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Long-Term Broads
The Broadland Flood Alleviation Project reaches its tenth year this spring. That is exactly halfway through the 20-year programme to bring this picturesque and environmentally significant East Anglian waterway and wetland region into tip-top condition. It is being carried out not only for the benefit of tourism, farming, fowling and other recreational use, but also for the important plant and animal life which inhabits the region.
Crucial embankments along 240km of the three major rivers and their subsidiary watercourses, which make up the Broads, have long been in need of repair, to protect and preserve the low lying flatlands which cover some 30,000ha along the banks. Much of it is classified as environmentally sensitive and is dotted with nearly 30 scientifically important sites or particular species habitats. All of it is one to two metres below sea level, and kept dry only by a network of drainage channels and dykes, pumped out since medieval times by still visible windmills and today by electric pumps.
But the traditional narrow embankments, some of which are centuries old, have become increasingly inadequate, sheet pile river edge reinforcement installed in the 1960s is dilapidated and distorted, threatening banks behind them, while special drainage dykes are clogged and choked. Villages and the farmland in the zone, including 21,000ha of "grazing marsh", are at risk. A number of inundations have occurred during high tides. There is also an important road route from Norwich to Great Yarmouth between embankments.
In the late 1990s the Environment Agency (EA) was growing increasingly frustrated by the results of piecemeal repairs on sections of the defences and embarked on a scheme to deal with virtually all of it at once. It would cost around £100M and be undertaken as a PFI, allowing money to come in up front from the private sector and be paid back over a long period.
The scheme was let in 2001 to Broadlands Environmental Services (BESL). This joint venture comprises consulting engineer Halcrow and contractor BAM Nuttall and is working closely with the EA client on a long-running series of works which have steadily been dealing with both the physical problems and the environmental consequences of the improvements.
Some 80% of the significant capital works on the scheme have now been completed, primarily strengthening 140km of flood banks, setting back and rebuilding another 44km, removing 13km of old river-bank sheet piling and installing 18.5km of assorted erosion protection systems.
If four-fifths completion seems a little premature for a 20-year scheme it is because the project is really in two parts. First is the upgrading and repair of nearly all the riverside embankments and shorelines, and a number of associated small-scale civil works to provide flood walls and protection to various properties and residential areas. These include several village areas and also individual houses, boatyards and other riverside facilities such as pumping stations.
All is done with detailed attention to environmental factors, particularly looking after animals such as water voles, adders and other reptiles, dragonflies and significant insect species. The project team has to constantly be aware of its impact and make provision for mitigation and transfer of populations. Of the 16-strong Halcrow team on the project, there are eight engineers and eight environmentalists, says BESL project director and Halcrow engineer Kevin Marsh. They share an office in Norwich with BAM Nuttall.
The second phase of the project is to service and maintain the defences, keeping the embankment vegetation in trim, extending the structures where necessary to cope with slow but steady rises in sea level, and providing repairs when and where necessary, including any emergency damage restoration. In the last eight years of the scheme capital work will wind down and this maintenance will become central.
'An interlocking series of service elements defines the project essentially,' explains Marsh. 'Early on we had to maintain the old existing embankments and defences while we carried out a very detailed survey and planned the programme of work for the major replacement programme. That involved aerial and lidar surveys and a detailed hydraulic model.
'Then there is a second maintenance requirement for the finished structures that obviously grows as the works are created.
'We also have a 24-hour emergency response function and are responsible for major emergency repairs under the direction of the EA if there is a need.'
So far there has not been too much call on this side. Even the dramatic storm surges seen in the North Sea in 2007, the worst since the Great Flood of 1953, did not breach any of the new works which overtopped 'gently'.
'Water ran down the back of embankments into floodable areas without any problems,' says Marsh, providing the "proof of the pudding" that the embankments were effective.
Payment is contingent on fulfilment of the various service functions on a mainly monthly basis along with an accumulating monthly payment against capital work. Each main capital project, around half a dozen annually, is agreed with the EA according to a tariff set up in the pre-contract negotiations. The cost is paid with a 70% capital amount on completion, while the rest of the money is distributed across the remaining time period of the project and paid back month by month by the EA.
'It is not a PFI in the traditional sense, more of a term contract system done as a private-public partnership,' says Marsh.
It offers big advantages, he believes, allowing the long-term build up of skills and experience both in design and construction and in relations within the JV and with the client. Works can be overlapped in the programme and scheduled flexibly, with resources redistributed if needed in a way that one-off contracts do not allow.
According to Dan Russell, his counterpart in BAM Nuttall, there has been almost 100% steady use of resources. The local suppliers for plant and materials also gain confidence, he says.
The development of experience is helped in that much of the work is the same throughout, strengthening and restructuring the embankments and reconfiguring the riverside. The overall goal is to eliminate steep embankments close to the water with a gently sloping river edge profile, populated by reed beds to bind the bank.
So-called 'soke dykes' running behind the embankments are cleared and widened, with new reed growth encouraged on a shelf on one side. The sokes are special perimeter water ditches which surround the freshwater field drainage network and prevent intrusion of any saline water.
Where possible, the best solution has been to create new embankments set back up to 50m from the existing bank line. The old riverside bank is cut down somewhat and the space behind it allowed to develop as reed bed, often as an enclosed lagoon space. Reed rhizomes proliferate and no deliberate cultivation has been needed.
If necessary the riverline is cleared of sheet piles using excavators running on the old embankment to pull them out or, in built up or narrower areas, working by crane from a barge. The team has learned to use the old piles for other protection works; the long-term programme allows them to be assigned to other schemes even at the design stage.
Eventually, around a year after the reed growth is established, the old embankment can be removed and the river edge is reprofiled to a 1 in 3 slope into the water, much already covered in reed.
Other areas use the existing embankments which are built up on the back slope with additional material. Behind them the soke dyke channels are reformed further back, partly to provide the fill material, and partly to widen the space behind the embankment to a minimum 9m for future maintenance access.
Different forms of erosion protection are used on the sloping water banks too. The primary method is an asphalt matting which allows the reeds to grow through. Stone-filled gabion mattresses are used in the fast water tidal areas, while upstream, 'soft' methods such as timber or reed faggot bundles are used.
A big advantage of the long-term approach has been in the understanding of the ground behaviour. Most of the area comprises a young weathered clay overlaying soft alluvial clay or sometimes peat and below that 'an even softer clay'.
Ground investigation took time initially. Halcrow developed a rotary mechanism for a standard percussion drill that allows relatively undisturbed core samples of the soft clays to be taken. 'The tubes were sealed with wax and wrapped in bubble wrap laid on air mattresses to go back to the lab,' says Marsh. The elaborate precautions were needed to keep samples of no more than around 20kPa shear strength in good condition, for a good result on settlement tests.
Results have been correlated with CPT records and almost all the testing is now done this way, along with some window samples, saving on bore costs. Experience and an accumulating bank of borehole data held in a Holebase system gives the team the confidence to proceed with designs. Embankments are built with the top clay material in 300mm lifts tracked once or twice with the excavator to compress it. Work crews know almost by feel how much is needed.
Two more years of this work will now complete the protection, after which the big shared office in Norwich will be wound down for the more limited maintenance phase.
Article courtesy of Ground Engineering - february 2011
Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
peter.bishop@bamnuttall.co.uk
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