The Environment Agency commissioned an independent investigation by CH2M to look at how water entered the Foss Barrier building on Boxing Day 2015.
Ref: LIT 10462 PDF, 1.81MB, 26 pages
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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. On 26 December 2015, 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. This was to prevent even more serious flooding as, 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 report was commissioned 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.
The exceptionally high water levels in the River Foss on 26 December 2015 exceeded the capacity of all eight 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 peak water level on the River Foss would have occurred some 18 hours earlier.
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.
The report recommends remedial actions to minimise the future risk of water entering the building all of which the Environment Agency is including as part of the upgrade work currently underway.