Landslides in eastern Ireland
Landslides in eastern Ireland
Mary C. Bourke, PSI
Martin Thorp, Department of Geography, University College Dublin, Ireland
Slope failures are important geomorphological agents in shallow periglacial and glacial soils, particularly on the steep sides of small upland valleys in northwest Europe (Newson, 1980). Given that global climate models predict increased precipitation for northern latitudes (IPCC, 2001), there is a pressing need for investigations into the response of slopes to current climatic extremes. Although, failures triggered by intense, mid-to-high latitude storms are likely to be spatially widespread (Brooks and Richards, 1994) they have received little attention in Ireland relative to those in bedrock and peat (e.g. Alexander, 1986; Colhoun, 1965; Coxon et al., 1989; Douglas, 1980; Dykes and Kirk, 2001; Prior et al., 1968; Tomlinson and Gardiner, 1982; Wilson and Hegarty, 1993) or those resulting from past anthropogenic land use practices (McGreal and Lamour, 1979; Moles et al., 1999) or deglacial processes (e.g., Prior et al., 1968; Whittow, 1974).
This project conducts a geomorphologic analysis of four slope failures triggered by a storm on the slopes of the Wicklow Mountains, Ireland in 1986. We describe the failures and the nature of the viscous flow using morphological, textural, geotechnical and facies data. We describe the modification of the valley floor slope deposits by concurrent fluvial activity. We identify factors that contributed to the location of the failures.
Figure 1. Location of the Cloghoge Catchment and landslides (A-D) in the Wicklow Mountains eastern Ireland (from Bourke and Thorp, 2005).
The Cloghoge Valley (53°5΄N, 6°16΄W) is located in the Wicklow Mountains, 28 km south of Dublin City, eastern Ireland (Fig. 1). The upper part of this 35 km2 catchment is a wide, gently sloping moorland valley underlain by the Paleozoic granites of the Caledonian Leinster Batholith. The lower part of the valley, in which the slope failures occurred, is a 250-300 m deep, trough-shaped glaciated valley underlain by mica schist of the granite metamorphic aureole and by the slates and grits of the early Paleozoic.
A rain storm (>1-in-200 yr) following high antecedent rainfall in August 1986 triggered four debris slide-flows on the slopes of the Cloghoge Valley in eastern Ireland. Failures occurred at the bedrock interface underlying shallow (~1 m) soils on slopes between 19° and 35°. In all of the Cloghoge failures, rapid mass movement of soil started with a slide which was transformed into high velocity (3 to 7 ms-1) debris flows that felled trees, stripped bark and smeared mud and pebbles up to a height of 5 m on the upslope side of trees.
Figure 2. Slip face of Failure A (see Fig 1 for location) (from Bourke and Thorp, 2005).
Cobble clusters collected at minor break of slope and small vegetated slabs. Water pipe from road is located at the base of dry stone wall at top right of photograph.
Resistance to shear of the soil ranges from 26 to 32 kN/m2 and liquid limits range between 34 %t and 58 % and these thresholds were exceeded. Factors influencing failure location include local slope morphology, soil depth, preferential groundwater seepage, and natural and anthropogenic surface runoff routing.
Figure 3 Oblique photographs of Failure D on c.19º slope (from Bourke and Thorp, 2005). Sediment in foreground is reworked by overbank flood from the same triggering rainfall event. Floodplain is locally aggraded by a thin debris flow matrix supporting boulders and rafts of soil. Photograph taken looking upslope from floodplain. Figure 4 is based on deposits in foreground of photograph.
Fluvial modification of debris flow deposits
The debris flows were followed by a more fluid phase but there is an absence of evidence for transitional facies found by others (Wells and Harvey, 1987). Two types of fluvial deposits were preserved in failure D. The first is associated with drainage from a gully which flowed from the source area to the distal lobe, depositing very poorly-sorted fine gravel in small (<0.3 m) levées, terminating in a small fan.
A second fluvial unit, composed of moderately well-sorted coarse gravel, is located between the lobe and the Cloghoge River bank. This bar, sourced from the debris flow and post failure gully deposits, was streamlined and winnowed by overbank flow from the Cloghoge River shortly after the debris flow had ceased (Fig. 3). Its sediments are coarser and more angular than the thin sandy overbank deposits on the floodplains upstream (Bourke, 1990). The debris flow sediments, reworked by both gully runoff and overbank flow have lower silt/clay and organic content (<1%) and a higher gravel content (55 to 70%) than the debris flow matrix. No channel change was observed downstream from the landslide. Others have reported the transportation of failure sediments to floodplains and channels (e.g. Alexander, 1986; Carling, 1987; Coxon et al., 1989; Harvey, 1986; Newson, 1975; Newson, 1980; Tomlinson, 1981). The runout of failure D onto the floodplain during the overbank river flood suggests that there is the potential for preservation of slope failure deposits in floodplain sequences in upland Ireland. Figure 4 illustrates the cross section stratigraphy of the debris flow deposit on the Cloghoge floodplain. It is representative of a rainfall triggered failure that is concurrently modified by overbank flow.
Figure 4 Schematised cross-section stratigraphy of debris flow deposits overlying the Cloghoge floodplain (from Bourke and Thorp, 2005). Sections i-iii reflect variations down the debris lobe. The stratigraphy includes large boulders and organic rich gravel and boulder-size inter-clasts, supported in a fine grained matrix. These overlie a fine grained vertical accretion floodplain.
Alexander, R. W., P. Coxon, and Thorn, R.H. (1986). A Bog Flow at Straduff townland, County Sligo. Proceedings of the Royal Irish Academy 86, 107-119.
Bourke, M. C. (1990). “The geomorphic effects of the August 1986 storm on a glaciated upland catchment in the Wicklow Mountains.” Unpublished Masters thesis, University College Dublin.
Brooks, S. M., and Richards, K. S. (1994). The significance of rainstorm variations to shallow translational hillslope failure. Earth Surface Processes and Landforms 19, 85-94.
Carling, P. A. (1987). A terminal debris-flow lobe in the northern Pennines, United Kingdom. Transactions of the Royal Society of Edinburgh:Earth Sciences 78, 169-176.
Colhoun, E. A., Common, R. and Cruickshank, M.M. (1965). Recent bog flows and debris slides in the north of Ireland. Proeedings of the Royal Dublin Society A2, 163-174.
Coxon, P., Coxon, C. E., and Thorn, R. H. (1989). The Yellow River (Country Leitrim, Ireland) flash flood of June 1986. In “Floods: Hydrological, Sedimentological, and Geomorphological Implications.” (K. Beven, and P. Carling, Eds.), pp. 199-217. John Wiley & Sons Ltd.
Douglas, G. R. (1980). Magnitude and frequency of rockfalls in Co. Antrim. Earth Surface Processes and Landforms 5, 123-129.
Dykes, A. P., and Kirk, K. J. (2001). Initiation of a multiple peat slide on Cuilcagh Mountain, Northern Ireland. Earth Surface Process and Landforms 26, 395-408.
Harvey, A. M. (1986). Geomorphic Effects of a 100 year Storm in the Howgill Fells, Northwest England. Zeitschrift fur Geomorphologie 30, 71-91.
IPCC. (2001). Climate Change 2001. Intergovernmental Panel on Climate ChangeThird Assessment Report.
McGreal, W. S., and Lamour, R. A. (1979). Blanket peat erosion: theoretical considerations and observations from selected conservation sites in Slieveanorra Forest National Nature Reserve, Country Antrim. Irish Geography 12.
Moles, R., Moles, N., and Leahy, J. J. (1999). Radiocarbon dated episode of Bronze Age slope instability in the south-eastern Burren, county Clare. Irish Geography 32, 52-57.
Newson, M. D. (1975). The Plynlimon floods of August 5th/6th 1973, pp. 58. Institute of Hydrology, Wallingford.
Newson, M. D. (1980). The geomorphological effectiveness of floods-a contribution Stimulated by two recent events in mid-Wales. Earth Surface Processes 5, 1-6.
Prior, D. B., Stephens, N., and Archer, D. R. (1968). Composite mudflows on the Antrim coast of north east Ireland. Geografiska Annaler 50, 65-78.
Tomlinson, R. W. (1981). A preliminary note on the bog-burst at Carrowmaculla, Co. Fermanagh, November 1979. Irish Naturalist Journal 20, 313-316.
Tomlinson, R. W., and Gardiner, T. (1982). Seven bog slides in the Slieve an Orra Hills, Co. Antrim. Irish Journal of Earth Science 5, 1-9.
Wells, S. G., and Harvey, A. M. (1987). Sedimentologic and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwest England. Geological Society of America Bulletin 98, 182-198.
Whittow, J. (1974). “Geology and Scenery in Ireland.” Penguin Books, Harmonsworth.
Wilson, P., and Hegarty, C. (1993). Morphology and causes of recent peat slides on Skerry Hill, Co. Antrim, Northern Ireland. Earth Surface Processes and Landforms 18, 593-601.
Bourke, M. C., and Thorp, M. (2005). Rainfall-triggered slope failures in eastern Ireland. Irish Geography 38, 1-22.
Bulmer, M. H., Barnouin-Jha, O. S., Peitersen, M. N., and Bourke, M. (2002). An empirical approach to studying debris flows: Implications for planetary modeling studies. Journal of Geophysical Research 107.
Bourke, M. C. (1990) The geomorphic impact of Hurricane Charlie on the Cloghoge Valley, County Wicklow. Geomorphic Hazards in Quaternary Terrains, Geological Survey of Ireland.
Bourke, M.C. (1990) The effects of high magnitude events on the environment, Sligo Regional technical College.
Bourke (1990) The geomorphic effect of the storm of August 25th/26th, 1986 on the Cloghoge river, Co. Wicklow.
Bulmer, M. H., Barnouin-Jha, O. and Bourke, M. (2001). Modeling Mass Movements – Validation for Planetary Studies. Eos Trans. AGU, Fall Meet. Suppl.
Bourke, M. C. (1990). The geomorphic effects of the August 1986 storm on a glaciated upland catchment in the Wicklow Mountains. Unpublished Masters Thesis, University College Dublin.