Water: Why Abingdon’s mega reservoir is making waves

Thames Water are not just the people who flood our rivers with sewage. They’re also planning to submerge a prime chunk of rural Oxfordshire under England’s second largest reservoir.

Could they be any more the pantomime villain? But don’t rush to judgement quite yet. The Thames Valley is short of water, much though this might seem unlikely to anyone who lives on Oxford’s flood-prone Abingdon Road – and the water company argues a mega-reservoir would fix this.

It would be the size of Gatwick Airport and cost £2.2 billion. As with Botley West Solar Farm, which we looked at yesterday, many locals hate the idea. While the proposals make their way through Britain’s labyrinthine planning system, Chancellor Rachel Reeves has already signalled her approval.

It is a watery dilemma worthy of Hans Christian Andersen's The Little Mermaid, with an epic cast of characters and dramatic setting to boot, with Thames Water cast in the role of Ariel. Or possibly Ursula. In this, the latest instalment of the Clarion’s Infrastructure Week, we take liberties with Andersen’s story (and the cartoon dramatisation of it) to explain why the proposed reservoir is making waves. Read on to decide for yourself if Thames Water is the hero or the villain of the piece.

I want to be where the people are

Oxfordshire is about as far away from the sea as you can get in the UK, We have water – sometimes too much of it, as our article on flooding explained. But water served through pipes at the times we want it?

That, claims Thames Water, is harder. They forecast that their customers face a shortfall of over 1 billion litres of water every day in the next 50 years, enough to fill around 400 Olympic sized swimming pools. This, they, say is down to population expansion and climate change, neither of which will come to any surprise to regular Clarion readers. Though not exactly California levels – or the Middle East, where commentators predict lack of water will increasingly become a weapon and driver of conflict – a severe drought could cost London's economy alone as much as £500m a day. Hosepipe bans are already commonplace.

I'll tell you a tale of the bottomless blue

The Abingdon Reservoir aims to address this. (You may also see it referred to as the South East Strategic Reservoir Option, or SESRO.) A long-term proposal for fresh water storage for the Home Counties, at 150 billion litres it is predicted to be the second largest reservoir in the UK. Only Kielder Water in Northumberland, at 200 billion litres, is bigger. (Rutland Water in the East Midlands has a greater surface area but is shallower, coming in at 125 billion. We’ll keep coming back to these two examples.)

The site is 3 miles southwest of Abingdon. It is close to the River Thames, has reasonably flat land, and the right geology and ground conditions for a reservoir. There is reasonable construction access by road and rail. Thames Water's vision for the site is lofty:

Building a reservoir is not just a matter of digging a hole in the ground – indeed, high banks would store much of the water above today’s ground level – but also building the supporting infrastructure. A pumping station; a conveyance tunnel to transfer flows to and from the River Thames near Culham; infrastructure to link the reservoir to the River Thames, for drawdown of the reservoir in case of emergency; an access road into the site; shifting the Steventon–Hanney road to the south; a temporary rail siding so construction materials can come in by freight train; diverting local streams to the east and west; construction of a “compensatory floodplain”… all are on the menu. There is even space reserved for a future Thames-to-Southern Transfer Water Treatment Works, which would enable reservoir water to be piped down to Hampshire, Southern Water’s territory.

Ever since Rudyard Kipling’s parents courted at Rudyard Lake in Staffordshire (yes, he is named after it), reservoirs have been sites of leisure and recreation. That’s true of Kielder and Rutland, Carsington and Vyrnwy, and Thames Water say it would be true of Abingdon too. They promise “public access, parking, recreational facilities, public education facilities, landscape and biodiversity habitat proposals”.

Habitat concerns are greater now than when Kielder opened in 1982. To get approval, Thames Water will have to convince planners that the reservoir, and its imposing earth sides, will be net positive for Oxfordshire’s environment. They promise:

“…embankment earthworks with varied slopes, combined with proposals for woodland belts and copses, hedgerows and pasture, which would help to integrate the reservoir into the surrounding landscape and to mitigate for loss of existing habitats. Watercourse diversions and associated wetlands, and replacement floodplain storage areas are proposed to the west and east of the reservoir to manage flood risk, contribute to biodiversity net gain and ensure Water Framework Directive (WFD) compliance. Extensive and ambitious habitat creation and enhancement is proposed to significantly improve the terrestrial and aquatic habitat available for wildlife locally and deliver biodiversity net gain.”

Which is a long-winded way of saying “we'll take the existing natural environment, destroy it pretty much entirely, and then put back something equivalent or better”.

Taking a leaf out of Kielder and Rutland Water’s book, popular with MTBers and family cyclists respectively, Thames Water also propose to create new “high- quality green space” for walkers, wheelers, cyclists and horse riders. There would be nature trails, a visitor centre, cafés, a water sports centre, recreational lakes and an education centre. And you would be able to reach all this by foot or bike, with new active travel provision including a segregated foot/cycleway along the diverted Steventon to East Hanney road.

I don’t see how a world that makes such wonderful things could be bad

As Thames Water’s expensively produced brochures portray it, this should be a no-brainer – a community amenity, along a beaten path (Kielder Water, Rutland Water), in the context of rapidly increasing consumer need.

But if you live in the shadow of the reservoir banks, you might think differently.

Enter GARD, or Group Against Reservoir Development. Their main concern is flooding. A Guardian article described the reservoir as a “giant flan case full of water dumped in the middle of a marsh”. The reservoir is to be built on arable land, which is normally pretty good at soaking up floodwater: building a reservoir therefore reduces the area of absorbent flood plain.

The area is already prone to flooding. In November 2024, the River Ock burst its banks, flooding the Tesco Extra store in southwest Abingdon and nearby houses. This is happening with increasing frequency as climate change has increased the number of days of exceptional rainfall by 20%: GARD also points to a major flood in 2007, for example. Right now, they say, this flat land at the eastern end of the Upper Thames catchment area is an essential soakaway.

They are also concerned about the design of the reservoir itself, including the extent to which it might leak and flood the area yet more. A petition against the reservoir has just shy of 5,000 signatures.

GARD recognise the water shortage facing the South East, and they propose alternative solutions. They say better options would include reducing leakage from existing pipes to conserve precious water; piping water in from areas with more rainfall (successfully delivered in Kielder Water, and further afield, proposed for Lake Chad); recycling waste water for reuse (a solution widely used in America); and extending desalination of coastal seawater and brackish estuary water (a solution used in the Emirates, where fresh water is at a premium).

Thames Water are sticking to their guns. They say their existing Water Resources Management Plan considers these solutions, with for example a water recycling project in West London:

“We’ve assessed a wide range of options, including tackling leaks, making the best use of our water resources and developing new sources of water including water recycling, regional water transfers and reservoirs. Given the scale of the water resources shortfall, we’ll need a combination of demand reduction and new water sources.”

Darling it’s better, down where it’s wetter

The Abingdon design is rather different from many of its forebears. Most large reservoirs in Britain are ‘impounding reservoirs’, where a valley is dammed to impound a stream of water (though water may also be pumped in from another river).

SESRO, however, would be a ‘fully bunded reservoir’, where artificial embankments are constructed on each side to keep the water in. Opponents say it would be the largest earth-banked reservoir in Europe.

The site for the Abingdon reservoir is on clay. Thames Water propose to extract and store the topsoil for use in future landscaping, dig a deep hole in the clay, and then use the excavated clay to form the embankments. Clay compaction trials have already taken place to explore the geology of the proposed site, and ascertain if the proposed reservoir design could work with the materials that exist naturally on site.

The SESRO site is about 7 metres above its planned connection to the Thames at Culham Reach (although it is actually closer to the sewage works in South Abingdon than to Culham). Most of the water will be stored behind embankments 15 to 25 metres high. Water will be pumped up from the Thames, particularly when flow is high. Then, when needed, it will be released either to a local treatment station, or back into the Thames to make its scenic way to London, where it may be abstracted for treatment and use at several points along the way.

What would I pay to spend a day warm on the sand?

Let’s return to Northumberland and Kielder Water.

Kielder's story started in the mid 19th century. Expanding industries, such as steelworks on Tyneside and Teesside, were demanding more water while domestic water usage was also rising. The answer was the “Kielder scheme” – not just a single reservoir, but a whole water transfer system making use of local rivers like the Tyne, Derwent and Tees. Water released from the reservoir can supply Tyneside and Teesside, more than 80 miles away.

Before Kielder Water came Kielder Forest, the largest man-made woodland in Britain. The coincidence of the two has created a sizeable visitor economy in what would otherwise be a remote part of upland Northumbria. There are water sports, walking and residential stays including luxury accommodation, and a Clear Skies camp site (Oxfordshire councillors, take note?). Ospreys, Scots pine and water voles have been reintroduced. There’s an observatory in the Dark Sky Park. All this brings an estimated £24m to the area each year. It’s an enviable precedent.

But in 1989, the Thatcher Government privatised the UK's water companies. Since then, only one new reservoir has been built – Carsington Reservoir in Derbyshire, in 1992. Does the UK still have the civil engineering skillset to pull off a project of this scale?

Thames Water did… once. The Thames Water Ring Main was completed in 1993 at a cost of about £650m in today's money. This vast project is a system of approximately 50 miles of concrete tunnels, transferring drinking water from water treatment works in the Thames and River Lea catchments for distribution within central London. A ring main, up t0 30m below sea level, connected 11 new pumping stations, three of them entirely underground. Quite some feat. But this was 32 years ago.

Thames Water burnishes its environmental credentials with the London Wetland Centre, opened in 2000. This joint project between the water company, Richmond Council and the Wildfowl & Wetlands Trust transformed four disused concrete reservoirs into what is now a world-renowned nature reserve. Over five years, 300,000 water plants, 8,000 wild flowers and 27,000 trees were planted by hand. Within two years of opening, London Wetland Centre was designated a Site of Special Scientific Interest thanks to a number of wintering shoveler and gadwall ducks. David Attenborough, who opened the reserve, called it “London's extra lung”. But again, this was some 25 years ago.

You're not getting cold fins now, are you?

The reservoir has some staunch opponents beyond GARD. There is little affection for Thames Water in Oxfordshire, even before you mention the reservoir. Much of this is to do with sewage and sharply increasing water bills, but the company’s role in the Botley Road debacle has hardly endeared it to Oxfordshire’s politicians. Councillors and MPs alike are far from convinced that Thames Water, based on its recent performance, can deliver a whelk stand – let alone an engineering project the size of the SESRO, in a flood plain, without flooding local villages.

Didcot & Wantage MP Olly Glover sums up much local opinion:

“There is huge local opposition to this reservoir. The local Liberal Democrats are urging the Environment Secretary to call a public inquiry into their proposals, to ensure transparency and rigorous scrutiny of decision making.

”Climate change and population growth are putting a serious strain on water resources. Thames Water already loses more than 600 million litres of water every day to leaks – a quarter of the water in its network. Water engineers make a compelling case that the £2.2bn cost of building a reservoir would be better spent on tackling water leaks and reducing water demand and waste.

“Given their shoddy record on sewage, leaks, and the dismal state of their finances, residents have no faith in Thames Water to properly use public funds to deliver such a huge and untested project.”

Can Britain build giant infrastructure projects like this any more? Looking at Kielder Water, clearly it once could. But time and sewage have eroded public trust in Thames Water's ability to deliver such a significant project.

Part of your world?

Thames Water has set out a 15-year timeline for delivering the reservoir. (We imagine it set out a timeline for the Botley Road pipe works as well.) It is being opposed at every step by GARD, which successfully saw off similar reservoir proposals first time round, in 2010.

We are currently two notches along the timeline. An informal consultation concluded at the end of August 2024. Its results (yet to be published) will inform the design. Later this year, Thames Water is required to hold a formal consultation on the detailed design, which should at that stage include robust engineering and biodiversity proposals. A formal decision is expected by the Secretary of State in 2028.

Life’s full of tough choices, innit?

55 years ago, Britain’s smallest county was festooned with posters pleading ‘Don’t Flood Rutland’. Just a few years later, Kielder locals (including landed gentry and retired Field Marshals) marched under the banner of the North Tyne Valley Preservation Society to oppose the reservoir plans.

In both cases, opposition continued even after the reservoirs opened. Both were dubbed “white elephants” for years, as water shortages failed to materialise.

Yet today, it is hard to find anyone with a bad word to say about Kielder or Rutland Water. The cycle trails and bird-watching hides are loved by locals young and old. The sharp edges and concrete structures have mellowed. Both are now vital parts of Britain’s infrastructure, supplying drinking water to millions of homes.

Thames Water could be tempted to brazen it out, telling the public inquiry that history proves worries are overstated. This would be a mistake. 2020s Abingdon is neither as remote as 1980s Northumberland nor as under-developed as 1970s Rutland. The bunded SESRO design is more artificial, and arguably riskier, than the natural slopes of the Tyne and Gwash valleys.

Above all, today’s expectations are higher, while confidence in Thames Water could hardly be lower. We have an urgent need for water – but Thames Water has a long way to go to prove it can be trusted to meet it.

Further reading

• Wikipedia summary of the Abingdon Reservoir
• Thames Water documents: project summary, dedicated site, fact sheet on managing flood risk, FAQ
• GARD, Group Against Reservoir Development
• Petition against the reservoir
• Considerations for the structural engineering of a reservoir
• The engineering of Kielder Water (Institute of Civil Engineering)
• Kielder Water Scheme; the last of its kind? An Oxford University paper examining the UK's largest reservoir and how it came to be.
• BBC report on the government approving Thames Water's plan
• Lake or mistake? Guardian article from 2023 examining the proposal
• London Wetland Centre
• Thames Water Ring Main

This is part of the Oxford Clarion’s Infrastructure Week series. Read our introduction to the series – and why it matters.