BUILD-A-REEF: COCKBURN'S INNOVATIVE TRIAL BEATING EROSION

BUILD-A-REEF:

COCKBURN'S INNOVATIVE TRIAL BEATING EROSION

by John Thompson, Journalist, Council Magazine

For those living in the suburbs of Perth, going to the beach is an essential reprieve to the seemingly everlasting heat. This is why the City of Cockburn undertook an innovative trial to remedy a rapidly eroding beach, installing an artificial reef to barrier the coastline against wind and wave, preserving both the natural and built environment, and contributing to valuable erosion-beating research.

Twenty years of erosion has reduced Cockburn’s beachfront by 50m, threatening the surrounding natural and built environment. Image: City of Cockburn. Aerial view of the artificial reef installed off the coast of C. Y. O'connor Beach in Cockburn, Western Australia. Image: City of Cockburn.

Twenty years of erosion at Western Australia’s C.Y. O’Connor Beach has seen a reduction of over 50m to its shoreline, threatening both the beach itself and nearby infrastructure.

The City of Cockburn’s battle against erosion at the beach began in 2019, after the community brought the issue to the Council's attention.

By then, the ongoing onslaught of rain and wind had decimated sand dunes, necessitated periodic sand replenishment works costing over $500,000, and submerged community assets.

Now, 100m of artificial reef shields these assets, in a project thought to not only reduce erosion, but also foster healthier ecosystems.

In a separate but complementary project, a 35m long geotextile sand container seawall was installed in the dunes adjacent to the reef, promoting increased stability along the shoreline.

Though the effectiveness of artificial reef solutions in reducing erosion is currently unproven, a team of researchers from the University of Western Australia (UWA) will monitor its effectiveness in a three-year trial.

The results of this trial will inform Australia’s response to the ongoing degradation of its beaches, accelerated by global climate change – offering insight and data to benefit all Australians.

BUILDING THE REEF

Balancing the need for community access, coastal aesthetics, and effective prevention would require an innovative solution to fighting erosion.

The answer? 135 precast concrete modules, carefully craned into place 100m offshore and together forming a unique engineered fringing reef, purposefully designed to reduce the energy and intensity of incoming waves, and thereby reducing erosion.

Overseeing this project was City of Cockburn’s Head of Sustainability and Environment, Chris Beaton, who is responsible for Council’s sustainability and climate change programs in addition to environmental education.

Preserving the coastline was fast becoming a necessity, Mr Beaton explained.

“We lost a dual-use path, which previously used to be about 50m from the water. It was washed into the sea probably about five, seven years ago.

“That path had to be relocated another 50m up the dune and we started to realise that things were only getting worse.”

Installed in March 2022, the solution will test the overall effectiveness of artificial reef initiatives in reducing erosion over a three-year period.

“That's when we'll have some results which can clearly state whether it's having a beneficial impact, or if it's actually having an adverse impact,” Mr Beaton said.

“Just a couple of storm events or similar is not something that we can just look at and say ‘it is having an impact’ but monitoring it over a three-year time span and looking at different storm events that come in, we'll get a good idea of what's happening there.”

The reef project was completed in partnership with Cockburn-based global ocean engineering company Subcon and the UWA.

It was made possible with $350,000 from the City and $214,349 from the State Government’s 2020-2021 Coastal Adaptation & Protection Grants program.

COMMUNITY CONSULTATION

The needs of the Cockburn community informed and motivated Council decision making prior to the artificial reef project, and was ultimately critical to both its construction and short term success.

Consultation conducted during the development of Council’s Coogee Beach Foreshore Management Plan, where respondents stressed the importance of consistent beach access and low-profile solutions to erosion.

“The community didn't want to see things like breakwaters and groynes established, Things that might reduce their ability to get access to the beach and reduce amenity. So that's where this idea came from.

“People didn't want to see hard structures, but structures that are working with nature rather than opposing it.”

Developed with the assistance of a local engineering firm, the artificial reef addresses both concerns, while also meeting the environmental needs of native flora and fauna.

“It’s a win-win, doing a project like this will not only address the beach erosion, but also increase habitat for marine life. The community – they like to get access to the beach, they want to see their beaches remain and not be turned into hard breakwaters,” Mr Beaton said.

BENEFITING THE ENVIRONMENT

Given enough time, the artificial reef is expected to begin developing the characteristics of a natural reef, as surrounding sea life begins acclimatising to its new home.

“We expect the reef to actually start to take on the role of a natural reef, and we'll probably see coral, seagrass and seaweed start to attach to it, and have more marine life coming in, using it for habitat and protection from other bigger predators,” Mr Beaton said.

This was evident in Council’s Coogee maritime trail, a unique diving experience that supplemented barren seabed with man made structures, planting the seed for a thriving natural habitat.

“So it'll be similar to our Coogee maritime trail which we've had for about five years now. It's been a wonderful thing and has added a lot of habitat, and brought in a lot of marine life into a barren seabed,” Mr Beaton explained.

GATHERING EROSION-BEATING RESEARCH

A UWA research team will oversee the monitoring of the project, following an additional investment of $433,540 from the Federal Government’s Australian Research Council Linkage Project and an additional $150,000 in research funding from Subcon.

The funding will support UWA researchers in developing a multifaceted research program to optimise the design of artificial reefs for coastal protection applications.

This will include establishing an extensive field monitoring program to assess the reef’s performance over three years and using the data to develop improved models to more broadly predict how artificial reefs can protect coastlines.

“Regular monitoring of the reef and surrounding shoreline in the years following installation will demonstrate the trial’s effectiveness, and whether it can be rolled out on a larger scale,” Mr Beaton said.

“Erosion will still occur to some degree, but we believe the reef will slow its rate, minimising the quantity and frequency of sand nourishment, and allowing for a greater, more stable sand dune buffer to establish along the shoreline.

“While this innovative trial will benefit one of Cockburn’s beaches, it will also benefit other local and State Governments around the nation, as communities deal with the increasing impacts of sea level rise and its impact on our coastline.”

Professor Ryan Lowe from UWA’s Oceans Graduate School, agreed, adding that the project would contribute to the development of a framework for predicting how artificial reef structures could be optimally designed to protect coastlines from erosion and flooding.

“The research will develop new theories and models to quantify how waves interact with complex reef structures to reduce wave heights and extreme water levels at the shoreline," Professor Lowe said.

“Expected outcomes include new practical tools and design guidelines that can be adopted by coastal engineers and managers to maximise coastal protection by reefs."

The artificial reef was constructed with 134 concrete modules, each individually installed by crane over 100m from the beachfront. Image: City of Cockburn