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Outlook changeable

Cover story: Weather

Extreme weather events are increasingly frequent and pose new challenges to engineers. On the 50th anniversary of the east coast floods, NCE explores the effects of climate change on civil engineering.

For whatever reason, the Earth's climate is warming up, and the weather is changing. Storm surge defences like the Thames Barrier are being deployed far more often than their designers ever imagined.

In the UK, winters are becoming milder and wetter, summers hotter and dryer. Meteorologists are predicting that full-blown hurricanes could hit Western Europe on a regular basis if sea temperatures continue to rise.

Extreme environmental events of all types are likely to become much more common and their consequences - scour, land slips, rock falls, embankment failure, dam and seawall overtopping and the like - could strike much more frequently.

Global temperatures have risen by about 0.6degreesC in the last century - the fastest rate of increase in 1,000 years - and temperatures are expected to rise by between 1.4degreesC and 5.8degreesC by 2100. If CO 2 emissions are curbed dramatically as the world attempts to control global warming, UK temperatures will rise by 2degreesC. If they continue at their current rate or higher, the UK will become 3.5degreesC warmer. The south and east will warm more than the north and west.

Summer rainfall could decrease by 50% or more by the 2080s and winter rainfall increase by up to 30%. Ground water levels are expected to drop by up to 40% during summer, leaving areas like the south east with an overall water shortfall.

Between 60% and 90% of snow in Scotland will by 2080 fall as rain. Across the UK, average annual rainfall will become between 5% and 20% heavier.

Gale frequencies will increase by up to 30%.

The impact on our coasts is expected to be significant. Sea level was 2m-6m higher than at present during the last interglacial period, 125,000 years ago, but up to 120m lower during the most recent ice age, 20,000 years ago. Sea level rose by up to 1.5mm a year over the last century.

Because the UK is rotating (the north west is rising and the south east sinking), by 2080, sea level in Scotland could be 20mm lower than at present, if global CO 2 emissions are checked fast. If high emissions continue, however, sea level will rise by 580mm in Scotland, regardless of tectonic movement. Sea level will rise by between 260mm and 860mm in London.

For some east coast locations, extreme tides may become 10 to 20 times more frequent.

Thirty one percent of the UK coast has a built up frontage.

26M people, 40% of UK manufacturing industry and 57% of grade one agricultural land are located in potential flood zones.

Engineers building now need to be thinking about the likely effects of climate change on their work 20, 50 and 100 years into the future.

For more information on climate change see www. ukcip. org. uk

Beating the retreat

Rising sea levels and higher rainfall are threatening many of Britain's coastal towns with extinction.

The sea devoured 60m of land in less than three years following the removal of cliff defences protecting the town of Happisburgh, North Norfolk, in 1998. So spectacular has been the sea's rate of advance that the small town has found itself besieged by 'erosion tourists' who come to stare at the collapsing lifeboat ramp and houses teetering on the cliff edge.

Erosion at Happisburgh following removal of the rock wall has happened unusually fast. Elsewhere, soft cliff made of glacial deposits similar to those at Happisburgh is typically eaten back at a rate of between 1m and 1.5m a year.

The town's sea defences were built at the toe of Happisburgh cliff 50 years ago, explains Bristol University lecturer in civil engineering Jim Hall, who is also an advisor on erosion to North Norfolk Council.

But over time, the protected section of coast fell severely out of equilibrium with its unprotected surroundings. Undefended cliffs either side of the town were worn away leaving the protected section as a bulge on the coastline.

Meanwhile, the beach and foreshore that had buffered incoming waves were stripped away - there was no new material from cliff collapses to replenish them. 'This led to very rapid reestablishment of equilibrium when the defences were removed, ' Hall concludes - 50 years' worth of erosion took place in 36 months.

Happisburgh's lost battle against the relentlessly attacking sea is a story that in the coming 20 to 30 years will be repeated many times around the UK, Hall warns.

Of the UK's 11,000km long coastline, 1,1000km is made up of soft cliffs. Of that 1,100km, around 860km is protected.

However, many existing sea defences are Victorian. They have been patched and improved, but most are now on their last legs.

Unless there are large populations or high value industrial installations to protect there is little economic justification for spending the hundreds of millions of pounds it would cost to renew them, Hall believes.

Meanwhile, melting glaciers and polar ice are expected to raise sea levels by about 320mm in the next 50 years, and by anywhere from 420mm to 650mm by the end of the century. Protective beaches will gradually disappear under advancing tide lines. And more water sloshing around in the oceans will produce higher, more powerful tides.

Rising sea levels will accelerate cliff erosion, softening and weakening porous materials. At the same time stormier weather will whip up bigger waves, carrying more abrasive debris with greater force - what is now a one in 50 year storm is predicted to become a one in three year storm by midcentury, and the number of severe weather events will go up from five a year to eight, says Association of British Insurers The rate at which cliffs are undermined will accelerate dramatically, says High Point Rendell geotechnical director Alan Clark.

Cliff erosion will also be affected by rainfall which is expected to be concentrated in the winter months in future, following long, dry, hot summers. In 50 years time average UK rainfall is expected to have increased by between 10% and 20%. In summer groundwater levels will be a good deal lower than at present but in winter they will be very much higher.

Greater pore pressure will dramatically increase the risk of cliff collapse. Cliffs behind many Victorian sea defences are oversteep, says Clark. Though most have not collapsed in the past, saturation caused as groundwater levels rise will trigger landslips, bringing down material from up high. As the century advances, the angle of most soft cliffs will become far gentler.

Rising groundwater levels are also likely to trigger new movement in historic landslides, says Clark. Towns like Scarborough in Yorkshire, Lyme Regis in Dorset and Ventnor on the Isle of Wight have been largely built on landslides thought to be stable.

But these are now accelerating seaward. During prolonged wet spells of three months or more, the rate of movement can speed from 300mm a month to 300mm a week. And these landslips can cover areas up to 20km long, extending 1.5km inland.

It is impossible to stop erosion or landslip completely, and engineers and government alike are facing up to the inevitability that both will become more acute, despite the fact that losses already cost £22bn a year.

Where cliff erosion must be slowed or put off, however, engineers have an armoury of protection techniques at their disposal: Slips can be piled, nailed, arrested with shear keys, buttressed and drained. The toes of cliffs and landslips can be protected with walls and rock armour. Beaches upon which waves break, dissipating energy, can be replenished and fortified with groynes.

Cliff faces can be clad in netting or shotcrete, battered back to a stable angle, anchored, dowelled, or supported with boards. Piled retaining walls can be installed behind the cliff face to provide a line of resistance.

But in the main, people living in vulnerable locations must face up to a harsh truth - that cliff defences have bought them time which is now running out.

Personal effects

In future building owners will be forced to stump up between £4,000 and £5,000 to cover insurance excesses on claims for damage caused by flooding, subsidence or high wind, warns the Association of British Insurers.

Piling specialists could see workloads soar as owners seek to counteract the effects of shrinkage in soils increasingly parched by torrid summers, while the design of new and replacement roofs will need to account for far more frequent extreme wind loading - one in 50 year storms could occur every three years in a couple of decades time.

But flooding remains the number one menace at present. In historic buildings, repeated soaking and drying out will cause masonry and plaster to crack, timbers to warp, and the onset of rot, says English Heritage conservationist Bill Martin.

While river management and flood defence will be important in minimising flooding, building owners must be encouraged not to rush the drying out process when inundation does occur.

Aggressive use of heaters dramatically increases risk of structural damage, Martin warns.

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