Flooding

Flooding

Flooding can damage property and infrastructure, injure or kill people and animals and contaminate water and land. Many parts of Christchurch are prone to flooding. Floods are usually caused by continuous heavy rain or thunderstorms that can overwhelm the drainage system and cause rivers and streams to breech their banks. Coastal storms and, occasionally, tsunami can also cause flooding.

Much of Christchurch is built on low-lying land on the old channels of the Waimakariri River, a massive gravel fan stretching from Brooklands Lagoon in the north to Te Waihora/Ellesmere in the south. These low-lying wetlands and swamps were often drained for agriculture and then used for residential development. Urban areas are also close to rivers and the coast, so unsurprisingly the city is naturally flood prone.


Managing flood risk

Hundreds of kilometres of open drains, timber or concrete-lined channels and pipelines were constructed after the Christchurch Drainage Board was set up in 1875. They also dredged and straightened the Ōtākaro/Avon, Heathcote and Styx rivers. Tide gates, stop banks and pumping stations were built to protect low-lying properties. Today, Christchurch City Council’s responsibility includes reducing flood risk through civil defence and emergency management, land-use planning controls and stormwater management. More recently, the Council has protected large natural ponding areas in the upstream catchments of these rivers to store and detain floodwaters.

Land Drainage Recovery Programme [CCC website]


The impact of the Canterbury earthquakes

Some communities were vulnerable to flooding before the earthquakes. When the ground shook in the earthquakes, it also dropped in many areas and this worsened the flood risk. These areas are generally older settlements and suburbs without adequate stormwater management and with houses built too close to waterways. There is literally no room for the natural function of the rivers. New subdivisions are designed to cope with an up to one in 50 year storm. Developments over the last decade or so typically have large stormwater basins.

The earthquakes changed the pattern of flooding and generally increased the flood risk from:

  • Extensive areas of land subsidence (dropping about 200mm to 300mm) and lifting in some areas
  • Reduced storage capacity in some stretches of rivers and streams
  • Damage to stormwater infrastructure.

Extensive post-quake maintenance and repair works have been undertaken to improve the city-wide capacity of drains and waterways.


Flood risk in the residential red zone

Almost all the flat land residential red zone are at some risk of flooding. The areas of greater risk are generally near the coast, the rivers or in particularly low-lying areas. Higher risk areas include Bexley, Brooklands, and parts of Dallington, New Brighton, Southshore, Burwood, Avondale, the Avon Loop, Wainoni, Avonside, and Richmond. These areas will experience extensive flooding from a one in 50 year flood or less.

Some land can be protected from flooding by flood protection works that may include stop banks and drainage pipes. If a particular piece of land that is prone to flooding is not protected by a stop bank or other measures, it is likely to flood.

The technical area summaries on our land information page have more detailed information about the flood risk in each residential red zone area.


Flooding - how likely is it?

Natural events like flooding do not occur with any regularity. A one in 50 year flood will happen regularly every 50 years, or only once in 50 years. In any given 50 year period, a 50 year event may occur once, twice, more, or not at all. We can estimate the probability of them occurring in a year period. This is the Annual Exceedance Probability or AEP. A "50 year flood" has a two per cent AEP (=1/50) or two per cent chance of happening in any one year. Similarly a "200 year flood" has a 0.5 per cent AEP (=1/200) or 0.5 per cent chance of happening in any one year.


Flooding information and statutory requirements


The flooding provisions (required standards) within the Christchurch District Plan include:

  • Adaptation within the Flood Management Areas by requiring floor levels above the one in 200 year flood event – mapped for fluvial (river), pluvial (rainfall) and tidal flooding across Christchurch city (in force)
  • Protection of the floodplain including Flood Ponding areas and overland flow paths by controlling filling within the FMAs (in force)
  • Prevention of intensification of development within High Flood Hazard Management Areas – areas subject to flooding more than one metre deep in a one in 500 year flood (proposed and awaiting a decision).

Maps showing the extent of floodwaters from a combination of rainfall events, rivers overflowing and tidal flooding are linked to below. These are based on modelling undertaken by Christchurch City Council.

The areas affected by the one in 200 year flood (including a one metre sea level rise) are mapped as Flood Management Areas (defined in the Christchurch District Plan) where minimum floor levels provisions apply to new buildings, or significant additions to existing buildings, to provide protection to property in a one in 200 year event. These provisions only apply if a resource consent is required. The Building Code requires minimum floor levels to above the level of the one in 50 year flood (with 0.5m sea level rise).

The Building Act requirements apply mainly to existing developments. The more stringent one in 200 year scenario in the Christchurch Replacement District Plan reflects the Regional Policy Statement (Policy 11.3.2) and applies to all new developments.

Flooding

Flooding can damage property and infrastructure, injure or kill people and animals and contaminate water and land. Many parts of Christchurch are prone to flooding. Floods are usually caused by continuous heavy rain or thunderstorms that can overwhelm the drainage system and cause rivers and streams to breech their banks. Coastal storms and, occasionally, tsunami can also cause flooding.

Much of Christchurch is built on low-lying land on the old channels of the Waimakariri River, a massive gravel fan stretching from Brooklands Lagoon in the north to Te Waihora/Ellesmere in the south. These low-lying wetlands and swamps were often drained for agriculture and then used for residential development. Urban areas are also close to rivers and the coast, so unsurprisingly the city is naturally flood prone.


Managing flood risk

Hundreds of kilometres of open drains, timber or concrete-lined channels and pipelines were constructed after the Christchurch Drainage Board was set up in 1875. They also dredged and straightened the Ōtākaro/Avon, Heathcote and Styx rivers. Tide gates, stop banks and pumping stations were built to protect low-lying properties. Today, Christchurch City Council’s responsibility includes reducing flood risk through civil defence and emergency management, land-use planning controls and stormwater management. More recently, the Council has protected large natural ponding areas in the upstream catchments of these rivers to store and detain floodwaters.

Land Drainage Recovery Programme [CCC website]


The impact of the Canterbury earthquakes

Some communities were vulnerable to flooding before the earthquakes. When the ground shook in the earthquakes, it also dropped in many areas and this worsened the flood risk. These areas are generally older settlements and suburbs without adequate stormwater management and with houses built too close to waterways. There is literally no room for the natural function of the rivers. New subdivisions are designed to cope with an up to one in 50 year storm. Developments over the last decade or so typically have large stormwater basins.

The earthquakes changed the pattern of flooding and generally increased the flood risk from:

  • Extensive areas of land subsidence (dropping about 200mm to 300mm) and lifting in some areas
  • Reduced storage capacity in some stretches of rivers and streams
  • Damage to stormwater infrastructure.

Extensive post-quake maintenance and repair works have been undertaken to improve the city-wide capacity of drains and waterways.


Flood risk in the residential red zone

Almost all the flat land residential red zone are at some risk of flooding. The areas of greater risk are generally near the coast, the rivers or in particularly low-lying areas. Higher risk areas include Bexley, Brooklands, and parts of Dallington, New Brighton, Southshore, Burwood, Avondale, the Avon Loop, Wainoni, Avonside, and Richmond. These areas will experience extensive flooding from a one in 50 year flood or less.

Some land can be protected from flooding by flood protection works that may include stop banks and drainage pipes. If a particular piece of land that is prone to flooding is not protected by a stop bank or other measures, it is likely to flood.

The technical area summaries on our land information page have more detailed information about the flood risk in each residential red zone area.


Flooding - how likely is it?

Natural events like flooding do not occur with any regularity. A one in 50 year flood will happen regularly every 50 years, or only once in 50 years. In any given 50 year period, a 50 year event may occur once, twice, more, or not at all. We can estimate the probability of them occurring in a year period. This is the Annual Exceedance Probability or AEP. A "50 year flood" has a two per cent AEP (=1/50) or two per cent chance of happening in any one year. Similarly a "200 year flood" has a 0.5 per cent AEP (=1/200) or 0.5 per cent chance of happening in any one year.


Flooding information and statutory requirements


The flooding provisions (required standards) within the Christchurch District Plan include:

  • Adaptation within the Flood Management Areas by requiring floor levels above the one in 200 year flood event – mapped for fluvial (river), pluvial (rainfall) and tidal flooding across Christchurch city (in force)
  • Protection of the floodplain including Flood Ponding areas and overland flow paths by controlling filling within the FMAs (in force)
  • Prevention of intensification of development within High Flood Hazard Management Areas – areas subject to flooding more than one metre deep in a one in 500 year flood (proposed and awaiting a decision).

Maps showing the extent of floodwaters from a combination of rainfall events, rivers overflowing and tidal flooding are linked to below. These are based on modelling undertaken by Christchurch City Council.

The areas affected by the one in 200 year flood (including a one metre sea level rise) are mapped as Flood Management Areas (defined in the Christchurch District Plan) where minimum floor levels provisions apply to new buildings, or significant additions to existing buildings, to provide protection to property in a one in 200 year event. These provisions only apply if a resource consent is required. The Building Code requires minimum floor levels to above the level of the one in 50 year flood (with 0.5m sea level rise).

The Building Act requirements apply mainly to existing developments. The more stringent one in 200 year scenario in the Christchurch Replacement District Plan reflects the Regional Policy Statement (Policy 11.3.2) and applies to all new developments.