Coastal erosion

What we know

Factors contributing to coastal erosion

Coastal erosion can be a long-term or a short-term process. In the longer-term erosion is largely caused by an imbalance in sediment supply, but in the short-term erosion can occur as a result of dynamic wave run-up during high tide or storm events, and is heavily influenced by the shape of the coastline. This is part of a natural cycle where coastlines erode during storms and recover by accretion between them . Further information on the process of coastal flooding in outlined in the Ministry for the Environment Fact Sheet: http://www.mfe.govt.nz/sites/default/files/media/MFE_Coastal_Fact%20Sheet%201.pdf


Short term erosion events can occur despite pattern of long term accretion

While the open coast of the Regeneration Strategy project area is currently in a cycle of long-term accretion, storm events will cause localised erosion. Increased intensity of storms could cause worsening of short-term erosion, and increased frequency of storms would result in less time for the beach to recover between storms. This changes the balance in the short-term cycle of erosion and accretion.

Long-term increases in sea level elevation will exacerbate both long-term and short-term erosion, and any changes in the sediment budget of the Waimakariri River could also change this balance over short and long term.


Erosion is different across the Regeneration Strategy project area due to the shape of the land

The south end of the Spit is subject to erosion/accretion cycles because it is affected by both open coast processes (breaking waves, tides and ocean currents) and estuarine processes (freshwater drainage, river flooding, sedimentation). The interactions of these two processes can be unpredictable.

Waves, and therefore wave energy, are highest on the exposed open coast and can undergo more dramatic erosion events during coastal storms than the more sheltered the Estuary/Ihutai. However, erosion of the Estuary/Ihutai shorelines is often very slow to repair naturally as, unlike the open coast, the more sheltered wave conditions in estuaries are not conducive to the movement of sediments onshore which can help a shoreline naturally recover from storm events.


Exposure to coastal erosion

Coastal erosion within the Regeneration Strategy project area is largely confined to open space areas directly adjacent to the coastline, with a maximum of 73 properties affected by 2120 in the highest RCP 8.5+ scenario .

Coastal erosion - number of properties affected

Here's a map showing the coastal erosion hazard for the years 2065 and 2120.


Why this is important

See Modelling coastal erosion and inundation for more information.

What we know

Factors contributing to coastal erosion

Coastal erosion can be a long-term or a short-term process. In the longer-term erosion is largely caused by an imbalance in sediment supply, but in the short-term erosion can occur as a result of dynamic wave run-up during high tide or storm events, and is heavily influenced by the shape of the coastline. This is part of a natural cycle where coastlines erode during storms and recover by accretion between them . Further information on the process of coastal flooding in outlined in the Ministry for the Environment Fact Sheet: http://www.mfe.govt.nz/sites/default/files/media/MFE_Coastal_Fact%20Sheet%201.pdf


Short term erosion events can occur despite pattern of long term accretion

While the open coast of the Regeneration Strategy project area is currently in a cycle of long-term accretion, storm events will cause localised erosion. Increased intensity of storms could cause worsening of short-term erosion, and increased frequency of storms would result in less time for the beach to recover between storms. This changes the balance in the short-term cycle of erosion and accretion.

Long-term increases in sea level elevation will exacerbate both long-term and short-term erosion, and any changes in the sediment budget of the Waimakariri River could also change this balance over short and long term.


Erosion is different across the Regeneration Strategy project area due to the shape of the land

The south end of the Spit is subject to erosion/accretion cycles because it is affected by both open coast processes (breaking waves, tides and ocean currents) and estuarine processes (freshwater drainage, river flooding, sedimentation). The interactions of these two processes can be unpredictable.

Waves, and therefore wave energy, are highest on the exposed open coast and can undergo more dramatic erosion events during coastal storms than the more sheltered the Estuary/Ihutai. However, erosion of the Estuary/Ihutai shorelines is often very slow to repair naturally as, unlike the open coast, the more sheltered wave conditions in estuaries are not conducive to the movement of sediments onshore which can help a shoreline naturally recover from storm events.


Exposure to coastal erosion

Coastal erosion within the Regeneration Strategy project area is largely confined to open space areas directly adjacent to the coastline, with a maximum of 73 properties affected by 2120 in the highest RCP 8.5+ scenario .

Coastal erosion - number of properties affected

Here's a map showing the coastal erosion hazard for the years 2065 and 2120.


Why this is important

See Modelling coastal erosion and inundation for more information.