The Council today passed unanimously the following motion, proposed by Gavin McLauchlan, Oxford County Councillor for Benson and Crowmarsh:
“Council notes that in July 2025 a judicial review upheld the Secretary Of State for the Environment’s decision to approve the proposed Thames Water Resources Management Plan which includes the South East Strategic Reservoir Option in Abingdon.
As a result, Thames Water has to have an Emergency Discharge facility in place including the ability to empty the reservoir at a rate of 1 metre per day, possibly over a period of 3 to 4 weeks. This would put water back into the Thames at a rate of 75 m³/s just south of Abingdon c.3x greater than its normal flow.
This Council makes clear its deep concern that we could be made responsible by default for safely managing such an emergency, leaving us with impossible decisions about which lives, homes and businesses to save in a crisis situation.
We therefore call on the Leader to write to the Secretary of State to request clarity on how such an emergency discharge would be managed and to provide a commitment that we will be provided with the resources to do so before any Development Consent Order is considered.”
What is emergency drawdown?
Emergency drawdown is the rapid release of water from a reservoir to lower its level in an emergency and prevent structural failure. Having an installed emergency drawdown is a fundamental safety requirement. It is not an optional feature, therefore it is unacceptable that SESRO does not yet have a definite demonstrated design, even in its present DCO (development consent order) consultation.
How difficult is it to design a system?
The proposed reservoir has vast above-ground scale, and presents an ‘all-round’ threat – the fault could occur anywhere around the 11km embankment. The rules for reservoirs state that the emergency drain has to lower the level by 1 metre in 24 hours. The vast area of this proposed reservoir means that this equates to the ability to safely release 75 cubic metres (75 tons of water) per second. This must be sustained until a safe level is reached, which, in the case of this proposed reservoir, would mean 10-15 days of constant operation.
How do they know it will work in an emergency?
The engineering design will have to be submitted to expert scrutiny, and extensive simulations must prove the principle before the project could proceed. Thames Water’s plans for the reservoir rely on an emergency drawdown system that has fundamental failings: it has not demonstrated, even on paper, that the proposed evacuation valves and siphons and the transmission tunnel to the River Thames at Culham can deliver the required 75 cubic metre per second discharge (the equivalent of emptying an Olympic swimming pool into the river in 33 seconds). Very importantly, once the system is built, it must be regularly tested in a full system in a sustained test. Thames Water’s proposed testing method avoids running the system at full capacity, leaving its real-emergency performance unproven.
Are there dangers in an emergency drain to the Thames?
There are obvious problems one could imagine with the emergency drain procedure. The average flow in the River Thames at Culham is only 17 cubic metres per second. A flow of 75 cubic metres per second corresponds to a river in full spate, on the verge of flooding – conditions that exist on less than one day in ten. Thames Water has not assessed the downstream flood risk of suddenly releasing such volumes into the river. Serious damage to the local river banks and downstream flooding are real risks. Until Thames Water can show by detailed design that the system works, model its impacts on communities, and commits to proper full-flow testing, the safety case for the proposed reservoir remains incomplete and the downstream public are at unassessed risk.
| Reservoirs like the proposed reservoir near Abingdon are not just benign boating lakes. A massive above-ground embanked reservoir of this size (six times the volume of any other embanked reservoir) carries unique risks. The lack of serious design effort on proper reservoir engineering and safety by Thames Water, while devoting its efforts to virtual-reality tours of proposed leisure facilities, is simply unacceptable. |
The OCC passed the motion at its meeting today.
