8 July 2009

Shallow landslides in County Mayo, Ireland

Posted by Dave Petley

Last Thursday (2nd July) an unusually strong rainstorm passed across the northwest of Ireland. Considerable damage was caused, as recounted in this Irish Times article, with floods occurring across quite a large area. The rainfall in the Castlebar area was particularly intense – there is a quite nice article on this in the Mayo news. This rainfall appears to have triggered a set of large but shallow slides at Croaghmoyle. Castlebar News has two images, shown below, of the landslides:

The top (overview) shot is quite interesting. The large slide that is most obvious appears to have started as a very localised slip in what I think is probably a blanket peat layer high up on the slope (see annotated image below). The main slide is of the shallow regolith layer on the boundary with the underlying bedrock. It appears that a comparatively small change in gradient low on the slope caused the slide to cease to erode and start to deposit, although the material has moved a considerable distance over the land surface and onto the track. This is not unusual in this type of slide. The other slides appear to just be on the regolith / rock boundary.

The second image shows the debris on the road, which appears to be mostly peat and some entrained bedrock pebbles and cobbles. This road is the access track for a large television transmitter – two technicians were trapped on the wrong side by the landslides.

The Irish Meteorological Service has a short article on the rainfall here. The peak rainfall intensity in nearby Newport was 41.6 mm in 24 hours, which has an annual exceedance probability of 1 in 150. 60 mm of rainfall was recorded in total, with a maximum one minute intensity of 2.8 mm, and a maximum five minute intensity of 9.8 mm. The rainfall was thundery, so this image of lightning strikes shows the rain bands quite well: