St Germans pumping station keeps fens flood-free

The 100m3/s capacity pumping station at Wiggenhall St Germans in Norfolk has now been formally opened by Lord James Russell, marking the culmination of this huge construction and flood defence project. Atkins’ John Sheppard explains.

Owned and operated by Middle Level Commissioners, the St Germans pumping station is no ordinary flood risk management pumping station. Designed by consultant Atkins, it is the largest ever constructed in the UK with a capacity 40% greater than the old station it replaces. Fed by 120 miles of main arterial water course and with 80 smaller pumping stations also pushing water into the system, it pumps the land drainage and flood flows from the middle level system into the tidal River Ouse, providing flood protection for 70,000ha of agricultural land – much of which is Grade A – and more than 20,000 properties altogether valued at £3.6B.

Historically, the Great Level Cambridgeshire Fens of East Anglia were wetlands, although they have been drained for hundreds of years. In the 1600s, the Earl of Bedford decided to drain the fens, initially to provide grazing land in the summer and later all year round. Drainage channels were carved into the ground and this gravity system with sluice gates operated until the 1930s when it was no longer effective due to the substantial lowering of the land level – the drainage of water causing the shrinkage of the peat.

In 1934, a pumping station was built at the St Germans site, initially with three pumps and a fourth added in the 1950s. In 1998, however, Cambridgeshire suffered its most significant flooding event for many decades and required all four of those pumps to be called to run at full pelt for 52-hours solid. With no standby in place, had any of these failed there would have been serious consequences and the area would have flooded.

Middle Level Commissioners decided soon after to build this modern facility with greater capacity. Atkins was contracted to design the facility and ECC project manage construction, which began on site in 2006. Teams from across the business have been involved, from planners through to flood modellers, civil engineers and mechanical, electrical and process specialists. Designed with a planned working life of 80-100 years, the Middle Level Commissioners have not taken any chances and the station has been built to cope with the worst possible disaster scenarios. Atkins’ design incorporates six huge variable speed pumps supplied by KSB Bosman. Each can be controlled independently of one another meaning that St Germans pumping station is effectively six pumping stations rolled into one.

The pumps each have a maximum capacity of 16.6m3/s at a static head of 4.25m. The pumps’ 2.1m gunmetal impellers, which are precision-engineered with just 1mm between the impeller and its housing, turn at about 128rpm. This is not particularly fast, but each pump can handle a phenomenal amount of water – about a double-decker bus worth of water every second. When all operate together, the facility is capable of pumping in the region of 100m3/s of water, which is the equivalent of five Olympic swimming pools in two minutes.

That is a considerable volume of water, requiring a special design and size of pump – and a more unusual approach to the construction of the pumping station. The volutes and suction boxes had to be cast in situ with the structure being built around them.

The station spans the river. To build it safely and to prevent disruption to the working of the existing station, the main civil contractor, Costain, constructed a huge cofferdam – big enough to fit Carrow Road stadium, home of nearby Norwich City football club, within – and diverted the river around it.

With the cofferdam in place, the foundations were put in: 469 CFA piles which, if laid end-to-end, span some 6km and ranged from 11-15km in length with diameters between 500-900mm. As the upstream part of the station is higher than the downstream, the 13,500m2 foundation slab was put in place in two halves. The steel superstructure followed, with its south-facing glass elevation housing the main pump hall. With construction complete, the diversion channel was blocked off, the cofferdam removed and transfer of operation from the old station to the new was performed in just four days.

The St Germans pumping station has been the project of a lifetime to work on. It is a beautiful example of the multidisciplinary approach that Atkins takes, covering all the bases from Defra grant application, environmental impact assessment and detailed design to implementation and project management.

John Sheppard is project manager at Atkins

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