Back Beach blues

UNDER THREAT: An aerial photograph taken in November of the Tahuna Beach Holiday Park and the Back Beach at high tide. The area through the centre and front of the photo is now about half as wide.
UNDER THREAT: An aerial photograph taken in November of the Tahuna Beach Holiday Park and the Back Beach at high tide. The area through the centre and front of the photo is now about half as wide.

The shifting sands at Tahunanui Beach are again prompting calls for the council to tackle nature head-on. Naomi Arnold investigates.

There is not much left of the Back Beach these days. A wander along at high tide is to wade through the shallows, dodging driftwood and giant tree stumps. Though they make great huts for children, they're stern reminders that there has been some serious erosion over the past few decades.

That's nothing new in New Zealand, of course; it's a natural process to which an island nation has to constantly adapt. But where is the line between letting nature take its course and stepping in at great expense to protect a way of life?

LAND AND SEA: Erosion on Tahunanui's Back Beach exposes the giant weathered roots of trees along the coastline.
LAND AND SEA: Erosion on Tahunanui's Back Beach exposes the giant weathered roots of trees along the coastline.

Last week, Tahuna Beach Holiday Park manager Ann Cumpstone raised her worries with the Nelson Mail, saying the popular camp would disappear into the sea if the council didn't help fund erosion protection work, but up until now, the council has had a hands-off approach to the area.

In stories about cities on the edge of the sea, you can usually count on the legend of King Canute popping up somewhere. Children's storybooks have it that the mighty king was so arrogant that he had his courtiers carry his chair to the shore to demonstrate how his command could hold back the rising tide - and was, of course, humiliated.

But the legend is generally misunderstood these days. The original source has it that Canute wasn't a proud and foolish git at all, but merely demonstrating deference to an unstoppable force: God, or nature, depending on your point of view.

FRUSTRATED:  Tahuna Beach Holiday Park manager Ann Cumpstone on the camp’s rock wall that helps prevent erosion.
FRUSTRATED: Tahuna Beach Holiday Park manager Ann Cumpstone on the camp’s rock wall that helps prevent erosion.

In what is possibly the first and last time the Nelson City Council will be compared to Viking royalty in the pages of the Nelson Mail, you could say that it is acting just like the latter version of King Canute in its "managed retreat" policy around Back Beach, which has been in place since 2001.

Blind Channel, that familiar rush of water that drains the Waimea Estuary, is intimately related to all of Tahunanui Beach, and has been slowly pushing eastward over the last 140 years, scouring away all before it.

Although we now enjoy the results of the council's efforts to fix erosion at the front beach in 2005, at a cost of nearly $3 million, NCC had decided there wasn't much it could do about the Back Beach.

It would simply get out of the way of the hungry sea, rearranging the deckchairs as it went. These days, during a big high tide, carparks at the Back Beach can be awash. In fact, one has already been removed under the council's policy when waves turned its edges to rubble.

Though annual monitoring is carried out, the council has not commissioned an in-depth survey of the area since 2004, despite Nelson City Council's erosion consultants Ocel recommending in 2010 that at least two be done - one into the impacts of sea level rise and one into Blind Channel's movement.

Now mayor Rachel Reese believes it's time to take another look at council policy around Tahunanui, and a new draft plan for the area will be presented to a council workshop in March. But can we expect ratepayers to hold back the sea?

Tahunanui Beach is known nationally as a jewel of Nelson - but so is Tahuna Beach Holiday Park. Built on flattened sand dunes in 1937, like the nearby sports fields, it is the largest in the southern hemisphere and hosts 200,000 people every year, as well as conferences.

Employing 25 staff, the camp operates as a non-profit

incorporated society, on land leased from the council.

Over summer, its numbers swell to 2500 visitors a night, though there are about 102 permanent residents year-round.

In 2012-2013 it made more than $2m a year from accommodation, and Companies Office records show its council levy was $234,318. Records show it paid out more than a million dollars in wages in the same period.

But its pretty, peaceful seaside location is also leading to its downfall. Fifteen years ago, it lost 28 sites to the sea, a loss of income of $14,000 a year. Another 14 sites were threatened, so in 2003 it built the rock wall, costing a quarter of a million dollars.

A flat top was built on top of the wall in the hope it would link up to a future cycleway, helping people from Richmond and beyond get to Tahunanui without going on the road.

In its 2010 report, Ocel said that the wall had worked well, and was "low key and reliable" protection.

But damage has been ongoing, Mrs Cumpstone says, and the park can't afford to fund any more work itself. She says the council had cut down unstable trees on a Back Beach sandspit a while back. The sand and the stumps had soon disappeared, leaving the holiday park more exposed.

"When the tide's in, [water] is coming straight into the side of the holiday park and hitting the rock wall now," she says. "Once it starts it doesn't take long. It undercuts very quickly."

Nearby, at the Nelson Golf Club, superintendent Brendon O'Hara says the sea has been chipping off pieces of its land in the eight years he's been there. In some spots they had lost about four metres along the waterfront.

"We've definitely lost substantial amounts in a couple of areas, and it's actually in a direct line with the [exposed stumps] on Back Beach."

He suspects the club will probably need external advice on how to slow or halt the erosion, and the issue will be discussed at the golf club's board meeting this month.

No-one is very surprised by all of this; it is a long-known problem and blindingly obvious to all but the dreamiest dog walker. The council's current management plan says erosion will continue to be a part of the coastal character of Tahunanui, having been an issue since the 1950s. It's difficult to predict where the beaches and dunes will end up in the future.

Ocel's 2000 report put erosion at between 3.5m and 6m a year, although in 2010 council staff estimated it had increased to an uneven 10-15m a year, with less pressure on Parker Cove and more on the outer edge of the Back Beach. There is no possibility of recovery in the short term.

Beaches do not stay in one place forever; the sand is always shifting around the system. At the Back Beach, you could almost say that Blind Channel is making its way home again.

Nelson landscape architect David Sissons has compiled much of the council's history of Tahuna Beach, which is less than 100 years old.

Maori used the area at the entrance to the Waimea Inlet and Nelson Haven to collect seafood, and the site is a former village, dating from about 1360, with artifacts suggesting it was used as a spot to make fishing gear and stone implements.

When the first European settlers arrived in Nelson, the beach wasn't there at all. Before the 1870s, the Waimea Estuary drained further east, through the Beach Cafe and the carpark.

The sandy strip at the base of Rocks Rd used to be nothing but gravel, and a long, wide sandbank reached from Rabbit Island nearly to the point where Back Beach meets the front beach now.

But that began to change in about 1875, when a new channel draining the estuary broke through the large sandbank. The giant chunk of sand from the tip of this sandbank eventually washed around to form the existing front and back beaches.

The channel alignment changes on a hinge point near Parker Cove, and though the new channel immediately started migrating back east, the old channel began to silt up as the scouring effect of swift tides disappeared.

Tahunanui Beach begun to form, and by 1900, much of what we now know as the beach and the playing fields was there. But it was sandbank, and most of it drowned at high tide.

Dunes had formed by 1904, and 10 years later the Back Beach was drying out, the first trees had been planted, the first playing fields created, and the first changing sheds built.

The airport and the golf course were built on sandbanks later on. A car racetrack in the middle of the embayment destroyed some of the vegetation there, and it is slowly revegetating now.

Since then, several spots at Tahunanui Beach have alternately built up and eroded away, helped or hindered by human efforts. One little-known consequence of the erosion is the loss of some ecological significance of national value, including rare estuarine tussock and an endemic beetle - known, naturally, as the back beach beetle.

Ocel's 2010 report says that the channel and the spit moved 170m in the four years between 2000 and 2004, the last time a water depth survey was done.

It says the potential consequences of these changes are significant, and if they continue, it will mean more sediment dumped near Port Nelson's shipping channel, as well as changes to the exposure of the front beach to coastal processes.

In fact, Sissons points out that about 250m of the western end of the front beach has been washed away over the past 60 years, and if the trend was to continue then the roller rink might be threatened in another few decades.

Then there's the added force of climate change; sea level rise was likely to be "a significant issue", Ocel said, and recommended a Tahunanui study be done as there was potential for storm tides and waves to breach the sand dunes, increase pressure on areas eroding now, and affect Blind Channel's stability.

Ultimately, Ocel suggested three ways to deal with erosion at Back Beach: do nothing and accept natural changes, which it said "was the ideal response" but required acceptance that we might lose the beach. This was the method NCC chose.

But a second option was to protect the top of the beach and stabilise Blind Channel, and a third option was to encourage estuary outflow through dredging the channel near Rabbit Island, and/or adding a groyne near the airport - though that had the potential to significantly change the shoreline and affect the entrance channel to the port.

These solutions may be revisited if a council review goes ahead, but the options at Back Beach seem to boil down to either accepting the impact and adapting to it, or spending a lot of ratepayer money on protecting the area - and possibly shifting the problems somewhere else, as has happened with the groyne at Port Motueka, which swiftly led to erosion on Jackett Island.

Whatever the council decides, it's not the first time Nelson has had to grapple with changes in the volatile area, which impacts on so much of our social and economic wealth.

When the new channel broke through in 1875, it caused the bay beyond Haulashore Island to shoal up so dramatically that by 1900 only small craft could make it through to the port at high tide. Four were grounded in one week.

In 1899 a Dunedin engineer proposed slicing out a chunk of the Boulder Bank and creating The Cut, which sparked violent debate in the local press, a high-profile court case, and several resignations. But it went ahead, and saved Nelson from becoming a truly isolated outpost in the world, during a time when shipping was even more of a lifeline than it is now.

The city has always debated whether or not it should tamper with nature to suit itself. It's just part of life when you build a city on the edge of ever-shifting sands.