Rapid methods of landslide hazard mapping : Papua New Guinea case study

A landslide hazard probability map can help planners (1) prepare for, and/or mitigate against, the effects of landsliding on communities and infrastructure, and (2) avoid or minimise the risks associated with new developments. The aims of the project were to establish, by means of studies in a few test areas, a generic method by which remote sensing and data analysis using a geographic information system (GIS) could provide a provisional landslide hazard zonation map. The provision of basic hazard information is an underpinning theme of the United Nations International Decade for Natural Disaster Reduction (IDNDR). It is an essential requirement for disaster preparedness and mitigation planning. This report forms part of BGS project 92/7 (R5554) ‘Rapid assessment of landslip hazards’ carried out under the ODA/BGS Technology Development and Research Programme as part of the British Government’s provision of aid to developing countries. It provides a detailed technical account of work undertaken in a test area in the highlands of Papua New Guinea (PNG) in collaboration with the Geological Survey Division. The study represents a demonstration of a methodology that is applicable to many developing countries. The underlying principle is that relationships between past landsliding events, interpreted from remote sensing, and factors such as the geology, relief, soils etc. provide the basis for modelling where future landslides are most likely to occur. This is achieved using a GIS by ‘weighting’ each class of each variable (e.g. each lithology ‘class’ of the variable ‘geology’) according to the proportion of landslides occurring within it compared to the regional average. Combinations of variables, produced by summing the weights in individual classes, provide ‘models’ of landslide probability. The approach is empirical but has the advantage of potentially being able to provide regional scale hazard maps over large areas quickly and cheaply; this cannot be achieved using conventional ground-based geotechnical methods. In PNG, landslides are usually triggered by earthquakes or intense rain storms. Tectonic instability and the extreme ruggedness of the terrain make the highlands very susceptible to landsliding, but the extent to which regional factors influence the distribution and severity of landsliding is uncertain. The report discusses the remote sensing and GIS methodology, and describes the results of the pilot study over an area of approximately 4 500 km2 in the Kaiapit/Saidor districts of the Finisterre mountain range. The landslide model uses geology, elevation, slope angle, lineaments and catchments as inputs. The resulting provisional landslide hazard zonation map, divided into 5 zones of landslide hazard probability, suggests that regional controls on landslide occurrence do exist and are significant. It is recommended that consideration be given in PNG to implementing the techniques as part of a national strategic plan for landslide hazard zonation mapping