to download report - Geological Survey of Ireland

More documents

Recommendations

Info

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
Due to the difficulty of estimating probabilities of landslides in Ireland, the estimation of the likelihood of anevent can at best be only qualitative. As alluded to by Aleotti and Chowdbury (1999) the challenges posed bythe probabilistic component of hazard assessment, coupled with assessing both vulnerability and theuncertainties associated with both of these aspects, frequently the best that can be achieved is an assessmentof susceptibility. They define susceptibility as the possibility that a landslide will occur in a particular area onthe basis of the local environmental conditions. In Ireland susceptibility assessment based on what can betermed an “environmental pre-condition” approach offers the best way forward for engaging with the hazardposed by landslide.5.1.3 Landslide susceptibility assessmentProcedureIrrespective of the form of hazard and risk assessment employed, whether quantitative, qualitative or a combinationof both, a number of steps are required for the successful implementation of a national strategy for landslidehazard assessment and mitigation. These are:• the development of a national inventory of landslide events.• the development of maps to show areas where the potential for landslides exists• the development of appropriate guidance and standards arising from the aboveInventoryThe development of a national inventory is vitally important as part of the initial investigation of landslides in anIrish context. It is generally accepted that landslides are more likely to occur in areas of previous occurrence.This is based on the fact that there are a number of key factors that are important in determining the occurrenceof landslides. These include geology - type and structure, soil type, topography and slope angle and form.These factors will generally occur in zones and the previous occurrence of a landslide event in such a zonemust be recognised as significant in indicating the potential of future occurrence, given a similar triggeringmechanism.Susceptibility MappingA key use of a landslide inventory map is the location of landslide events relative to these factors, whichfacilitates the analysis of potential causative factors. By determining the spatial relationships between landslideoccurrence and pre-existing environmental conditions the complex interaction of these factors in the causativesequence of slope failure can be better understood. The recognition of these environmental factors also meansthat their occurrence elsewhere can be determined, leading to the development of maps of potential futurelandslide occurrence. As such the susceptibility of other areas to landsliding can be established. The developmentof landslide susceptibility mapping is a key component in landslide hazard assessment.Susceptibility mapping also allows the incorporation of the knowledge and experience gained by the geotechnicalengineering community into landslide hazard assessment. A substantial body of work exists in the area ofslope stability analysis. While the results of such analysis, e.g. Factor of Safety indices etc., are often drivenby site-specific investigation or laboratory experimentation, incorporation of these elements into a broadermapping framework allows assessment at a regional level. While exposed to the potential criticism that thesestudies are developed on a local scale and should not be used outside of the scale of their development, in theabsence of raw data to drive an empirical approach, their inclusion gives hazard assessment a defensiblestarting point for the spatial allocation of susceptibility outside the local scale.Appropriate guidance and standardsThe ultimate aim of any inventory and mapping effort is to ensure the reduction of risk to human life andproperty. It is essential therefore that the results of the inventory and susceptibility mapping be taken accountof in both the planning and regulatory frameworks and by individual citizens. The provision of information to thegeneral public facilitates informed decision making and the incorporation into the planning system ensures thatthe recognition of risk is formalised and acknowledged in the planning process with the aim of mitigating risk.The issue of planning is dealt with in Chapter 6.35
Page 1 and 2: Landslides in IrelandGeological Sur
Page 3 and 4: LANDSLIDESinIRELANDEditorRonnie Cre
Page 5 and 6: CONTENTSPreface ...................
Page 7 and 8: Fig. 4.3 Forces caused by water in
Page 9 and 10: PREFACEUntil recently Ireland has b
Page 11 and 12: een very important in populating th
Page 13 and 14: involves a report on the current st
Page 15 and 16: Main Objectives• Increase public/
Page 17 and 18: 1. INTRODUCTIONRonnie Creighton1.1
Page 19: 1.3 The Landslides PublicationThis
Page 22 and 23: Peat flows are not nearly so well d
Page 24 and 25: In Ireland the Quaternary or uncons
Page 26 and 27: Journal - published papersLetter -
Page 28 and 29: Fig. 3.1 Two images showing the use
Page 30 and 31: Table 3.1 Landslide Events Per Coun
Page 32 and 33: Fig. 3.3 Location map of the Pollat
Page 34 and 35: 3.7.4 Quaternary GeologyQuaternary
Page 36 and 37: 3.8 The Derrybrien Landslide - 2003
Page 38 and 39: • The classification scheme shoul
Page 40 and 41: The force causing the block to slid
Page 42 and 43: Plate 4.1 Herringbone drainage syst
Page 44 and 45: 1210Number of events864Raised bogBl
Page 46 and 47: It is well known by research at Uni
Page 48 and 49: 5. LANDSLIDE SUSCEPTIBILITY MAPPING
Page 52 and 53: 5.1.4 Mapping and GISBy definition,
Page 54 and 55: Platform / sensor pixel resolution(
Page 56 and 57: For this initial run, the criteria
Page 58 and 59: 42Fig. 5.5 Peat and peaty podzols f
Page 60 and 61: for susceptibility mapping purposes
Page 62 and 63: elationships between developed susc
Page 64 and 65: The methodology used to derive the
Page 66 and 67: 50Fig. 5.12. Landslide susceptibili
Page 68 and 69: 52Table 5.6. Landslide classificati
Page 70 and 71: The percentage of events occurring
Page 72 and 73: with slope, overall 20-25% of all e
Page 74 and 75: As the dataset was not available at
Page 79 and 80: CHAPTER 5.2 - TABLE APPENDIXBedrock
Page 81 and 82: 6. LANDSLIDES AND PLANNINGAileen Do
Page 83 and 84: 6.2.4 Development ManagementThe phy
Page 85 and 86: 6.5 Current Practice on Landslides
Page 88 and 89: 7. LANDSLIDES IN NORTHERN IRELANDTe
Page 90 and 91: 2. MudflowsPlate 7.1 Rotational Lan
Page 92 and 93: 7.2 Carboniferous Cliff Lines (Co F
Page 95 and 96: 8. RESEARCH ON IRISH LANDSLIDESKoen
Page 97 and 98: 8.4 Research. Post-2003 AbstractsTa
Page 99 and 100: Identifying, recognizing, and predi
Page 101 and 102:
In the case of vulnerability mappin
Page 103 and 104:
Steep slopes in glacial tillMichael
Page 105 and 106:
9. RECOMMENDATIONS FOR FUTURE WORK9
Page 107 and 108:
Priority 1 Peat slides and peat str
Page 109 and 110:
TEXT REFERENCESAcreman, M., 1991. T
Page 111 and 112:
Skipper, J., Follett, B., Menkiti,
Page 113 and 114:
APPENDIX 2Glossary of TermsBlanket
Page 115 and 116:
APPENDIX 3Nomenclature for Landslid
Page 117 and 118:
APPENDIX 4101
Page 119 and 120:
103
Page 121 and 122:
105
Page 123 and 124:
Hammond, R. 1979. The Peatlands of
Page 125:
APPENDIX 7Useful Web Linkswww.gsi.i
show all

to download report - Geological Survey of Ireland

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?