Illustrative sections depicting landslide susceptibility of the Illawarra Escarpment

Andrew Leventhal and Phil Flentje

The challenge that landslides pose to infrastructure and to domestic and commercial development in the Illawarra region has been recognised in land-use planning for decades. The seminal regional mapping undertaken by Bowman (1974) and, before him, the work reported by Shellshear (1890), set the technical benchmarks for others to follow. The challenges presented to development and road and rail infrastructure were recognised by both Wollongong City Council and major infrastructure providers in the NSW Roads and Maritime Services (previously NSW Roads and Traffic Authority) and NSW Rail Corp (previously State Rail Authority). Continuing support from this group has permitted research and work in the field throughout the Illawarra by Flentje since 1993, which has brought together the collectively faced issues into a composite landslide inventory.

The landslide issues are well recognised by those who are familiar with the Illawarra area – in particular, the typically steep terrain of the 45 km long Illawarra Escarpment, the presence of a colluvial mantle draped over the steep terrain, the presence of many sub-horizontal coal seams throughout the stratigraphy, past and present underground coal mining throughout the region and intense rainfall events generated by virtue of its location and also as a consequence of the escarpment’s influence upon local meteorology with the orographic rainfall.

One of the challenges of the application of Bowman (ibid) is the mapping base available at that time. Bowman reported mapping at 8 chains to the inch (1:6,336) but appears to have used a basemap enlarged from a much earlier base (possibly enlarged from 2 inch to the mile, viz: 1:31,680, or 1 inch to the mile, viz: 1:63,360). Bowman also noted the poor edge matching of his basemaps in his work. The NSW Central Mapping Authority 1:4,000 scale 2 m contours generated in the late 1970’s provided a significant enhancement to the mapping base, though draping of the Bowman mapping over this improved basemap faced obvious challenges. Recently, the availability of the contours in electronic format and the rise of both Geographic Information System (GIS) capability and expertise, have greatly facilitated mapping in the Illawarra. Subsequent Airborne Laser Scan (ALS) digital terrain mapping at high resolution has also recently become available, which together with Flentje’s detailed mapping (Flentje, 1998) and access to landslide records of Wollongong City Council (through council’s Geotechnical Engineer, Peter Tobin) has permitted enhanced understanding of the specific conditions and characteristics that influence the Illawarra landscape. This area is also the scene of the Engineering Geology course run biennially on behalf of the Australian Geomechanics Society (2010).

As no doubt is often the case, a simple question triggers a line of action, and the raison d’etre for this paper fits that pattern. In this case, the question asked of themselves by the authors was “Is there a type-section of landslide issues within the Illawarra?” This paper, and the development of the illustrative sections herein, is a response to that somewhat rhetorical question.