̽��ֱ�� of Cambridge - storms

Salt marsh plants key to reducing coastal erosion and flooding

sc604 — Thu, 02 Oct 2014 10:07:00 +0000

̽��ֱ��effectiveness of salt marshes – wetlands which are flooded and drained by tides – in protecting coastal areas in times of severe weather has been quantified in a study by researchers from the ̽��ֱ�� of Cambridge.

In the largest laboratory experiment ever constructed to investigate this phenomenon, the researchers have shown that over a distance of 40 metres, the salt marsh reduced the height of large waves in deep water by 18%, making them an effective tool for reducing the risk of coastal erosion and flooding. Sixty percent of this reduction is due to the presence of marsh plants alone. ̽��ֱ��results are published in the journal Nature Geoscience.

One of the most noticeable effects of climate change is the increasing frequency and severity of storms, such as the series of storms which battered parts of south west England last winter. As the climate continues to warm and sea levels continue to rise, the effects of these storms could be devastating, putting these and other coastal communities worldwide at risk.

While the important role of salt marshes in protecting against coastal erosion is well-known, their effectiveness in mitigating the effects of extreme weather, when water levels are at their maximum and waves are at their highest, had not been understood or definitively quantified.

Recreating a salt marsh in a large wave tank and subjecting it to realistic storm conditions, the researchers found that it significantly ‘buffered’ the effects of the waves. Similar to wind blowing through a forest, the plants reduce the energy of the water as it flows through and around them. Even when the waves flattened and broke the marsh’s vegetation, the soil surface beneath remained stable and resistant to surface erosion.

Salt marshes are found throughout the world, particularly at middle to high latitudes. In addition to their role in protecting against coastal erosion and reducing flooding, they also act as nurseries and refuges for many species of marine animals, and protect water quality by filtering runoff.

Given increased rates of global sea level rise, there are concerns about losing salt marsh on many coasts, particularly where there is insufficient sediment and space to allow marshes to build upwards and landwards.

“While we have long known that salt marshes and other natural defences such as sand dunes or mudflats can help protect our coastlines, a lack of data on their effectiveness in extreme conditions has meant that they often are not included in flood risk assessments,” said Dr Iris Möller of Cambridge’s Department of Geography (Cambridge Coastal Research Unit), who led the research. “But we’ve shown that even in extreme conditions, salt marshes are a vital defence for our coastlines and protect against more frequent storms.”

̽��ֱ��researchers used large sections of salt marsh, cut from a natural marsh in northwestern Germany. ̽��ֱ��team then rebuilt the marsh in one of the world’s largest wave tanks, located in Hannover, and subjected it to water depths and types of waves that are typical in storm surge conditions. Even after the waves flattened the plants, the marsh was still an effective barrier against erosion, demonstrating the importance of natural flood defences alongside manufactured defences such as flood walls.

̽��ֱ��flooding which hit south west England last winter was the worst in nearly 20 years. A series of 12 major storms between December and February caused huge waves, strong winds and hide tides to pummel large parts of Cornwall, Devon and the southwest, causing millions of pounds worth of damage. Many homes and businesses were flooded multiple times, and major flooding in the Somerset Levels forced many families to evacuate their homes and many farmers to evacuate their livestock.

As part of the government’s attempts to mitigate the effects of future storms, salt marshes have been re-created in several locations around the UK coast: a large new salt marsh on the Somerset’s Steart peninsula was recently completed, and several more are planned for locations throughout the UK.

̽��ֱ��research was supported by the European Community’s 7th Framework Programme and a grant from ̽��ֱ��Isaac Newton Trust, Trinity College, Cambridge.

̽��ֱ��researchers are blogging about their work at thesaltmarshexperiment.wordpress.com.

Study finds that natural flood defences such as salt marshes can reduce the height of damaging waves in storm surge conditions by close to 20%.

Even in extreme conditions, salt marshes are a vital defence for our coastlines

Iris Möller

James Tempest

Storm on a rising tide, Orplands, Essex

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Cambridge researchers learn lessons from recent storm surge

jfp40 — Thu, 16 Jan 2014 12:20:31 +0000

̽��ֱ��wet and windy weather that has battered Britain’s coasts this winter has brought misery to many, but for researchers at the ̽��ֱ�� at Cambridge the storm damage is providing vital data that could help improve future flood warnings and emergency planning.

On 5 December 2013, coastal communities along the North Norfolk coast were threatened by a significant storm surge – the result of low atmospheric pressure (which causes sea level to rise) combined with high winds pushing up the sea surface as they blow water towards the coast.

According to Dr Tom Spencer of the Department of Geography’s Cambridge Coastal Research Unit: “ ̽��ֱ��southern North Sea is very vulnerable to storm surges because of its shallow water, and winds blowing from north to south funnel the sea into the narrowing basin near the Straits of Dover.”

Predicting the impact of storm surges on coastal areas like North Norfolk, which is lined with barrier islands and gravel spits – and cut by tidal inlets bordered by mudflats and saltmarshes, is challenging. To find out how these features affected the water levels and waves that hit the area’s coastal settlements during the storm, the team from Cambridge and Birkbeck, ̽��ֱ�� of London took to the road.

Immediately after the surge, they travelled the 45km between Holme-next-the-Sea and Salthouse measuring maximum water levels with a satellite-based survey system able to resolve positions and heights to an accuracy of less than 50mm and often less than 20mm. “We looked for debris lines, erosion marks on earthen banks and floodlines on buildings – or in some cases car windscreens,” Dr Spencer explained.

They found that maximum surge heights here differed by almost 2m, depending on whether the site was exposed or sheltered, differences much larger than previously thought. “At some sites this was the critical difference between a business or a home being flooded or not,” said Dr Spencer. If these results can be incorporated into surge models and flood forecasts, they could help improve early warning systems and evacuation planning.”

̽��ֱ��December 2013 event that Dr Spencer studied came almost exactly 60 years after the disastrous storm that hit the region in 1953, claiming more than 2,000 lives around the southern North Sea – the largest death toll from flooding in Europe for 100 years.

“Looking at the atmospheric pressure charts, the 2013 and the 1953 events look similar: both were characterised by a deep low pressure system that came down the long axis of the North Sea. But while the 2013 storm was short-lived, producing waves of around 3.8m offshore from North Norfolk, in 1953 gale force winds blew for several days ahead of the surge, producing waves probably close to 8m high off eastern England,” he says.

Like the 2013 event, the 1953 storm arrived under cover of darkness. But unlike 2013, it hit over the weekend, with devastating results.

“Most people in the UK and ̽��ֱ��Netherlands were asleep in bed when floodwaters broke into their houses, many of which were single storey chalets and bungalows. Some people managed to get onto their roofs but, with the storm still raging, they died from exposure or slid into the sea,” he said. “In ̽��ֱ��Netherlands, where over 1,800 lives were lost, the radio ceased transmitting at midnight on the Saturday and although warnings were issued by telegram, these arrived at offices that were shut over the weekend.”

Much was learned from the 1953 storm: coastal defences were heightened and strengthened, and there have been major advances in storm surge forecasting and emergency planning. But, as Dr Spencer’s results show, there are still lessons to be learned from today’s floods that could help prevent tomorrow’s victims.

̽��ֱ��team’s initial assessment is published in Nature.

For researchers at the ̽��ֱ�� at Cambridge, recent storm damage is providing vital data that could help improve future flood warnings and emergency planning

These results could help improve early warning systems and evacuation planning

Tom Spencer

I Moeller

Stranded boat at Blakeney after storm surge

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