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2019 Cliff Safety Action
Following a 2017 limestone risk assessment modifications were made to stairs at Gracetown and Surfers Point, and additional signage installed throughout the Shire. In 2019, further investigations were undertaken at Gracetown, Surfers Point and Gnarabup. This has resulted in 4 key actions being recommended:
A temporary fence has been erected across the beach at Riflebutts to manage limestone stability risks until a permanent fence can be manufactured and installed. Various other options to manage limestone risk at Riflebutts were thoroughly investigated including removal of the overhang and battering back the slope, installation of fencing parallel to the cliff edge, installation of rockfall mesh, and additional signage, however were discounted as they would not sufficiently reduced the risk to an acceptable level. The permanent fence is likely to consist of 125mm stainless upright posts with 2-3 high tensile stainless horizontal wires. Interpretive signage is also planned to be installed.
The Shire will run information sessions at Gracetown and Prevelly/Gnarabup prior to other works occurring.
Copies of the full report and Council minutes are available below.
For more information on this work and its staging over the next three years, please contact the Landcare and Environment Team at the Shire on (08) 9780 5255.
2017 Limestone Cliff Stability Assessment report
A Limestone Cliff Stability Assessment was prepared in 2017. Key drivers of the study were to provide a better understanding of how limestone cliffs will react to coastal processes i.e. from sea level rise, storm surges, wind, waves etc. and to investigate how these coastal processes may affect assets in close proximity to the coast. The assessment covers Gracetown, Prevelly and Gnarabup, and is based on a 100 year planning timeframe. A key outcome of the assessment was to refine hazard mapping identified in CHRMAP where limestone cliffs are present through hazard mapping.
A copy of the Limestone Cliff Stability Assessment mapping can be found here, and the full copy of the Limestone Cliff Stability Assessment Report can be found here.
Lower Extent Hazard Mapping
The lower extent of hazards (blue line represents recreational users and red line depicts Shire and assets) defines the seaward side that may be impacted from rockfall collapse.
Upper Extent Hazard Mapping
The upper extent shows the landward extent of the zone considered hazardous to Shire assets due to clifftop rerogression over a 100-year period. It considers a notional 0.9 m of sea level rise over a period of 100 years as a significant trigger to initiating slope and cliff instability that ultimately results in cliff-edge retreat. The report follows a process outlined in the State Coastal Planning Policy of risk identification, risk analysis and risk evaluation; in order to make recommendations for risk management and adaptation.
2016 Coastal Hazard Risk Management and Adaptation Plan
A Coastal Hazard Risk Management and Adaptation Plan (CHRMAP) was prepared in 2016. The purpose of the plan is to provide strategic guidance on management and adaptation in areas potentially exposed to coastal processes, specifically sea level rise and storm erosion. The plan focuses on seven study areas, including Gracetown, Prevelly, Gnarabup, Hamelin Bay, Molloy Island, Augusta North and Augusta South. The report follows a process outlined in the State Coastal Planning Policy of risk identification, risk analysis and risk evaluation.
The limited topographic and geotechnical data available means the analysis of inundation and influence of coastal processes (particularly along limestone cliffs) is limited to a very broad scale assessment. As a result, a more conservative approach has been adopted resulting in the consideration of a larger coastal exposure area and more coastal assets. The intended outcome is to prioritise investment for further assessment and monitoring in these areas.
A series of maps were prepared for each location, and risk categories developed as follows:
Low coastal exposure (potential exposure to coastal processes in 20 -100 years)
Medium coastal exposure (potential exposure to coastal processes in 10 – 20 years)
High coastal exposure (potential exposure to coastal processes in 0 – 10 years)
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