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Bog Bursts

Definition:
In its simplest form a bog burst occurs when a mass of bog or peat moves. In the case of blanket bogs, this usually happens when the top layer of the bog literally starts to slide. In raised bog movement occurs when the edges give way, this is usually in areas where intensive cutting has taken place.

Bog burst in pre-drained forestry area in Co. Mayo blanket bog. Liquified peat was discharged into the Owenmore River. Late 1980's. (Photo: P.Foss)

A bog burst can occur at any time of the year, though the majority on blanket bogs tend to be most frequent during the autumn and winter months. This is due to the high levels of precipitation in the mountains of Western Ireland. However, on areas of raised bog, which tend to be situated more inland, these seasonal patterns are not quite so obvious.

Ireland, however, like the rest of the world is experiencing some of the effects of global climate change, such as periods of very heavy rain following unseasonably long spells of hot, dry weather. These changes in weather patterns are having seriously deleterious effects on some of the countries most fragile ecosystems.

However, climate change alone cannot be held responsible for these bog bursts and events such as these owe much more to the increasingly invasive presence of human activity on an already fragile ecosystem. These intrusions on the landscape include the effects of industrialised peat cutting, the intensive draining of land for farming and agriculture and, of course, the ever increasing number of wind farms which are being built on ecosystems already under extreme pressure.

Bursts are often naturally occurring events, though often exacerbated by human activity, They usually occur during times of extreme weather conditions. Such weather conditions include extreme heavy rain or prolonged spells of dry weather followed by flash flooding. During these dry spells, the intricate root systems of the upper layer of the bog or acrotelm, which under normal conditions acts as an anchor for the bog, starts to shrink and die back. This, in turn coupled with adverse human activity on the bog surface can cause a resulting shift of the bog mass. With nothing left now to anchor the bog a burst can occur. This shift can take the form of a slow moving mass or conversely a free flowing avalanche of soil, which can often resemble a lava flow by its shear force of movement.

Peat situated on mountain slopes is much more at risk of bursting due to its occurrence on often-sloped areas and its sensitivity to gravity. The risk of a bog burst is further increased after heavy rainfall. The deluge lubricates the base layers, the upper layer becomes lightened the resulting effect is that the upper layer of the bog becomes separated from its base and floats off. A slope of 4 degrees is sufficient for this event to take place but it has been known to happen in areas with a slope of as little as 2 degrees.

Once a burst has occurred the flow rate depends on several factors, such as the gradient of the slope and the volume of material involved. For example the 1984 burst in Stranduff, Co. Sligo comprised of 4,200 tonnes of peat and 73,000 tonnes of water.

A farmer in Co. Mayo gave an account of a bog burst in 1931

'The lighter, more coherent upper peat was carried on top of the black slurry, breaking up into a series rafts and deep chasms as liquid peat spread out. The avalanche charged down the hill to the stream below at great speed'.

After a burst the areas effected by the deposits of the semi-liquid peat have shown surprisingly little ill effects after several years. However, in cases where the peat has been transported to rivers and lakes, as in the 2003 Galway burst at Derrybrien, the damage done can have adverse and long term effects especially on aquatic invertebrate and fish populations. Also, these bursts can have serious implications on human water supplies where contamination occurs.

Bog bursts have been and always will be naturally occurring events and, with the effects of global climate change they are set to become more frequent in Ireland in the long term. However, with increased human activity these naturally occurring events could become more serious as human intervention slowly erodes the delicate balance of the bog landscape. It is important in light of this, that bogland conservation needs to be supported by a much larger community awareness of the issues and the consequences of what could potentially happen if we do not take greater care of the fragile upland bog landscape.

Jayne Byrne

Reference and Further Reading

Feehan, John and Dovan, Grace (1996) The Bogs of Ireland ­ an introduction to the nature, culture and industrial heritage of Peatlands Walsh / UCD

www.ireland.com for Irish Times press releases on the 2003 bog burst at Derrybrien

www.nationaltrust.org.uk for details of the National Trusts Soil Protection Strategy 1999