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Fens are looking up

FLOOD PROTECTION

Concrete revetments are protecting embankments from flood water erosion on the UK's largest man-made flood storage system, reports Max Soudain.

In 1630 the 4th Earl of Bedford formed 'The Adventurers', a group of 13 noblemen and landowners in the Cambridgeshire Fens who saw the potential of draining this bleak and often flooded part of eastern England to create a huge farming area.

Armed with this vision, the Adventurers hired Dutch embankment engineer Sir Cornelius Vermuyden to transform the landscape into one that could be farmed all year round.

What he eventually created in the Fens is still the UK's largest man-made flood storage reservoir, the 32km long Ouse Washes between Earith and Denver, and some of England's most fertile agricultural land. The system of channels, sluices, pumps and embankments protects towns and villages and 29,000ha of farmland.

Work to upgrade and strengthen the system has continued since the washes were formed. The latest scheme aims to protect the reservoir banks from wave erosion in stored flood water using Armorflex, a concrete revetment system from MMG Civil Engineering Systems.

As a Dutchman Vermuyden was well versed in methods for keeping out the sea and had been knighted for repairing the banks of the River Thames at Dagenham, east London in 1626.

In 1630 his first task in the Fens was to improve the flow of floodwater out to the sea. A 30km long straight channel, the Bedford River, was dug between Earith in the south and Salters Lode. When completed in 1631, it replaced the meandering Bedford Ouse River, shortening the distance to the sea by 16km.

Despite some improvement, flood water discharge was still held back by high tides and further work was needed. But plans were postponed with the outbreak of the English Civil War in 1642.

Parliament ordered dykes to be broken and the land flooded to stop a royalist army advance.

After the war finished in 1649, Vermuyden restarted work.

To counteract the effect of the high tides, he dug a second 30km long channel parallel to and about 1km east of the first.

The New Bedford River (or Hundred Foot River) opened in 1656, creating the Ouse Washes between the two channels. This 'washland' - an area periodically flooded by overflow water from a river, stream or the sea - includes a 1900ha flood storage reservoir bounded on the west by the Middle Level Barrier Bank (MLBB) and on the east by the South Level Barrier Bank (SLBB).

The integrity of the 30km long banks is vital to protect the Fens from flooding. The Environment Agency (EA) says that failure of the SLBB could affect 230 homes, up to 11,000ha of arable land and threaten road and rail infrastructure, with a potential damage bill of £23M. Failure of the MLBB could be even worse, with more than 600 homes and 18,000ha of agricultural land flooded at a cost of £42M. Failures of both banks in 1947 led to flooding of thousands of hectares of farmland.

But maintaining the banks is extremely difficult. The EA says it is a constant battle to keep the Fens free of devastating flood.

The geology is a major problem.

The Fens are underlain by a peat layer that compresses under the weight of the embankments and shrinks because the surrounding area has been drained for agriculture. This, combined with rising sea level, has resulted in a 5m relative fall in land level since the banks were built. There is also the risk of deep-seated slips occurring in the peat.

The MLBB is under prolonged wave attack every time the washes flood. There is fetch distance (the open area over which waves build up and travel) of nearly 1km between the banks and south westerly winds generate significant wave action in the stored water, which has led to serious erosion of the slope face.

By the early 1990s, the MLBB was at a low level and was unstable and cracking in places. The EA spent £14M raising and reprofiling the embankment to create a 3m wide crest using 0.5M. m 3ofimported fill. It also strengthened and raised the SLBB where it was unstable and cracking using 150,000m3 of fill at a cost of £6M.

Haul roads built during the work now form permanent access ways.

The timing of the work was fortuitous, because in Easter 1998 severe flooding again threatened the Fens. The restored banks held firm, holding 60M. m3 of water at 5m above surrounding ground level, says EA project manager Peter Cowie.

But the EA wanted a 'permanent' solution to the erosion problem on the MLBB, in the form of a protection system. Any system had to be environmentally friendly because the Ouse Washes have become an internationally important wildlife sanctuary where birds, plants and invertebrates have benefited from the lack of farming activity.

The area has been designated a site of special scientific interest, it has Ramsar convention status as an internationally important wetland area, it is a special protection area for several bird species and part of the Old Bedford River is a candidate for status as a special area conservation for the spined loach, a freshwater fish.

A number of conservation bodies with a strong interest in the washes include the Royal Society for the Protection of Birds, which has a reserve there, the Wildfowl and Wetland Trust at Welney and the Bedfordshire, Cambridgeshire and Huntingdonshire Wildlife Trust.

Another consideration for the EA is the sheer scale of the task.

The MLBB is 30km long and construction work can only be done between July and September each year, not only because of flooding but to avoid disturbing the birds during the nesting and migration seasons.

After considering a number of options, including planting willows along the toe of the bank (this would have created a habitat for predatory birds and so was rejected), the EA and its consultant Halcrow chose the Armorflex 140, a concrete revetment system.

Funded by the Department for the Environment, Food and Rural Affairs, the first phase of the £2.5M scheme, to protect 17km of the most seriously eroded sections of the MLBB, was carried out in 2001.The EA's Emergency Workforce laid Armorflex along 2km of the bank at the north eastern end of the washes near Welney.

The first stage was to excavate and profile the slope. A 100mm thick bedding layer of drainage sand (1mm to 7mm grain size) was then placed and covered with a woven mono-filament polypropylene geotextile to prevent the sand being washed out. 'The geotextile has a high flow rate, allowing water to flow through it and preventing pore pressure build-up beneath, ' says MMG Civil Engineering Systems' area manager Jonathan Sturt.

The concrete revetment mattress was dry laid on top.

Armorflex comprises strips of cable linked concrete blocks, typically 2.4m wide, although at Welney they were supplied in 1.2m wide, 6m long strips, 1m short of the full slope length, to aid installation. The cables are gathered together at the top of the slope and buried up to 1m down.

These, and a pegged board at the toe, help prevent the revetment mattress sliding downslope.

Angular gravel is then swept over the revetment and down between the blocks. Sturt explains that this, and the double tapered edge on the sides of the blocks, is the critical element of the system, preventing movement of the mattress.

'The cables are more a delivery and installation aid, although of course, if there was a failure, they would help, ' he says. 'But it is the double taper and gravel between that enhances the interlock between the blocks and prevents movement and stops the revetment lifting out from the slope. With a straight-edged block, the gravel would wash out.'

Once the slope is prepared, the system is quick and easy to install. Up to 200 strips were installed each day at Welney, with a long reach hydraulic excavator working from the top of the slope to lift them into place. Sturt says the Armorflex blocks were made in Lincoln to cut down transport costs.

The cellular revetment design means that up to 20% of the protected area is open, allowing water to drain freely and vegetation to establish itself.

At Welney, the mattress was covered with topsoil and seeded with a variety of grass species.

Sturt says that when used for sea defences, the cells are usually filled with gravel, but here vegetation was needed to create an attractive environment and to provide winter grazing for widgeons, as these ducks feed off wetland grass.

The 2002 work programme is being planned ready to start next month and the entire scheme is expected to finish in summer 2005, providing erosion protection for the next 100 years.

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