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.
Types of erosion
There are two types of coastal erosion:
- Cut and fill erosion occurs on coasts made of loose sediment such as sand, gravel or shells, which are freely exchanged by water action between land and sea. Known as soft coasts, these are mainly beaches. Eroded material may be replaced over time.
- Permanent erosion occurs on rocky or hard coasts. Eroded material cannot be replaced.
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:
- Before the storm, waves are small, and the beach accretes as sediment moves onshore.
- As the storm approaches, waves become higher and the beach erodes.
- During the storm, as waves get higher and more frequent, erosion increases.
- After the storm, waves become flatter and less frequent, and the beach accretes once more.
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.