to download report - Geological Survey of Ireland

EXECUTIVE SUMMARY1. IntroductionLandslides are a major cause of substantial damage to property and loss of life every year across the globe.They are a major geohazard and can be triggered by earthquakes, volcanic eruptions, heavy rainfall, or indeedby man-made activities. Ireland is not a high risk area for major landslide events and in fact is a relatively benignenvironment in this regard compared to other countries. However the historic record does contain a few seriousevents such as that at Castlegarde in Co. Limerick when twenty-one people died. The events at Pollatomishand Derrybrien in 2003 brought this issue to the fore, and it was clear that there was no collated body of dataeither on the historic record or the susceptibility of areas to landslides in the future.In early 2004 the Geological Survey of Ireland (GSI) established a multi-disciplinary team, the Irish LandslidesWorking Group (ILWG), with expertise in geology, geomorphology, geotechnical engineering, planning, andGIS. The main objectives were:-1. Build a national database of past landslide events.2. Examine geotechnical parameters with regard to landslides.3. Assess the potential for landslide susceptibility mapping in Ireland.4. Make recommendations on the integration of landslide hazard issues into the planning process.5. Promote landslide research in Ireland.6. Raise public awareness about landslide hazard in Ireland.The Group did not have the resources within its timeframe to document submarine slope failures or coastallandslides. Also it did not have the remit to do site-specific studies at landslide events.The Group was also of the view that the work should be done on an all-Ireland basis and welcomed theparticipation of the Geological Survey of Northern Ireland in the project.2. Landslide ClassificationAs with many natural phenomena landslides have proved difficult to classify because of their inherent complexitywith regard to movement and material types. The classification used is based on that of the British GeologicalSurvey (BGS) and Varnes, 1978. Movement types are listed as flows, slides (rotational and translational), falls,topples, spreads, and complex. Earth materials range from clay-size particles up through boulder-size to solidbedrock. This grading classification is also combined with water content to give a fuller description. The materialsclassification used is also that adopted by the BGS in their Geohazards Programme. There is one crucialaddition to it for the Irish context and that is the inclusion of peat as a significant material. The main materialtypes are therefore rock, debris, earth, mud, and peat, which are defined in detail. The causes of landsliding arealso complex and are the subject of substantial research worldwide. The myriad of factors can be divided intotwo groups. Firstly there are the conditioning factors which relate to the inherent nature of the slope in question– rock/soil type and their geotechnical properties, slope gradient and profile, slope drainage and permeability,and land cover. Secondly there are the triggering factors which act on the slope to initiate the landslide. Theseinclude earthquakes, volcanic eruption, heavy rainfall, natural erosion, and man-made causes such asundercutting and land drainage.3. The Irish Landslides DatabaseCrucial to the study of landslide hazard in Ireland is data on past landslide events through the creation of alandslides database for the island of Ireland. An exhaustive search has been made of a wide range of sourcesand an Access database has been developed in GSI. Using this baseline information and modern landscapedatasets, areas which might be susceptible to landsliding can be identified. GIS has been a very useful tool indefining more accurately the location, and determining the conditioning factors at any site. Digital maps ongeology, topography and drainage, and land cover among others, as well as digital aerial photography, haveii