Groynes not the answer

Groynes are still well used around the world, but generally not for large-scale erosion projects, a senior coastal engineer says.

Dr Tom Shand, from engineering firm Tonkin and Taylor, says he does not have first-hand knowledge of Dunedin's Ocean Beach but on beaches where sand depletion is a major problem, groynes tend to trap sand on one side and cause erosion on the other.

They are a very ''site-specific'' solution, and in general sea walls are preferred, as they add more surety for protection and, in the long run, tend to be cheaper.

Where groynes are used to combat erosion, they tend to be combined with ''large-scale beach nourishment'' - constant resupplying of sand - which means significant ongoing costs.

Yet in a big storm the added sand will probably still be sucked away, he says.

Adding to the high cost of sand replacement is the high cost of groyne construction in the 21st century, as today's groynes tend to be built from rock.

That rock construction is needed in ''high energy environments'' where timber groynes were not able to cope.

''There haven't been many built out of timber that I've seen for a very long time.''

University of Otago coastal processes specialist Dr Mike Hilton says groynes have their place and are widely used in Europe.

But they are not suitable for many coasts, including St Clair, where there is no steady drift of sand along the shore in one direction.

''Different waves and surf-zone currents move sand onshore and offshore, primarily, with some sand also moving to the east, towards St Kilda, and back again.

''That is, sand moves in all directions, not just alongshore, so a groyne might or might not get a chance to trap sand.''

That movement of sand in all directions at St Clair, particularly onshore and offshore, means groynes are ''probably not'' likely to work effectively there.

Dunedin City Council parks, recreation and aquatics group manager Richard Saunders says the ''coastal experts'' like those at Tonkin and Taylor are the best placed to discuss whether groynes will work today at St Clair.

When asked whether the council is open to groynes being built again at the beach, he says the council's position ''would be that all options will be considered when long-term solutions are reviewed''.

• The Otago Daily Times acknowledges the assistance of Dunedin City Council and Toitu Otago Settlers Museum archive staff.

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