Slope instability, hazard and risk associated with a rainstorm event - a case study

Over the last few decades there has been an increasing awareness of landslide hazard and risk in many coastal regions of Australia. Urban communities in hilly areas are, from time to time, adversely affected by rainfall-induced landsliding. However, acute awareness of hazard may be absent during periods between significant rainstorm events. In general, the serious consequences of landslides to property and life have been underestimated in Australia. It is now known that at least 80 deaths can be attributed to a number of landslides (Leiba, 1998). Some of these events have focussed the attention of the public, the most important being the Thredbo tragedy associated with the loss of 18 lives at the Thredbo in the Snowy Mountains region of New South Wales on July 30, 1997. That landslide was, however, not the consequence of a rainstorm event unlike most of the slope instability that occurs in Australia. This paper is concerned primarily with landsliding associated with the August, 1998 rainstorm event which affected the hilly suburbs of Wollongong along the Illawarra escarpment south of Sydney (Fig.1). The importance of protecting the escarpment is highlighted by the report of a recent commission of enquiry ordered by the State Government of New South Wales in recognition of widespread and sustained public concern (Commission of Inquiry, 1999)