Collaroy and Stockton communities weigh up the benefits and costs of seawalls

This is meant to be one of the jewels in the crown of Sydney northern coastal strip. Instead the long-term local says he feels like he's in a maximum security prison as he gestures to the giant concrete seawall towering over him.

The 13-metre high by 100 metre long brutalist structure that spans 10 properties sprung up during lockdown 2021.

It is more than a wall - it has become a symbol of how panic, commercial and self interest are, in many cases, leading the response to coastal erosion up and down the state's coastline.

Ask the property owners who each contributed $300,000 towards it and they will tell you it's been a worthwhile investment. Not only does it protect them from the sea but it has increased their property frontages by about 25 per cent.

Elsewhere, the structure is despised within the wider community for making a section of the beach practically impassable at high tide.

"I've yet to meet anyone who isn't protected by it who likes it," Donohoe, who is president of the Northern Beaches Surfrider Foundation branch, said.

"You really have to stand in front of it to appreciate how hideous it is. If it was built 100 or 200 years ago you might think 'well that's what we did back then'. There are so many beaches around the the world that no longer exist because of this type of idiotic development."

Love them or loathe them, seawalls of one kind or another have been a reality for many Australian coastal communities for more than a century. But their role and, more importantly design, is now in the spotlight due to the unrelenting forces of erosion driven by climate change.

About 150 kilometres north of Collaroy, the costs and benefits of seawalls are being weighed up carefully in the debate about how to protect the treasured Newcastle seaside suburb at Stockton - one of the state's 18 erosion hot spots.

Stockton presently has three seawalls - two are rock revetments and another constructed from sandbags - have appeared since the late 1980s.

Work will commence on a buried seawall at the southern end of the existing Mitchell Street seawall in the new year.

Stockton resident and University of Newcastle honorary Associate Professor Ron Boyd acknowledged that while they may not be popular, they definitely serve a purpose.

"They protect the structures and the infrastructure that's immediately behind them. If you've got a house behind the seawall you're pretty happy about the situation," he said.

At the same time, Professor Boyd noted coastal communities in the United States are looking at alternatives because of the significant drawbacks associated with seawalls.

"In the case of Stockton, for example, you can't use the beach where the seawall is because all the stairs that were constructed over it have all been destroyed," Professor Boyd said.

They also require significant maintenance. To this end, a significant portion of City of Newcastle's $27.5million Coastal Management Program fund will be spent on maintaining existing infrastructure.

Like Stockton, the Collaroy seawall has resulted in the loss of the beach immediately in front of the wall caused by the dissipation of wave energy.

But unlike a rock revetment at Stockton, which gradually collapses into the beach, Collaroy's sheer concrete wall relies on deep foundations to stabilise it.

Retired Public Works engineer Angus Gordon OAM said the wall's face enhanced erosion and made the beach even more unsafe.

"Not only have they alienated the beach but they have made it extremely dangerous for anybody who walks along there," Dr Gordon said.

"It's not easy to increase the height of a vertical wall like that to accommodate climate change. So in my view, the Collaroy seawall is really a solution from a past century."

While, Northern Beaches Council and the state government have defended the consultation and approval process for the Collaroy seawall, locals say they were misled about its design.

"We all thought it was going to be rock revetment until we saw the concrete formwork coming out of the ground," Mr Donohoe said.

"We are not in favour of seawalls, but if you have to do one, the rock revetment is the lesser of two evils."

Regardless of their design, most experts agree that seawalls are a necessary part of a complex and expensive solution to stabilising the coastline.

Sand renourishment, which is used on the Gold Coast, is regarded as another equally important part of the puzzle.

And for Stockton locals it can't come soon enough.

A 2021 City of Newcastle analysis found an offshore dredging program to provide sand for ongoing sand renourishment was the most cost effective and efficient long term solution to address beach erosion at Stockton.

It's a process has been used successfully in Australia at the Gold Coast and internationally in the Netherlands over the past decade.

Former deputy premier and resources minister John Barilaro paved the way for offshore dredging in NSW in 2020. But progress has stalled in recent times due to an impasse over whether the government or the council should hold the mining lease. Newcastle Lord Mayor Nuatali Nelmes said last week that her expectation was that the government would take on the work.

Dr Gordon led a team of state government engineers who studied sand movements in Stockton Bight in the 1970s.

The team's research showed the beach was continually receding as a result of the natural movement of sand northwards.

Dr Gordon said he was confident there was enough sand offshore to sustain a dredging and renourishment program.

"There's no question in my mind, based on the work we did all those years ago, that there's more than adequate sand out there. It's kind of really a question of the cost and who's going to pay for it," he said.

"If you don't have a terminal wall of some description there then you're going to have to put not just enough sand to manage the average conditions, but you're going to have to put enough sand on to deal with a severe storm."

Meanwhile, communities up and down the coast are watching the community-led push for offshore dredging and sand renourishment at Stockton with interest.

"We have been promoting sand nourishment for the beach for a long time," Mr Donohoe said.

"We have been working with the government and hopefully, considering what's going on at Stockton, we can get that going here too. A simple stroke of the pen will allow it to happen."

Looking to longer term solutions, Dr Gordon said one of the biggest challenges at Stockton was the size of the bight, which stretches to Anna Bay at Port Stephens.

"The reason the problem is so significant is because of the size of the embayment," he said.

"You've got to think about this not just in terms of Stockton, you've got a look at the whole Newcastle bite and because it's all part the one system. As long as you've got sand eroding into the dunes at the northern end, you're going to continue to have Stockton eroding."

In order to achieve the long-term stability of the southern end of the 30 kilometre bite, the entire dune system needed to be managed in a holistic manner.

Engineers took a similar approach to arrest severe erosion at the Cronulla embayment in the late 1970s.

The embayment was receding at about 2.8 metres a year before a renourishment program began to complement a newly constructed seawall.

But the key to the project's overall success was a soil conservation program to stabilise the foredune system.

"Since that's happened the environment has stopped receding. The soil conservation service still has to maintain those dunes but nothing like what they initially did when they reshaped and replanted them. Cronulla is actually a one tenth model of what you might try to do at Stockton," Dr Gordon said.

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