A 30 Year review of Rockfall Frequency and Coastal Cliff Regression on the Northern Beaches of Sydney, Australia

Sydney’s coastline is characterised by spectacular cliffs and headlands, comprising near vertical exposures of Triassic sedimentary strata that range in height up to 80 metres. Natural regression processes are constant and largely manifest as rockfalls of various sizes. These rockfalls have the potential to impact people and property that are located both around the base and across the crests of coastal cliffs and headlands. Recreationalists accessing parks, reserves, walkways and coastal swimming pools can be at risk of rockfall impact from above. Cliff-top developments that have encroached towards the crests of cliffs can be at risk of subsidence from rockfalls below.

The former Pittwater Council and the recently formed (May 2016) Northern Beaches Council in Sydney, have sought to minimise possible adverse effects of naturally occurring rockfalls from coastal cliffs in their local government area. In association, the authors have reviewed from an engineering geological perspective, all significant rockfalls that have become known to these two Councils – over the last 32 years – in order to inform the rockfall hazard management process. This paper presents the collated rockfall data, along with assessments of rockfall frequency and coastal cliff regression. It is hoped that this information can assist with the future management of developments and the maintenance of public safety in the coastal cliff environment.