Natural barrier to invasion by the tides: Salt marshes instead of concrete barriers could hold the key to combating the encroachment of farmland caused by rising sea levels. Nicholas Schoon reports

BRITAIN'S salt marshes, which help to defend hundreds of miles of coastline from invasion by the sea, are being squeezed out of existence by rising sea levels.

This is caused not by man-made global warming - which may come into play in the next century - but by the entire nation tilting, with the northern half rising and the South sinking. Britain is still finding a new equilibrium following the end of the last ice age when a vast weight of frozen water was removed from the land.

Until a few centuries ago the salt marshes could have retreated gradually inland, keeping pace with the ocean's slow but remorseless rise. But the low sea walls built behind them, which act as a secondary line of flood defence, have blocked their retreat.

Large areas of salt marsh have now eroded all the way back to the sea walls. Once the thick layer of vegetation has gone the walls alone have to absorb the power of waves and currents at high tide with no help from nature, and they quickly become eroded. They have to be rebuilt, taller, thicker and stronger, because the salt marsh is no longer there to help.

But there is an alternative, now being taken very seriously, of letting the sea carefully on to farmland behind the sea wall. There it creates a new salt marsh which acts as a new line of defence. This would be a strategic surrender to the sea, which is rising as rapidly as five millimetres a year along parts of the South Coast.

It is all part of the new thinking on coping with sea-level rise, which rejects the 'concrete culture' of massive sea walls and groynes. Advocates talk instead of 'managed retreat' and 'soft defences', of learning from nature's ways of keeping sea and earth separate.

At the National Trust's Northey Island on the Blackwater estuary in Essex, the Government's conservation arm, English Nature, and the National Rivers Authority have created the first experimental salt marsh.

The salt marsh in front of a 300 yard (274m) stretch of sea wall had almost completely disappeared and was in need of rebuilding. But just over a year ago, the wall was lowered by several feet to allow one in every seven high tides to flood over.

At the same time a lower and cheaper sea wall without concrete was built on slightly higher land 30 metres (33yds) behind the existing wall. A channel was cut into the old wall to allow most high tides to flood on to the pasture behind.

Now, after one growing season, the grass has been destroyed by the incoming sea. In its place is a healthy crop of the specialised, salt-tolerant plants which make up a salt marsh - like glasswort, sea purslane, sea blight, sea aster. There are the first signs of the network of muddy little creeks which are characteristic of salt marshes.

The new marsh will take over the coastal defence of the one which has been lost. The new sea wall behind it can be low and cheap because it will not have to absorb the force of high tides, waves and currents alone. Richard Leafe, English Nature's coastal geomorphologist, said: 'The marshes are very good at killing the energy in waves.'

The bill for the new sea wall was pounds 22,000 compared with the pounds 55,000 cost of repairing the old one to the required standard.

The Ministry of Agriculture, responsible for coastal defence, is now starting to take seriously the 'managed retreat' of the kind pioneered at Northey.

The main reason is money - rebuilding a sea wall can easily cost over pounds 1m a mile. Turning farmland into salt marsh and building lower sea walls further back should cost less than half as much. The savings should be more than enough to compensate farmers for the loss of land, especially at a time when thousands of acres are being set aside because of crop surpluses.

(Photograph omitted)