History of slope instability in the Oldina plantation, Tasmania

Abstract

Background: This paper describes a landslide swarm generated by exceptionally high two-day rainfall (c. 300 mm) associated with a stationary cut-off low pressure system over northern Tasmania in early June 2016 and investigates evidence for previous slope instability. The landslide swarm occurred in a recently harvested plantation in the Inglis River catchment at Oldina, south of Wynyard in north-west Tasmania. Within a relatively small area of plantation underlain by weathered Permian tillite and minor siltstones more than thirty rapid earthflows, rotational and translational landslides occurred. Many landslides also occurred in the nearby Forth and Mersey River catchments. Methods: Field observations combined with a digital elevation model produced from high-resolution drone imagery were used to describe the morphology of the Oldina landslides, and to calculate the mass of soil, sediment, and woody debris displaced. Radiocarbon dating of charcoal exposed in landslide backwalls enabled palaeo-landslides and periglacial activity at Oldina to be dated. Results: An estimated 48,400–72,310 t of soil, sediment and woody debris was carried downslope by the major landslides but has been retained within the plantation area. The total sediment loss from the affected upper catchments is likely to be greater than the above estimate as the contribution from small riparian landslides, sheet, gully, and rill erosion has not been accounted for, nor has streambank erosion and sediment transported off the study site been measured. Radiocarbon dating of charcoal in sediments indicated that two landslides had evidence of previous instability in the Holocene. Most ages indicated that previous instability dated to 35–15 cal ka BP, i.e., to a time when the climate was cold and dry and freeze-thaw processes in a sparsely vegetated landscape were active. Conclusions: During planning for harvest the soils developed on Permian tillite were correctly described as having a low to moderate risk of landslide erosion. This study concludes that the landslides initiated in June 2016 resulted from exceptionally heavy rain falling on harvested steep and hilly land coupled with the decline in root strength of the harvested trees. The frequency of such a combination of circumstances may increase if high-intensity rainfall increases in Tasmania as the result of climate change. To improve the long-term stability of this terrain and the overall sustainability of plantation forestry it is recommended that landslides and riparian areas are seeded with native vegetation, and that the current assessment of landslide risk for this terrain is re-evaluated.