Coastal erosion is the retreat of the shoreline due to water currents, waves and wind. It is a natural process that can be influenced by human activities.
There are two types of coastal erosion:
The public perception of coastal erosion is of a sandy beach being washed away, threatening nearby houses with the same fate. This is just one effect, caused by natural processes that move sediment on- and offshore. The coast is a dynamic environment; the flow of water and wind constantly shifts sediment from one place and deposits it somewhere else.
Waves are the main cause of soft-coast erosion. They may be wind-generated surf, boat wakes, tsunamis or tidal currents. On sandy beaches the sand is often transported just offshore. But on coarser, gravel beaches erosion may occur when waves carry the gravel inland.
Whether sand is deposited on, or eroded from, a beach depends on the type of surf; in turn, the type of surf may depend on the slope of the beach. High waves erode sediment, while flatter waves deposit it (a process called accretion).
The introduction of fast ferries to cross Cook Strait led to complaints that their wakes were increasing coastal erosion in the Marlborough Sounds. Because fast-ferry wakes tend to produce waves higher than those of conventional craft or the natural waves produced by winds, they can shift more sediment. But they can also deposit sediments. As the wakes tend to have long gaps between them, they can move sediment onshore. And since each passing ferry only makes a few wakes, the beach’s water level is never greatly elevated, making coastal erosion less likely. Researchers concluded that high-speed ferry wakes did not cause significant erosion, but they did change beach shape and the distribution of sediment along the beaches.
New Zealand beaches erode rapidly, while it may take several decades for material to be deposited. If a series of storms occur within a short period (a few weeks to a couple of years), their cumulative effect can be severe, even if the individual storms are minor.
Erosive conditions vary as storms pass:
A sandy beach that has undergone a long period of accretion is quite steep, and much of the wave energy hitting it is reflected back to sea. (This is called a reflective beach.) By contrast, an eroded beach is flat, and most of the incoming wave energy transports sediment and generates currents (a dissipative beach). Between these extremes there are a number of complex beach shapes that typically have currents running along the shore (intermediate beaches).
Changes in ocean and atmospheric circulation in the Pacific Ocean (such as El Niño, when warm ocean waters spread across the eastern tropical Pacific) cause long-term variations in New Zealand weather patterns. As changes in climatic conditions modify the direction of onshore winds, there are episodes of increased erosion and accretion. For example, a combination of climatic conditions was associated with severe erosion along New Zealand’s north-east coast from 1948 to 1978, and was predicted to cause more erosion between 1998 and 2030.
Coasts made of solid rock, or sediment deposits with tightly packed or cemented grains, are known as hard coasts and are relatively resistant to erosion. When hard coasts erode they form cliffs, which is evidence of constant wearing away over thousands of years.
The erosion of rocky coasts is not reversible, and loss is permanent. However, the rate of erosion is less than for sandy shores. Hard coasts erode at a rate predominantly controlled by the strength of the rock; waves play only a minor role.
Some people think that the rise in sea level associated with global warming has caused coastal erosion. But erosion occurs naturally whether sea levels are rising, falling or static. However, if sea levels continue to rise as they have in the last century it is likely that vulnerable coastal areas will continue to erode, possibly at an accelerating rate. And coasts as yet unaffected by erosion may begin to experience it.
The strength of a rocky coast depends mainly on how many breaks there are between the blocks of rock. The spacing between breaks, their angle, length and width, and what material (if any) they are filled with all influence how susceptible cliffs are. Lots of breaks close together result in increased erosion that can form sea caves, arches, and eventually rock pillars known as stacks. The rate and magnitude of erosion is also affected by the strength of intact blocks, the amount of water present within them, and the degree to which they have been weakened by weathering.
The stability of steep slopes, and the climate – particularly rainfall – are factors in erosion. Earthquakes can trigger slope failures.
Some cliffs made of soft sedimentary rocks, such as papa (mudstone) can retreat inland up to 1–2 metres each year, while hard rock erosion rates can be imperceptible over a year. Cliffs of soft sedimentary rock are particularly vulnerable to waves. The cliffs retreat as waves undermine the base, causing rock faces to slip into the sea.
The rate of cliff erosion along most of the New Zealand coast is very slow, and its cliffs have been shaped by processes that occurred before the sea reached its present level, about 7,200 years ago. For example, glaciers carved the fiords of Fiordland, the East Cape’s headland bays are the result of valleys flooding as sea levels rose, and the Marlborough Sounds are a combination of inundated river valleys and land subsidence.
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.
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.
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.
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French, P. W. Coastal and estuarine management. London: Routledge, 1997.
Goff, James R., Scott L. Nichol, and Helen L. Rouse, eds. The New Zealand coast: te tai o Aotearoa. Palmerston North: Dunmore Press/Te Whitireia/Daphne Brasell Associates, 2003.
McKelvey, P. J. Sand forests: a historical perspective of the stabilisation and afforestation of coastal sands in New Zealand. Christchurch: Canterbury University Press, 1999.
Payne, George, and others. New Zealand's sandy coasts. CD-ROM. Hamilton: NIWA, 2003.