Procurement description
From January 2026, the South East Strategic Reservoir Option (SESRO) project name is changing to White Horse Reservoir, with a strapline of 'a space for nature, a place for people.'
As our reservoir proposals have evolved, so too has the potential legacy it could have. The proposed reservoir is a critical drought insurance policy for the next century and beyond - but it's also being designed as a place for people to use and enjoy for many years to come.
At this exciting stage in development, it's important that we give the project a new identity: one that better reflects the crucial role it will play in shoring up water supply, as well as the broader benefits it will deliver for future generations.
It is the same project, with the same vision, but with a new name that will help people recognise it more easily. For us, the name White Horse Reservoir roots this project in local heritage and in the natural world around us. Equally important as the project's new name, the strapline tells us more about where the project's strengths lie; a space for nature, a place for people.
To stay up to date and find out more, please visit www.thames-sro.co.uk/whitehorsereservoir
Thames Water is leading the development of the new reservoir project, in partnership with Affinity Water and Southern Water.
Thames Water is responsible for delivering the development phase, developing the commercial model for the project and for running the procurement processes to appoint the project's Main Works Contractor (MWC) and later, the appointment of an Infrastructure Provider (IP).
Thames Water is delivering the project under the Specified Infrastructure Project Regulations, or "SIPR", first established and used for the Thames Tideway Tunnel project. Under this approach a new regulated entity, known as an IP will be established to finance and deliver the project, holding a Project Licence granted by Ofwat.
Delivery under SIPR is subject to a notice to specify an infrastructure project, which is currently under development between Thames Water and Ofwat.
Following the approach taken on the Thames Tideway Tunnel project, Thames Water is separately procuring the Main Works Contract and the IP Contract.
This tender notice relates to the procurement of the Main Works Contract only, which comprises the award of a multi-supplier Framework Agreement and a single Call-Off Contract.
The Framework Agreement will be a bespoke framework agreement which is described further in the document associated with this tender notice.
The Call-Off Contract will be based on the NEC4 Engineering and Construction Contract June 2017 (with amendments January 2023) using main option C, secondary options including clause X22 (early contractor involvement ("ECI")), and bespoke amendments, which are described further in the documents associated with this tender notice.
The Call-Off Contract two-stage ECI approach comprises a design and construction planning stage ("Stage One"); and a detailed design and build stage ("Stage Two"). The two-stage ECI Call-Off Contract includes a key decision point that is based on conditions precedent that must be satisfied before a Notice to Proceed to Stage Two can be issued.
The Development Consent Order (DCO) reference design (provided by others) will inform the requirements and constraints within the Call-Off Contract scope (Scope). The MWC will develop the design for construction to comply with the Scope, which will be integrated with the discharge of certain planning requirements "secondary consents" and construction planning for Stage Two.
Tenderers will be required to submit a Target Price to deliver the Call-Off Contract requirements included in the tender documents. As part of the Call-Off Contract, the Pricing Information will set out the circumstances where the tendered Target Price can be adjusted during Stage One ("Adjustment Events"), for example, if the Client changes the Scope. Following a Notice to Proceed to Stage Two, the Target Price (established during Stage One) will be subject to amendment in accordance with the compensation events included in the Call-Off Contract.
The decision gate to proceed to Stage Two will be dependent on a number of matters, including: 1) appointment of the IP; 2) an accepted design (insofar as it has been developed in Stage One) that meets the requirements of the Call-Off Contract; 3) confirmation of the Target Price in accordance with the Pricing Information; and 4) the granting of the DCO.
At the end of Stage One, the Call-Off Contract will be novated to the IP from Thames Water. From the point of novation, the IP will become the MWC's Client and contract counterparty throughout Stage Two, administering the Call-Off Contract and making payments to the MWC accordingly.
Subject to the issue of the Notice to Proceed, Stage Two will commence with an initial period to discharge any remaining secondary consenting issues, followed by mobilisation to commence construction. Following construction, a testing and commissioning phase will be required with a target date for the reservoir to be operational from 2040 (although filling the reservoir with water could take several years). Some construction of visitor facilities, as well as landscaping activities will likely continue until 2043.
The Call-Off Contract involves the design, build, test and commission of a new reservoir with an operational capacity of 150 million cubic metres (Mm³) and water surface area of circa 6.7km2, situated to the South-West of the town of Abingdon in Oxfordshire, which will support long-term drought resilience across the South East of England.
White Horse Reservoir is a proposed fully bunded, non-impounding raw water storage reservoir in the upper catchment of the River Thames.
Water will be abstracted from the River Thames during periods when sufficient river water is available and conveyed to the reservoir via a pumping station and circa 4km of tunnels.
White Horse Reservoir is being developed in full compliance with the Reservoirs Act 1975. A Construction Engineer and Reservoir Advisory Panel have been appointed to oversee the design and construction, ensuring adherence to safety standards and regulatory requirements throughout the project lifecycle. The Construction Engineer appointment will continue through the entire construction phase, through first filling and the initial years of operation. The Reservoir Advisory Panel will provide regular review of the design and construction activities and draw upon best practice from around the world.
The reservoir embankment and associated structures will be designed to national and international standards and guidance documents.
To deliver the reservoir, watercourse diversions and replacement floodplain storage are required.
A conveyance system will be implemented to transport water to and from the River Thames, and to the interface points with the Thames to Southern Transfer (T2ST) and Farmoor Transfer projects.
The reservoir includes passive design provisions for future integration with other strategic water resource and flood alleviation schemes.
Other associated infrastructure forms part of the works, including roads, rail sidings, recreational facilities, utilities diversions, renewables and power supplies, and drainage.
The scope of the Call-Off Contract will include, but not be limited to:
· Design and construction planning.
· Mobilisation and enabling works (incl. utility diversions, site clearance and demolition, environmental and archaeological surveys, utility connections, securing the site boundary, habitat creation and species relocation, geotechnical investigations and various other activities).
· Creation of a temporary rail siding and materials handling area on the Great Western Main Line, required to facilitate the delivery of certain materials by rail freight and therefore reduce the total volume of material imported and exported by road.
· Creation of watercourse diversions, floodplain and drainage, as well as initial works relating to the future restoration of the Wilts and Berks Canal.
· Reservoir earthworks, excavation to create the borrow pit and embankments, Replacement Flood Storage, other components of the landscape scheme and road embankments.
· Tunnelling and pipelines including the reservoir tunnels and river tunnel.
· Road construction (including overbridges).
· Concrete works for the operational structures, such as the reservoir towers and pumping station.
· River Thames intake/outfall structure, requiring a sheet-piled temporary cofferdam in the River Thames.
· Construction of ancillary buildings.
· Mechanical and electrical infrastructure and fit-out.
· Building construction and fit-out.
· Landscaping.
· Land and habitat management.
Note: The total value (estimated) stated in this Tender Notice is estimated at Q2 2025 prices and therefore excludes any allowance for inflation over the life of the contract from this date.