capture figure and no published spill performance. Any claim that the tunnel has hit or missed its design target is, as at September 2026, unevidenced in either direction.

What happens when the super sewer is full?

It overflows, as it was always designed to. The Thames Tideway Tunnel was never intended to capture every spill; the 2011 case forecast a residual discharge of around 2.6 million cubic metres a year, with the intercepted overflows still operating 3.7 per cent of the time and on a maximum of four days a year at commissioning.

1. Rain falls across a combined sewer catchment, and roofs, roads and paved surfaces send runoff into the same pipes that carry foul flow.
2. Flow exceeds the capacity of Bazalgette's interceptor sewers, at a threshold that in 2011 was as little as two millimetres of rainfall.
3. Rather than spilling to the river, the excess drops through a shaft into the Tideway tunnel.
4. The tunnel stores it and passes it forward through the Lee Tunnel to Beckton, where it is pumped out and treated, under the operating strategy set out in the project's own engineering literature.
5. Once both the storage and the treatment works' pass-forward capacity are exhausted, the intercepted overflows operate as they did before the tunnel was built.

The design forecasts rest on a rainfall record that has since moved. More intense short-duration rainfall raises the peak a combined catchment delivers, and a fixed storage volume absorbs a smaller proportion of a bigger peak. The tunnel's 120-year design life will span a climate materially different from the one its sizing assumed. That is the same pressure now driving the 2025-30 London surface water strategy.

Why was a tunnel built instead of sustainable drainage?

Because a tunnel was assessed as the option that would meet the environmental standards then in force, and because the alternatives were never costed to the same standard. The National Audit Office found that Thames Water's analysis of alternatives was less detailed, and that its costing of alternatives was not independently scrutinised. That is a finding about process rather than a verdict on the alternatives themselves.

The argument for sustainable drainage was made at the time and has not gone away. Separating surface water from the combined network at source, through permeable surfaces, green roofs, rain gardens, swales and attenuation, reduces the peak the sewer has to carry rather than storing it after the fact. That is the case Unda sets out for nature-based drainage in modern planning, and it is an argument about where the water is stopped, not whether it is. London Waterkeeper's position, recorded when the tunnel was completed, was that the tunnel is an improvement but that natural drainage solutions need to be prioritised alongside large-scale engineering.

Policy has since caught up with that view. Policy F8 of the National Planning Policy Framework, as rewritten in August 2026, applies the National Standards for Sustainable Drainage Systems to development through planning policy, and the 2025 rewrite of those standards is what a drainage strategy is now judged against, and the discharge hierarchy those standards set places a combined sewer at the bottom of the list of acceptable destinations for surface water. Discharging surface water to a combined sewer is now the option of last resort, to be justified only when everything above it has been ruled out on evidence.

What the super sewer means for a development in London

Very little changes at the site boundary. The Thames super sewer is downstream infrastructure serving the tidal Thames, not spare capacity at a connection point, and none of the obligations that sit on a London scheme were relaxed when it opened.

• Capacity still has to be evidenced. A water company's confirmation that a connection can be made is not confirmation that treatment capacity exists for it, a distinction set out in Unda's guide to sewage capacity and planning.
• Surface water still comes off the combined sewer wherever the hierarchy allows. Policy F8 and the 2025 National Standards apply to development generally, rather than to major schemes alone.
• Runoff rates are still restricted. A discharge rate agreed with the lead local flood authority is unaffected by storage 60 metres below the river.
• Critical drainage areas still bite. A site in a critical drainage area carries its own evidential burden regardless of what has been built downstream.
• Exceedance still has to be routed. The design event is not the worst event, on a site or in a tunnel, and surface water flooding remains the most widespread flood source in England.

Which water company holds the network, and what it will and will not confirm, shapes all of this — Unda's guide to water companies and the planning process sets out where those decisions actually sit. For a scheme in a combined catchment the practical work is the same as it was before February 2025: establish where foul and surface flows can go, evidence that the receiving network can take them, and design the surface water solution down the hierarchy rather than defaulting to the sewer. The drainage information a planning authority expects has not changed because a tunnel opened downstream. That is the substance of a surface water drainage strategy built to the 2025 standards, and of the foul strategy that sits alongside it.

Unda prepares flood risk assessments and drainage strategies for developments across England and Wales, including sites throughout the Thames Water region and inside London's combined sewer catchments. For a fixed quote on a flood risk assessment for planning or a drainage strategy for a London scheme, or for advice on evidencing capacity before an application goes in, get in touch.

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