Coastal erosion, flooding and rising groundwater

Coastal flooding (also called coastal inundation) occurs when normally dry, low-lying land is flooded by the sea.

Coastal flooding is primarily caused by severe weather events such as storms where low-pressure weather systems, large waves driven by strong winds, and heavy rain combine to raise water levels.

The worst flooding occurs when larger-than-normal tides and storms occur at the same time.

As the level of the sea rises, land that currently floods during storms or extreme high tides may experience more frequent and severe flooding. Slightly higher areas or low lying areas further inland may also begin to flood over time.

In the Christchurch District, there are some differences in how floors happen along different parts of the coast.

Areas most at risk are low-lying areas in the east of the city close to the Ihutai Estuary, Brooklands Lagoon and the lower reaches of the Ōtakaro Avon and Ōpāwaho Heathcote rivers, as well as some harbours, bays and coastal lakes in Te Pataka o Rakaihautu Banks Peninsula.

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.

Over years and decades, depending on the balance of short-term erosion events and recovery periods, some coastlines grow out towards the sea (accrete), some stay in about the same place (equilibrium) and some cut back inland (erode). 

As the sea rises, and storms happen more often due to climate change, it is expected that erosion will happen faster and recovery periods will shorten. This means that eroding coastlines will erode faster or have larger individual erosion events, and accreting coastlines could grow seawards more slowly, stop accreting, or even become eroding coastlines. Even a shoreline that is accreting in the long-term can still be at risk of erosion, which could cause damage to nearby property, infrastructure and activities.

Erosion of a beach in a storm followed by accretion. If there is less accretion compared to the erosion, the beach narrows with time and it becomes an eroding coastline. 

For more information on coastal erosion generally, see the Ministry for the Environment’s factsheet(external link)

Coastal erosion and accretion happen differently along different parts of the coast due to the varied coastlines and coastal processes.

Christchurch's open coast from the Waimakariri River through to Southshore is a dynamic and constantly changing coastline. It is currently in a cycle of long-term accretion because of continued sediment supply from the Waimakariri River. This could continue for the next 100 years even with sea level rise. However, risk from erosion is still present from short-term storm events and long-term climate change impacts.

Enclosed coastal waters such as within the Avon-Heathcote Estuary or Lyttelton and Akaroa harbours are low energy environments, which change much more slowly. However, softer sediments and steeper slopes and banks of the shoreline mean they are also less able to recover once eroded.

The coastal cliffs around Christchurch and Banks Peninsula are typically made of harder volcanic rock so are less likely to erode, but if they do, they are unable to naturally build back.

Groundwater is water that exists in saturated zones beneath the land surface.

Near waterways, the groundwater connects to the surface water. When closer to the coast, the groundwater level fluctuates with the level of the sea, including twice-daily tides.

This means groundwater is often very shallow in low lying areas near the coast, estuaries and rivers.

Due to this connectivity, rising sea levels are expected to lead to rising groundwater, and in areas where the groundwater is already close to the surface, this can be a hazard.

Changing weather patterns may also result in less frequent but more intense heavy rainfall which soaks into the ground and temporarily brings groundwater even closer to the surface.

The hazards that occur as a result of rising groundwater are not limited to coastal areas as much of Christchurch has a high water table.

However, away from the coast groundwater levels do not fluctuate with the changes in sea levels and so will not be affected by climate change-related sea-level rise or tidal patterns. This means the risk of rising groundwater and worsening effects are most significant in coastal areas and those close to waterways.

Another coastal hazard is tsunami - a sustained elevated sea-level caused by underwater earthquakes, landslides or volcanoes that can flood coastal areas.

A tsunami large enough to cause flooding of land In Christchurch is most likely to come from a distance source, for example, an earthquake off the coast of South America than from local or regional sources. This is because a distant source tsunami travels directly towards our coast, as opposed to the oblique angle created by a regional source wave.  

Changes in long term sea level will have no impact on the occurrence of tsunamis, so will not affect the likelihood of a tsunami occurring. However, increases in the water level that the tsunami is superimposed on could mean the impacts of tsunami, even smaller and more likely tsunamis will be greater in the future.

See our website for more information on how tsunami risk is currently identified and managed through evacuation zones and warning systems.

River and rainfall flooding

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

Other types of flooding include:

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

More information on how flood risk is identified and managed.

Liquefaction

Liquefaction is a natural process where earthquake shaking increases the water pressure in the ground causing some soils to behave like a fluid, resulting in temporary loss of soil strength.

More information on how liquefaction risk is identified and managed.

Slope instability

For coastal areas in the Port Hills and Banks Peninsula, slope stability hazards can also be present.