Engineers Australia

Analysis of rainfall-induced landslides in northern New South Wales, Australia

Sinnappoo Ravindran, Ivan Gratchev and Dong-Sheng Jeng


Although rainfall-induced landslides are common phenomena in Northern New South Wales (NSW), no systematic studies have been performed to date to better understand this natural disaster. This study seeks to determine the common characteristics of these landslides, including geology, slope geometry, rainfall distribution, and soil properties. The study area includes mountain passes such as Mt Lindesay, Mallanganee Range, Ramornie–Cangai Bluff, Gibraltar Range, Dorrigo Mountain and Mt Seaview; and riverbanks, embankments and cuttings where more than 100 landslides occurred between 2009 and 2017. Field survey of sixteen landslide sites was carried out to collect soil samples, which were tested in a shear box apparatus. This study reveals that many natural slopes affected by landslides consist of weathered sedimentary rocks (mostly sandstone) while the soil from the landslide mass contains a significant amount of coarse material. In addition, landslides tend to occur every two years on natural slopes which are inclined at about 35􏲓. Analysis of rainfall characteristics enabled the development of an indicative rainfall threshold associated with landslide occurrence in the studied area. Rainfall index studies show that landslides could occur at Mt. Lindesay and Dorrigo Mountain if daily rainfall reaches 88 mm and 136 mm respectively.