By Kim Lawry
As another summer cyclone crashes into the east coast of the Coromandel Peninsula, it is a good time to think about just why our sand dunes are important and why we work so hard to restore and protect them.
Coastal squeeze Our coastal margins, that small strip where the land meets the sea, are an extremely dynamic zone. Extreme weather events, king tides, storm surges, high winds, long periods of settled weather, all affect what happens along the coastal margin, and these effects are most clearly seen on the beach.
Interestingly, the effects are often seen most dramatically where a town, city, property or infrastructure sits directly behind the beach. There are two reasons that this causes a problem. The first is that in the development of the town, the dunes are often flattened out or effectively removed to provide unrestricted sea views. Secondly, the proximity of houses or amenities to the beach means a loss in the ability for the beach to retreat inland during periods of erosion. This is known as coastal squeeze, and we see a great example of it here in Whitianga.
Our beaches have been moving inland, and back out towards the sea again for thousands of years. Shoreline migration is a natural process and it happens within dynamic cycles. The problem is we never know how long the cycle may last. It could be 5 years, 20 years or 150 years. If we happen to be in the middle of a very long cycle, we might see the beach retreating inland for our whole lifetime, but that still doesn’t mean the erosion is permanent. The sand is moved away from the beach during storm events and is deposited on the sea floor offshore. When the conditions change, favourable calm periods may allow that sand to be returned to the beach. Sometimes this happens soon after the storm event, but it may take many, many years for the beach to recover.
Paramount to that recovery being successful is a healthy sand dune sitting behind the beach. The dune works as a buffer zone, a body of sand that can and does lose ground to the ocean in storm events, but which can rebuild itself over time after the erosion has taken place.
Just how effective the dune is in rebuilding itself depends on the vegetation that is present and its protection, and this is why we are trying to restore the dunes and plant them with the native species that are a critical part of this process. The dune plants that go in the ground trap sand that is brought in by the waves and blown inland as that sand dries.
Unlike houses, these native plants thrive in shifting sands. Plants on the foredune, that is the front of the dune closest to the water, can be completely covered with windblown sand and they will keep growing and capturing more sand to rebuild the dune seawards. We plant Spinifex and Pingao in this area, and a wide margin of these plants can be seen on any naturally healthy dune system in New Zealand.
Behind this, in the mid-dune zone, we find Knobby Club Rush (Wiwi) and Meuhlenbeckia (Pohuehue), which grow taller and help to capture that sand which has blown over the top of the lower growing foredune plants. Ideally, behind this, we would have taller hardy trees and shrubs in a zone called the backdunes, but a wide coastal margin is needed for this to establish. A great example in our area is the natural dune along the middle section of Otama Beach.
The Spinifex on the foredune sends runners down the beach towards the ocean. These take root and as the plant grows it captures sand and helps grow the dune towards the water. However, if the plant is constantly battered by erosion and can’t establish further down the dune, it will turn around and send runners inland, re-establish itself further up the dune, and then begin the process again. In this way the dune can move backwards and forwards over time depending on the prevailing conditions. Dunes have been doing this for thousands of years.
Arrival of humans However, humans arrived and interfered with the natural coastal system. Coastal squeeze has meant that we don’t have enough room for a healthy dune to fluctuate naturally. The native plants we put along the coastal margins struggle to survive because they don’t have the room they need to move inland and bring the beach with them. People are often critical of the native plantings that we do, saying that a few plants will never stop the erosion. But the important point is that we do not plant these plants to stop erosion. We plant these native plants so that the dune can self- repair and recover from the storm events. The natural rebuilding of the dune system seaward increases the buffer zone and that will mean that we will continue to have a beach. Where we have no dune, there is no natural system to repair erosion damage, and this is most often where we see the land continuing to crumble into the sea.
Kim Lawry Chairman
Mercury Bay Environmental Trust