Characterisation of the 2020 Drumkeeran peat landslide: a large peat slide in Ireland.&#160;
Authors: Connolly John , Holohan Eoghan , Bourke Mary , Cruz Charmaine , Farrell Catherine , Foyle Fearghus , Habib Wahaj , Halpin Robert , Henry Tiernan , Hrysiewicz Alexis , Long Michael , Johnson Paul , McKeon Charise , Trafford Andrew 
Published: March 2021

&lt;p&gt;Mass movements in peatlands are poorly understood. This is because of the unusual geotechnical properties of the materials (organic soils) and a paucity of well-constrained case studies. At the end of June 2020, a large peat slide occurred on Shass mountain, several kilometres northeast of the village of Drumkeeran in Co. Leitrim, north-western Ireland. The source area of the peat slide is an area of blanket bog within a Special Area of Conservation (SAC). This area is characterised by a topographic slope of 3-5&amp;#176;. On recently published Landslide Susceptibility Maps it was classified as &amp;#8216;moderately low&amp;#8217; to &amp;#8216;low&amp;#8217;.&lt;/p&gt;&lt;p&gt;To understand this peat slide&amp;#8217;s genesis and impact on the landscape, post-slide site investigations and aerial surveys were undertaken in the following days and weeks. These included: photogrammetry and LiDAR surveys via UAVs and crewed aircraft; Ground Penetrating Radar (GPR) profiling; in-situ peat depth measurements, soil coring and a vegetation survey. &amp;#160;These data were complemented by pre-and post-slide radar satellite data (Sentinel-1) and were compared to high-resolution pre-slide aerial imagery and digital surface models (DSMs) captured in August 2017 and April 2020.&lt;/p&gt;&lt;p&gt;Mapping and DSM differencing show a source area of 7 ha, from which ~ 171,000 m&lt;sup&gt;3&lt;/sup&gt; of peat flowed 6.6 km down a river channel. The height/run-out ratio was 0.035; the run-out/volume ratio was 0.038. Peak flow or run-up heights near the source area were &gt;4 m. Video, field and satellite evidence indicates that the peat was highly liquified. It deposited in three zones: upstream of a small bridge, which acted as a partial dam and on two floodplain areas. About 45 ha were covered with peat up to 1-3 m thick, these deposits generally thin downstream. Radar intensity data support local accounts that most of this material failed retrogressively and redeposited within 24 hours.&lt;/p&gt;&lt;p&gt;Data from the nearest meteorological station, 27 km to the west, show that the region experienced a relatively dry period (118 mm of precipitation) in the 2.5 months before the landslide, and a period of exceptionally high rainfall (53 mm) three days immediately beforehand. Flow pathway analysis indicates a natural drainage convergence in the upper catchment. The landslide possibly started here and regressed upslope into ~5 ha of well-drained bog, afforested in 1996, located at the head of the catchment. The areas to the south and east comprise of a mosaic flushes, wet heath, and blanket bog vegetation.&lt;/p&gt;&lt;p&gt;The peat depth was assessed by both GPR data (calibrated by coring), which shows the base of the peat and probing. It ranged from 2-5 m. This accords with a typical 2-4 m thickness of failed peat from DSM differencing. Coring also revealed a ~50cm thick layer clay at the base of the peat. These preliminary results highlight the potential importance of local drainage patterns and localised clay layers in increasing peat-slide susceptibility on low-angle slopes. This characterization underpins further investigation into the multifarious factors causing peat slides, which may be exacerbated by climate change.&lt;/p&gt;