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Saving the South East

Flooding & coastal management - Coastal defences: At Pevensey, East Sussex, the Environment Agency is using the private finance initiative to launch a new approach to coastal flood defence. Andrew Mylius reports.

William the Conqueror landed his invading army on a very different Pevensey shore to that enjoyed by the blue rinse residents of the South East's retirement belt today. His ships would have sailed on a high tide between shingle shoals and islands, and picked their way through the hinterland of inter-tidal salt marsh.

He found terra firma roughly where the ruins of Pevensey Castle lie, nearly 2km inland.

The smooth curve of Pevensey Bay between Eastbourne and Hastings in East Sussex and the lusciously grassy Pevensey Levels, were created through generations of sea defence and reclamation work from the early 13th century. It is those same defences - a 9km long shingle bank fortified with timber groynes - that prevent the tide surging back to inundate 5,000ha of land that is now claimed by housing.

Roads, a rail line and rich grazing pasture are all established on the Levels. So too is such a variety of unusual flora and fauna that Pevensey Levels is a designated Site of Special Scientific Interest and protected by European Union statute.

Ever since the locals started pushing back the sea however, it has been attempting to re-assert itself. Flooding has been a problem for centuries and, though in recent years has been fluvial, not coastal, the risk remains high, says water manager for the Environment Agency's southern region, Ken Allison. 'Three quarters of flood risk in the South East is coastal and tidal, ' he notes.

So far, this winter is the first in five years when the Agency has not been worried that combined high tides, surge and storm conditions would overtop or breach the shingle bank protecting Pevensey. Over time, action of waves and ocean currents has eroded Pevensey's steep beach, while the ageing timber groynes are at the end of their useful lives. In 1999, it was estimated the defence would be overcome by any storm exceeding a 1:20 year severity.

The Agency is also concerned by the long term effects of climate change. At best, sea levels in the South East, which have been creeping up by 3mm to 5mm a year over the last century, will accelerate to 7.5mm a year within the next decade, according to Allison.

By 2050 mean sea levels could be over 0.5m higher. Hence, there is considerable urgency in the work now afoot to beef up Pevensey's fortifications.

Under a £27M, 25 year private finance initiative scheme, height of the shingle embankment, which in places stands no more than 10m tall above the high tide mark, is being increased to 22.5m. Distance from crest to toe will be - give or take - 55m, says Environment Agency project manager, Frank Chester.

Contractor Pentium Coastal Defence, a joint venture between Westminster Dredging, contractors Dean & Dyball and Mackleys, and consulting engineer Mouchel, started on site in June 2000 and must complete the upgrade by December. It is in the process of placing 300,000m 3of new shingle on the beach, sourced at licensed offshore dredge sites, and pushing it into place using bulldozers. Pentium will then be responsible for maintaining the mountainous barrage, finding an additional 25,000m 3a year to replace material lost to the elements.

Once complete, the enhancements should make Pevensey beach impregnable to storms up to a 1:400 year event.

Pevensey Bay is a landmark scheme in two ways. It marks a departure in coastal management strategy from providing hard defences such as concrete or masonry walls and groynes in favour of 'soft' and 'open beach' solutions. And, with a sister scheme, Broadlands, in Norfolk, it exemplifies a new approach to financing and planning coastal management.

Hard defence solutions look robust, says Allison, but are intrinsically flawed. In the process of bouncing back, waves are amplified by a factor of two. As they return, they scour the sea bed and undermine the wall foundations, precipitating collapse.

By contrast, a shingle beach absorbs wave energy, dramatically reducing the ocean's destructive power. It cannot be undermined and is easily maintained, Allison enthuses. Some heavy plant is all that is needed to shore up sections of the bank eaten away by tide and current.

At Pevensey, another strategy is at work. The Environment Agency aims to improve the marine environment as well as safeguard the environment inshore. Groynes are being removed, leaving an uninterrupted sweep of beach. Though more material will be claimed by the sea, a study of coastal regimes shows that it is likely to contribute to accretion at other points along the coast.

Letting a 25 year concession for maintaining Pevensey Bay's flood defence has introduced a hitherto unprecedented degree of certainty over future security.

Coastal flood defence funding for the UK is still allocated annually, making it difficult for the Agency, maritime district councils and unitary authorities to plan ahead. By contrast, funding for the Pevensey PFI has been fixed and underwritten by government for the next 25 years.

Setting performance criteria for the contractor was difficult, says Chester. There was no definition of how extreme a 1:400 year storm in Pevensey Bay is.

The Agency, with the help of hydraulics and maritime specialist HR Wallingford, therefore set out to model conditions using available data. Storm conditions are gauged by collating data from the Proudman Oceanographic Laboratory surge model, Admiralty tide tables, and the Meteorological Office's European wave model - a 1:400 year event will produce wave height of 7m, 20km offshore.

Bidding for the project, Pentium paid close attention to prevailing ocean conditions to determine the rate at which shingle would be lost. The contractor also insisted on measuring the volume of shingle already on Pevensey beach to +/-5,000m 3: Pentium carries the cost of replacing material lost due to all conditions up to a 1:50 year event. Above 1:50, the contractor splits costs with the Agency, which shoulders risk alone for the cost of repairs following extreme weather of greater than 1:400 magnitude.

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