Computer based landslide hazard and risk assessment (Northern Illawarra Region of New South Wales, Australia)

Quantitative risk assessment should complement, if not replace qualitative assessments as the preferred approach for hazard and risk assessment and management of land instability. Therefore, a comprehensive Landslide Risk Management Procedure has been proposed in this thesis. In a significant step towards a quantitative approach for risk assessment, a hazard ranking of 328 documented landslide sites within the study area has been achieved during this research project. This hazard ranking of existing landslides is considered to be unique and the first quantitative landslide hazard assessment in Australia. A new series of geology and land instability maps has been prepared, the Geotechnical Landscape M a p Series (GLMS). These maps have been produced using the Wollongong City Council\u27s ( W C C ) Geographic Information System computer package. The maps can be easily updated as new information becomes available. A total of 328 sites of land instability have been documented in a landslide inventory of the study area. A Site Reference Code numbering system has been used to identify the sites of instability. The inventory comprises of two separate but essential and interrelated components: •a land instability m a p record, one map layer of the Geographic Information System computer package based Geotechnical Landscape Map Series • a computer-based database summarising 60 aspects of each site of land instability. The GLMS and the accompanying Land Instability Database will provide a greatly improved graphical representation and database of geology and land instability hazards within the local government area. The importance of this information as a land management tool cannot be overstated. The excellent results achieved in the production of the G L M S and a subsequent spatial analysis of the geology and landslide areas confirms the viability and resourcefulness of the GIS computer package for this project. This thesis is not only concerned with the spatial distribution of landsliding within the study area, but also with recurrences of instability over time. Therefore, the analysis of historical recurrences as well as that of recorded shear movements at depth has been undertaken in relation to detailed rainfall records. A unique an innovative procedure has been developed which calculates the percentage exceedance (determined using a base 20 year period of daily rainfall) of antecedent rainfall for five specified (7, 30, 60, 90 and 120 day) daily rolling periods. Percentage exceedance (magnitude and period) of antecedent rainfall which correlates with slope \u27failure\u27 or with accelerated landslide movement can be examined. This approach is called the Antecedent Rainfall Percentage Exceedance with Time (ARPET) technique. Landslide movement, specifically rates of shear displacement at depth have been determined with borehole inclinometer monitoring records for a total of 19 sites within the subject area. Following a comparison of these inclinometer monitoring records with daily and antecedent rainfall it has been concluded that the timing of \u27acceleration\u27 and \u27failure\u27 of a landslide show the best correlation to antecedent rainfall periods of 90 and 120 days. On some occasions, a correlation between 60 day antecedent rainfall and landslide movement is apparent. It has been shown that acceleration of a landslide can be expected when the 60, 90 and 120 day antecedent rainfall magnitudes increase above 275mm, 340mm and 400 m m respectively. Furthermore, a landslide \u27failure\u27 m ay be expected if the 60, 90 and 120 day antecedent rainfall magnitudes increase above 500mm, 825mm and 980 m m respectively. ARPET values, or exceedance probabilities corresponding to these antecedent rainfall values have been determined. Three case study landslide sites have been examined in detail. Computer based stability analyses of these sites have been completed. These case studies have demonstrated many of the characteristics and complex inter-relationships between landsliding and the environment in the Illawarra