About 70% of New Zealanders live in coastal towns and cities, most of which lie along the quarter of the country’s coast that is eroding. People seldom heed warnings not to build on unstable land; the demand for houses close to beaches, with sea views, means that many have been built on low-lying sand or coastal cliffs. But houses and roads can be undermined when natural erosion occurs. Local councils have often been blamed for permitting construction on such land in the first place. Coastal erosion is a frequent cause of disputes between landowners and local authorities. The Resource Management Act 1991 established a national coastal policy to guide local authorities in their day-to-day management of coastal areas.
Sea walls and groynes
A common response to erosion has been to construct sea walls and groynes (low barriers built out into the sea). Sea walls are common around the Waitematā Harbour in Auckland and the Wellington waterfront. Although these structures stabilise the coast, they cause more erosion at adjacent coasts, merely shifting the problem or creating a new one. They also cause the loss of any beach in front of the structure at high tide (this occurred at Wellington’s Oriental Bay). In recent years it has been argued that constructing sea walls and groynes is inconsistent with the principles of the Resource Management Act.
In the 1870s a breakwater was built at Caroline Bay, Timaru, to shelter the harbour. Not long after, the sea undermined the railway line on the clay cliffs just to the north, requiring expensive repair work. In 1879 colonial engineer J. Macandrew wrote to the Marine Department, pointing out where he thought the blame lay:
‘Before any breakwater was erected, the sea-beach was covered with a coating of shingle … When the breakwater was erected, it acted as a stop to the flow of shingle … laying bare the beach beneath.’ 1
A form of coastal protection that has fewer negative side-effects than sea walls is beach re-nourishment – adding sand to the beach. The most successful projects have been at relatively sheltered beaches such as Mission Bay on the Waitematā Harbour, Tauranga Harbour, and Oriental Bay on Wellington Harbour. Beach re-nourishment does not prevent coastal erosion, so sediment must be occasionally replenished. There may be difficulty finding a source for the sand, and many people oppose sand mining because it causes coastal erosion somewhere else.
Zoning is a regulatory step to control coastal development. The first zones were established after the introduction of the Town and Country Planning Act 1977, which allowed for the identification of areas vulnerable to natural hazards. The Resource Management Act 1991 extended this, taking account of a wider range of hazards and their probability.
A hazard zone is defined as the width of shoreline that is likely to be affected by coastal erosion or flooding by the sea during a specified period. Although in some areas flooding is the greater hazard, most zoning is based on erosion. Once a coastal zone has been defined, restrictions may be placed on development, including building houses, to minimise inundation. However, coastal protection can be threatened when landowners, who see zoning as limiting development options, put pressure on local authorities.
Other options for managing erosion
A combination of sea walls and re-nourishment has proven effective. Sea walls may be buried in the sand, or T-shaped groynes or submerged breakwaters may be used to stabilise the re-nourished shore. A cheaper alternative is to restore the natural beach by rebuilding sand dunes, using simple structures and vegetation to trap the sand that moves onshore during fair weather. Plants have been used successfully to bind sand along parts of New Zealand’s coast.
Internationally, managed retreat (the do-nothing approach) is becoming an accepted method of dealing with coastal erosion. However, because of pressure to build houses close to the sea, this has generally not been accepted in New Zealand.