Observational approach for urban landslide management

The occurrence of landslides in urban hillside areas poses significant challenges for risk management within residential suburbs as well as along road and railway lines. The main focus of this paper is the development of an “observational approach” for landslide risk management and its application within an important landslide study area, the Wollongong Local Government Area, in the state of New South Wales, Australia. Monitoring of subsurface shear movement and pore water pressures at a number of places in the study area during the last decade has already proved very useful in combination with rainfall data from a number of existing rainfall stations. Analyses of observational data have led to the assessment of rainfall triggering thresholds for the occurrence of landslides. Four new real-time monitoring landslide field stations have now been established for obtaining continuous data on subsurface shear movements, pore water pressures and rainfall in real time. The data is transferred automatically to a web-server and displayed on the web. These recent developments have resulted in a vast improvement in research techniques for understanding the initiation and progression of landslide movement triggered by rainfall. The data will prove to be very useful in the management of risk during rainstorms in real time or near real time. Moreover, over the medium to long-term, such accurate data will facilitate improvements in the planning and management of assets and land-use

Risk assessment of sloping land using GIS-based mapping

In this paper, attention is drawn to GIS-based techniques for mapping and risk assessment for slopes and landslides. The authors focus on an approach developed recently for mapping of geology and slope stability in the Northern Illawarra (or the Northern Suburbs of Greater Wollongong Area) in New South Wales, Australia. The use of databases of geology and land instability and use of other GIS-based information relevant to sloping land for the development of a valid hazard and risk assessment procedure is explored. The developed approach will be applicable to other sloping areas and, in particular, urban areas both nationally and internationally

Bulli Pass Landslide Risk Management Part 1 – Hazard Assessment

The Princes Highway along Bulli Pass is a narrow, heavily trafficked two lane section of the Princes Highway that traverses steep slopes on a grade of 9H:1V on the Illawarra Escarpment, about 11 km north of Wollongong, and 75 km south of Sydney in New South Wales (NSW), Australia. It is an important arterial road for the northern suburbs of Wollongong, connecting Mt Ousley Road (M1 Princes Motorway) at the crest of the escarpment to the suburb of Thirroul on the coastal plain at the base of the escarpment. Bulli Pass has a long history of landslide and rockfall events, some of which were reported as early as 1890. One of the most significant of these events occurred on 17 August 1998 during a 1 in 100 year rainfall event. The 1998 landslide event comprised approximately 38 debris flows and slides and numerous rockfalls which partially inundated a number of cars and trapped about 15 cars on the pass. More recently, in early 2015, a small rockfall penetrated the windscreen of a car travelling up the pass. Transport for New South Wales (TfNSW) commissioned an investigation into slope instability hazards affecting the road in late 2011. This was followed in 2015 by a Risk Mitigation Options study and the detailed design of risk mitigation works in 2016. This paper provides an overview of the methods used to investigate hazards and assess risk at the site over a five year period. This has included research into the landslide history, geomorphological mapping, acquisition and review of airborne laser scanning (ALS) data, review of rainfall data and the development of a landslide volume frequency model. The development of this model allowed hazards to be readily communicated and risks to be assessed. The actual design and construction of the Shallow Landslide Barriers and the Debris Flow Barriers that followed on from these assessments will be discussed in a subsequent companion paper

Massive landsliding in Narrabeen sandstones in the Watagan region

The Narrabeen Group is a thick sequence of bedded Triassic sandstones which occupies the stratigraphic interval between the Permian coal measures and the Hawkesbury Sandstone in the Sydney Basin. In the northern Sydney Basin it is laterally extensive, extending throughout the southern Hunter Valley from the Central Coast to beyond the Great Divide. In the Watagans region, a unique situation arises where a disconformity causes the Narrabeen group to be underlain directly by Permian upper marine sediments. The result is a dramatic occurrence of ancient massive landsliding, leading to steep sandstone slopes that break to an elevated detrital plane, of considerable lateral extent. This paper describes the unusual geomorphic features associated with the landslide mass and interprets a variety of individual mass-movement and rockfall mechanisms which have contributed to these impressive features in the various stages of its development

Prehistoric landslides: significance, recognition, examples

Prehistoric landslides with a wide range of ages and sizes exist worldwide in both rock and soil. Many are thought to have occurred during Pleistocene time when climates in some areas were harsher and wetter. Subsequent weathering and erosion have subdued topography and other features of prehistoric landslides, often making them difficult to recognize. Recognition is the key to dealing with prehistoric and other old landslides. Old slide masses are usually only marginally stable because past movements reduced available shear strength on their failure surfaces to residual levels. These masses are susceptible to reactivation by construction activities, heavy precipitation, and earthquakes. If prehistoric landslides are recognized, they can be avoided or steps taken to minimize interference with them. Where they cannot be avoided, they must be stabilized or lived with. Stabilization typically involves robust retaining structures, large buttress fills, or excavation of most of the slide mass - all of which are expensive. Living with old landslides may involve continuing maintenance for distress caused by creep or other movements; this can also be expensive. Problems arise when prehistoric and other old landslides are unrecognized, then reactivated during or after construction. Then, unexpected ground movements cause damage, increase costs, set back construction schedules, and disrupt partially completed or completed facilities and operations. Geologic considerations, features of prehistoric and old landslides, and guidance for recognizing them are presented. Then examples of prehistoric landslides in the United States, Papua New Guinea, and Australia are given

An evaluation of airborne laser scan data for coalmine subsidence mapping

The accurate mapping of coalmine subsidence is necessary for the continued management of potential subsidence impacts. The use of airborne laser scan (ALS) data for subsidence mapping provides an alternative method to traditional ground-based approaches that affords increased accessibility and complete spatial coverage. This paper evaluates the suitability and potential of ALS data for subsidence mapping, primarily through the examination of two pre-mining surveys in a rugged, densely vegetated study site. Data quality, in terms of mean point spacing and coverage, is evaluated, along with the impact of interpolation methods, resolution, and terrain. It was assumed that minimal surface height changes occurred between the two pre-mining surfaces. Therefore any height changes between digital elevation models of the two ALS surveys were interpreted as errors associated with the use of ALS data for subsidence mapping. A mean absolute error of 0.23 m was observed, though this error may be exaggerated by the presence of a systematic 0.15 m offset between the two surveys. Very large (several metres) errors occur in areas of steep or dynamic terrain, such as along cliff lines and watercourses. Despite these errors, preliminary subsidence mapping, performed using a third, post-mining dataset, clearly demonstrates the potential benefits of ALS data for subsidence mapping, as well as some potential limitations and the need for further careful assessment and validation concerning data errors

A photographic essay on landslides across southeastern New South Wales triggered by the rainfall events of 2022

Eastern Australia has experienced a significant magnitude rainfall event of extended duration in the first 7 months of 2022. Across the east coast of New South Wales (NSW) a series of troughs and East Coast Lows occurred during a La Nina weather cycle bringing above average rainfall to the region. As this first half of 2022 La Nina event was drawing to a close the Indian Ocean Dipole entered a negative phase which coincided with another intense East Coast Low in early July 2022 impacting the Illawarra region of NSW. These events caused widespread flooding and significant landslide damage to road and rail infrastructure across the state networks and local government infrastructure across NSW. During this extended wet period in the first 7 months of 2022 more than 200 landslides have been recorded across the Illawarra, Southern Highlands and Blue Mountains regions of NSW whilst many more have occurred across the north coast region. This paper presents a brief and albeit preliminary summary of the rainfall and provides a series of photographs with very brief descriptions of some of these landslide events within southeastern NSW. The intent of the paper is to provide early guidance to AGS members of the nature and form of landslides that have occurred across the Illawarra region. This paper does not discuss the dual fatality resulting from the Wentworth Falls area rockfall of the 5th April

Mitigation of landslide impacts, strategies and challenges for the 21st century

Reliable methods for mitigation of landslide impacts must be based on the latest developments in knowledge and advanced methods of analysis and synthesis. An interdisciplinary approach is essential for effective solutions to landslide problems. Prevention or mitigation of landslide disasters requires understanding the factors which often lead to catastrophic landsliding and the simulation of conditions under which such failures may occur. The analysis of rainfall-triggered landsliding in saturated soils requires a proper understanding of how the factor of safety decreases with increase in pore water pressure. On the other hand, for landsliding in unsaturated soils it is necessary to understand the decrease of factor of safety with decrease in soil suctions (negative pore pressures) which is associated with increase in the field water content during rainfall infiltration. Landslide risk management requires understanding and assessment of susceptibility and hazard. In particular, for regional assessment, landslide susceptibility and hazard mapping and zonation are necessary. Mitigation of landslide impacts is facilitated by the use of research -based thresholds of rainfall and/or pore water pressure and/or displacement. These thresholds can be used for the development and application of early warning systems. Monitored data on landslide movements and pore water pressures can be very useful for updating hazard and risk scenarios. Such data also contribute to the capacity for landslide management in near real-time. The paper refers to some of the findings in the regional case study from Wollongong region, New South Wales Australia

High-Resolution Topographic Data for Subsidence Impact Assessment and SMP Preparation: Methods and Considerations

Corporate and social requirements relating to sustainable mining practices have resulted in an increasing need for identification and assessment of natural features that may be susceptible to coalmine-induced subsidence. Natural features such as, cliff lines, watercourses and steep slopes, that are typically susceptible to subsidence-induced impacts can often be identified and quantified using high-resolution topographic data and a geographic information system (GIS). Once identified, digital representations of these features can be used in the impact assessment process and for Subsidence Management Plan (SMP) preparation. This paper demonstrates the use of topographic data for site characterisation and feature identification purposes by mapping susceptible areas for a study site, including valley floors, steep slopes, drainage lines, and erosion-prone areas. It also discusses the potential use of topographic data and GIS for assessing subsidence impacts through knowledge- and data-driven approaches. The assessment of pre- and post-subsidence hydrological conditions is also shown for two swamps within the study area. The area over the proposed Dendrobium Area 2 operation in the Southern Coalfield was chosen as a case study site, and high-resolution airborne laser scan data were acquired for the site from BHP Billiton