4 Jan 2016

Coastal erosion: people versus nature

From Our Changing World, 9:20 pm on 4 January 2016

On May 13 2013 raging seas battered Dunedin’s St Clair beach, and in areas usually filled with visitors and walkers, sink holes large enough to swallow park benches began to appear.  Several years later University of Otago science communication student Sam Fraser-Baxter heads to the beach to investigate what’s going on there now.

New Zealand’s coast is an unpredictable and relentless source of energy. Situated between Antarctica and the tropics we are constantly battered by wild seas.  But in our desire to live close to the beach and the sea we have fortified ourselves with static, coastal structures, such as the concrete seawall at St Clair.

However, in a world of changing climate and rising sea levels structures such as seawalls are beginning to fail, putting a spotlight on coastal management.

Seawalls work by reflecting wave energy, unlike normal sandy beaches which dissipate wave energy. Seawalls are static, so when waves break on them the water is reflected back out to sea. And those reflected waves transfer sand offshore, which lowers the beach, and reduces the volume of sand over time. Then, as the beach is lowered, larger waves can happen. So the problem perpetuates itself, as the wall becomes subjected to more energy.

“The esplanade wasn’t like this originally, when we moved here 10 years ago,” says local Starfish Café and Bar owner Cushla Sullivan-Dodds.

So what can be done if we still want to live this close to the beach?

“It’s a matter of how you balance the assets against nature,” says University of Otago coastal geomorphologist Wayne Stephenson. “It’s a catch-22 situation, because while there are lots of things you could do to prevent this, no matter what structure you build, it will get damaged and require repairs.”

“Everything we do costs money. It just depends on how much we want to spend”.

Unfortunately, there is nothing unique about the issues surrounding St Clair’s seawall; it is a generic and costly problem of a “static structure in a dynamic environment”. And so the story of St Clair is just a potent reminder of the power of the coast.