GIS mapping of landslide susceptibility and life risk in Blue Mountains National Park

Blue Mountains National Park is part of a world heritage-listed conservation area managed by New South Wales National Parks and Wildlife Service, spanning over one million hectares of protected wilderness on the western outskirts of Sydney. The park encompasses an uplifted tableland of Triassic sandstone, underlain by shale, claystone, and coal measures. The escarpment is incised by rivers and streams, forming deep valleys surrounded by cliffs. Some of the most popular walking tracks receive hundreds of thousands of annual visitors and follow steep routes that connect the top of the escarpment to the valleys below via switchbacks, stairways and ladders that pass under high cliffs and overhangs. Typically, a new rockfall or debris slide is reported in the park about once per month, and over the last decade, landslides have infrequently caused serious injuries and fatalities; more frequently, they have caused major damage to walking tracks and fire trails.

Previous landslide risk assessments in the park have usually been focused on life risk for bushwalkers, with assessments undertaken on a site-specific basis in response to past instability or incipient slope failure. Under the National Parks Landslides and Rockfalls Procedures, the results of quantified risk assessments are used to estimate societal risk and develop appropriate risk mitigation strategies. Despite many years of reactive and site-specific landslide risk assessment, the park contains hundreds of kilometres of trails and walking tracks, and many locations have never been subject to any form of landslide risk assessment. This paper presents methods and findings of a proactive GIS investigation of landslide susceptibility and risk across 3400 km² of Jamison and Grose Valley, encompassing the most visited regions of the park. A landslide susceptibility map was produced from analysis of digital elevation models, expected rainfall intensity, bedrock geology, and hillslope erosion datasets. The concept of a “synthetic landslide return period” is introduced to link GIS-based landslide susceptibility scores to potential life risk for track and road users. The resulting susceptibility and risk maps can be interrogated to rank the relative landslide risk exposure of linear infrastructure, and identify locations that may require additional administrative risk management, site investigation, or engineered slope risk mitigation works.