A Shore Thing: not enough money to “hold back the sea”, but defense is possible

Auckland has 3,200 km of coastline, and as sea levels rise and cliffs erode, some 17,600 homes in the area are at risk. A shore thing is a Things series that speaks to people whose properties are under threat, to scientists trying to warn us, and engineers trying to hold back the ocean.

Over the past five years, Auckland Council has spent over $ 5.5 million on new large coastal defense structures.

These include a $ 2.35 million project to replace the mudcrete groynes that protect Shelly Beach in Kaipara, a $ 950,000 effort to build two new groynes and protect the masonry seawall at Huia Estate. in Waitākere, and an effort of $ 680,000 to strengthen the Te Atatū dike.

Other coastal defenses have been built by developers and landowners, which often, when completed, become the responsibility of the council to maintain.

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One of the largest projects where work continues is the remodeling of the Muriwai dune.

Muriwai experiences big swells and shorter, more aggressive waves generated by the Tasman Sea.

Chris McKeen / Stuff

Muriwai experiences big swells and shorter, more aggressive waves generated by the Tasman Sea.

Natasha Carpenter, responsible for coastal management practices at the council, says that since the 1960s the beach has lost about 65 meters of land.

“It threatened some of the park infrastructure that we had in place here, particularly the south parking lot, the toilet block and the road.”

The beach experiences large swells and short, aggressive waves generated by the Tasman Sea, and as threats increase so do the population. The continued development means that 2,500 people now live in the region.

Natasha Carpenter, responsible for coastal management practices at Auckland Council, Muriwai Beach.

Chris McKeen / Stuff

Natasha Carpenter, responsible for coastal management practices at Auckland Council, Muriwai Beach.

In an example of a managed retreat, the southern parking lot at Muriwai was moved back 40 meters.

Over 4,000 cubic meters of sand have also been moved to reshape the beach and create a safe pathway, and the dunes have been planted with more resilient plants that better capture and hold windblown sand.

Carpenter says that a five-year project recently launched by the council is a testament to the fact that not everything can be protected.

Instead, creating shoreline adaptation plans for 16 coastal regions will prioritize council assets to be safeguarded.

Muriwai lost around 65 meters of land due to coastal erosion.

Chris McKeen / Stuff

Muriwai lost around 65 meters of land due to coastal erosion.

The plans will take into account private ownership, as how homes are protected and who is responsible for their protection will not be known until the new Resource Management Law and the Law on Resource Management come into force. adaptation to climate change.

Carpenter says the board is trying to find a balanced response because protecting everything isn’t an option.

Some defenses that people may take for granted may not be maintained in the future, she says, as science and experience find that some inherited structures may be less important to maintain than others.

Carpenter says some old sea defenses may not be maintained in the future as the council prioritizes different jobs.

Chris McKeen / Stuff

Carpenter says some old sea defenses may not be maintained in the future as the council prioritizes different jobs.

Cannot “hold back the sea”

Council Technical Resilience Officer Ross Roberts says you can’t ‘hold back’ – there just isn’t enough money available to do it.

“If we were Singapore with a very small coastline and a fairly wealthy population compared to that coastline, then you can invest more and do different things.

“We have to choose very wisely where we spend the money to make sure we get the most out of it. “

The budget for coastal renewals is slowly increasing, although it has declined due to Covid-19, from $ 3.5 million to $ 11 million next year.

Roberts says the amount spent on renewing and creating new defenses (which are addressed with a business case on a case-by-case basis) is likely to increase in the future.

A pedestrian and vehicular access route is threatened by erosion in Muriwai.

Chris McKeen / Stuff

A pedestrian and vehicular access route is threatened by erosion in Muriwai.

New materials may be needed

Roberts says the physics aren’t changing, so many of the old methods of coastal defense would likely stay the same.

However, since so much depends on concrete, which produces a lot of greenhouse gases during its manufacture and transport, the materials used may need to change.

“We don’t want to defend ourselves against climate change by building things that create more climate change. So it’s a consideration that is high on our priority list at the moment.

“It’s fair to say that no organization anywhere can control the situation. Nobody has the whole situation in hand, ”says Roberts.

Roberts says anyone looking to buy property should consider the dangers, whether it’s earthquakes or rising sea levels.

“Everyone has a role to play in making sure they are ready for the future. “

Every mitigation has a trade-off

Paula Blackett, an environmental and social scientist at Niwa, says that while cost is usually a big factor, it’s not the only limitation when it comes to mitigating erosion.

“Every adaptation has a compromise. If you build a seawall you will protect the property behind it, but you will lose the beach, ”she says.

Paula Blackett, an environmental social scientist in Niwa, said communities will need to weigh the pros and cons of erosion mitigation versus managed withdrawal.  (File photo)

Stuart MacKay, Niwa / Supplied

Paula Blackett, an environmental social scientist in Niwa, said communities will need to weigh the pros and cons of erosion mitigation versus managed withdrawal. (File photo)

Levees can impact nearby areas as well, as waves bounce off the hard structure and won’t last forever, Blackett says.

“As the sea rises the wall becomes less effective and you have to do something else because you can’t build mega walls everywhere. “

Groynes can have limited effectiveness, while even gentle work like beach replenishment can negatively impact the beach where the sand comes from, Blackett explains.

“It’s really, really important to think about how effective they will be and how long they will last with rising sea levels. Indeed, we are leaving it to future generations to make some really tough decisions.

University of Auckland coastal scientist Dr Mark Dickson agrees that every mitigation comes at a cost, including the lack of information on the environmental effects of sand mining and replenishment.

“A wide range of coastal engineering options have been tried around the world to stop coastal erosion, and while there are examples of success, there are also many examples of problems, including long sections of coast in places like the UK and Japan, and elsewhere, where beaches have been lost or significantly reduced in size.

Most defenses will last less than 50 years, but they can give people false confidence, leading to more, higher-value coastal properties – increasing overall risk, he says.

Is Dickson therefore advocating a managed retreat rather than building coastal defenses? Not necessarily, but constantly updated adaptive planning will be necessary, he says.

“Another change that I think is needed in our conversations about coastal management is a shift from a binary ‘retreat or defense’ position. For many coastal sites, we will probably end up switching from one management option to another.

He gives the example of a community using beach food to save time to plan a managed retreat – where houses could be moved when erosion reaches a certain threshold.

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