Wave goodbye to coastal defences?
Should we give up defending the coast from the sea?
The National Trust is currently finalising plans to abandon miles of coastline, allowing hundreds of properties to be flooded.
One area under careful scrutiny is Mullion Cove, on the edge of the Lizard Peninsula in Cornwall. Owned mostly by the Trust, the small harbour is proving increasingly expensive to protect from the advancing sea. Mullion Cove is one of a number of Trust sites short-listed for abandonment in a recently-completed study.
This is a policy known as managed retreat (or sometimes by the less defeatist slogan 'managed realignment'). It is a response to coastal flooding and erosion that been gaining currency for several years now. In 1998, a controversial report from the Commons agriculture committee criticised the widespread 'deluded belief that we can maintain indefinitely an unbreachable Maginot line of towering sea-walls and flood defences.' (The Guardian, August 06, 1998)
There are 1.14 million properties in the UK classified as at risk from coastal flooding, and abandoning some of them will certainly help to reduce the annual cost of maintaining sea defences. However, questions over the costs of abandonment have yet to be resolved. In this instance, the Trust proposes to find new homes for people living in threatened houses and must consult with farmers about how to adapt to flooding fields. Clearly there are new costs here, but the Trust, having conducted its own cost-benefit analysis, believes this is a better way to spend money than labouring endlessly with sea walls.
A test-case has already been proved successful at Porlock in west Somerset, where nature has been allowed to take its course and agricultural land has been left to flood. Saltmarsh has developed there, with halophytic (salt-loving) species such as eel grass, salicornia and spartina thriving in the new environment which is regularly inundated by high tides. By allowing this natural process of vegetation change known as a halosere to take place, coastal managers are actually nurturing a highly effective natural coastal defence. Incoming waves lose energy as they cross the saltmarsh, thereby helping to safeguard land further inland from marine processes such as hydraulic action and abrasion.
Why is flood risk increasing?
DEFRA (the department for the environment, food and rural affairs) recently estimated that 83,000 commercial properties and 400,000 hectares of agricultural land were at risk of flooding, worth a total of £140 billion. However, estimates of risk keep needing to be revised upwards due as there is now widespread certainty that sea levels are currently rising, particularly in the south of England. For instance, there is plenty of evidence of settlements in the Solent area (near the Isle of Wight) that have been abandoned to the sea over the last two thousand years, with archaeological remains from Roman and Saxon-occupied areas now found well below the high tide mark.
The Hadley Centre (part of the Meteoroogical Office) predicts that sea levels will rise by 24cm over the next 50 years, due to thermal expansion of the oceans and melting glaciers as global warming occurs (these are known as eustatic changes). The problem is exacerbated by the gradual tilting of the UK, due to earth movements associated with the last ice age which ended 10,000 years ago. Following the initial melting of the great mass of ice that covered Scotland and the North of England, land levels there have been rising by several millimetres a year (this is known as isostatic recovery). Unfortunately, this has led to the south being tilted downwards at a similar rate!
Currently, our best defences have been engineered to withstand floods that are thought likely to recur only once every thousand years, according to historical records. However, the combined effect of eustatic and isostatic changes makes it likely that such catastrophic events may occur more frequently in future (they will have a shorter 'return period'). Faced with escalating costs of upgrading defences and uncertain knowledge of future flood levels and their likely recurrence intervals, managed retreat appears an increasingly attractive option for much of our coastlines.
Of course, some areas will always have to be protected if enough people live there or important amenities need to be safe-guarded. Cost-benefit analysis will have to be conducted on a case-by-case basis and some communities will inevitably lose out. In the case of Mullion Cove, some of the residents are upset and cannot understand the proposal, but others accept the inevitable. One resident, John Pascoe, is reported as saying 'the sea is the master, isn't it? In time, one day the walls will crumble and that will be that.' (The Observer, November 09, 2003).