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for susceptibility mapping purposes. The table shows that the newly defined peat class for input to thesusceptibility modelling process would potentially include some peaty podzol categories which are a mineralsoil with a peaty surface horizon. This is primarily due to the fact that these classes mapped by soil surveyinclude both peaty podzols and peat which, in hill and mountain environments, often intergrade into each otherover short distances.Similarly it was decided to include all peat derived from this selection process despite the criteria suggestinguse of peat greater than 50cm in depth. This was again deemed justifiable on the basis of the inherent variabilityof soil/peat classes in the field. Combined with the established field mapping specification for peats whichrequires them to be >40cm in depth in an undrained state it was considered that for conservative purposes insusceptibility mapping all peat classes derived from this modelling process would be included.The model process for the second run susceptibility map is described in Fig. 5.7. The second susceptibilitymap is shown in Fig. 5.6. Here the susceptible areas have not been adjusted for display purposes. It isapparent that there is a significantly greater area classified as susceptible in this output.Fig. 5.7 Model schematic for second susceptibilty mapFor the final model run both criteria for landslide hazard assessment were included in the map process. In thisfinal run of the susceptibility map, the output of the second run susceptibility map was combined with allsubsoils mapped as peat. Once again, all peat mapped in the subsoils map was included on the basis that inthe majority of cases most mapped peat will approach or be greater than the 50cm depth limit specified by thehazard criteria. The results of this model run are shown in Fig. 5.8.Once again there is a significant increase in the area mapped as susceptible to landslide. Fig. 5.9 shows theareas mapped as susceptible by all three approaches as percentages of the total land area of Co. Mayo.Whereas previously the areas mapped as susceptible were confined to predominantly upland areas, whereslopes would be expected to be in excess of 15º, the inclusion of the criteria relating to peat greatly extendsthe area and range of susceptible areas, accounting for 42% of the total land area in Co. Mayo. While appearinglarge, this figure is not necessarily an incorrect or exaggerated result. When viewed in light of the eventsrecorded in the landslide database and their coincidence with susceptible-mapped areas, the results suggestthat there is merit to the development of susceptibility mapping in the manner described here. This is furthersupported by the fact that in all three model runs the Pollatomish area is identified as being susceptible tolandslide hazard. However, uncertainties inherent in the model output should be investigated fully.It is important to note that this case study focuses only on organic soils and their susceptibility to landslide.Mineral soils and rock areas have not been evaluated. Furthermore, the issue of run-out zones, which are areasoccurring in landslide fall-lines, has not been examined here and such areas have not been incorporated intothe map. It could be argued that the investigation and mapping of run-out zones should be deemed a veryimportant area for future research.Susceptibility maps tend towards mapping landslide initiation sites only, which are those sites where theenvironmental conditions prevailing suggest the possibility of a landslide hazard. However those areas downslopeof the initiation site will not be mapped in many susceptibility efforts as these will not exhibit conditions knownto be associated with landslide initiation. These areas in the fall-zone are more likely to be inhabited ordeveloped and therefore where the proper assessment of risk of landslide events should be targeted.44