Nefyn Bay, North Wales

Landslide case study

On 2 January 2001, a fatal landslide occurred in the coastal village of Nefyn on the Lleyn Peninsula. A series of slides swept two cars over the cliff and onto the beach below. Tragically one of the occupants was killed and another seriously injured in the incident.

Nefyn Bay is between Penrhyn Nefyn and Penrhyn Bodeilas and has been subject to minor landsliding activity for many years. The landslides of January 2001 occurred after a period of heavy rain along a stretch of coast that comprises weak superficial (drift) deposits. The amount of water and weak nature of the deposits contributed to the instability.

The Nefyn landslide is National Landslide Database ID 2749/1 and 2749/2.

Nefyn Bay, North Wales
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Nefyn Bay, North Wales.

Geology

The Lleyn Peninsula has high ground formed from igneous granite-type rocks and low ground formed by superficial deposits (drift). For more detailed information on the bedrock geology of the Lleyn Peninsula, see our report: Ordovician of Snowdonia and the Lleyn Peninsula.

In Nefyn Bay the superficial geology comprises two different till units, clays and silts as well as glacio-fluvial outwash deposits of sand and gravels. The glacio-fluvial deposits were probably deposited at the margin of a retreating ice front, at the end of the Devensian glaciation.

Landslides in Nefyn Bay

There are a variety of landslide types within Nefyn Bay including rotational failures, flows, falls and debris slides. The slopes are covered in weathered debris and this is particularly susceptible to shallow landsliding, especially when saturated by water.

Record rainfall

The period between August 2000 and February 2001 was one of the wettest recorded periods for this region with one and half times the long term average of rainfall. As a result, groundwater levels would have been affected by this prolonged rainfall and could have been a factor that contributed to this fatal landslide event.

Seepage erosion

Failures in the bay have also been initiated through seepage erosion caused by the juxtaposition of sands over a less permeable silty clay layer. Water seeps through and erodes the geology causing the material to collapse, leaving a depression at the surface. This depression becomes filled with debris, which is then able to move as a landslide given the right climatic conditions.

The landslides of January 2001 were probably translational in nature but degraded to a mudslide due to the steep slope and composition of the landslide material. As the landslide progressed, it also flowed down the slope.

References

Gibson, A D, Humpage, A J, Culshaw, M G, Forster, A, and Waters, R A. 2002. The geology and landslides of Nefyn Bay, Gwynedd. In Nichol, D, Bassett, M G, and Deisler, V K. Landslides and landslides management in North Wales. Geological Series No 22. National Museums and Galleries of Wales.

Jenkins, G O, Gibson, A D, and Humpage, A J. 2007 Climate change and evolution of landslide hazard at Nefyn Bay, North Wales. In: McInnes, Robin, (ed.) Landslides and climate change: challenges and solutions. Proceedings of the International Conference on Landslides and Climate Change. Taylor and Francis, 113–119.

Statham, I. 2002. Failure on the coastal slope above Lon Gâm, Nefyn, Llyn Peninsula: mechanism and remedial measures. In Nichol, D., Bassett, M.G. and Deisler, V.K. Landslides and landslides management in North Wales. Geological Series No 22. National Museums and Galleries of Wales.

Contact the Landslide Response Team

Tel: 0115 936 3143 Email: landslides@bgs.ac.uk

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