Flooding

Flooding from rainfall (pluvial) and rivers (fluvial) is the most common natural hazard in Canterbury. The low-lying characteristics of Christchurch, and the Regeneration Strategy project area, mean that there have always been some areas prone to flooding during heavy rainfall.

The magnitude of a flood depends on many factors including the intensity and duration of rainfall, the shape of the land and surface features, the plants and soil characteristics, how saturated the area is before a rain event, and evaporation. The impact of a flood is a combination of the ability of the water to drain away, and the vulnerability of the assets and people exposed to the floodwaters.


What we know

Flooding in the regeneration strategy project area is primarily from rainfall and tides

The Regeneration Strategy project area most commonly experiences local runoff flooding. This is flooding from localised high intensity, short duration rainstorms or more prolonged, lower intensity rainfall events. The flooding is caused where the water in smaller rivers or streams, or flowing over land, is unable to drain away and collects in low lying areas.

The tides also have a strong influence in and around the Regeneration Strategy project area. If a high tide occurs with localised heavy rainfall or higher than normal river flows from rainfall further up the catchment, it can restrict drainage and cause backflow up the river or the stormwater infrastructure, or flood low lying areas around the estuary edge.

See the Environment Canterbury website for more information on flood dynamics in Canterbury https://www.ecan.govt.nz/your-region/your-environment/natural-hazards/floods/causes-of-flooding/


Flood risk will increase over time with climate change

The risk of flooding is likely to be exacerbated by climate change and sea level rise (in tidally affected areas) which will likely increase the extent and intensity of flooding. Longer duration rainfall predominantly comes from low-pressure systems to the east of Christchurch, with highest rainfall more likely to occur during late autumn through winter. Current climate change projections suggest that there will be lower annual average rainfall in eastern Canterbury, but that the intensity of extreme rainfall events will increase.


Different sized flood events

In Christchurch 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.

The Council is legally required to assess and manage areas most at risk of flooding, in particular the Canterbury Regional Policy Statement requires assessment and management of areas most at risk from damaging and dangerous flooding, where in a major flood event the velocity or depth of water could be damaging to people or property. In these areas restrictions on new development apply.

Flood probabilities can be described as AEP (Annual Exceedance Probability) which is the probability of a certain size of flood occurring in a single year (or being exceeded), or ARI (Average Recurrence Interval) which is the average period between floods of a certain size. This does not mean that a 1-in-200-year flood will happen regularly every 200 years, or only once in 200 years. In any given 200-year period, a 200-year event may occur once, twice, more, or not at all. While the probability of a major flood occurring in any one year is relatively low, the probability of a flood of that size occurring over the lifetime of a person (or building) is much higher.


Widespread exposure to flooding

This map shows that most of the residential area of Southshore and South New Brighton is at risk from flooding in a large flood event. Areas around the estuary edge and along the Rocking Horse Road where the lowest lying land is located are at risk from potentially damaging depths or velocities of flooding in a major event. These areas are also susceptible more regular flooding in smaller events.

Why this is important

Complex interactions of rainfall and tides are accounted for in modelling

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% because of climate change) and tides, with an allowance for 1 metre sea level rise . These reflect the influences on flooding which can occur separately, cumulatively, or over time (100 years) to increase the risk. All models are also run on the assumption that there are no stopbanks (i.e. that they have failed in an extreme event).

In addition, to calculate the High Flood Hazard Management Area, only those areas with flood depths or velocities which meet the Canterbury Regional Policy Statement definition are modelled.


Likely effects of flooding

Flooding can have significant consequences for communities and individuals. These include community trauma and disruption, damage to property and infrastructure, changes to the natural environment, business losses and economic hardship. These effects will be different depending on the source of flooding; the depth and location of the water on a property (i.e. above floor flooding affecting the interior of the house, underfloor flooding affecting the building footprint, and flooding on the section); the velocity (i.e. whether the water flows through a property or ponds); and the duration of flooding .


What we don’t know

Exposure to small scale, regular flooding

Christchurch City Council is doing more work to identify areas at risk of flooding during smaller scale, more regular flood events. This work involves combining assessments of the risk of flooding of more frequent events (such as a 1 in 10-year event) and whether this would be expected to flood above floor levels for each site.

Flooding from rainfall (pluvial) and rivers (fluvial) is the most common natural hazard in Canterbury. The low-lying characteristics of Christchurch, and the Regeneration Strategy project area, mean that there have always been some areas prone to flooding during heavy rainfall.

The magnitude of a flood depends on many factors including the intensity and duration of rainfall, the shape of the land and surface features, the plants and soil characteristics, how saturated the area is before a rain event, and evaporation. The impact of a flood is a combination of the ability of the water to drain away, and the vulnerability of the assets and people exposed to the floodwaters.


What we know

Flooding in the regeneration strategy project area is primarily from rainfall and tides

The Regeneration Strategy project area most commonly experiences local runoff flooding. This is flooding from localised high intensity, short duration rainstorms or more prolonged, lower intensity rainfall events. The flooding is caused where the water in smaller rivers or streams, or flowing over land, is unable to drain away and collects in low lying areas.

The tides also have a strong influence in and around the Regeneration Strategy project area. If a high tide occurs with localised heavy rainfall or higher than normal river flows from rainfall further up the catchment, it can restrict drainage and cause backflow up the river or the stormwater infrastructure, or flood low lying areas around the estuary edge.

See the Environment Canterbury website for more information on flood dynamics in Canterbury https://www.ecan.govt.nz/your-region/your-environment/natural-hazards/floods/causes-of-flooding/


Flood risk will increase over time with climate change

The risk of flooding is likely to be exacerbated by climate change and sea level rise (in tidally affected areas) which will likely increase the extent and intensity of flooding. Longer duration rainfall predominantly comes from low-pressure systems to the east of Christchurch, with highest rainfall more likely to occur during late autumn through winter. Current climate change projections suggest that there will be lower annual average rainfall in eastern Canterbury, but that the intensity of extreme rainfall events will increase.


Different sized flood events

In Christchurch 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.

The Council is legally required to assess and manage areas most at risk of flooding, in particular the Canterbury Regional Policy Statement requires assessment and management of areas most at risk from damaging and dangerous flooding, where in a major flood event the velocity or depth of water could be damaging to people or property. In these areas restrictions on new development apply.

Flood probabilities can be described as AEP (Annual Exceedance Probability) which is the probability of a certain size of flood occurring in a single year (or being exceeded), or ARI (Average Recurrence Interval) which is the average period between floods of a certain size. This does not mean that a 1-in-200-year flood will happen regularly every 200 years, or only once in 200 years. In any given 200-year period, a 200-year event may occur once, twice, more, or not at all. While the probability of a major flood occurring in any one year is relatively low, the probability of a flood of that size occurring over the lifetime of a person (or building) is much higher.


Widespread exposure to flooding

This map shows that most of the residential area of Southshore and South New Brighton is at risk from flooding in a large flood event. Areas around the estuary edge and along the Rocking Horse Road where the lowest lying land is located are at risk from potentially damaging depths or velocities of flooding in a major event. These areas are also susceptible more regular flooding in smaller events.

Why this is important

Complex interactions of rainfall and tides are accounted for in modelling

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% because of climate change) and tides, with an allowance for 1 metre sea level rise . These reflect the influences on flooding which can occur separately, cumulatively, or over time (100 years) to increase the risk. All models are also run on the assumption that there are no stopbanks (i.e. that they have failed in an extreme event).

In addition, to calculate the High Flood Hazard Management Area, only those areas with flood depths or velocities which meet the Canterbury Regional Policy Statement definition are modelled.


Likely effects of flooding

Flooding can have significant consequences for communities and individuals. These include community trauma and disruption, damage to property and infrastructure, changes to the natural environment, business losses and economic hardship. These effects will be different depending on the source of flooding; the depth and location of the water on a property (i.e. above floor flooding affecting the interior of the house, underfloor flooding affecting the building footprint, and flooding on the section); the velocity (i.e. whether the water flows through a property or ponds); and the duration of flooding .


What we don’t know

Exposure to small scale, regular flooding

Christchurch City Council is doing more work to identify areas at risk of flooding during smaller scale, more regular flood events. This work involves combining assessments of the risk of flooding of more frequent events (such as a 1 in 10-year event) and whether this would be expected to flood above floor levels for each site.