Natural sand dunes play a vital role in protecting our beaches, coastline and coastal developments from coastal hazards such as erosion, coastal flooding and storm damage. Sand dunes protect our shorelines from coastal erosion and provide shelter from the wind and sea spray. Sand dunes also provide a future supply of sand to maintain the beach. The wider the band of dunes, the larger the reservoir of sand. The height of natural dunes also provides protection from coastal flooding from storm surge and wave action.
Waikato’s coastal dunes are under pressure from a wide range of human activities, which have already changed over 70 percent of all dunelands in our region. Many coastal areas in the Waikato region are under threat because their natural vegetation has been altered, exposing them to the effects of erosion. Damage to our beaches’ natural features has reduced many dunes to narrow bands along the shoreline.
Global warming could see some of our Coromandel beaches move 15 to 20 metres inland in a hundred years' time. Sand dunes are one of many our coastline’s natural features that protect our shorelines from erosion.
During long periods of settled weather, sand builds up on the visible part of the beach, including the dune. However, short-term erosion can happen during storms, as waves erode the beach and the dunes closest to the sea. This often leaves a near vertical cut in the face of the dune (an ‘erosion scarp’) as shown by the diagram below. The eroded sand is carried offshore into the surf zone, where it forms shallow bars that help absorb storm wave energy.
Significant dune erosion can occur in just a few hours, but full sand dune and beach recovery can take years.
After the storm, the beach area repairs itself first before the dunes can recover. Gentler wave action moves sand back to the shore, slowly rebuilding the beach. Dry sand is then blown further inland and trapped by sand binding vegetation to repair the eroded dune. The diagram below shows how dunes are repaired following beach recovery.
Natural dune repair depends on a good cover of native sand binding grasses, such as spinifex and pingao, to trap moving sand.
Erosion through climate cycles sees shorelines on Waikato beaches move naturally over periods of decades, with the largest change s usually seen near estuary and river entrances. Though periods of erosion can continue for years, in most cases it is not permanent. When viewed over a long period, such as a hundred years, the shoreline is simply shifting backwards and forwards.
Very large shoreline fluctuations (often more than 100 m) occur on sand spits near some river and estuary entrances.
The Port Waikato spit has grown northwards over the last 150 years, eroding large volumes of dunes from the north side of the river. Similar changes cause problems where they threaten coastal development. At Mokau, such processes have resulted in the loss of several properties and threaten many more. It is important to understand these changes and use this information to plan future development carefully .