17 November 2020
Mount Eagle: another peat slide in Ireland
Posted by Dave Petley
Mount Eagle: another peat slide in Ireland
Amongst the focus on the major peat slide in Meenbog in the Republic of Ireland over the last few days, it slipped my attention that another event occurred, this time at Mount Eagle in County Kerry. The Farmers Journal has an article that provides the detail. The landslide has occurred on a Sitka spruce plantation in a peat bog area of County Kerry on land owned by Coillte, the state owned forestry company.
The best imagery that I have seen has been posted to Twitter by Save Kerry (@SaveKerry):
On this occasion the peat slide appears to have started in the middle of the Sitka spruce plantation. It has displaced a large amount of bog, and as observed previously movement is shallow, consisting of rafts some of which include upright trees. The image below is a screenshot showing these rafts:
As the above image shows, the deformed land is highly chaotic, without the ordered structure seen in the Meenbog slide. I suspect that this may indicate that the this is not an area that has been affected by retrogression of the landslide. I would like to see an image that captures the whole of the source area of the Mount Eagle peat slide.
Downslope, the Mount Eagle peat slide has transitioned into a channelised flow with a long runout, in common with the many other failures of this type that I have described on this site. There are concerns for drinking water quality and for the ecological impacts downstream.
The trigger for these landslides is undoubtedly the recent heavy rainfall. The spate of events that we have seen this year so far reflects the periods of extremely wet weather that have occurred over the British Isles in 2020. But the precise mechanism of the initiation of the failures is not clear. I think that there is a particular question to be asked about the role that the spruce plantations play in initiating the slides (this may be related to the way that the trees create rafts of material and/or the ways that they change the drainage of the bogs), as well as the impact of wind farm construction in triggering instability.
As I have noted previously, global heating will mean that these prolonged spells of rainfall, and shorter duration high intensity rainfall events, will increase, so we might expect to see more failures of this type. Blanket bogs are an important habitat in the UK and in Ireland, and of course they are a major carbon store. Restoration is a very slow and difficult process. So, in my view, there is an urgent need to undertake research to understand these landslides better, and to ensure that the land is managed properly. A combination of detailed field monitoring and novel lab testing should yield some fascinating results.
This is an interesting situation: Trees possibly facilitating mass wasting. Here in NW California and the Pacific NW of USA, the opposite is often seen as the case thru canopy buffering of rainfall spikes, increased evapotranspiration (ET), and root reinforcement.
Tossing out an idea – these benefits may be negated by the high rainfall – ET drops in the winter and these slopes appear to be what we would call (here) flat to gentle. Maybe lateral permeability is poor and the water simply cannot escape fast enough, creating unusually high pore pressures at the aquitard boundary that allow liquefaction. A trigger could be as simple as a wind event flexing the spruce and their shallow root systems (also an indicator of high groundwater). If the entire slope is supersaturated and near failure, this could trigger a domino-like event.
A loose analogy would be areas of forest blow-down we saw here in 2005 when there were 95+ mile per hour jet stream winds. In some places, all it took was a tree or two to fall and start start a cascading blowdown event.
Has there been any monitoring to see if these sites reactivate? Reactivation may not readily occur with the newly opened face area and well-defined channels facilitating drainage.
I was wondering do you have coordinates of this peat slip as we are nearby in Coollegrean, Brosna where the wind farm is. It was built above a previous slip of which we was ignored by the council. There would be about 1 House in extreme high risk and about 4 or 5 more. My immediate neighbours are not at risk as have the valley the peat slide was in between myself us and the wind farm. Thanks.
[I don’t know, sorry. D.]
Sitka Spruce may have been planted because it grows where there is high rainfall and high winds along the NW Pacific Coast, USA. However, the roots cannot anchor and stabilize a peat bog in Ireland to prevent it from sliding. One would think of blanket peat bog as something the tree roots would grow penetrate to lower, more solid layers, preventing it from moving. It would seem very disappointing that this concept did not work out as hoped for. Is something in Irish peat, such as a fungus, weaken the roots? Or had movement within the peat ruined the roots while they grew? Or, did the weight of the Sitka Spruce trees cause shear within or on the contact of the peat? If a lighter weight native tree that customarily grew on peat had been planted, would this have prevented sliding?
Thanks for posting the additional detail on this Dave. I was informed about this one shortly after Meenbog but there was little information on where it was and some indication it was being conflated with another event. Mount Eagle is near the coast whereas the Mount Eagle Bogs are northeast of Glountane and where some commenters noted the failure was likely to have occurred. More information would be useful (as you note).
Re: the trees, coniferous plantations have very shallow root plates set well above the base of the deep peat in which the shear surface or failure zone (if liquefaction) of these failures is likely to occur. Forest drains cut into the bog prior to planting are often cut to the substrate and can be larger in dimensions than typical moor drains or grips e.g. (cut in open fells). There is also significant disturbance from the double mouldboard ploughs that are used to create the furrows along which the trees are planted. Water table drawdown by trees is commonly observed, increasing cracking in the surface of the peat, one possible avenue for water ingress.
Both the effect of the weight of tree mass (supported by the soil) and the change in moisture content (caused by uptake by the trees) need to be considered when evaluating the effects of afforestation on peat. Drain alignment (across slope vs downslope) also needs to be considered since these can act as lines of weakness, compartmentalising the slope and removing toe support where cut along / oblique to the contour.
In response to James (above) lateral permeability is very limited (see the work of Prof Joseph Holden at Leeds), with most lateral flow in pipes or in the acrotelm (vegetated surface). Both of these hydrological systems are likely to be heavily disrupted by ground preparation for forestry.
It seems very interesting. I’m wondering if there is available coordinate for approximate centre location of the Mt Eagle peat slide.
The location of the approximate centre of the slide is x = 47955.1 m and y = 5786261.5 m in the EPSG: 32629 reference frame. It may be helpful to people who eager to see the exact location.