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Fenlands defender Flood control is essential to protect the fertile fenlands of East Anglia. Lisa Russell reports on reconstruction of a vital part of the system.

Alot depends on the effective operation of a structure taking shape inside an impressive circular cofferdam near Downham Market in Norfolk. Welmore Lake Sluice is vital to the protection of tens of thousands of hectares of prime agricultural land, hundreds of homes and an important wildlife habitat.

This is where the two major flood threats to the Fens meet - the River Delph and the tidal Hundred Foot River. The sluice serves the dual role of holding back flood waters stored on the Ouse Washes until the worst has passed, and keeping out saline water from the tidal river (see box). The scale of the danger was apparent this spring, when the area between two channels held 80M.m3 of floodwater.

Client the Environment Agency has been carrying out work on projects such as embankment strengthening for a number of years. However, in 1995 a study also identified as a high priority the pounds 5.2M reconstruction of the sluice, funded by the Ministry of Agriculture, Fisheries & Food and the Great Ouse Local Flood Defence Committee.

The old sluice's position encouraged siltation, which prevented the tidal flap gates from opening, so dredging was needed, and the gates themselves had been damaged by debris. The sluice openings were not letting the floodwater escape fast enough, and there were no pumps to evacuate the extra summer floods which have been a feature of recent years.

Lewin, Fryer & Partners was appointed to look at how best to replace the sluice. The new

sluice will do exactly what the old sluice did, explains resident engineer Mike Wakelin, but

better. Being bigger - three gates as opposed to two - it will evacuate the Washes quicker, and won't silt up as badly as it is in a better location and has more effective pumps.

Jackson Civil Engineering's initial civils work is finished, and M&E contractor Waterlink is now on site. The final stage will involve Jackson coming back to finish off work, divert flow through the new structure and demolish the old for completion in July next year.

A big part of the project's cost has been spent on creation of a single skin sheet pile circular cofferdam encompassing the works. Cost of the cofferdam is about pounds 1M, and with a diameter of 46m, it is believed to be the largest of its kind yet built in the country.

Lewin, Fryer & Partners director Graham Fryer had felt from the outset that it would have been difficult to work in a cheaper, square cofferdam, because of all the bracing.

Jackson contracts manager Adrian Judge recalls: 'When we got the drawing at time of tender, we looked at the circular cofferdam and British Steel Piling's handbook and blanched a bit because it was beyond the range.

'So we also looked at rectangular cofferdams, but as had already been deduced by Graham Fryer, the amount of strutting meant that the working space would have been terribly disjointed.'

Two insitu reinforced concrete ring beams were constructed, one about 2.5m from the top and the other about 6m down. A complication was that the loading on the cofferdam was uneven. Part of the circle was bounded by silt and water, the remainder went into the flood bank. 'For a ring beam to work in compression you really want a nice uniform load all the way round,' says Judge.

'We had to take the out of balance load out and we ended up putting in a series of ground anchors in the top and bottom ring beam.' These were purely temporary works, some 50m long, and were installed by Keller Colcrete.

The steel piles themselves are up to 17m long, 10m of which is in the ground. They are a mix of LX20s and LX25s, because some are being re-used in the permanent works. A jetty was built for the pile driving, allowing the 100t crane to reach right round. Piling contractor Fussey Engineering managed the difficult task of completing the circle using standard sections, with the final pile fitting an exact gap.

Once the base had been cast, the lower ring beam could be removed by cutting it into 6t lumps and lifting it out. The top ring beam is still in place.

The permanent structure is founded into the peat and clay on a grid of 357 by 357 steel bearing piles. Design of the sluice is conservative, explains Wakelin, with thick concrete walls and piers. 'The techniques used are well tried, even traditional, methods,' he adds.

The three openings each have two types of gate, all made by M&E contractor Waterlink, which also had the task of designing the transmission system.

Steel vertical lift gates will discharge the floodwater, while pairs of timber mitre gates provide a straightforward method of keeping the tide out, explains Waterlink engineering manager Alan Downham.

Built to a traditional design, hinged at the side, the gates are large, blocking a clear waterway of 7.3m and standing about 7m high. The frames are of the African hardwood ekki, while the planking is of pitch pine. 'The maintenance is very much less than would be needed for a steel structure,' says Downham.

One of the terms of reference from the design stage was to avoid blockages by silt. 'We have devised two systems,' explains Wakelin.

One is that discharge from land drainage pumps is diverted sideways across the faces of the gates, tending to wash silt away.

In addition, a high pressure silt jetting pump is being built in. Each mitre gate will have four nozzles. In attempting to solve this notoriously difficult problem Lewin, Fryer has worked with nozzle manufacturer Delavan, trying different configurations.

'All you need to do is move enough silt so that the gates can open. Once you've done that the rest will go when you let the flood out,' explains Judge.

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