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18th of January 2018

New Zealand

The sea will always win

The power of the ocean is unfathomable, its pounding against the shore unrelenting. The one thing you can always be sure about when it comes to the sea is that it will inevitably win.

Across the centuries, people have tried to tame its influence by building all kinds of structures that will hopefully stop erosion here, or help build up the coastline there. Some have even been moderately successful for a period of years. Ultimately, however, the sea will have its way.

Coastlines are the world's most constantly dynamic landscapes. In New Zealand, our coasts get an even more perpetual battering from the force of the sea, thanks to our location in the heart of the ''roaring forties'' westerly wind belt and the big swells rolling in from the southern ocean.

The clamour to own a house next to the sea also puts pressure on what is already a fragile environment. Not only can coastal developments physically exacerbate beach erosion, but also they can encourage a mindset that if we choose we can ''tame'' the sea for our own ends whenever it suits. Future sea-level rise due to climate change will be a further nail in the coffin of such thinking.

Even a quick look at historic photographs of the beach at St Clair reveals how vigorously the Pacific Ocean has been eating away at the shoreline since Dunedin was settled. Similar examples of rapid erosion can be found around the country, including along the Canterbury coast, down the West Coast and in parts of Hawke's Bay. Only some of this damage is inflicted by storms - a great deal is actually due to never-ending tidal action.

St Clair resident Jules Radich has been encouraging the Dunedin City and Otago Regional councils to look again at building a groyne where the beach's decaying wooden poles are. Mr Radich believes such a groyne at right angles to the beach would allow sand to build up, helping mitigate sea-level rise, and says the three structures previously built were successful each time, although they had not been properly maintained.

However, Dr Wayne Stephenson, a coastal geomorphologist at the University of Otago, is worried a groyne could exacerbate the natural movement of the sand and possibly cause erosion of nearby beaches. He also points out that perpendicular groynes are most effective where the movement of sediment is along the shore, not periodically on and offshore like at St Clair.

There may be other issues too. Dr Stephenson says a large groyne could block the view from shore of people in the water, with obvious repercussions in bad weather or when the sea conditions are dangerous. And a badly designed groyne may actually generate rip currents.

Mr Radich's proposal has received a cautious response from the city council. Three Waters group manager Tom Dyer has met Mr Radich to discuss his idea and says while it ''has merit'', the idea would need to be discussed with other groups and individuals too.

It is encouraging to see that technology has lent a hand in the fight to keep the land from being nibbled away. Sand sausages along the beach from St Clair appear to have been successful in reducing the erosion of the sand dunes. Contractors have been improving beach access and the sausages installed by the city council in August last year remain undamaged after their first full winter on the job. The tidying up work has been largely cosmetic to fix-up erosion from high tides in the winter.

Engineers are still investigating ways of reducing the effects of the waves. Some of these efforts will enjoy some medium- to long-term success, and there is truth to the old adage that, if you can pay for it, engineers can design and build just about anything.

But in the end, time and tide wait for no man.

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