Coastal erosion is a hot topic around town. Some of Raglan’s most well-loved and well-used areas are visibly eroding at what can feel like a worrying rate. One of the solutions being floated is building seawalls – using rock or concrete to halt the threat. But are seawalls the solution they seem or would they just make the problem worse?
Erosion has long been a feature of life in Raglan. We live on a dynamic coast, and residents are used to sand that comes and goes – especially at Ngarunui Beach, which can transform from full sand cover to a rock-strewn platform within a matter of months.
Recently, there is a feeling that erosion may be accelerating, threatening some of the town’s most iconic areas. The popular family beach at Papahua is being scoured of its sand; the lifeguard tower at Ngarunui has had to be dismantled before the sea collapsed it, as were the toilets; areas of Cliff Street are being undermined and chunks of the shoreline adjacent to the airfield are being eaten away.
Understandably, people want to do something about it, and in some places, building a seawall might seem an effective and immediate answer. If we can keep the sea away from a vulnerable area of land, then it can’t erode it.
Dr Shaw Mead from eCoast, however, cautions that seawalls are not necessarily a good idea, especially in areas of open coastline.
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“Seawalls are a land protection structure. If you put them on open coast that’s exposed to waves,, when those forces then interact with the hard structure, they just increase and accelerate erosion, because they bounce off it. The walls cause reflections, and those reflections take more sand offshore.”
Shaw explains that if you were going to put a seawall on an open coast you would have to cover the whole stretch of coastline. If you wanted to erect one on Ngarunui Beach, for example, to protect the surf tower, it would have to stretch right from the southern surf club end to the harbour entrance to prevent detrimental “end effects”.
“On a natural beach, when you get a storm, it wants to erode it, drag sand off. That sand forms a near-shore bar that knocks out the wave energy, so it's actually a natural process to slow down the erosion. But if you put a wall in there, that sand can't be pulled offshore to form that natural protection.
“What also happens is that there's then a deficit where the wall is. So the demand gets greater on either end, and you get a big erosion scarp, known as an ‘end-effect’.”
Shaw says that it’s far better to have a natural dune system. Dunes are a depository for sand, which will be eroded by storms and high tides. Then, in calmer periods, the water will shift the sand back on shore, building the dunes back up and creating the opposite of erosion – accretion.
“You can get accretion events that happen just as fast as erosion events,” says Shaw.
This is not to say it is never appropriate to build a sea wall. Shaw says they do have their uses, often within inner harbours where there is a hard stand, such as a rock shelf, to construct them on.
Old footbridge helped halt erosion
One area of our coast where a hard structure acted effectively to retain sand – even if that was not its intended use – was the old footbridge and footpath linking town with Papahua.
The old footbridge, replaced in 2011, had a form of abutment at its end and a low-profile footpath on the Papahua side. Together, they acted as a form of groyne, trapping the sand and shifting the shoreline outward.
“It acted like a terminal groyne [and] over the years, the 60 years or so that it was there, it's a very benign environment, and slowly, slowly it built and held sand there.”
Shaw tried to warn the regional council when the bridge was replaced, and since, that the groyne needed to be replaced or the sand would erode, but for various reasons his message did not get through. Now the bridge extends all the way to the grass, he says we are seeing “chronic loss of sand” and that, as the waves and water more consistently reach what used to be the flat platform of the beach, that process will speed up.
“A thin layer of water is enough to make little waves to suspend the sand and the current is taking it away … Once you get to that period where it's getting wet all the time, it'll move faster.”
Shaw continues to try to alert the council, and is hopeful that if a similar low profile groyne structure can be reinstalled on the beach that, over time, the sand will start to build back up.
Our shifting coastline
Shaw has photographs from 1940 that show that erosion along the harbour entrance at that time was far more pronounced than it is today. And in 1974, erosion along the main beach was so severe it went all the way back to the cliff, about 12 metres further inland than more recent erosion events.
“We got close a little while ago, but then it recovered again,” says Shaw.
The fluctuating fortunes of Elephant Rock will be a familiar marker for many people of exactly how far the sand on Ngaranui can shift. At times the rock towers over people’s heads, standing up to 2 metres above the surrounding sand, while at others it is completely buried, leaving no trace.
Shaw has taken a series of photos of the rock to capture this cycle (you can see them on Raglan Community Radio website). The photos were taken during a particularly significant erosion event a few years ago.
“That sand on our beaches usually moves North. In the photos you can see the erosion scarp moving up the beach. In one of the photos, Elephant Rock is still buried, but in the distance you can see the rock platform showing and all the boulders. Then in the next photo Elephant Rock is starting to show again.”
The process is both gradual and fast – during that erosion event, within 3 months Elephant Rock was fully exposed again.
Natural solutions provide better options
So if seawalls are not effective to manage erosion, what is?
Shaw says natural solutions tend to work better than more hard engineered options.
“People talk about tipping rocks as being better than building a wall because it dissipates energy. But even on the small scale of each rock, you're getting reflection happening. It's far steeper than a natural beach, and it's hard. So if the structure is interacting with waves on a daily basis it's very difficult for sand to build back up and the beach to reform itself.”
A better option is planting, which Shaw describes as both more vulnerable and more dynamic, but ultimately more effective, as over time plantings enable natural dune systems to reform in areas where they have been removed or eroded.
A good example is the planting that has taken place at the end of Papahua Domain.
“Some substantial waves come through the harbour entrance and that area gets really eroded sometimes, but the spinifex is still there and the pīngao is still there and it recovers very well.”
Spinifex and pīngao are both halophytes – salt-tolerant plants that will withstand an occasional assault by the sea. Shaw explains that they work to trap sand and hence build up dunes naturally over time. Unfortunately, though, they are also very attractive to goats, hares, sheep and other grazing animals – the reason they were stripped from many coastal areas initially, and that dune replanting programmes remain so crucial.
“There are success stories from all over the country where spinifex and pīngao have been planted on our coasts, and the dunes are now rebuilding, especially on our East Coast.”
A constant state of change
Amongst other things, our eroding coastline acts as a reminder of the need for Raglan, and other coastal communities to prepare for future sea-level rises.
Whether you attribute it to climate change or something else, Shaw says there is no doubt that sea- levels are going up and the rate of that rise is accelerating.
What many people may not know is that most of New Zealand is also actually sinking, due to movements of the tectonic plates it straddles.
“I think we're going down about two or three millimetres a year here in Raglan, but there are some parts around Wellington where that rate is 9 or 10 millimetres. so the land is sinking faster than sea levels are moving in those areas at the moment, effectively doubling the rate of sea level rise.”
All of which requires comprehensive long-term planning to determine how best to protect our coast and the communities that live there.
“It's looking at dynamic adaptive planning pathways. So what we're doing is saying we know the seas rising. How do we look at our whole coast and how are we going to manage it as things go on. … To use it for planning, we don't use the years we use triggers. We say well, when the erosion gets back to here or when this marker pole falls in the sea, then we’re going to do this, and we have choices about what we want to do.”
Shaw says the main and most important thing is to bring the community along. “So the community is at the centre of all that decision making.”
By making a plan – one that the community has created – Shaw says you give people confidence that they understand what’s going on. Natural and hybrid solutions are used to advance the shoreline, buying time for vulnerable communities and allowing them to organise a managed retreat. Many of these communities know it is inevitable that they are going to have to retreat, but by planning how sea-level rises will be managed in the short and longer term they are given more time to deal with it.
Halting or mitigating erosion will be a big part of this work – just not by building seawalls.