Modelling coastal erosion and inundation

Why this is important

Requirement to identify areas potentially affected by coastal hazards

National direction through the New Zealand Coastal Policy Statement (NZCPS) requires councils to identify areas potentially affected by coastal hazards over at least the next 100 years, particularly areas at high risk of being affected.


Modelling reflects best available information

Tonkin and Taylor modelled coastal inundation and erosion risk on the open coast and harbour coast (the Estuary/Ihutai) 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.


Coastal erosion modelling

Separate methodologies, recognising the different coastal processes have been used to assess and map coastal erosion along the open and harbour (the Estuary/Ihutai) coast.

  • For the open coast, a probabilistic model is used to map erosion where the rate of erosion for small sections of the coastline is calculated and averaged for stretches of the coast that exhibit similar characteristics. This means that within the line indicating the potential erosion, there may be localised differences in the potential erosion risk. Two different sediment budget scenarios were used for the long-term erosion or accretion patterns. The ‘average sediment budget’ reflects the current pattern of accretion on the open coast, and a ‘reduced sediment budget’ reflects a potential future pattern if changes in the Waimakariri River sediment supply were to occur.


  • For erosion within the Estuary/Ihutai, one value of potential erosion is used for the whole area for each time horizon. This is largely due to uncertainties with the modified shoreline and the effects this will have on future erosion. The lower energy environment is accounted for in the inputs of storm cut, sea level rise effects, and long-term erosion rates. However, existing estuary edge protection structures were not factored into the modelling as many are damaged or not engineered to specific design standards. Instead the erosion lines represent potential erosion effects if these structures are not retained in good working order. It is recognised that there will be localised differences in erosion along the estuary edge.

Erosion at the end of the Spit was not modelled due to the complex dynamics in this area. Instead the inlet migration curve is used. This represents the most landward extent that each section of the Spit has been during recent recorded historical events, and is a conservative estimate of the area potentially at risk of erosion in the future.


Coastal inundation modelling

Modelling of coastal inundation is a combination of estimating changes in water levels because of various coastal processes, and the topography of the land which affects the extent and depth of flooding.

Two approaches are used to assess inundation. On the open coast a bath tub method is used to map the extent of flooding inland to the same level as at the coast. This assumes even displacement of water across the inundated area. In the Estuary/Ihutai, a hydrodynamic model method uses flood and tide simulation software (called TUFLOW) that defines inundation more accurately in low lying and wide flat areas and factors. This factors in flow paths and the interaction of different hydrological processes.


National, regional and district context

The National Institute of Water and Atmospheric Research (NIWA) undertook a national overview of exposure to hazards in coastal areas in 2015 as part of a wider project by the Parliamentary Commissioner for the Environment on Preparing New Zealand for rising seas . This work looked at the potential exposure of people and assets based on elevation above MHWS, which is a simplified approach but provides high level context of the national distribution of risk.

National coastal inundation exposure (NIWA, 2015).


National and regional exposure and potential effects

The analysis summarised in the graphic above found that nationally, there is significant population (133,265 residents) and assets (with an estimated replacement cost of $19 billion for all buildings) potentially at risk from inundation due to their location within 1.5m elevation of the coast . Canterbury is the most exposed region in regards to residential buildings (10,284) and residential population (30,580), with an estimated replacement cost of residential buildings of $2.5 billion .


District exposure

The Coastal Hazard Assessment for Christchurch and Banks Peninsula (2017) provides a more detailed analysis of risk to Christchurch city. While it doesn’t specifically calculate assets at risk, it highlights the extent of the city likely to be affected by inundation, shown in this map, which could potentially affect (either in part or fully) around 13,600-24,800 properties in the District by 2120.


Implications of wider area at risk

The wider modelling of coastal inundation risk highlights the scale of potential impacts nationally, regionally and across the city. While there is evidentially a large area potentially affected by coastal inundation within the regeneration strategy project area, the hazard is not isolated to this area and is an issue that will need to be dealt with in the broader context.


What we don’t know

Modelling of hazard risk is not a static process, but a reflection of the best available estimates at any point in time. Further work currently being undertaken on groundwater levels, extreme sea levels, and future sediment budgets will inform the assessment of coastal hazard risk in the future and may require modelling to be recalculated as the understanding of these processes develops. Despite this, as the 2017 Coastal Hazard Assessment reflects the best currently available information it is appropriate to use this for the purposes of the Regeneration Strategy project.

Why this is important

Requirement to identify areas potentially affected by coastal hazards

National direction through the New Zealand Coastal Policy Statement (NZCPS) requires councils to identify areas potentially affected by coastal hazards over at least the next 100 years, particularly areas at high risk of being affected.


Modelling reflects best available information

Tonkin and Taylor modelled coastal inundation and erosion risk on the open coast and harbour coast (the Estuary/Ihutai) 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.


Coastal erosion modelling

Separate methodologies, recognising the different coastal processes have been used to assess and map coastal erosion along the open and harbour (the Estuary/Ihutai) coast.

  • For the open coast, a probabilistic model is used to map erosion where the rate of erosion for small sections of the coastline is calculated and averaged for stretches of the coast that exhibit similar characteristics. This means that within the line indicating the potential erosion, there may be localised differences in the potential erosion risk. Two different sediment budget scenarios were used for the long-term erosion or accretion patterns. The ‘average sediment budget’ reflects the current pattern of accretion on the open coast, and a ‘reduced sediment budget’ reflects a potential future pattern if changes in the Waimakariri River sediment supply were to occur.


  • For erosion within the Estuary/Ihutai, one value of potential erosion is used for the whole area for each time horizon. This is largely due to uncertainties with the modified shoreline and the effects this will have on future erosion. The lower energy environment is accounted for in the inputs of storm cut, sea level rise effects, and long-term erosion rates. However, existing estuary edge protection structures were not factored into the modelling as many are damaged or not engineered to specific design standards. Instead the erosion lines represent potential erosion effects if these structures are not retained in good working order. It is recognised that there will be localised differences in erosion along the estuary edge.

Erosion at the end of the Spit was not modelled due to the complex dynamics in this area. Instead the inlet migration curve is used. This represents the most landward extent that each section of the Spit has been during recent recorded historical events, and is a conservative estimate of the area potentially at risk of erosion in the future.


Coastal inundation modelling

Modelling of coastal inundation is a combination of estimating changes in water levels because of various coastal processes, and the topography of the land which affects the extent and depth of flooding.

Two approaches are used to assess inundation. On the open coast a bath tub method is used to map the extent of flooding inland to the same level as at the coast. This assumes even displacement of water across the inundated area. In the Estuary/Ihutai, a hydrodynamic model method uses flood and tide simulation software (called TUFLOW) that defines inundation more accurately in low lying and wide flat areas and factors. This factors in flow paths and the interaction of different hydrological processes.


National, regional and district context

The National Institute of Water and Atmospheric Research (NIWA) undertook a national overview of exposure to hazards in coastal areas in 2015 as part of a wider project by the Parliamentary Commissioner for the Environment on Preparing New Zealand for rising seas . This work looked at the potential exposure of people and assets based on elevation above MHWS, which is a simplified approach but provides high level context of the national distribution of risk.

National coastal inundation exposure (NIWA, 2015).


National and regional exposure and potential effects

The analysis summarised in the graphic above found that nationally, there is significant population (133,265 residents) and assets (with an estimated replacement cost of $19 billion for all buildings) potentially at risk from inundation due to their location within 1.5m elevation of the coast . Canterbury is the most exposed region in regards to residential buildings (10,284) and residential population (30,580), with an estimated replacement cost of residential buildings of $2.5 billion .


District exposure

The Coastal Hazard Assessment for Christchurch and Banks Peninsula (2017) provides a more detailed analysis of risk to Christchurch city. While it doesn’t specifically calculate assets at risk, it highlights the extent of the city likely to be affected by inundation, shown in this map, which could potentially affect (either in part or fully) around 13,600-24,800 properties in the District by 2120.


Implications of wider area at risk

The wider modelling of coastal inundation risk highlights the scale of potential impacts nationally, regionally and across the city. While there is evidentially a large area potentially affected by coastal inundation within the regeneration strategy project area, the hazard is not isolated to this area and is an issue that will need to be dealt with in the broader context.


What we don’t know

Modelling of hazard risk is not a static process, but a reflection of the best available estimates at any point in time. Further work currently being undertaken on groundwater levels, extreme sea levels, and future sediment budgets will inform the assessment of coastal hazard risk in the future and may require modelling to be recalculated as the understanding of these processes develops. Despite this, as the 2017 Coastal Hazard Assessment reflects the best currently available information it is appropriate to use this for the purposes of the Regeneration Strategy project.