All erosion options in DCC focus

The Dunedin City Council says it is looking at all options as it plans the defence of the city's most popular beaches from erosion caused by pounding Pacific waves.

• Study of the shape of sands to come

A multimillion-dollar extension of the St Clair seawall as far as Lawyers Head, to protect St Kilda and Middle beaches, is among extreme "hard options" council staff say cannot be ruled out, while a host of "soft solutions" each come with their own problems.

Council parks and reserves team leader Martin Thompson said yesterday a series of projects aimed at gathering data about the state of the three beaches was either finished, nearing completion or soon to start.

The work began in the months after a dramatic series of storms in 2007 caused significant erosion in the area - repeated when the storms returned last year - and threatened to send parts of Kettle Park crumbling into the sea.

The projects launched in response included the work of two University of Otago students, who were conducting a monthly GPS survey, mapping the form and volume of sand on the beaches and how it changed over time.

Also planned to start soon was a survey using ground-penetrating radar to locate the edges of the old landfill under Kettle Park, parts of which had been exposed by the severe storms in 2007, Mr Thompson said.

The projects tied in with work already completed, including mapping the seafloor offshore from the beaches, ongoing monitoring of rip tides - which influenced erosion "hotspots" - and weather information from automated cameras at St Clair.

Mr Thompson said the data would then be used in hydrodynamic computer modelling, mapping the impact of possible solutions - from a seawall extension to simply restocking the area with sand - on the beaches over a 50-year period.

Other measures to be modelled included the construction of an offshore artificial reef, to dissipate the power of waves, he said.

The modelling was expected to begin once the council established a new project team to co-ordinate the next stages of the work, possibly as soon as June this year, he said.

However, it could take the best part of a year to identify the preferred solutions and prepare a draft report recommending the best options to councillors, he said.

The initiatives would then be presented for public consultation, and possibly become the subject of a resource consent hearing, meaning any construction could be several years away, he said.

The data collection and modelling work had a budget of $335,000 for this year, but some of the possible solutions could be "quite expensive".

Extending the St Clair seawall - considered an unlikely example, but one that could not be ruled out - was likely to cost "tens of millions of dollars" and raise significant aesthetic questions, he warned.

"That would be a huge cost and maybe quite a structurally intrusive solution."

Smaller "soft solutions", such as continually re-stocking the beaches with dredged sand, would be cheaper and less intrusive, but might mean "sacrificing" some vulnerable parts of the coastline to the sea, he said.

It was too soon to say what the likely measures taken would be, or what the exact cost could rise to.

"We are not ruling anything out and we are not ruling anything in."

In the meantime, the council had an annual budget of $400,000 to maintain a "holding pattern" of remedial work in the area, but the beaches remained vulnerable to further erosion in another series of big storms, he said.

"We are at the mercy of the sea, basically, in terms of what sort of storm events we get . . .

"If we get a run of storms again this year, we could be back in a situation where we are getting a lot of erosion."