Coastal erosion and inundation

Climate change and sea-level rise is likely to increase the risk of coastal hazards of inundation (flooding by the sea) and coastal erosion. It's a challenge facing coastal communities here, and around the world.

Coastal inundation

Coastal inundation is when normally dry, low-lying coastal land is flooded by the sea. Coastal inundation is primarily caused by severe weather events (storms) along the coast, impacting on estuaries and rivers

Storm surge is one of the main causes of coastal inundation. A storm surge is a rise in water level from low pressure weather systems, over and above the predicted tide height, caused by a severe storm.

These severe storm events bring strong wind and heavy rain. The wind drives large waves and heavy rain raises water levels in rivers and streams. The worst flooding occurs when larger-than-normal tides (king tides) and storms occur at the same time.

Factors causing inundation

Coastal erosion

Coastal erosion is a natural and ongoing process that occurs when the sea wears away the land. Some shorelines undergo short term periods of erosion but then recover (i.e. build out again) while other shorelines may continuously erode with no cycle of recovery.


Coastal inundation is one type of flooding that Christchurch and Banks Peninsula settlements are susceptible to.

Other types of flooding include:

  • Fluvial flooding: occurs when streams and rivers are at capacity and water flows over the banks
  • Pluvial flooding: occurs when the stormwater drainage system cannot cope with extremely heavy rain.


Another coastal hazard is tsunami - a sustained elevated sea-level that can flood coastal areas.