Coastal defence

Recently I mapped the existing vegetation areas at out Tūmai farm park to prepare grant applications to restore the forest that was cleared nearly 200 years ago for farming.  I was shocked to discover that 1.03 hectares of our 39-hectare communal property had been lost to erosion into Te Hakapupu’s (Pleasant River’s) estuary since the maps were first drawn up (probably around 70 years ago).

The signs of land loss were there, like this derelict fence running off a cliff at our farm park boundary, but we had not understood the full extent of the land loss until this mapping.  Erosion destroys the land and disrupts the ecology of the estuary.

The rate of erosion is very likely to be greatly accelerated by deforestation, sea level rise and increasingly stormy weather. New Zealand’s sea level is predicted to rise anywhere between 0.46 and 1.05 meters by 2100.  This wide uncertainty makes it hard to decide what to do, especially whether to withdraw investments and rebuild infrastructure (houses, roads, farmland, fences etc.). Adjustments will be very costly for private individuals and our community. So who picks up the costs, and how can society better start recognising and factoring in the cost of doing nothing to prevent or reduce climate change in future?

Defence of an area like Tūmai farm park is not simply a matter of building the wall up by another 0.46 – 1.05 meters.  It’s the interaction of high tides and storm surges that does serious damage to the coastline. But the consequences are minor at Tūmai compared to low lying areas around Dunedin city.  One thing we can do is to prohibit further development of coastal areas that will be encroached when sea level rises, but this still leaves the thorny political question of what to do with the areas already developed – do we retreat or defend?  Sustainability and environmental justice depend on have good governance and healthy community discussion to decide what to do.