In a major submission to the regulators, Ofwat and RAPID, on their draft Gate 3 decision on the South East Strategic Reservoir Option (SESRO), GARD warns that the proposed Reservoir now represents a serious failure of governance, transparency and public protection.
GARD’s submission to Ofwat/RAPID on the SESRO Gate 3 draft decision (available here) shows that the capital cost of the proposed Reservoir has tripled since Gate 2, rising from around £2.2 billion to over £6.6 billion. The scale of this cost escalation means that the Reservoir can no longer be presented as “best value” for customers or the environment. Ofwat/RAPID recognise the issue and are asking the SESRO promoters to re-evaluate the “best-value” case by the end of this May.
Yet the public has not been told clearly or openly that the economic case has failed. Instead, consultation materials continue to imply that the need for the Reservoir is settled. This is unacceptable for a project that would expose communities to long-term safety, flood and water-quality risks and burden customers with billions of pounds of cost.
We are therefore calling on Ofwat/RAPID and Defra to take control of the process in the public interest by:
- Making it clear in their final decision and with a public declaration that the tripling of costs invalidates the previous cost–benefit analysis and “best-value” conclusions, and must be revisited.
- Requiring a formal, transparent reassessment of need, alternatives and affordability before any Development Consent Order (DCO) is allowed to proceed.
- Ensuring that the DCO timetable reflects the need for life-critical safety, flood and environmental risks to be independently assessed and publicly disclosed before further decisions are taken.
Flood risk
Built on a floodplain, the proposed Reservoir would significantly alter both river and groundwater behaviour. Yet comprehensive river and groundwater flood modelling has still not been completed or published, despite having been raised many times with Thames Water by numerous bodies. Key issues such as groundwater modelling and the feasibility of the proposed perimeter drainage system remain unresolved. Communities are being asked to accept permanent changes to flood risk without seeing the evidence that this can be handled.
Reservoir safety
The consequences of a dam breach or the need for an emergency drawdown have not been openly analysed, with potentially catastrophic implications for downstream communities along the Thames Valley. Independent application of Defra’s breach-assessment methods indicates that failure could result in rapid flooding, major damage extending far beyond the immediate vicinity of the Reservoir, affecting villages, towns, and critical infrastructure, and even loss of life.
Despite this, SESRO’s promoters want to defer full breach modelling and inundation mapping until after the project is approved. Withholding life-critical information from the public and local authorities , while also asking them to accept the construction of one of the largest earth-embankment reservoirs in the country, is indefensible. Regulators have a clear duty to require that these safety risks are independently assessed and publicly disclosed before any consent process is allowed to continue.
Deployable output and drought resilience
The claimed supply benefit of 271 Million litres/day is significantly overstated, according to our expert’s technical analysis. Errors in drought modelling, climate change assumptions, river-flow constraints and inadequate design allowances for unused and emergency storage mean the Reservoir’s true reliable yield during long, severe droughts could be much lower. If the deployable output has been exaggerated, the fundamental justification for the scale – and possibly even the existence – of the Reservoir collapses.
Water quality
Serious unresolved risks to the water quality in SESRO are predicted, by Thames Water’s own reports, from polluted winter inflows, sewage discharges upstream, algal blooms and cyanobacteria. Current modelling does not adequately reflect future climate conditions or the interaction between nutrients, temperature and low-flow operation. Proposed mitigation measures, such as aeration and advanced treatment, have not been demonstrated to be effective at the scale required, raising concerns for public health, operational reliability and downstream river quality.
Crucially, these water-quality constraints are also likely to limit when the Reservoir can be filled at all. The periods when flows in the River Thames are high enough to permit abstraction are increasingly the same periods when pollution from sewage overflows and agricultural runoff is at its worst. If abstraction has to be curtailed to avoid introducing heavily contaminated water, the number of safe Reservoir filling time windows will be reduced, slowing recovery after droughts and further undermining the Reservoir’s claimed deployable output and drought resilience.
Environmental and community harm
The area of land needed for the Reservoir has expanded significantly and the biodiversity value of the site has been considerably underestimated by Thames Water. Ancient and veteran trees, priority habitats and areas now identified for wild habitat recovery by the Oxfordshire Local Nature Recovery Strategy are at risk, while local communities face decades of construction and permanent transformation of their landscape.
| Our message is clear: this is now a matter of public protection. A project of this scale, cost and risk cannot be allowed to proceed on the basis of outdated economics, incomplete safety analysis and opaque decision-making. Regulators have a statutory duty to act in the interests of customers, communities and the environment. They must now intervene decisively, require full transparency, and ensure that no further steps are taken until the properly modelled costs, risks and alternatives have been openly and independently examined. |
