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Age of reason

Norfolk Broads - A 20-year contract is giving engineers time to develop flood defences best suited to protect the Norfolk Broads, writes John McKenna.

Taking three years to relocate a 300m long, 17m wide soft silt clay embankment may seem rather slow, but that is how they do things in Norfolk, UK.

It is not that engineers for Broadlands Environmental Services (BESL), a joint venture between contractor Nuttall and consultant Halcrow, do not have much to do - they are responsible for the maintenance, repair and upgrade of the 240km of flood defences that protect 30,000ha of farmland and national park, 60% of which lies below sea level. It is just that, thanks to their 20-year public private partnership with the Environment Agency, BESL is able to spend time finding the best possible way of improving the earthen embankments that line the Broads' waterways.

Nuttall and Halcrow won the £100M ($170M) contract in May 2001 and have 12 years to improve defences, then a further eight years to maintain them. At the end of the contract they must hand back the flood defences to the Agency with a working life of a further seven to 10 years.

Their first two years on the project were spent surveying the area, testing soil samples and carrying out emergency repairs on sections of banks in severe distress. Many of the embankments are hundreds of years old and years of underinvestment meant BESL had to leap into action from day one.

'When we started the scheme we carried out condition surveys to see what state the flood defences were in, ' says BESL technical manager and Halcrow principal engineer Kevin Marsh.

'We found the piles in very poor condition, and that we had inherited poor flood bank structures - these were continually deteriorating as we carried out the works and it's an ongoing problem as we try to bring defences up to a set level.' The defined protection level is a one in seven year storm.

To put that in context, the flood defences in New Orleans were designed to withstand a one in 300 year event, while the Thames Barrier has a defence level of one in 1,000 years.

Agency project manager Paul Mitchelmore says that a one in seven year level of protection was cost-effective and, while some overtopping will occur, this is acceptable and manageable.

'We don't mind overtopping as long as the bank is sound. But we don't want breaches. A breach is a catastrophic situation.' The Bure, Yare and Waveney rivers are all tidal, and if defences were breached sea water would flood fields lying behind them, damaging grazing and crops, destroying the natural habitats of local wildlife.

Overtopping is managed, says to Marsh, by knowing where banks are going to overtop and designing them to take it. The main solution to overtopping on this project has been the development of a flood bank set back, where a new flood bank is built 30m-50m behind the original on the outside bend of the river (see box). Marsh describes the set back's function as similar to 'an old-fashioned flood plain', because it allows water to spill over from the river onto a reed covered area before it reaches the new flood bank. Even if the high waters then gush over the new bank, there is a drainage ditch behind it to catch the overflow.

Marsh freely admits that the set back method is, so far, their greatest engineering innovation on the project: 'On the ground the rest of the work we're doing in engineering terms is fairly low key - it's bank strengthening, relocating dykes, earthworks, and moving muck about site.' But the set back solution offers innovation because, although it is more time consuming than piling - each set back can take up to five years - it is more sustainable.

'It's moving away from a canalised system to a much more natural river system, ' says Marsh. 'It's a river system that will manage itself rather than one that man's got to manage.

We're trying to move away from putting in very expensive heavyduty piling and moving back to something much more natural.' Around 45km, or a sixth of the existing flood bank length, will be set back, either because of poor existing pile condition or narrow eroding ronds - the flat reeded area between the river and the flood bank. Each set back takes so long to construct because work on the banks can only be carried out in the summer - the ground is too soft in the winter - and time must be allowed for the earth on the new bank to settle and for the reeds, which act as a buffer, to grow.

The piles that are being removed do not act as a flood defence; they act as erosion protection for the banks, which are subjected not only to tidal wash, but also boat wash on this popular destination for holidays afl oat.

The old flood bank and reeds, therefore, act as the new erosion protection and are, according to BESL project manager Bob Lancastle, far more economical than simply replacing the piling.

'There's about 60km of sheet piles and quite a lot of that's in poor condition. The ratio of doing set back to putting piles in is probably five to one and the reality is that if we had replaced all the piles as we had before, that would have become unaffordable, ' he says.

According to BESL environment manger Jeremy Haus, there are clear environmental benefits too. 'In terms of wildlife habitat it's better to have a nice wide reed margin rather than just a very narrow piled edge, ' he says.

The previous narrow piled edges were also a lot harder to maintain, but with set back, the new banks are profiled with a one in three gradient, allowing the grass and other vegetation that will grow there to be cut and kept under control. And it lets tourists walk along the banks and enjoy a rare elevated view of the Broads.

Moving a flood bank Each flood bank set back on the Broadlands project takes a minimum of three years to complete, dictated by the time it takes for reeds to bed in and grow densely enough to act as part of the flood defence system.

Behind each original flood bank is a ditch from which clay used to build it was excavated.

New flood banks are set back from the existing structures by 30m-50m, and built up from 300mm deep layers of clay. The 18m wide ditches created in building them act as interceptors for water overtopping the bank.

New ood banks are monitored for settlement and built up where necessary. Reeds are planted just in front of the new bank to act as a buffer protecting it from erosion. They are also planted just behind the original piled flood defence.

Reeds require a tidal flow of water to grow properly, so a hole is cut in the piling and a temporary sluice lets water flow from the river to the existing drainage ditch.

In the third year piles are removed from the original flood bank and it is reprofiled. The original bank acts as the first buffer to the river, with second and third lines of defence provided by the dense mat of reeds, new bank and ditch.

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