South East Strategic Reservoir Option (SESRO) – a new reservoir for the south east

We’re proposing a new reservoir which would be located to the south west of Abingdon in Oxfordshire.

Reducing leaks

When river levels drop, or demand for water increases, water would be released from the reservoir back into the river for re-abstraction downstream.

Our proposals are at an early stage and we’re planning, over the next couple of years, to carry out further public consultations and engagement, design and environmental assessments.

As well as providing a resilient water supply for the south east, the reservoir would also provide opportunities to create new habitats and increase biodiversity, as well as provide new leisure and recreation facilities.

The reservoir would provide water to customers in London and the Thames Valley as well as customers served by Affinity Water and Southern Water.

A new reservoir - the South East Strategic Reservoir Option (SESRO)

Reservoir map View full image

Following a public consultation last year on our draft Water Resources Management Plan (draft WRMP), which set out the actions and investment needed to make sure we’ll have a resilient and sustainable water supply for the next 50 years, we published our revised draft WRMP at the end of August. This is currently with the Secretary of State for the Environment, Food and Rural Affairs for a decision on whether to publish in final form.

The proposed new reservoir is a key part of this revised draft plan, along with targets to halve leakage by 2050 and install a further one million smart water meters in customers’ homes.

We’ve published a project update, which you can find here: View SESRO summary brochure.

We held two community information events in November 2023, which were well attended. Thank you to everyone that came.

In February and March 2024, we hosted a series of workshops with stakeholders, including those interested in water, land, environmental and community benefits the reservoir could provide. In these workshops we shared our initial plans for the reservoir and listened to feedback on what local communities want and the challenges they face, so we can shape our plans to reflect these needs. We are now planning our summer consultation which will give the local community an opportunity to have their say on our latest designs and the reservoir benefits.

We are also undertaking surveys and investigations across the proposed reservoir site to support our design development and environmental assessments. You can find out more about this work below.

Ground Investigations

We’re now carrying out our next phase of surveys to inform our ongoing design work.

These surveys started at the end of March 2024 and will run through the course of 2024 and 2025. This includes checking for ecology and carrying out ground investigation works to assess local ground conditions.

Part of our work will include ground investigations on fields within the proposed new reservoir site. This will involve measuring and extracting soil and rock samples from boreholes, small pits and trenches. We will also be undertaking archaeological surveys.

Our ground investigations will start at the end of April 2024, with our first set of investigations throughout May within fields to the north of the Hanney Road, between Steventon and East Hanney.

We’ll share further updates on the progress of our ground investigations through the course of the year.

Clay Compaction Trial

As part of our assessment work, we’re planning to undertake a clay compaction trial. Using land we already own, we plan to create test embankments at a height of up to approximately 3 metres to see how the strength and water content of the local Kimmeridge clay changes when it’s compacted.

The clay compaction trial requires planning consent and in March 2024 we received planning consent for our work from the Vale of White Horse District Council. We are now working on discharging the planning conditions.

We will begin the trial in Summer 2024. It will take between 6 and 12 months to complete the trial, depending on weather conditions. We will confirm the start date nearer the time.

You can find out more about the clay compaction trial and ground investigations here.

Larger reservoir is needed

Why do we need a new reservoir?

Our water resources are under pressure and we need to plan ahead to ensure we can continue to provide a secure water supply while protecting the environment. We’ve considered 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 as well as new water sources.

Working with WRSE, the technical assessments and modelling have shown that a new reservoir will form an integral part of the proposed plan for the south east.

In our draft WRMP we explained that the decision around the size of the reservoir was finely balanced between 100 Mm3 and 150 Mm3. We’ve taken into account feedback from regulators, stakeholders and our customers and completed further modelling work in collaboration with WRSE which has concluded that the larger, 150 Mm3, reservoir is needed.

It’s hard to predict what all our challenges might be over the reservoir’s expected life span (up to 250 years), but a larger reservoir provides a more resilient source of water, and so provides the ideal base of an adaptive plan for an uncertain future.

Our proposals for the new reservoir have been overseen by RAPID, a consortium of water industry regulators. RAPID have implemented a ‘gated’ regulatory process to make sure that all new strategic water supply options are considered in a fair and consistent way, with transparency, and that our customers’ money is spent wisely.

More information about RAPID and the gated process can be found here, where you will also find the technical reports, additional information provided to RAPID and feedback from RAPID relating to the project.

We’ll now develop our reservoir proposals and seek the necessary powers to build it through a Development Consent Order (DCO). Find out more about the DCO process in our factsheets in the Document Library. More information is also available on the Planning Inspectorate website here.