Could more extreme weather speed coastal erosion?
10:04 05 January 2013
Following one of the wettest years on record, the Met Office has warned of more extreme rainfall events in future – which could speed the erosion of our already-fragile coastline.
According to the nation’s official weather forecasters, the UK experienced its second wettest year on record in 2012, with warnings that we face a future of increasingly intense downpours.
Analysis published by the Met Office this week shows that the country is experiencing more “extreme rainfall” events, and suggests this is likely to be linked to climate change.
And a separate study by the British Geological Survey says the deluge has resulted in a significant increase in landslides and slope failures, many located around the coast.
The BGS’s landslides team has been monitoring reports of landslides in the media and in scientific publications since 2006 and recording them in the organisation’s National Landslide Database, which contains around 15,000 records.
Although new reports are being processed all the time, the initial figures indicate a four to five-fold increase in the number of landslides for July and December, when compared with previous years.
The majority of those incidents happened in the south-west, where the prevailing winds leave the shoreline more exposed to severe rainfall.
But here in East Anglia, it serves as a warning of what the future could hold if this year’s weather trends are continued.
The erosion of the Norfolk and north Suffolk coastline, already estimated at an average rate of about one metre per year, could accelerate if cliffs are undermined by a combination of more rainwater seeping into the ground, coupled with added wave activity from the North Sea, driven by increasingly stormy conditions.
A spectacular example of what could happen was reported in the EDP on Thursday, after a seafront pathway was closed at Corton, near Lowestoft, following a cliff slippage.
Waveney District Council sealed off the pathway due to public safety concerns, and said the cliff probably crumbled away due to recent high rainfall.
Hydrogeologist Dr Vanessa Banks, a team leader at the British Geological Survey, who used to live in Norfolk, said it was reasonable to predict an increase in the rate of coastal erosion – but said many factors were at work.
“The increased incidence of landslide activity appears to be closely linked to the development of saturated ground conditions associated with higher than average rainfall,” she said.
“In areas of glacial sediments like north Norfolk, the cliffs will recede in two ways.
“The landslides are composites of slope failures as rainwater infiltrates from the top, and marine erosion from the bottom.
“Many are a response to severe erosion at the toe of the slope, and those events are driven by storm activity in the North Sea.
“The increased rainfall will increase erosion at the top, but we will also see short-term responses to the infiltrating rainfall. When you walk along the cliff in Norfolk you often see cracks forming which we call ‘tension gaps’.
“It is where the face of the cliff is starting to move outwards, and they enable water to be taken into the top of the cliff. Some of that water, through gravity, will move through the slope towards the water table, which may be some considerable depth.
“It is a reasonable statement to predict an increase in the rate of [shoreline] recession and it will be influenced by all these factors in the geology and in human engineering.”
As well as shoreline management plans, sea defences and groynes, Dr Banks said other “human engineering” to protect cliffs could include artificial drainage projects.
“But it can bring its own problems if it is not managed properly,” she said. “If you started to drain a slope and then it starts to recede, the pipes can become impacted and then you have got a source of water feeding directly into the cliff.”
Between 2000 and 2006 a coastal erosion research project at three survey sites in Norfolk found an average of one metre per year had been lost for the last 5,000 years – although at Happisburgh in particular, the cliffs were found to be crumbling at a much faster rate of up to seven metres per year.
The survey report says: “During the six-year monitoring programme the cliff at the Happisburgh test site retreated by over 40m to form a new embayment. The erosion here appears to be influenced largely by direct mechanical abrasion from the sea, but also significantly by run-off and groundwater seepage.”
Dr Banks added: “It is reasonable to assume this rate will increase if these weather patterns continue – although we should be careful about jumping to any conclusion. To solve the equation, you need an understanding of how the storms in the North Sea will change.”
Dr Clare Goodess, at the UEA’s Climatic Research Unit, said current “medium” projections for sea level rises on the Norfolk and Suffolk coasts show an increase of between 0.13 and 0.36 metres by 2050, with a most likely estimate of 0.25m. By 2100, the most likely estimate rises to 0.5m.
“In terms of a general rise in sea level, we have a fairly robust picture,” she said. “But it is much harder to predict changes in storm surges or extreme sea levels.
“The uncertainties in storm surge and wave height estimates are very large.
“There is some indication that the size of surge expected to occur on average about once every 50 years may increase slightly around the UK but these changes are small in relation to natural variability.
“The projections do not indicate any significant changes in surge or wave height for the east of England.”
The Met Office report says rising global temperatures could be playing a part in increasing rainfall, as a warmer atmosphere can hold more moisture and increase the potential for heavy rain.
The world has seen temperatures rise by around 0.7C since the beginning of the Industrial Revolution, which would equate to around a 4pc increase in moisture in the atmosphere.
Dr Goodess said: “There is nothing in the projections that indicates the current pattern of change will be reversed. The question is whether the change in the intensity of rainfall goes beyond the changes we would expect from that simple relationship [linking temperature and air moisture].”
As a contributor to the Natural Hazards Partnership, the British Geological Survey issues colour-coded assessments regarding landslide risk, based on Met Office weather forecasting and observational data regarding slope instability.
Last week, an amber warning was issued in the south-west following a combination of saturated ground conditions, forecasts of more heavy rainfall and multiple reports of slope instability.
Dr Banks said the same set of circumstances was less likely on the east coast, which is less exposed to the prevailing south-westerly winds.
“One of the advantages of living in East Anglia is that you are not so badly affected by this rainfall, but there are cases when there are storm surges, when Norfolk can be more vulnerable to landslides,” she said.
“There are still weather systems where you can get high intensity rainfall, but not to the same degree as the south west.”
For more on the BGS landslide database, see www.bgs.ac.uk/science/landUseAndDevelopment/landslides/November2012.html