What is coastal erosion?
Coastal erosion happens when wind, waves and water currents wear away the shoreline.
Shorelines erode in different ways, depending on whether they are sandy (soft) or rocky (hard).
The flow of water and wind on sandy beaches constantly shifts the sand from one place to another. This means that sand can be lost from a beach, or it can be washed or blown there.
Waves are the main cause of erosion, and may be storm waves, boat wakes, tsunamis or tides. High waves take sand away, while flatter waves can deposit it.
Shorelines made of rock take thousands of years to wear away. When rocky beaches erode, they form cliffs.
How quickly cliffs erode depends on their strength. Cliffs full of closely spaced cracks wear away quickly, and form sea caves, arches, and eventually rock pillars. Rock can be weakened by earthquakes, heavy rain, or the constant crashing of waves. Waves hitting the base of a cliff can undermine it and cause it to collapse.
New Zealanders love to live by the sea, but coastal erosion can be a big problem if houses are built on low-lying sand, or on cliffs. A zoning system has been set up to prevent buildings being put up in areas which are likely to flood or erode.
One way that people have tried to make the coast stable and protect buildings is to build sea walls. There are many around the Waitematā Harbour in Auckland and the Wellington waterfront. Beaches can be restored by bringing in sand from somewhere else. Planting native grasses that trap sand can stop sand dunes from being blown or washed away.