Coastal Cliff Stabilisation using innovative aesthetic matching solution. Scarborough, Australia
Updated: Sep 19
Coastal cliff erosion is a common problem worldwide, including at the iconic 320m stretch of cliffs at Scarborough in Moreton Bay. They have been suffering from long-term retreat due to natural coastal processes. The retreat of these cliffs not only threatened the loss and damage to infrastructure and public amenities, including parkland, paths and road, but also the iconic aesthetic of the cliffs for which the Redcliffe Peninsular were named after. Without an effective coastal protection solution, erosion of the cliffs would continue and likely accelerate due to sea level rise and climate change impacts.
Moreton Bay Regional Council (MBRC), Queensland, initiated this project to stabilise and protect the cliffs from coastal erosion, whilst also preserving the iconic aesthetic of the historically and culturally significant cliffs.
Cliff erosion threatening safety & public amenities
Investigations into coastal processes and geotechnical conditions at the site found that the retreat of the cliffs was largely driven by slow notch erosion at the cliff base due to wave and tidal impacts, with some rotational collapse of the crest. The notch erosion at the base was then resulting in translational block failure of the overlying laterized cliff face, destabilisation of the upper cliff and undermining of vegetation.
Aesthetic integrity. A key design factor.
While stabilising the cliffs was a key objective for project, it was essential for the design solution to also:
preserve the iconic aesthetic of the cliffs as much as possible,
minimise impacts to the foreshore environment,
maximise usable beach width and
provide adaptability to sea level rise and climate change.
Solution
While multiple conventional design options were investigated (rock revetment walls, rock gabions and large-scale precast blockwork walls) these options were not able to suitably meet each of the project objectives, or required a significant footprint to be effective. The solution required an 'out of the box' and site-specific design approach to achieve the design objectives. ICM developed an innovative two part solution to protect the base of the cliff from notch erosion (Lower Cliff Works) and enhance the resilience of the upper cliff (Upper Cliff Works).
Lower Cliff Design
An innovative coloured and textured, fibre-reinforced shotcrete wall was designed to best mimic the natural form and aesthetic of the cliff, whilst protecting the lower cliff from erosion. The wall included soil nails to provide attachment to the cliff face and a bull nose wave return at the crest to reduce overtopping impacts to the upper cliff. Due to the extreme exposure to the marine environment, the structural reinforcement and soil nails were all comprised of non-corrosive Glass-Fibre Reinforced Polymer (GFRP).
The drainage system was comprised of vertical strip-drains behind the wall with PVC weep holes to release water from the cliff. A geotextile wrapped ‘bladder’ of drainage aggregate was included at the toe to dissipate wave impact and prevent sediment loss due to water ingress through the weepholes.
To provide some flexibility to the shotcrete, the wall was designed as a segmented structure, allowing for controlled shrinkage and displacement at specified intervals. Each of these intervals included a geotextile filter layer to prevent sediment loss, and GFRP dowels to allow for lateral expansion and contraction.
The footprint of the design was significantly smaller than a conventional rock wall and maintained significantly more usable beach width, whilst producing an aesthetic that resembled the iconic cliffs.
Upper Cliff Design
A low impact and aesthetically friendly solution was required to improve the stability of the upper portion of the cliffs, whilst preserving the vibrant red earth. This included identifying all existing unstable vegetation, soil blocks, near vertical and overhanging sections of the cliff for removal; re-profiling the cliff crest to a more stable configuration and spray applying an environmentally safe soil stabiliser. The soil stabiliser was applied to reduce surface soil erosion due to rainfall run-off and wave overtopping, as well as limiting regrowth of unfavorable vegetation.
The site-specific design provided a low impact and cost-effective stabilisation solution that retained the natural and iconic aesthetic of the red Scarborough Cliffs.
Effective resilience against storm conditions
The ‘hardening’ solution was designed to replicate the natural form and aesthetic of the existing cliff and included a ‘bullnose’ wave return at the crest to reduce overtopping and reflect wave energy similar to the existing conditions at the cliff.
Several storm events have occurred since construction of the works was completed, including in December 2020 and a significant rainfall event in February 2022 due to ex-tropical cyclone Seth. These events have provided an opportunity to observe the performance of the structure against wave conditions that would have previously contributed to notch erosion at the base of the cliff.
The post-construction monitoring has shown the following:
The works have been successful in protecting the lower cliff from further erosion and preventing the retreat of the cliff.
The drainage system has been successful in allow release of water from the cliff whilst preventing the loss of cliff sediments due to wave attack.
The wave return has been successful in reducing wave attack and overtopping to the upper cliff.
The shotcrete wall remains well integrated with the cliff face.
Monitoring of the works is on-going.
Advanced technical design and monitoring
Terrestrial Laser Scanning of the cliffs has undertaken by MBRC both prior to the works and following the works to serve as a ‘digital twin’ of the site. In conjunction with LiDAR survey and models, conceptual designs were able to be digitally tested for efficiency.
Ongoing monitoring using advanced survey methods allows for accurate measurement of the site for detailed analysis.
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Biological benefit of mimicking natural cliff face
The shotcrete wall was carefully constructed to not only match the colour of the existing cliff face but also the texture.
This mimicking of the natural conditions made the site not only visually cohesive but also allowed for the natural process of marine habitat creation, specifically for local mollusk species that have taken refuge in the texture of the shotcrete.
Site Specific vs. Conventional
While there were a variety of conventional coastal engineering solutions that would have been suitable for the site from a coastal protection standpoint, a 'beyond conventional' design approach was required to best achieve the objectives required by Moreton Bay Regional Council.
For ICM, this is where our design capacity thrives. With a 'bespoke' approach to site specific design and coastal erosion solutions, we are able to continually achieve successful projects that require 'out of the box' approaches.
For a comparative example, a site specific approach vs. conventional design approach is highlighted in the figure below.
Leading the way in innovative coastal engineering designs
For over 35 years, International Coastal Management has been working to push the coastal engineering industry beyond conventional methodologies.
Working with governments, private and public organisations across the globe we been bringing together the latest in technology and knowledge with coastal engineering experience and passion to derive the best and most cost effective solutions.