Science Spin July 2009

Landslides, Ireland's greatest natural hazard

By Sean Duke

Irish people, unlike say Italians or Icelanders, are not regularly threatened by very large earthquakes or volcanic eruptions. That's not to say there are no natural dangers here, there are some, and chief amongst them are landslides, which are increasing in frequency due to climate change and man's activities. Landslides occur in all areas of the country, but occur most in areas covered in bog, and subject to occasional very heavy rainfall. When peat, bog or other sediments move, it can be very dangerous, and anything in the vicinity, including homes, roads, bridges and people can be swept away in an instant.

BACKGROUND

Landslides are probably the major 'natural' threat to lives and property in Ireland, but we know very little about them. That, however, has begun to change, and the GSI has initiated a project, supported by the National Development Plan to the tune of €500,000 to draw up 'landslide susceptibility maps' that will be integrated into the planning process. The idea is to send out a warning to those that might be considering developments in areas that are susceptible to having landslides.

"It's waving a red flag, we are not saying don't build there, but we are saying be aware of the area's susceptibility to landslides. You can build anywhere, but cost comes into it," said Dr Ronnie Creighton, GSI Landslides Susceptibility Mapping Project Manager.

The GSI is working with consultants Mouchel Parkman Ireland Ltd on the project. The Mouchel engineers are working under the guidance and supervision of the GSI. The work began in 2008, and will run for four more years, said Dr Creighton. He explained that part of the work is to determine the frequency of landslides in particular areas, and the reasons why they occurred.

The initial focus is on Greater Dublin and Cork, as population centres, but the plan is to draw up susceptibility maps for all of Ireland, north and south. "In each area of study, an inventory of past events (landslides) is studied, so a pattern can be established," explained Dr Creighton. "This can be related to slope, vegetation and other ground conditions, and from that do some mathematical modeling and you can get a landslide susceptibility map. It is the first stage and the end stage is risk assessment and that brings in cost of damage and frequency."

"A lot of the emphasis is on peat because that is where the slides are happening with heavy rain and so on. We want to look at landslide hazard to commercial buildings, and settlements. The consultants have been given the brief to do the coastal area as well. That works well with the coastal protection strategy so there is synergy with other agencies."

FREQUENCY

The landslides project is highly relevant for many reasons, most notably because of the danger to homes, property, and to human life. And frequency with which landslides are occurring is increasing, and this is due to climate change and economic development, so this is a natural event that we are going to see a lot more of in coming years in Ireland.

As Ireland's climate changes with global warming, experts are predicting that we will see wetter winters, particularly in the west, and drier summers, notably in the east. There will also be more intense downpours of rain. Furthermore, with sea levels to rise, as predicted, this will lead to bigger waves hitting our 'soft coasts' causing erosion and cliff collapses.

Compounding the natural factors is the impact of the Celtic Tiger, which is roughly defined as the period between 1996 and 2008. The level of construction all around Ireland went through the roof in these years, and building works - whether it was a small cluster of homes, or a group of wind farms - began to penetrate into remote areas, even on top of bogs. If the soils or sediments in the vicinity of the construction works were undercut, this would render them unstable and more prone to landslides when rains came.

Development has slowed, but when it picks up again, this will again become an issue.

EVENTS

A total of 148 landslides have been recorded so far during the landslides project, and the information entered into the National Landslide Database (the information in which will be freely available). A further 264 landslides have been incorporated into the National Database as a result of the work of the Breifne Landslide Susceptibility Project. This was a GSI project that ran from 2005 to 2006 and included Sligo, Leitrim and Roscommon.

The historical information on landslides in Ireland has been gathered from all available written records, including journals, newspapers, local authority reports, and books. The most notable book source used by the GSI is, The Bogs of Ireland, by Dr John Feehan and Dr Grace O'Donovan, published by the Environmental Institute, UCD, 1996.

The records indicate that some landslides that have occurred in Ireland have been fatal. The most lethal landslide on record killed 21 people at Castleguard Co Limerick in 1708.

Records from the time state that:

"The bog moved along a valley and buried three houses, which at the time contained about 21 people. The landslide was a mile long, a quarter mile broad and 20 feet in depth in some parts. The landside ran for several miles crossed the high road at Doon and broke through several bridges before flowing into the Lough of Coolpish."

The second most lethal landside was at Knocknageesha Co Kerry in 1896, when 8 people were killed.

It was reported that: "The event occurred on the morning of December 28th 1896. The bog gave way along the line of a 1-3m deep turf cutting. It carried off a cottage in which Lord Kenmare's quarry Stewart, Cornelius Donnelly and his family (His wife and six children) were sleeping. All eight lives were lost."

"The cause was attributed to a dry summer followed by a wet autumn. A heavy downpour of rain set in on December 27th accompanied by a SE gale."

Dangerous landslides, however, are not confined to the distant past. For instance, a lot of destruction occurred following a slide at Pollatomish Co Mayo in September 2003.

Describing the Poolatomish event, Dr Creighton said: "Several houses were just wrecked. They came down the slope. There was a high road above the coast, a lot of modern bungalows on it and the material swept like a tide, there was a lot of damage to property, and the bridge was taken out."

Pollatomish is a good example of the type of areas that are vulnerable to landsides. In this area of North Mayo there was a steep slope, with blanket bog perched on top, and peat coming down the sides. Following a very dry summer in 2003 the peat dried out and cracks appeared. When heavy rains came in September the water got into the cracks and saturated the peat. The peat became 'buoyed up' and heavy and shot down the slope due to gravity. When the peat moved it moved quickly, and its movement downward could be described as something akin to a hovercraft skimming over water, said Dr Creighton.

Another dangerous recent slide happened near Ballincollig Co Kerry in September 2008, when peat became unstable, moved, and took out a bridge and a road, as well as dumping slurry into a nearby river. And in April 2007, one km of road on the scenic Slea Head Drive, again in Kerry, was cracked and damaged following a cliff collapse - which is another form of landslide. The road had to be re-aligned subsequently and moved inland.

DATA

The susceptibility maps will be drawn up using existing data sets, as well as integrating new data sets. The plan is that maps will be easy to read, with three colours to represent 'high', 'medium' and 'low' susceptibility. The areas that need to be most careful are those with peat deposits, where there has been a dry summer followed by heavy rain.

"For each data set that you have, rock type, soil and slope angle, for example," explained Dr Creighton. "You create a series of polygons, and you overlay them and create a new series of polygons. Then you apply a wetness index to the ground, based on the drainage pattern and the slope, and you have your previously known sites, and you put them all on top of one another and run a mathematical model on it."

There are the conditioning factors, such as rock type, slope angle, vegetation, land use, and aspect, explained Dr Creighton. Then there are the triggering factors, the things that cause a landslide to happen, such as heavy rainfall over a short time period, or intense construction work that undercuts a slope, particularly one that is made up of peat.

All of this information is fed into a computer model which uses a specific algorhythm to come up with a map. This has to be done for lots of locations in order to come up with a map, and that is why the project will take several years to complete. It's time consuming.

This is the first time in Britain or Ireland that such susceptibility mapping has been done on a large scale, covering the greater Dublin and greater Cork regions. Up to now, this kind of mapping has only been done in site specific areas, such as around wind farm locations in Scotland, so that susceptibility in one local area can be determined.

The methodology for what's being attempted in Ireland has to be adjusted to take account of the bigger area, and it has to be tailored to suit existing data sets and photo imagery. Also, unlike Britain, the maps are intended for general use, and will be freely available. In Britain the trend is for these maps to only be available to the insurance industry.

GOALS

By the end of the project in 2013, Dr Creighton said he would like to have developed useful susceptibility maps, to have applied the scientific methodology to all of Ireland, to have identified areas of higher vulnerability, and to have created public awareness about landslides in terms of what they are, why they happen, and how dangerous they can be.

When a landslide happens all hell breaks loose," said Dr Creighton. "People are at risk, it is a hazard. Roads are taken out, houses may be damaged - life may be lost." The aims are to make people more aware of the potential of landslides to cause damage, and to bring this to their attention through the planning system.