Natural Hazards

Natural events happen all the time and make up the atmospheric, earth or water-related processes that shape our environment in which we live. They only become a hazard when they cause damage to the human environment such as people, property and the economy.

There are a range of hazards which could affect the Southshore South New Brighton Regeneration Strategy project area such as flooding (from rainfall, higher sea levels, or shallow groundwater), land damage (from liquefaction, vertical displacement or coastal erosion) or risk from tsunami or earthquake. These could occur at the same time and can lead to cumulative effects.

Flooding from rainfall, coastal inundation (during extreme tide events or coastal storms), or due to shallow groundwater pose the main risks to the Regeneration Strategy project area, and are most likely to affect the largest proportion of land. Low lying areas around the estuary edge are the most exposed to these hazards. This exposure is expected to increase over time with sea level rise.

Because of the multiple hazards and cumulative hazard risks, any response to these hazards needs to consider the range of potential risks and effects.


Climate change

Our climate is changing and will continue to change in the future. While the extent and rate of change is dependent on the extent that greenhouse gas emissions are reduced globally, some change to our climate is already in motion and inevitable. The Ministry for the Environment recognises the Intergovernmental Panel on Climate Change (IPCC)’s Fifth Assessment Report provides the most up to date climate science outlining what New Zealand can expect as a result of climate change.

Climate change is not, in itself, a natural hazard. However, it does worsen hazards such as flooding from rivers, rainfall and rising groundwater, the potential for liquefaction, and coastal erosion and inundation.

The way we measure the potential effects of climate change is influenced by national guidance. Four future climate scenarios, known as Representative Concentration Pathways (RCPs) are used to estimate the range of changes in sea level that might be expected.

While not all people agree with climate change projections, the RCP 2.6 scenario is globally accepted as inevitable, and the RCP8.5 scenario is a reflection of current emissions. Even if we were to reduce emissions substantially there will be some sea level rise and some effects that will need to be managed.

The key uncertainty in climate change projections is not if, but when changes will reach a certain threshold, and what the extent of the changes will be. Read more.


Coastal inundation and erosion

Coastal processes can create hazards onshore either by flooding (coastal inundation), or wearing away or removing sediment from the coastline (coastal erosion).


Coastal inundation

Coastal inundation is primarily caused by storm surges, high or extreme tides raising sea levels, and larger than normal waves or swell.

These effects can be exacerbated when storms occur at the same time as extreme tides. The extent and depth of flooding is also influenced by the physical characteristics of the land adjoining the coast.

As sea levels continue to rise the frequency, duration and extent of coastal flooding will increase. This will cause king tides, storm surges and waves to reach higher up the shores than they used to and increase the likelihood of cumulative effects where they occur at the same time.

While coastal inundation from a 1 in 100-year event is considered rare in todays’ terms, this can be expected to occur once a year in Christchurch with as little as 0.3m of sea level rise. If sea level continues to rise, the same inundation event can be expected to occur every tide.For a rise in sea level of 0.3m (the minimum likely to occur in the next 50 years), the water levels currently modelled as 1 in 100 year inundation event can be expected to occur once a year in Christchurch. If sea level continues to rise, the same inundation event can be expected to occur every tide.

Coastal inundation which would flood from the estuary side is likely to affect a large percentage of the Regeneration Strategy project area with as little as 0.3m sea level rise (the minimum likely to occur in the next 50 years). Read more.


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.

Long-term increases in sea level 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 open coast and end of the Spit are subject to more dynamic and dramatic erosion/accretion cycles than the more sheltered estuary coast, but are also slower to naturally recover from storm events.

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


Modelling coastal erosion and inundation

Tonkin and Taylor modelled coastal inundation and erosion risk on the open coast and harbour coast (estuary) on behalf of the Christchurch City Council in the Coastal Hazard Assessment for Christchurch and Banks Peninsula (2017) using two time horizons:

  • 50 years - which reflects the likely extent of hazards based on a more certain estimate of sea level rise, and
  • 100 years - which reflects the possible extent in the longer term future.

For both timeframes and processes, sea level rise estimates for all four RCP scenarios recommended in the Ministry for the Environment guidance , and two sediment supply scenarios (for the open coast erosion models) were mapped. Modelling was based on no estuary edge protection.

While there is a large area potentially affected by coastal inundation within the Regeneration Strategy project area, other areas and communities in Christchurch district are also at risk from this hazard. Read more.


Flooding

Flooding is the most common natural hazard in Canterbury.

The Regeneration Strategy project area most commonly experiences local runoff flooding caused when the water flowing over land, is unable to drain away and collects in low lying areas. If a high tide occurs, either with a localised heavy rainfall event, or higher than normal river flows from rainfall further up the catchment, it can restrict drainage and cause backflow back up the river or the stormwater infrastructure, or flood low lying areas around the estuary edge.

While many properties are at risk of flooding at the present, this risk is likely to be exacerbated by climate change and sea level rise, which will likely increase the extent and intensity of flooding.

There have always been some areas prone to flooding during heavy rainfall. Areas around the estuary edge and along the middle of Southshore, where the land is lowest, are particularly at risk in a major event. These areas are also susceptible to more regular flooding in smaller events.

Flood hazard is measured by calculating the probability of a flood of a particular size happening in any given period. The modelling of areas at risk of flooding are based on a combination of rainfall (including predicted increase in rainfall intensity of 16% as a result of climate change) and tides , with an allowance for 1 metre sea level rise. This reflects the variety of influences which can occur either separately, cumulatively, or over time to increase the risk.

Flooding can have significant consequences for communities and individuals. These effects will be different depending on the source of flooding, and the depth and location of the water on a property, the velocity, and the duration of flooding.

In Christchurch flooding from a range of different sized events are identified and managed. The stormwater drainage network, flood ponding areas and overland flow paths are designed to manage the effects of flood waters in frequent and small flood events, while flood risk to new buildings is managed through requiring floor levels to be raised above the height of flood waters expected in large and rare flood events. The expectation is that flood mitigation schemes will provide a reduction in street flooding in more frequent events but would be overwhelmed or fail in larger events. Floor levels will need to manage this risk. Read more.


Groundwater flooding

Groundwater becomes a hazard when it is close to or above the ground surface for long periods of time, causing rising damp, impacting infrastructure and saturating the ground. This can lead to physical damage, and can also affect health and wellbeing, particularly if people are exposed to it long term, or on a regular basis.

Areas along the estuary edge and Rocking Horse Road have the shallowest groundwater at less than one metre below the ground surface.

As much of the flat area of Christchurch has shallow groundwater, this is not an isolated problem. However, in the Regeneration Strategy project area the additional influence of tidal fluctuations and future sea level rise exacerbates the hazard risk. The groundwater levels across the Spit are already shallow, so it is likely that more properties will begin to experience shallow groundwater with as little as a 0.3m increase in sea level rise (the RCP 2.6 scenario that is globally accepted as inevitable by 2065 ). Read more.


Liquefaction

Liquefaction occurs when loose silts and sands below the water table become suspended in the groundwater as the ground shakes during an earthquake. The sediment grains are then no longer supporting the ground above, but the weight of that ground falls back upon a layer that has now basically become a liquid. With continued ground shaking pressure builds up and forces the water to the surface through cracks and crevices in the ground. The water carries with it the suspended sediment and ejects it at the ground surface, and can lead to subsidence as this material settles, and lateral spreading, where stretch or translation of land occurs adjacent to waterways.

The worst affected areas of observed liquefaction were generally red zoned after the earthquakes. However, most of the land along the estuary edge and river mouth either side of Bridge Street has also experienced ejected material during the earthquake and is more vulnerable to future events than the rest of the Regeneration Strategy project area. Read more.


Tsunami

Tsunamis are waves created when the ocean floor is displaced by earthquakes, landslides or volcanoes. 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.

The latest tsunami modelling from Environment Canterbury shows the areas susceptible to a distant source tsunami originating from a magnitude 9.485 earthquake at the Peru subduction zone. If the wave arrived at high tide, all of the Regeneration Strategy project area could be inundated with a wave height of up to 12m at the coast. Most flooding would come from the estuary side with about 14 hours of warning time. However, the modelling shows that if the wave arrived at low tide, this would make a very big difference to the amount of damage the tsunami would do.

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 are greater in the future. Read more


Natural events happen all the time and make up the atmospheric, earth or water-related processes that shape our environment in which we live. They only become a hazard when they cause damage to the human environment such as people, property and the economy.

There are a range of hazards which could affect the Southshore South New Brighton Regeneration Strategy project area such as flooding (from rainfall, higher sea levels, or shallow groundwater), land damage (from liquefaction, vertical displacement or coastal erosion) or risk from tsunami or earthquake. These could occur at the same time and can lead to cumulative effects.

Flooding from rainfall, coastal inundation (during extreme tide events or coastal storms), or due to shallow groundwater pose the main risks to the Regeneration Strategy project area, and are most likely to affect the largest proportion of land. Low lying areas around the estuary edge are the most exposed to these hazards. This exposure is expected to increase over time with sea level rise.

Because of the multiple hazards and cumulative hazard risks, any response to these hazards needs to consider the range of potential risks and effects.


Climate change

Our climate is changing and will continue to change in the future. While the extent and rate of change is dependent on the extent that greenhouse gas emissions are reduced globally, some change to our climate is already in motion and inevitable. The Ministry for the Environment recognises the Intergovernmental Panel on Climate Change (IPCC)’s Fifth Assessment Report provides the most up to date climate science outlining what New Zealand can expect as a result of climate change.

Climate change is not, in itself, a natural hazard. However, it does worsen hazards such as flooding from rivers, rainfall and rising groundwater, the potential for liquefaction, and coastal erosion and inundation.

The way we measure the potential effects of climate change is influenced by national guidance. Four future climate scenarios, known as Representative Concentration Pathways (RCPs) are used to estimate the range of changes in sea level that might be expected.

While not all people agree with climate change projections, the RCP 2.6 scenario is globally accepted as inevitable, and the RCP8.5 scenario is a reflection of current emissions. Even if we were to reduce emissions substantially there will be some sea level rise and some effects that will need to be managed.

The key uncertainty in climate change projections is not if, but when changes will reach a certain threshold, and what the extent of the changes will be. Read more.


Coastal inundation and erosion

Coastal processes can create hazards onshore either by flooding (coastal inundation), or wearing away or removing sediment from the coastline (coastal erosion).


Coastal inundation

Coastal inundation is primarily caused by storm surges, high or extreme tides raising sea levels, and larger than normal waves or swell.

These effects can be exacerbated when storms occur at the same time as extreme tides. The extent and depth of flooding is also influenced by the physical characteristics of the land adjoining the coast.

As sea levels continue to rise the frequency, duration and extent of coastal flooding will increase. This will cause king tides, storm surges and waves to reach higher up the shores than they used to and increase the likelihood of cumulative effects where they occur at the same time.

While coastal inundation from a 1 in 100-year event is considered rare in todays’ terms, this can be expected to occur once a year in Christchurch with as little as 0.3m of sea level rise. If sea level continues to rise, the same inundation event can be expected to occur every tide.For a rise in sea level of 0.3m (the minimum likely to occur in the next 50 years), the water levels currently modelled as 1 in 100 year inundation event can be expected to occur once a year in Christchurch. If sea level continues to rise, the same inundation event can be expected to occur every tide.

Coastal inundation which would flood from the estuary side is likely to affect a large percentage of the Regeneration Strategy project area with as little as 0.3m sea level rise (the minimum likely to occur in the next 50 years). Read more.


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.

Long-term increases in sea level 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 open coast and end of the Spit are subject to more dynamic and dramatic erosion/accretion cycles than the more sheltered estuary coast, but are also slower to naturally recover from storm events.

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


Modelling coastal erosion and inundation

Tonkin and Taylor modelled coastal inundation and erosion risk on the open coast and harbour coast (estuary) on behalf of the Christchurch City Council in the Coastal Hazard Assessment for Christchurch and Banks Peninsula (2017) using two time horizons:

  • 50 years - which reflects the likely extent of hazards based on a more certain estimate of sea level rise, and
  • 100 years - which reflects the possible extent in the longer term future.

For both timeframes and processes, sea level rise estimates for all four RCP scenarios recommended in the Ministry for the Environment guidance , and two sediment supply scenarios (for the open coast erosion models) were mapped. Modelling was based on no estuary edge protection.

While there is a large area potentially affected by coastal inundation within the Regeneration Strategy project area, other areas and communities in Christchurch district are also at risk from this hazard. Read more.


Flooding

Flooding is the most common natural hazard in Canterbury.

The Regeneration Strategy project area most commonly experiences local runoff flooding caused when the water flowing over land, is unable to drain away and collects in low lying areas. If a high tide occurs, either with a localised heavy rainfall event, or higher than normal river flows from rainfall further up the catchment, it can restrict drainage and cause backflow back up the river or the stormwater infrastructure, or flood low lying areas around the estuary edge.

While many properties are at risk of flooding at the present, this risk is likely to be exacerbated by climate change and sea level rise, which will likely increase the extent and intensity of flooding.

There have always been some areas prone to flooding during heavy rainfall. Areas around the estuary edge and along the middle of Southshore, where the land is lowest, are particularly at risk in a major event. These areas are also susceptible to more regular flooding in smaller events.

Flood hazard is measured by calculating the probability of a flood of a particular size happening in any given period. The modelling of areas at risk of flooding are based on a combination of rainfall (including predicted increase in rainfall intensity of 16% as a result of climate change) and tides , with an allowance for 1 metre sea level rise. This reflects the variety of influences which can occur either separately, cumulatively, or over time to increase the risk.

Flooding can have significant consequences for communities and individuals. These effects will be different depending on the source of flooding, and the depth and location of the water on a property, the velocity, and the duration of flooding.

In Christchurch flooding from a range of different sized events are identified and managed. The stormwater drainage network, flood ponding areas and overland flow paths are designed to manage the effects of flood waters in frequent and small flood events, while flood risk to new buildings is managed through requiring floor levels to be raised above the height of flood waters expected in large and rare flood events. The expectation is that flood mitigation schemes will provide a reduction in street flooding in more frequent events but would be overwhelmed or fail in larger events. Floor levels will need to manage this risk. Read more.


Groundwater flooding

Groundwater becomes a hazard when it is close to or above the ground surface for long periods of time, causing rising damp, impacting infrastructure and saturating the ground. This can lead to physical damage, and can also affect health and wellbeing, particularly if people are exposed to it long term, or on a regular basis.

Areas along the estuary edge and Rocking Horse Road have the shallowest groundwater at less than one metre below the ground surface.

As much of the flat area of Christchurch has shallow groundwater, this is not an isolated problem. However, in the Regeneration Strategy project area the additional influence of tidal fluctuations and future sea level rise exacerbates the hazard risk. The groundwater levels across the Spit are already shallow, so it is likely that more properties will begin to experience shallow groundwater with as little as a 0.3m increase in sea level rise (the RCP 2.6 scenario that is globally accepted as inevitable by 2065 ). Read more.


Liquefaction

Liquefaction occurs when loose silts and sands below the water table become suspended in the groundwater as the ground shakes during an earthquake. The sediment grains are then no longer supporting the ground above, but the weight of that ground falls back upon a layer that has now basically become a liquid. With continued ground shaking pressure builds up and forces the water to the surface through cracks and crevices in the ground. The water carries with it the suspended sediment and ejects it at the ground surface, and can lead to subsidence as this material settles, and lateral spreading, where stretch or translation of land occurs adjacent to waterways.

The worst affected areas of observed liquefaction were generally red zoned after the earthquakes. However, most of the land along the estuary edge and river mouth either side of Bridge Street has also experienced ejected material during the earthquake and is more vulnerable to future events than the rest of the Regeneration Strategy project area. Read more.


Tsunami

Tsunamis are waves created when the ocean floor is displaced by earthquakes, landslides or volcanoes. 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.

The latest tsunami modelling from Environment Canterbury shows the areas susceptible to a distant source tsunami originating from a magnitude 9.485 earthquake at the Peru subduction zone. If the wave arrived at high tide, all of the Regeneration Strategy project area could be inundated with a wave height of up to 12m at the coast. Most flooding would come from the estuary side with about 14 hours of warning time. However, the modelling shows that if the wave arrived at low tide, this would make a very big difference to the amount of damage the tsunami would do.

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 are greater in the future. Read more