Tackling coastal erosion in Onkaparinga

Ms Nina Keath1, Mr Salvador Juardo

1City Of Onkaparinga, Noarlunga, Australia

City of Onkapringa is home to 31kms of coastline, much of which is comprised of highly erodible soft sedimentary cliff-faces. With roads, infrastructure and housing at risk, council has recognised the need to actively monitor coastal erosion and work with local communities to identify appropriate responses.

In 2015, City of Onkaparinga used aerial photography and photogrammetric modelling (the science of making measurements from photographs) to create a high-resolution, 3D computer model of the entire coastline. Local company, Aerometrex, used a helicopter to capture 25,000 photographs and added 125 ground control points to generate a detailed 3D mesh accurate to 50 millimetres. Each photograph represents 36 million data points, so 25,000 photographs creates trillions of data points.

As the first ever application of this technology to the problem of coastal erosion, it has proved to be highly effective for:

–              establishing an accurate baseline of the coast,

–              monitoring erosion over time (a follow up hot-spot scan in 2018 revealed a quantifiable rate of erosion),

–              modelling sea-level rise and future development,

–              safely and efficiently collecting the data required for designing coastal infrastructure, and

–              effectively presenting reliable and compelling information to local communities.

The success of this approach has resulted in councils across Australia and the world adopting this method.

Watch 3D modelling of the Onkaparinga cliffs at: https://www.youtube.com/watch?v=2M4gw4LlHH4


Biography:

Nina Keath is a Senior Strategic Planner at City of Onkaparinga.

Salvador Juardo is a Coastal Planner at City of Onkaparinga

Innovations in coastal stability assessments – case sstudy at Robe

Mr Lyndon Sanders1, Mr  Jason  Barnett1

1Tonkin Consulting, Adelaide, Australia

In 2018, Tonkin was engaged by the District Council of Robe to assess landslide risk along cliffs near the Robe township.

Traditional landslide risk assessment relies on accessing the top and base of cliffs but this comes with serious risks of falls from cliff tops and dangerous sea conditions.

Tonkin used a drone to capture high quality video of the cliffs supplementing a physical inspection from coastal walking trails. The drone allowed around stability assessment of 1.5 km of cliff in less than a day.

The video included low level oblique views of the undercut cliffs and high level (aerial) views. The footage was reviewed immediately after flight, allowing rapid preliminary assessment of the risks to be made by a principal geotechnical engineer. Advice to Council on the same day saw Council immediately close access to some parts of the cliff tops. Other risk mitigation measures are being implemented.

Post-processing of the drone footage integrated its data with a Digital Terrain Model so that comparison with future surveys will allow quantitative assessment of the rate of coastal recession, a critical input into Council’s Coastal Access Management Strategy.

Drones are not the whole answer, however. While inspecting the cliff top, signs of ongoing sinkhole development were observed near the known ‘Blowhole’. This required improved risk management and would not have been detected by drone flights alone.

We expect that, combined with expert analysis, drone surveys will form an inexpensive input into future landslide risk and shoreline recession assessments.


Biography:

Lyndon Sanders’ involvement with coastal stability includes the 2000 study of the City of Onkaparinga coastline, stability assessments at the Head of the Bight viewing platforms, in Robe, Hallett Cove and various other sites. He has been involved in riverbank stability assessment along the River Murray, River Torrens, Onkaparinga and Gawler Rivers, and worked on DEWNR’s Riverbank Collapse Risk Assessment Technical Committee. His paper ‘Examples of Landslides in the Adelaide Metropolitan Area’ includes coastal landslides.

Jason uses alternative technologies in specialist inspections, including landslides and storagetank internals. Jason combines video recordings with GIS to present data intuitively, reliably and cost-effectively.

Aldinga beach cliffs – coastal erosion protection

Miss Joanna Garcia-Webb1, Ms Astrid Stuer1, Ms Linda Rijkenberg1

1Water Technology, Adelaide, Australia

The township of Aldinga Beach is fronted by elevated coastal cliffs along the length of the town. Despite their elevation, the township is subject to increasing social, environmental and economic vulnerability due to ongoing erosion of the cliffs. This study assessed a range of cliff toe protection options to enable Onkaparinga City Council to implement the most appropriate erosion protection works for a section of the cliff.

The options are as follows:

– Option 1: Rock seawall located at the cliff toe

– Option 2: Rock breakwater offset from the cliff, acting to reduce energy at the toe of the cliff without resting directly against the cliff face

– Option 3: This option is a hybrid of Option 1 and Option 2.

— It aims to eliminate the disadvantages of both Options 1 and 2. It incorporates the use of the lower risk offset bund throughout the areas of steeper cliff, whilst at either end of the seawall, where the cliff is less steep and prone to failure, a transition to the seawall approach is employed. This hybrid option saves cost and material from Option 2 whilst also not increasing the risk of working directly adjacent to steep cliffs.

When analysing options, it is important to consult all relevant stakeholders. This presentation will provide an overview of the options and adjustments incorporated into the design to ensure the best possible outcome for all stakeholders.


Biography:

Joanna is a principal coastal engineer with over 15 years’ experience. She is passionate about sustainable management of the coastline. She is particularly interested in coastal processes assessments, design of coastal management solutions and assisting local and state governments with coastal planning and management within the context of relevant state coastal policies. She has extensive experience in numerical modelling of waves and hydrodynamics, and has applied these in numerous studies.

Joanna enjoys working with the community to raise awareness of coastal management and climate change. Joanna places high value on working collaboratively with internal and external stakeholders to achieve project success.

Securing Adelaide’s metropolitan beaches, the next stage

Dr Murray Townsend1, Mr Jason Quinn1

1Department For Environment And Water, Adelaide, Australia

Adelaide’s beaches have been actively managed since the Coast Protection Board undertook the first beach replenishment in 1973.  Beach replenishment continues to be the primary means for addressing ongoing and serious erosion along Adelaide’s beaches.  Without intervention, coastal development and infrastructure would be threatened as erosion progressed, starting at Kingston Park and Seacliff and progressively moving northwards.

In 2005, the Government adopted a beach replenishment strategy pumping sand slurry from accreting areas to address beach erosion and maintain dune buffers.  The pipeline system was not completed and left the West Beach and Henley Beach South foreshores vulnerable to erosion once the strategy became partially operational in 2013.

On 3 June 2019, the Premier announced funding of $52.4 million over 4 years for South Australia’s coast.  $4 million is being provided to regional councils for high priority coastal protection and adaptation projects, $20 million for replenishing West Beach and Henley Beach South with externally sourced sand and $28.4 million to complete the sand recycling pipeline from Semaphore to West Beach.

The sand and pipeline project will settle the management of Adelaide’s beaches for decades to come.  The project will largely restore lost beach volumes at West Beach and Henley Beach South and allow them to be maintained using sand recycling with the new pipeline.

The presentation will provide a description of the project, progress to date and the opportunities for community engagement and dune and ecological restoration along the coast.


Biography:

Dr Murray Townsend manages the Coastal Management Branch within the SA Department for Environment and Water.  He and his team manage Adelaide’s beaches, provide advice and guidance on coastal policy, engineering and management and climate change adaptation, and provide support to the Coast Protection Board.  The Branch is a specialist group of coastal engineers and scientists, urban and regional planners, surveyors and administrative staff.

He was the South Australia representative on Engineers Australia’s National Committee on Coastal and Ocean Engineering (NCCOE) from 2002-2016 and served two terms as Chair.   NCCOE publishes guidelines on climate change and sustainability on the coast.

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