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 earlier this 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.

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, which were well attended. Thank you to everyone that came.

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 November 2023 we submitted a planning application for our work to the Vale of White Horse District Council for their consideration. The application has now been validated by the Council and will be considered by them over the coming weeks.

You can view and make comments on the application directly to the Council via their website

The application reference is P23/V2559/FUL.

If consent is granted, we aim to begin the trial in Spring 2024. It will take between 6 and 12 months to complete the trial, depending on weather conditions.

Archaeological works

As part of the planning application for the clay compaction trial, we need to dig a series of trial trenches in the field we own at Cow Common to help us understand the potential for on-site archaeology. The trenches, which will be up to 0.6metres deep, will be dug by an excavator, and archaeologists will check to see if there is any important archaeology within the trenches.

Our findings will be written up in a report to be issued as an addendum to our planning application for the clay compaction trial.

We had originally planned to undertake the archaeological trial works in early January 2024, but this work has been postponed until ground conditions are better. We now expect these works to take place in February 2024.

On site will be a welfare unit, excavators to dig the trenches, matting and fencing. We expect the works to take between 2 and 4 weeks to complete. Once completed, the trenches will be backfilled and the ground reinstated.

Ground Investigations

From the end of January 2024, we'll also be undertaking ground investigations on land we own on fields located either side of Hanney Road. This will involve measuring and extracting soil and rock samples from boreholes, small pits and one small trench. The samples will help us better understand local ground conditions, supporting our design work. The work will take place over an approximate 4-week period and will involve the temporary erection of a small rig to extract the samples.

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 alongside 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 adaptive 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 have 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.