to download report - Geological Survey of Ireland

Two simplified hypothetical examples serve to mark the extremes of high and low risk. An overhanging cliff facethat is considered to be active, in the sense that rock falls are a frequent occurrence, situated in a remote andinaccessible mountain valley, will have a negligible risk associated with it. However to build a school under acliff face even if the cliff has been stable for as long as records have been maintained would pose an unacceptablyhigh risk because of the adverse impacts on human life and property that would follow a failure in the cliff face.The upper right cell in Fig. 5.1 represents the first case while the latter case of the school would be situatedtowards the lower left corner.Fig. 5.1 Example of qualitative risk matrix (after Lee and Jones, 2004)The extent of risk occurring between these two extremes is harder to determine and yet is arguably much moreimportant to fully understand. Low to medium magnitude events will often attract less attention than highmagnitude events, and the probability of occurrence of all magnitude events is often not well understood. Putanother way, just because we have no record of damaging landslides in an area does not mean that theycannot occur. The situation can therefore arise that human development and activity will occur in areas of highlandslide risk, primarily because the hazard has not been identified and consequently the risk not assessed.Types of Risk assessmentThere are two main approaches to presenting an assessment of risk. These are:Qualitative risk assessment. This involves the expression of the likelihood of an event occurring and theextent of its adverse consequences being expressed qualitatively. The most common representation of qualitativerisk assessment is in the form of risk matrices as shown in Fig. 5.1.Quantitative risk assessment. This involves quantifying the probability of an event occurring and expressingin real terms the losses that would arise from such an event.It might be possible to include a third broad category here which would span these two approaches. This formof risk assessment might include an expression of the probability of an event occurring with a qualitativerepresentation of the adverse costs, or a qualitative expression of the likelihood of an event with a quantificationof resulting costs.While at first glance, quantitative risk assessment would appear to be the most desirable, in practice theattainment of an accurate estimation of risk in this form is extremely challenging, if not impossible in an Irishcontext. The requirement of extensive amounts of data to estimate the probability factor alone most likelyexcludes its use in this country. Fundamentally as a method it is extremely vulnerable to the criticism of falseprecision, where the expression of risk in numerical terms makes it appear more accurate than it actually is.On the other hand the benefit of qualitative risk assessment is that it is simply expressed and thereforeperhaps more easily understood. This too has its weakness in that it could be argued that the method isgrossly over-simplistic as an approach to dealing with landslide hazard. It may be the case that in Ireland themost appropriate, and importantly, the most pragmatic approach would aim towards a semi-quantitative methodwhere a qualitative expression of likelihood is combined with a detailed estimation of the potential costs arisingfrom a landslide.Such a method would potentially result in a very powerful tool to assist the appropriate authorities in dealingwith landslide hazard in Ireland. In areas where a strong case for the potential of landslide hazard is identified,local authorities could adjust planning guidance as necessary. Similarly, infrastructure providers could takeaccount of the estimated potential loss in real terms when deciding on the location of assets such as powerlines. Even agencies charged with matters such as the management of natural resources would have valuableinformation to aid in their management strategy and to guide them in a cost-benefit analysis of implementingmitigation measures to the potential damage caused by landslides. The peat landslide in Derrybrien in 2003 isestimated to have killed over 50,000 fish (SRFB, 2003).34