Reservoir slog
After 30 years of delays and dithering, plans to build a new reservoir in Oxfordshire are finally coming to fruition. Utility Week heads to the site to see how Thames Water is progressing with its plans, despite a last ditch effort from objectors to thwart the project once more.
By Ruth Williams, water correspondent
Damned if you do
After 30 years of delays and dithering, plans to build a new reservoir in Oxfordshire are finally coming to fruition. Utility Week heads to the site to see how Thames Water is progressing with its plans, despite a last ditched effort from objectors to thwart the project once more.
By Ruth Williams, water correspondent
Thames Water has been working on plans to build a reservoir in Oxfordshire since the 1990s. During the three decades since the idea was first floated, it has been endlessly debated and development on plans has repeatedly hit the buffers. Now, however, the project finally looks set to be delivered – save for a last-ditch attempt from objectors to thwart it once more.
With a forecasted deficit of one billion litres of water in the region by the middle of this century, the need for a reservoir now has the full support of government and regulators alike. The Oxfordshire scheme, given the snappy name of the South East Strategic Reservoir Option (SESRO), is one of nine reservoirs in the works across the country.
In fact, with reservoirs firmly back in the picture many are asking why it’s been so long since any were built – 33 years to be precise.
Cost of the scheme
Circumference of the reservoir
Litre capacity of the reservoir
The current picture
The site near Steventon that is earmarked for the country’s second largest reservoir covers approximately 7 square kilometers, bordered to the south by the Great Western Main Line railway.
The £2.2 billion SESRO will be a non-impounding reservoir, which means it will be filled by pumping water from the River Thames over the winter when flowrates are higher to store for use during the drier parts of the year.
Leonie Dubois, Thames Water head of engagement land and consents, explains that while the project is not a new idea, it remains a hot topic in the indusrty. “This reservoir has been talked about for many, many years and hasn’t been built but the need for it is just getting stronger and stronger,” she says.
It is one of several actions that Thames, together with other water companies, is taking to ensure there is sufficient water for the population in the surrounding areas.
“The site is going to be here for hundreds of years,” Dubois says. “So there will be options to add to the site in the future for different needs.”
She explains the timeline of a project of this size may not seem speedy, but there is ample to do. Due to the sheer size of the scheme, government approval must be sought. A Development Consent Order (DCO) application is to be submitted to the Planning Inspectorate in the autumn next year, with the hope that permission will be granted to start on site from 2028 and construction to proceed in earnest by 2029.
“The site is going to be here for hundreds of years. So there will be options to add to the site in the future for different needs.”
Leonie Dubois, head of engagement land and consents, Thames Water
An extensive test site has been dug, which will provide evidence to support its application to the Planning Inspectorate
Clay compact trial
To minimise the traffic to and from the site and the impact of the project, the reservoir embankments will be formed from clay which already exists at the site.
Dubois explains that the reservoir will be built by digging out clay to form the pit of the reservoir, with that clay used to form the embankments. These clay walls will stand between 15 and 25 metres around the 10km circumference of the reservoir.
Trials have taken place around the site to understand the geological make-up of what the teams will be working with. Thames, and its joint venture partner Arup Binnies, has carried out the clay compacting trial to understand what material is available, and how it behaves.
Mark Job, integrated major projects leader at Arup Binnies, explains that every clay is different so the trials are important groundwork. “For a big earthworks project like this, we want to use what’s already on-site rather than bringing trucks in.”
He says that because the reservoir will not be lined, the impermeability of the clay is important and while Kimmeridge clay is pretty good for that, there are variations.
“When you dig it out, it all behaves differently,” he explains. The core must be strong to hold the mass of water but also malleable and without air bubbles. On top of that, Job says, supporting clay will be built up as embankments with different clays needed for the different functions.
The team will find these clay types on-site, he explains. “We can condition the clay to do what we need but the less we have to condition it, the better.”
Finding the right type of strong and natural clays on site is the intention. “Doing this early in the planning stage allows us to do the design and the early stage planning before a full embankment is built,” Job says.
James Barrett, project manager from Thames’ construction partner Costain, explains that each of the compaction trials reaches down 10 metres into the earth, with compaction run at 3 metre intervals. To do this, between 1.5 and 2 metres of superficial clay is lifted, then the bands of clay below that are used for the trial. These lower layers, which are more compact, are what will be used for the embankments.
The testing determines how the clay at different levels will behave; the lower down the team dig the more compacted the earth is and the moisture levels at each layer will be different. Three trials have been carried out to test the clay at shallow, medium and deep levels. There is one large open excavation to understand the geology of the earth as well as boreholes dotted around to find the right kind of clay to form the embankments.
The area comprises Kimmeridge clay, as is found across much of the UK. Within that, Barrett explains the team wants to find compacted, malleable clay with fewer air bubbles to form an impermeable core for the reservoir.
Information on how well the layers of clay perform will feed into the DCO submission process. Reservoirs require a full embankment trial, which can be carried out following the DCO. It is hoped that getting the testing underway early and gathering information about the geology of the site now will pave the way for smoother sailing down the line.
The trials have shown that the clay is slightly harder than the geotechnical engineers were expecting it to be. Through the trials, moisture levels and groundwater are taken into consideration for how the clay performs and how it would react under a variety of conditions and whether moisture conditioning would be required.
“It’s simple technology, we just need to understand the clay,” Barrett says. “It’s very heavy, strong clay layered up in a tried and tested technique.”
Despite so much time having passed since a major reservoir was built in this country, the team is confident that relevant skills and experience are not lacking as these are similar for other groundworks schemes.
Tunnelling will also be a big part of the project, with a 3km tunnel needed to connect to the River Thames, so tunnelling techniques honed on major infrastructure projects such as Crossrail and HS2 will be a big part of the job to get the water in and out.
“It’s simple technology, we just need to understand the clay. It’s very heavy, strong clay layered up in a tried and tested technique.”
James Barrett, project manager, Costain
Leonie Dubois, head of engagement land and consents, Thames Water
Local engagement
Debois explains that the difference between this project and reservoirs developed in the past is the desire to create an asset of value for the community: “We want to embrace people coming and enjoying the asset we are creating that will fundamentally secure their water supply for the future.”
Positioning it as a community asset is integral to the development, Debois says.
One concern raised locally has been around flooding risks. To combat that, water management considerations have been made. A settlement pond is included in the design to act as a sustainable drainage system (SUDS) to soak up groundwater and minimise risks from groundwater flooding.
The Environment Agency will also be required to review the reservoir’s DCO application to ensure that such risks are mitigated.
Last year the first public meeting was held and technical information shared to address the groundwater concerns, Thames said, but they have yet to win over everyone.
The Group Against Reservoir Development is mounting a last-ditch legal challenge with the High Couirt
Legal challenge
Travelling from the nearby train station through the village of Steventon to the site, it is impossible to miss banners hung by residents opposing the development. Its need has long been contested, with campaigners challenging both the chosen location and necessity of the project in general.
Derek Stork, chair of the Group Against Reservoir Development (GARD), says that despite his organisation’s name, they are not against reservoirs per se. “We are not NIMBYs, this is not a NIMBY group,” Stork says.
The group believes the water company should instead be investing in fixing leaks and transferring water rather than creating the new asset. Across its region, Thames Water has plans for a major transfer scheme in recognition that a variety of resources are needed to ensure future resilience.
GARD has been active for many years, its work has included a public inquiry in 2009 which Stork explains concluded that the company’s plans “were deemed to be not fit for purpose”. At that stage, he says, the justification for the reservoir of that size was questioned and the need to properly explore alternatives was voiced.
“They were told to go away and think again,” Stork explains. Now, with mounting pressures for water supply resilience in the face of climate change, the chancellor Rachel Reeves and environment secretary Steve Reed have backed the scheme.
Stork is one of several passionate local campaigners with technical backgrounds and qualifications in GARD. The group has joined forces with parish and district councils, local groups and the Council for the Protection of Rural England (CPRE). Together they have launched a challenge through the High Court on the grounds that Reed gave the project the greenlight without a public inquiry. It has requested that the court adjudicates on this and is awaiting a decision.
It is clear that the dedication of this group will not waiver and the company will have its work cut out to sway the court of public opinion, regardless of the upcoming decision of the High Court.
While the local opposition group remain dogged in its approach, others challenge why no new reservoirs have been built in recent decades. Droughts were declared as early as May this year and a dry summer is forecast to continue putting the stark reality of a changing climate and the urgent need for water resourcing into sharp focus.
