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Results from a recently completed Limestone Cliff Stability Assessment provides clear direction for the Shire to act in relation to cliffs that pose ground instability or rock fall risk. Sites include the coastal areas of Gracetown, Prevelly, and Gnarabup, with action staged over the next two financial years due to the cost of the work required.
Read the reports here:
For more information on this work and its staging over the next two years, please contact the Landcare and Environment Team at the Shire on (08) 9780 5255.
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:
A copy of the CHRMAP mapping can be found here
A full copy of the CHRMAP report can be found here
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
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.
A copy of the Limestone Cliff Stability Assessment mapping can be found here
A full copy of the Limestone Cliff Stability Assessment Report can be found here
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