The results of an independent investigation commissioned by the Environment Agency to look at how water entered the Foss Barrier building on Boxing Day 2015 says the event has identified a lack of resilience in the asset’s performance.

The Environment Agency commissioned the independent report from CH2M in February 2016 to look at how water got into the pumping station building and to recommend remedial actions to minimise the risk of water entering the building in the future.

On 26 December 2015 over 600 properties along the River Foss in York flooded. The Environment Agency operates the Foss Barrier and pumping station which is designed to prevent flooding by stopping the River Ouse (when in flood) backing up the River Foss. When the barrier gate is closed the River Foss is pumped around the gate with 8 high capacity pumps.

The pumping station was unable to cope with the extremely high flows coming down the River Foss and Environment Agency staff turned off the 8 pumps and lifted the barrier between the Rivers Foss and Ouse. The aim was to prevent even more serious flooding - had the gate remained in the lowered position, water would have built up behind the gate causing the water levels in the Foss to be higher, putting more properties at risk of flooding.

The extreme nature of the Boxing Day floods also led to water leaking into the Foss Barrier control room, putting the power supply to the building, and the ability to raise the barrier, at risk.

The exceptionally high water levels in the River Foss on 26 December 2015 exceeded the capacity of all 8 pumps. The report states that had the barrier not been raised, then the levels in the River Foss would have continued to rise until it spilled over the top of the barrier gate. Without raising the barrier the report concludes that peak water level on the River Foss would have occurred some 18 hours earlier.

Leakage via a construction joint and open access cover two main routes to water ingress

In addition to exceeding the capacity of the pumps, the extreme nature of the flood caused water to enter the building’s service tunnel drainage system. The report found that leakage into the building through a construction joint and through an access cover, opened to install a mobile pump, were the 2 main routes of water entering the building.

On the morning of 26th December 2015 the Foss Barrier was in the lowered position and all pumps at the Barrier were operating to pump water from the River Foss into the River Ouse. This had been the situation for the previous five days, a planned operational response to high water levels in the River Ouse.

Within the pumping station, power, control cables and drainage pipes are routed beneath the building through a service tunnel. The service tunnel extends beyond the building, providing a route for services to the barrier structure from the building. The investigation found that at the junction where the service tunnel meets the barrier structure a difference in settlement has opened up the construction joint, providing a route for water to leak into the service tunnel.

Th e report says that understanding of the mechanism by which water entered the Foss Barrier Pumping Station on 26th December 2015 has informed the identification of remedial steps to minimise the risk of water entering the pumping station in the future. These include:

• Access openings between the building and the service tunnel should be sealed so that when water enters the service tunnel it cannot rise and flood the building.
• The drainage could be configured in a way which eliminates the need to pass through the perimeter of the building below flood level i.e. by having a small pumping system to pump it up and over the flood risk level.

Even after adopting these remedial steps, the report concludes that there will “always be a residual risk of water entering the building because it is located below extreme river levels” – which could be eliminated by relocating water sensitive equipment above the flood risk level.

There is no evidence that the cause of water entering the building at the Foss Barrier is a result of anything other than the leakage into the service tunnel and flood water entering the service tunnel drainage, the report says.

Event has identified a lack of resilience in the asset’s performance

The report states:

“The event has identified a lack of resilience in the asset’s performance. There is a risk that without undertaking the remedial steps a repeat of the event would compromise the facility. Remedial steps should focus on operations and maintenance as well as structural measures.”

The Foss Barrier has been fully operational since the end of December. The Environment Agency said the remedial actions recommended in the report to minimise the future risk of water entering the building are all being included in the £17 million upgrade which is currently underway.

This includes addressing water leaks in the service tunnel and new pumps with an increased capacity which will be installed by winter 2016. By the end of 2017 the pumping station will have been raised to ensure the barrier is more resilient in the long term.

Agency is taking a catchment wide approach to improve flood resilience in York

Commenting on the report’s findings, Sir James Bevan, Chief Executive of the Environment Agency, said:

“I welcome this report, which will help us make the Foss Barrier even more resilient in future to extreme floods and greater flows on the River Foss. The upgrade work, which began in April, will see the pump capacity increase, providing a higher standard of protection for local people.”

“Work on the Foss Barrier is just one component of a wider programme of work, following the Government’s announcement of £45 million for York. We are taking a catchment wide approach to improve flood resilience in the city, looking at how we can slow the flow in the upper reaches of the Ouse and Foss catchments as well as new flood defences.”

Residents and other interested parties will have the opportunity to find out more about the package of work the Environment Agency is planning for York, the upgrade of the Foss Barrier and the independent report, at a public exhibition to be held on 20-21 May at the Hotel 53, Piccadilly, York.

Click here to download the report in full Foss Barrier and Pumping Station - factual report on flooding on 26th December 2015

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