Roberts, Dr Nicholas1
1Mineral Resources Tasmania, Rosny Park, Australia
Tasmania experiences frequent, diverse landslides ranging from extremely slow failures that gradually damage structures to extremely fast ones capable of claiming lives. Their impacts in recent decades might give the false impression that Tasmanian landslides threaten only infrastructure and not lives. However, life-loss risk is poorly constrained – and generally underappreciated – because the state’s landslide fatalities have not been fully inventoried. Details from a growing catalogue of Tasmanian landslide fatalities (excluding underground failures) show that deaths, although sporadic, do occur and were surprisingly common during the nineteenth and earliest twentieth centuries. Landslides have injured at least 23 people in the last 100 years. Fourteen of the injuries and four of the five confirmed landslide fatalities from that period occurred in 2001 when a road shoulder collapsed under the weight of a bus. Although sometimes conflated with mass movement, the rainfall-triggered burst of a rock-filled concrete dam above Derby in 1929 that claimed 14 lives was unrelated to landsliding. Tasmania’s at least eight landslide fatalities prior to 1920 provide additional insight into life-loss potential. Deadly landslides occurred during construction of Port Arthur’s Convict Church (ca. 1836), quarrying in Hobart (1848) and Queenstown (1905), railway expansion through Kelly Basin (1889), and open-pit mining at Mount Bischoff (1900). All but the 1848 failure, which killed four, were single-fatality events, providing a stronger historical basis for calibrating quantitative estimates of individual risk compared to group risk. Commonalities between these early events highlight settings and processes of particular concern as well as a population of elevated exposure. Each failure affected very recently excavated slopes, involved extremely rapid sliding and flow of soil-strength materials, and exclusively claimed lives of workers (convicts before 1850, employees thereafter). Coronial Inquests into each of the events from 1848 to 1905 provide more detail about failure metrics and behaviour than is commonly available for landslides of that era. All eight deaths were ruled accidental despite knowledge that many sites were failure-prone. Several close calls prior to 1920 highlight risks of members of the public being engulfed by debris flows or impacted by landsides at home. However, fatalities from hazardous phenomena that commonly influence (bushfires) or accompany landslides (flash flooding) remain more common. Although Tasmanian landslide fatalities superficially appear to have decreased, determining and explaining trends in these records is complicated by data sparsity. Advances over the past century in engineering, workplace safety, and regulation undoubtedly affect risk levels, but evaluating other possible influences such as long-term drought-rain cycles requires further work. Decreases in per-capita landslide fatalities elsewhere are attributed to progressive improvements (e.g. British Columbia) or drastic reforms (e.g. Hong Kong) in local risk management; circumstances in Tasmania are closer to the former, although the low number of deaths complicates comparisons. Notwithstanding these challenges, it is noteworthy that fatal landslide in Tasmania show several commonalities, are generally underestimated, and are possible in the future, particularly as population and development increase.
Nick is a natural hazards geologist and Quaternary geoscientist in the Geological Survey Branch of Mineral Resources Tasmania. A large part of his work centres on characterizing landslides and their impacts across Tasmania through a combination of field investigations, diverse remote sensing techniques, analytical tools, and reviewing historical documents.