Coastal rockfall characterisation and risk management on Wadjemup - Rottnest Island, Western Australia

Tourists visited Wadjemup – Rottnest Island in record numbers in 2023, and visitor numbers are forecast to continue to rise. Its 24 km of coastline is characterised by alternating bays and rocky headlands. The rocky headlands comprise Pleistocene-aged Tamala Limestone which present a variety of geohazards to the public including the potential for rockfalls and debris falls. Tamala Limestone is an aeolian calcarenite with a high variability in the amount of calcareous cementation (and therefore strength characteristics) both vertically and laterally. The erosion potential of wind, rain, and wave action that affects Rottnest Island makes for a constantly changing coastal landscape in which rockfalls are common. This paper presents a case study of the rockfall hazard characterisation, risk assessment and risk management implemented in the interests of public safety around the coastline of Rottnest Island, Western Australia. Five different cliff types have been identified around the island, the physiography and rock characteristics of which determine the predominant failure modes and frequency of rockfalls and debris falls. Each are potentially hazardous to the public who may inadvertently put themselves at risk. In accordance with the Landslide Risk Management Framework (Australian Geomechanics Society, 2007), a qualitative analysis of rockfall frequency based on observation and review of historical photographs, and an assessment of potential consequences has been carried out. In close collaboration with the Rottnest Island Authority, the Hierarchy of Controls has been used to guide decision making on which risk mitigation options to undertake, the corresponding priority levels, and the implementation of an ongoing monitoring regime.