Some of NZ's most beautiful cliffs and beaches are threatened by erosion, sweeping great swathes of shimmering black sand northwards from the foothills of Mt Taranaki, undermining some homes and farms, swamping beaches. Are these the rising waters of climate change — or just nature reminding us who's boss? Phil Vine investigates

Fine-looking homes perched on crumbling sea cliffs. "The terrifying face of coastal erosion". They loomed large in the foreign pages of my morning paper just after we bought the place at Piha. On the coast. By the sea.

Enough to give any privileged new bach owner the twitchy eye. Not to mention climate change and rising sea levels - 1m over the next 100 years, is what they say. I wonder if our compulsive desire to own seaside property might be something of a generational folly.

First thing I find out is that at Piha, the sea is beating a retreat. The sandy beach, far from being eaten away, is doing this thing called accretion. That's a technical term - meaning it's getting bigger. Surfies are having to schlep their boards an extra half a metre down the sand every year.

"We have to pack a lunch and take a thermos to get to the water at low tide," says Steve "Roachy" Davis. He's been surfing at Piha for 44 years. "When I was a grommet, we used to be able to jump off the edge of the car park into the water."


His worries are about what's happening beyond the shoreline. He invites me to his apartment behind the sand dunes. Amazing view of the beach. The picture window frames a perfect Sunday. Scorching waves. Fifty surfers out back.

"I'm gutted." Why, Roachy?

He describes surf breaks that don't exist any more. "The Ditch", a right-hander that used to peel north to south from the middle of the beach, and "the Bar", a left-hander.

"An iconic break, a wave people identified with. They've both disappeared," he laments.

Roachy plays me a surf doco from 1968 called Children of the Sun. Kiwis with bleached Beach Boy haircuts. Boards strapped to the roof. Falcons dancing over gravel roads.

Roachy reads along with the hilariously pompous voiceover. "There's freedom out there, more freedom than you've ever had."

Roachy points out the Ditch and the Bar. He speaks as if they were dead friends. There's no question what killed these two surf breaks. Sand. In the past 60 years, the sea has deposited a colossal 1.2 million cubic metres of sand at Piha.

Jim Dahm has been up to his armpits in sand, scientifically speaking, since the 70s. His report on Piha last year called for more work on the sand dunes to mitigate, among other things, the threat of erosion.


Erosion? Really?

"Coastal erosion is a natural process. It has been treated in the past with fear and ignorance."

Dahm is a scientist who helps advise councils on where to draw coastal hazard lines; identifying the seaside spots where people might end up with a surprise fish dinner.

When discussing buying coastal property he uses that grown-up term "due diligence". I make a mental note to check out the hazard lines near our bach, something I should have done before settlement, undoubtedly. (But no worries, Piha beach is fattening like a Christmas turkey, is it not?)

Well, yes, says Dahm, but it won't be forever. Why? In a word, dynamics. The west coast beaches are all part of a single dynamic system that stretches from New Plymouth to Cape Reinga. Those dark minerals that give the sand its distinctive hue are volcanic particles from Mt Taranaki. They're restless little beggars. Washed down to the sea by rivers and streams, they hit the Tasman and hitch a ride north on the waves.

As the sandy train rolls northwards, it turns each beach into either a winner or a loser. Some beaches, like Piha, are winning sand (accretion) and other beaches are losing it (erosion).

All good for us, though? Right. We've picked a winner. A beach that is gaining sand. Not exactly. Over time losers become winners, winners become losers. Dynamics.

South of Piha is a great big slug of sand at Whatipu. It's on a journey up the West Coast towards the Cape. Since the middle of last century it's been pumping Piha full of sand. But it will run out.

The good news is that it won't happen for many years, decades, Dahm reckons. The less good news: the modern scientific view of coastal erosion is that, where practical, we should just let it happen. Don't fight it. Lie back and think of England: East Anglia in particular, where they are relocating whole villages inland because of the encroaching sea.

Dahm insinuates coastal erosion isn't so much about people having an issue with the sea, as the sea having an issue with us. "Coastal erosion only becomes a hazard when something human gets in its way. If you put a tent up on the Southern Motorway sooner or later you are going to have a car problem."

All very well unless that "tent" happens to be your house.

The closest thing we've had to those frightening pictures in the paper is Mokau, a pretty little spot on the Waikato/Taranaki border. Sitting on a river mouth, it has beautiful views of the volcano and sparkling sea but in Dahm's words "the subdivision was based on an extremely dynamic feature" - a sandpit. "A lot of people have lost a lot of money there," says Dahm.

In the 50s, the Lands and Survey department, against local advice, decided to slap in a subdivision. To Dahm's mind, a classic case of pitching a tent in the fast lane.

In the early 90s, the sandpit entered a destructive period of erosion turning into one of the worst cases of house destruction from coastal erosion. Six to eight homes were lost to the sea.

Landowners asked for a seawall to protect their investments but the local authorities baulked at the cost. They followed a do-nothing policy. Dahm's advice was controversial. "Jim Dahm came down here and told us leave it alone," says 74-year old Ray Christiansen. "Us residents should have been given the right to stop the sea."

Christiansen, a retired mechanic, and his fellow homeowners threw up illegal constructions at the bottom of their sections to protect their property. "We put in sandbags, all the locals helped sew up the bags. It was good, it stopped the waves but then they slumped." They got a quote to build a seawall privately. Two and a quarter million dollars.

"For us fellas, that was dreaming. We laughed at them and put in our own rocks." They trucked them in from Egmont Village. Huge rocks weighing two to four tonne each.

"Environment Waikato told us rocks were alien to the environment. That's people coming out of university who don't know what they're talking about. Since the rocks went in five years ago we haven't lost any more houses."

So far. Dahm believes the sea "is going to win there because the value of the properties doesn't justify a properly engineered seawall, which would stop the waves".

He's not opposed to seawalls in general - "sometimes they're appropriate and necessary". He just says 70 to 80 per cent of the time they're not useful. You often lose the character of the coast and sometimes the beach itself.

"Shorelines move, they're dynamic and we shouldn't always play God and try to stop that movement." He claims the overuse of seawalls is a legacy of having engineers shape coastal management.

"Like Pavlov's dog, the bell rings and they bark 'rock rock'."

I'm not sure how I would feel if the Tasman started lapping around our deck. There's every chance I would bark "rock". In extreme cases Dahm makes it clear he would agree.

What's the alternative? Something called a managed retreat. Let nature have its way. In the case of Mokau, Dahm believes the Government should have bought out the owners. At the time it would have cost less than the quote for the seawall.

He points to Muriwai as a great example of managed retreat.

The council shifted a road, a carpark then moved the surf life saving club 500m inland. "The sea was getting closer and closer," says club chairman David Butt. "If it wasn't for the council we have would have been sitting there with water lapping around our knees."

"It's the first example in New Zealand of major coastal erosion being managed by moving assets," Dahm says proudly.

Those sorts of decisions will have to made all around our coastline as global warming heats up.

So how will that predicted 1m jump in sea levels affect the west coast beaches?

Dahm can't say. He can tell me fairly definitively though about the other side of the island.

"On the east coast, rising sea levels are very likely to cause widespread erosion. On the west coast, it's anyone's guess."

On her way out the door for a surf, Roachy's 25-year-old daughter Mischa gives a big thumbs-up to serious erosion. "That'd be great. We'd get our Ditch back again."

Viv Rutherfurd has lost about 5 per cent of his family farm to erosion and sand drifts. Photo / Doug Sherring
Viv Rutherfurd has lost about 5 per cent of his family farm to erosion and sand drifts. Photo / Doug Sherring