Impacts: Sea level rise Waitaha Canterbury case studies
Conway Flat, North Canterbury image: Sonny Whitelaw
Impacts
- Irreversible tipping points
- Feedback effects
- Aotearoa’s changing climate
- How hot could it get?
- – The forecast for Canterbury
- Rising sea levels
- – Canterbury sea levels
- Floods bigger & more often
- Ocean heating
- Ocean acidfication
- Ocean currents changing
- Wildfires bigger & more often
- Food insecurity
- Losing our glaciers
- Black carbon & ash on snow
- Animal & plants moving or dying
- Biodiversity vanishing
- Broken life-support systems
- More diseases and pandemics
- What will it cost us?
- Arctic ice loss impacts on NZ
- Antarctic sea ice loss impacts NZ
Home > Climate wiki > Impacts > Sea level rise: Canterbury
Sea level rise: Waitaha Canterbury case studies
Before exploring these case studies see Rising Sea Levels to understand the multiple ways the ocean rises and falls relative to the land (RSLR). Note: online mapping tools used below may be updated at any time.
Background
The coastal area of Waitaha Canterbury is long and varied. Some coasts have been built over millennia by rivers delivering gravel from the Southern Alps. Others were created by volcanoes or from layers of peat and mud in wetlands and lagoons. Many areas have been lifted by earthquakes exposing ancient seabeds and coral reefs. And most coasts are rising or sinking relative to the ocean due to vertical land movement (VLM); if the land is falling (-VLM) the effects of sea level rise (SLR) will be felt sooner than areas which are rising (+VLM).
SLR is not a stand alone problem. It exacerbates existing problems and creates new ones. As global temperatures rise, storms become stronger and waves reach further inland. ‘Soft shore’ coastlines—sand, gravel, mud, and pyroclastic material (stuff that explodes out volcanoes, not lava)—erode faster. These ‘soft’ coasts include cliffs such as those at South Taranaki and parts of Canterbury. Rain percolating into the soil at the tops of cliffs can also contribute to the sudden collapse of coastal cliffs. Sometimes waves and currents carry eroded sediment to other beaches. But this may bring new problems, blocking the mouths of rivers, preventing tuna (eels) from migrating and exacerbating inland flooding (Tab 3).
Geological history provides a window into the deep past that helps inform the future. Modelling can also help. But every storm has the potential to re-arrange coastlines in ways that cannot always be modelled. The decisions made by land owners, communities, local and regional councils, rūnanga, and the Government play a crucial role in this rapidly evolving story travelling in an irrefutable trajectory: sea level rise is irreversible, it’s accelerating, and it may rise much faster than previously expected.
The case studies below illustrate some of the complex effects of SLR, and how different online mapping tools are needed to evaluate risk. There are many other online tools, only a few are used here to illustrate the process.
Impacts
- Irreversible tipping points
- Feedback effects
- Aotearoa’s changing climate
- How hot could it get?
- – The forecast for Canterbury
- Rising sea levels
- – Canterbury sea levels
- Floods bigger & more often
- Ocean heating
- Ocean acidfication
- Ocean currents changing
- Wildfires bigger & more often
- Food insecurity
- Losing our glaciers
- Black carbon & ash on snow
- Animal & plants moving or dying
- Biodiversity vanishing
- Broken life-support systems
- More diseases and pandemics
- What will it cost us?
- Arctic ice loss impacts on NZ
- Antarctic sea ice loss impacts NZ
Before exploring these case studies see Rising Sea Levels to understand the multiple ways the ocean rises and falls relative to the land (RSLR). Note: online mapping tools used below may be updated at any time.
Background
The coastal area of Waitaha Canterbury is long and varied. Some coasts have been built over millennia by rivers delivering gravel from the Southern Alps. Others were created by volcanoes or from layers of peat and mud in wetlands and lagoons. Many areas have been lifted by earthquakes exposing ancient seabeds and coral reefs. And most coasts are rising or sinking relative to the ocean due to vertical land movement (VLM); if the land is falling (-VLM) the effects of sea level rise (SLR) will be felt sooner than areas which are rising (+VLM).
SLR is not a stand alone problem. It exacerbates existing problems and creates new ones. As global temperatures rise, storms become stronger and waves reach further inland. ‘Soft shore’ coastlines—sand, gravel, mud, and pyroclastic material (stuff that explodes out volcanoes, not lava)—erode faster. These ‘soft’ coasts include cliffs such as those at South Taranaki and parts of Canterbury. Rain percolating into the soil at the tops of cliffs can also contribute to the sudden collapse of coastal cliffs. Sometimes waves and currents carry eroded sediment to other beaches. But this may bring new problems, blocking the mouths of rivers, preventing tuna (eels) from migrating and exacerbating inland flooding (Tab 3).
Geological history provides a window into the deep past that helps inform the future. Modelling can also help. But every storm has the potential to re-arrange coastlines in ways that cannot always be modelled. The decisions made by land owners, communities, local and regional councils, rūnanga, and the Government play a crucial role in this rapidly evolving story travelling in an irrefutable trajectory: sea level rise is irreversible, it’s accelerating, and it may rise much faster than previously expected.
The case studies below illustrate some of the complex effects of SLR, and how different online mapping tools are needed to evaluate risk. There are many other online tools, only a few are used here to illustrate the process.
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Millions of years ago the coast of Te Waipounamu the South Island was at the base of Kā Tiritiri o te Moana, the Southern Alps. Over time, glaciers bulldozed through the mountains and braided rivers carried the glacial sediment to the coast, where they built new land. The rivers fanned over the new alluvial plains they’d created, ultimately forming the Canterbury Plains.
Fast forward to 20,000 years ago during the Last Glacial Maximum (LGM). Sea levels were around 120m lower than today, the coast was ~65km east of Ōtautahi Christchurch, and Banks Peninsula was a rocky mountain in the middle of the vast plain.
Fig. 1: Present day geology. The Canterbury Plains (yellow areas) were built entirely by braided rivers carrying glacial sediment from the mountains. The pink areas south of Christchurch are the volcanic Banks Peninsula. Some is basalt (from lava) but some is from pyroclastic flows that compressed into easily erodable breccia. Most is capped with loess soil accumulated over millions of years. Together, this makes parts of the peninsula vulnerable to erosion. Image: GNS/geology 2.0.0 with simplified key added for context: click to be taken to the website.At the end of the LGM the sea was rising was so fast that the rivers couldn’t deliver enough sediment to the coast to keep pace. Land that had been built over millions of years was inundated and eroded by wind, waves, and currents. While it’s not possible to tell exactly how the coastline reshaped as it withdrew (Fig. 2), sediment cores and an extensive area of submarine ridges and swales tell a story of dunes, bays, lagoons, hāpua, and estuaries forming along the retreating coast, then being drowned and reforming inland, over and over as the seas continued to rise.
Fig. 2: By 16,000 years ago sea levels were around 100m lower than today. The green area hides what would have been wide braidplains through which rivers flowed across a complex tapestry of wetlands, lakes, forests and springs. The location of the coast is approximate as it’s based on the current 100m bathymetry contour. Image: Composite Google Earth and bathymetry data from Canterbury Maps.
Fig. 3: Based on morphology and sediment cores (Forsyth et al 2009; Renwick 2010; Kirk 2018). Image base map: Google Earth; coloured overlays based on Begg and Cope 2018 and present day geology.The orange lines in Figure 3 show the approximate coastline of Kaitotere Spit 7,500 years ago. The yellow lines mark the coastline 6,000 years ago, at which time sea levels had stopped rising. The pale green overlay shading the land is the area over which the braided Waimakariri, Waikiriki, and Rakaia rivers wandered back and forth, depositing sand and gravel to the coast, rebuilding some of what had been drowned or eroded during the earlier rise of sea levels.Lake Ellesmere [Te Waihiora] was originally a bay, formed as the post-glacial sea level rise inundated the north-eastern sector of the broadly fan-shaped Late Pleistocene plain of the Rakaia River. The initiation of Kaitorete, fed by gravel funnelled northeast by long-shore drift along the Canterbury Bight coast, then started to enclose the bay.
There was considerable input of Waimakariri River water to the Ellesmere area c. 3000 to 6000 years ago…the Selwyn River plains is as much as 8 km wide where it adjoins the Ellesmere basin, but this surface is built out into the lake basin, indicating that during its formation, the Selwyn River may have had a wide-ranging course across its plain, but was not carrying much sediment. The Rakaia River built an extensive plain of Halkett-age [the cream coloured area in Figs. 1 & 2] sediments into the western corner of the Ellesmere basin, with a few raised channel distributaries extending into the basin near Lakeside [Fig. 2].- p38 Begg et al 2015
By the mid-twentieth century all three rivers had been confined by stopbanks and levees, preventing them from spreading new sediment across the vast areas they once freely roamed (Fig. 1). Now when the rivers run high in their narrow confines, sand and gravels are disgorged into the ocean. Some falls into waters too deep to be carried back to shore by gentler beach building waves. Some is carried alongshore by waves and currents where it accumulates along Kaitorete Spit. As sea levels rise, the dynamics of those movements are changing (Tab 3).
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By definition, braided rivers are composed of water channels weaving around temporary low lying shingle islands across a braidplain that can be dozens of kilometres wide (Tab 1a | Fig. 1). In some places, when the rivers reach the coast, they are confronted by waves and currents that force them to drop their sediment load. This leads to the creation of hāpuas (Figs. 4 & 5).
Along prograding coasts (those building seaward), rivers fan out into deltas that keep building new lands (Tab 1a | Fig. 1 light yellow areas). As sea levels rise, hāpuas are forced to retreat, along with the entire coastline (Fig. 6).
Fig. 4: The Māori term for river-mouth lagoons on mixed sand and gravel beaches that form at the river-coast interface where a typically braided, although sometimes meandering river interacts with a coastal environment that’s significantly affected by longshore drift. Image: Sonny Whitelaw for the Ashburton Hakateri Dotterel Point lookout).
Fig. 5: The Rakaia River and hāpua. Both rivermouths and the paddocks are part of the river’s braidplain. image Phillip Capper, May 2007 Wikipedia CC BY 2.0.The dynamic configuration of hāpuas change with floods and storm waves, so that from year-to-year, the hāpua and the gravel bars that contain them may migrate north (or south along other coasts if the prevailing currents are in that direction) of the main river channel/s.
Rising sea levels are eroding the alluvial material (sand and gravel) that make up much if the Canterbury coast. South of the Banks Peninsular, due to its aspect to waves and currents, the southern part of the Canterbury Bight is eroding quickly. Some of the eroded material is carried north to Kaitorete Spit. But some provides an ongoing supply of gravel to the barrier shown in Figure 4. So, along this stretch of the coast, hāpua and their rich biodiversity and sources of mahinga kai will likely migrate inland while maintaining their general (albeit highly dynamic) configuration (Fig. 6).
Fig. 6: Slide from Murray Hicks’ presentation at the 2019 Braided Rivers seminar outlining the most likely changes in the planform (shape, size, and location in two dimensions) of the Hakatere Ashburton River hāpua as sea levels rise (SLR) 1m. The ‘cliffs’ are highly erodable layers of sand and gravel. The estimated ‘100-125m retreat over 100 years’ is based on 2013 IPCC Report. Subsequent research and observations show that sea levels along this stretch of the Canterbury Bight coastline could reach 1m as soon as 2065 (see NZ SeaRise).
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Compared to the rest of Te Waipounamu South Island, Waitaha Canterbury—the teal-coloured uppermost lines in Figure 7—faces the greatest exposure to the effects of rising sea levels across all sectors. The other coloured lines are for the remaining South Island Districts. Download the full report.
Modelling published in 2022 indicates that wave heights along the east coast will be less than currently experienced. This may reduce the scope and scale of short-term erosion. However, this may be more than offset by more powerful storm events including cyclones reaching further south due to a rapidly warming ocean.
Fig. 7: Click on any graph to see the full report from NIWA.
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This illustrates that when evaluating the effects of sea level rise multiple factors must be considered as they compound one another. See also: Societal vulnerability to flooding: Little River/Wairewa case study.
Lake Ellesemere Te Waihora is not a lake. It’s a large hāpua (Tab 1b) formed by three braided rivers (Tab 1a | Fig. 1). Today, only the Waikiriki River flows into Te Waihora. The Waimakarari River has been banished to the northern side of the Banks Peninsula, held in place by stopbanks and levees to prevent Christchurch from flooding. The Rakaia River has also been confined to the south, where it now forms a narrow hāpua (Tab 1b | Fig. 5).Unlike the mobile gravel bars of most hāpua, the eastern end of the 25km long Kaitorete Spit that encloses Te Waihora is anchored to the volcanic rocks of Banks Peninsula, which acts as a natural groin.As summarised in Tab 1a, rivers can no longer deliver enough sediment to to Canterbury Bight coast to keep pace with rising sea levels. So the coast is in a long term state of erosion (see also Fig. 8 and Tab 2b | Fig. 6). As they did for millennia, waves and currents carry some of this eroded material to Kaitorete Spit. But this erosion has also changed the shape of the coast, pivoting the south-western end of Kaitorete Spit is that it’s now under attack from prevailing waves and currents. And the more it erodes, the more it’s exposed. By 2023, the ‘pivot point’ between erosion and deposition had moveed to middle of the Kaitorete Spit. This movement has been tracked over time in the Coastal Change tool (Fig. 8). Note that the National-led Government scaled back this project, putting communities at risk.
Fig. 8: Screengrab from the online tool ‘Coastal Change’. The area along the front of the spit where the red dots become white and then blue, marks the hinge point between erosion and accretion in 2023. That hinge point is moving north east. The black dialogue box in the centre shows that by 2023 the rate of erosion at the location was 0.48m/year. At the north eastern end, where the spit is anchored to the rocky Banks Peninsula, it’s accreting at ~0.59m/year. Click on the image to be taken to the website to see how this area has changed over time. The National-led Government scaled back the ‘Coastal Change’ project, so the data in some locations may be absent.Currently, enough sediment is being delivered by waves and currents to the narrow entrance of Te Waihora that it sometimes needs to be opened to prevent the lands surrounding the hāpua from flooding (see Tab 1b | Fig. 4).
Meanwhile, the north-eastern end of Kaitorete Spit that’s anchored to Banks Peninsula is accumulating so much gravel that it now prevents floodwaters from Little River, which flows into Wairewa (Lake Forsyth), from draining into the ocean. A canal has been dug to enable the migration of tuna (eels), however it must be opened using diggers to clear the build up of gravel. But it’s not always opened in time to stop the nearby settlement of Little River from flooding and with it, closing the main road to Akaroa and other inland settlements.
How Kaitorete Spit responds in the coming decades, whether Te Waihora opens permanently and transforms into an estuary, and whether Lake Forsyth become permanently landlocked, will depend on a three-way race between erosion, deposition, and accelerating sea level rise.
Fig. 9: Screengrab 2 May 2026: NIWA Coastal Flood Layers based on current sea levels and present-day climate conditions. This ‘bathtub’ model of coastal flooding shows the effects of a large storm tide (estimated 1% annual exceedance probability) where the entrance to Te Waihora is breached, leading to extensive flooding of lands surrounding Te Waihora Lake Ellesmere. The yellow area that covers Te Wairewa / Lake Forsyth in the valley below Little River does not mean it’s flooded, as it’s already a lake. The modelling does indicate if the canal between Lake Forsyth and the coast could be breached. In a large coastal storm, excess gravel may be pushed into the canal, effectively closing it at a time when it would be too dangerous to dredge. But it’s not possible to predict the pattern of extreme erosion/deposition likely to occur during such a storm. Click on the map to be taken to the website Coastal Flood tool.When evaluating the effects of sea level rise multiple factors must be taken into consideration. Coastal flooding from a storm tide (Fig. 9) is not the only hazard. Inland flooding and landslips due to the increasing intensity of rainfall, another manifestation of climate change, is already leading to a cascade of additional hazards including inland flooding (Fig. 10) and landslips. Together, these hazards affect ecosystems, infrastructure, and people. See: Societal vulnerability to flooding: Little River/Wairewa case study.
Fig. 10: Screengrab 2 May 2026: Flood Hazards mapping tool based on current sea levels and present-day temperatures in the event of 1%AEP rainfall (not coastal storm) event. The dark blue areas are existing water bodies. The lighter blue shows flooding around Te Waihora / Lake Ellesmere and between the lines of dunes on Kaitorete Spit, and that Little River could be flooded to a depth of 1-2m. Flooding has occurred multiple times in the last few years, with the February 2026 event being the worst. The grey areas overlaying the map have not been modelled. Click on the map to be taken to the website.IMPORTANT NOTE: If you intend use the NZ SeaRise tool to assess the future of level of SLR Kaitorete Spit or any part greater Christchurch, use the post-earthquake VLM (PDF) not the SeaRise VLM.
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Working out the relative sea level rise (RSLR = SLR+VLM) along the Ōtautahi Christchurch coast over time is complicated by the varied topography and different rates of VLM (Fig. 11). Banks Peninsula, made up of rocky volcanic cliffs and deep narrow bays that flooded the last time that sea levels rose (Tab 1b), is sinking faster in some places than others. VLM along the 25km-long sand and gravel Kaitorete Spit is barely measurable. But most of the central city was built on the low lying Waimakariri River delta. The following 4 steps illustrate the process of determining how high sea levels at one location, Southshore Spit, could rise by the year 2080, and how to use this to determine three possible effects: coastal erosion, saltwater intrusion, and flooding.
Step 1: determine VLM. The dark red dot in Figure 11 indicates that VLM at Southshore Spit is -7.5mm/year. Relative to the year 2020, by 2080 that part of the coast will have dropped 45cm (60 years x 7.5mm).
A recent study by GNS Science has shown that sea-level rise in some parts of the Christchurch District is happening up to twice as fast as previously thought, as a result of increased land sinking following the Canterbury and Kaikoura earthquakes. The rate of this land sinking is yet to slow down. This means the impacts of sea-level rise – flooding, coastal erosion and rising groundwater – will happen sooner, reinforcing the urgency and importance of planning in advance of these impacts. – CCC October 2025
Fig. 11. The dots on the map of post-earthquake vertical land movement (VLM) along the Ōtautahi Christchurch coastline. The ocean-side of Southshore Spit is sinking as much as 7.5mm/year (-VLM 7.5mm/yr). These figures are based on VLM data from satellites between 2015 and 2024. These differ to those used in the NZ SeaRise tool (Fig. 12), which is based on pre-earthquake data. Click on the image for the PDF fact sheet explaining VLM and the GNS report.
Fig. 12. DO NOT USE THIS; IT’S FOR ILLUSTRATIVE PURPOSES ONLY. This is a screengrab of the SeaRise tool at on the ocean side of Southshore Spit, site #4312. The white dot indicates the rate of VLM is negligible. The text box state ‘0.1’ +/- 2.5mm/ year. However, this VLM is based on data collected prior to the earthquakes.Step 2 determine SLR: Go to the NZ SeaRise tool. Launched in 2022, it’s VLM figures for Ōtautahi Christchurch (Fig. 12) are based on pre-quake VLM, so do not use their VLM figures for greater Ōtautahi Christchurch. SeaRise clearly states that newer local and regional VLM should be used where available. The SeaRise tool will be soon be updated, so be aware of any changes.
Step2a: Zoom into this area and select Southshore Spit (Fig. 13 site #4312). Under the ‘Projection to 2150‘ box, select the SSP5-8.5 climate scenario. Don’t select +VLM. Then select the date 2080. The pink area shows the range of potential SRL above 2005 levels. The red dotted line shows a ‘median’ (p50) value of 0.558m SLR. The maximum (p83) is 0.714m SLR.
Fig. 13. Two screengrabs of the SeaRise tool merged into one 04 May 2026 of site #4312 on the ocean side of Southshore Spit. For reasons explained above, +VLM is not selected. Compared to the baseline year of ~2005, two time periods are shown: 2080 and 2130.Step 3: add VLM to SLR.
- Under an SSP5-8.5 median scenario, 45cm VLM + 56cm SLR = 101cm (1.01m) RSLR by 2080
- Under an SSP5-8.5 H+ scenario (worst case) 45cm VLM + 71.4cm SLR = 1164cm (1.164m) RSLR by 2080
Why use the ‘median SSP5-8.5 M’ or ‘worst case SSP5-8.5H+’ climate scenario? The following recommendation is from the Ministry for Environment Coastal hazards and climate change guidance:
This [MfE] guidance takes a risk-based approach to the use of projections and/or increments of RSLR. In this respect, the upper-range SSP5-8.5 H+ should continue to be used in screening and detailed hazard and risk assessments to identify coastal areas potentially affected (Policy 24, NZCPS, DOC, 2010) and high-end stress testing of adaptation options and pathways (step 6). Furthermore, using the RSLR projection based on SSP5-8.5 M allows RSLR to be linked to the other climate drivers (e.g., rainfall) if a multi-hazard and risk assessment is being done.If the risk is underestimated, the consequences will be severe with lasting social, cultural and economic effects. If the risk is overestimated fora specific timeframe, using relative sea-level rise projections based on higher emission scenarios, this will be temporary (decade to multi-decadal timescales). This is because sea level will continue to rise, even as emissions are reduced, and it is only a matter of time before the adaptation threshold is reached for those exposed to the risk. While overestimation places costs today, the observed and increasing climate change impacts mean both current and future generations are, and will be, paying the costs. It is also important to consider how the costs and risks experienced by future generations may be affected by decisions taken today…Temporary adaptation options like seawalls, filling land or raising buildings above flood levels may buy time if they can be implemented quickly, but they can entrench development and limit access to communities, making it harder to transition to options like managed retreat, while also increasing ongoing adjustment costs.Step 4: go to the Christchurch City Council’s (CCC) Coastal Hazards online portal (Fig. 14). The figures in this tool use the earlier ‘2017 Ministry for the Environment (MfE) guidance for local government’ recommended RCP 8.5 climate scenario and do not include VLM. Consequently, if you click on Step 1 in that tool, you will see that sea levels are projected to rise 40cm by 2080. This is much less than the more recent updates. Instead, start with Step 2. There you can choose the year 2080 and insert the ‘median’ RSLR figure of 101cm (~1m) or ‘high’ RSLR figure of 1.164m
Fig. 14. Screengrab of the CCC coastal hazards portal 03 May 2026. Click on be image to be taken to the website and READ THE DISCLAIMER before using.The CCC Hazards portal is designed to map three effects of SLR during so-called ‘rare events‘ (1-in-100 years):
- Flooding. The tidal flood maps show only the tides, that is, without any of the meteorological influences that temporarily raise or lower sea levels. The ‘daily tide’ relates to an average high tide and the ‘monthly tide’ is our spring tide (not fortnightly in Canterbury). These maps should be used to give a sense of the persistent impacts (daily and monthly) you might expect as sea levels rise and should be used in conjunction with the coastal flood mapping tool plus ideally the flood hazards tool used in Tab 3: Kaitorete Spit.
- Erosion
- Groundwater
Why not use the CCC Hazards Portal RCP 8.5 climate scenario?The basis for higher projections of global mean sea level rise in the 21st century has been considered and it has been concluded that there is currently insufficient evidence to evaluate the probability of specific levels above the assessed likely range. – IPCC Fifth Assessment Report 2013The CCC Hazards portal does not map the many other local and short term causes of SLR, such as the known seiche effect in Pegasus Bay, any or all of which could compound RSLR in different circumstances.Ultimately, which figure is used depends entirely on the appetite for risk and ability to self-insure in the event of insurance retreat. See also the April 2026 Climate Change Commission 2nd National Climate Change Risk Assessment.
The impacts of RSLR will depend in part on what actions are taken and when. The following screen grab (Fig. 15) is the first page of ‘Community engagement on climate change adaptation Case studies August 2020‘. This was published 2 years prior to publication of the SeaRise tool and five years prior to 2025 GNS report showing that negative VLM is far worse than previously thought. However, it provides insight into the challenges faced by councils and communities alike.
Fig. 15: Local Government New Zealand report ‘Community engagement on climate change adaptation’ includes a case study on Southshore and South New Brighton. Click on the image to download the full report.
Case studies of the Waimakairi and Amberley Districts’ coasts
are currently being compiled.
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The Waimakariri River is one of several Alpine-fed braided rivers that formed the Canterbury Plains. Today, the river enters the ocean north of Ōtautahi Christchurch, just south of the equally low lying town of Kaiapoi. he shoreline over the past 4,500 years when eustatic (global) sea levels were relatively stable (Fig. 3).
To protect Ōtautahi Christchurch and Kaiapoi from floods—or from a path being torn through multiple towns and farms if the Waimakariri tries to once again migrate south of the Peninsula—the river has been confined by engineering so that it now flows through a vastly restricted corridor. By the time it reaches the ocean it’s confined to just one channel with an outlet to Brooklands lagoon. Confined, sand and gravel that once build up the coastline through natural flooding, is now carried out into the ocean. Some falls into water that’s too deep for beach-building waves to carry ashore. Some is still carried onshore by currents and waves that distribute it along the bay. But the dunes and dune plants that once held this sediment in place have largely been removed or replaced with buildings, farmlands, and radiata pine plantations.
After 4,500 years of extending seaward, the coast stopped growing in the 1990s. Since then, the rate of sea level rise has more than doubled.
As sea level rise accelerates, the effect along different parts of Pegasus Bay will differ for reasons explained here (see also Fig. 4).
See how biodiversity is being restored along the coastline at Tuhaitara Coastal Park between the Rakahuri Ashley River and Waimakariri River, to help mitigate and adapt to the impacts of climate change.
Fig. 3: The changing coastline of Pegasus Bay: 9,500 years ago (‘before present’ or ‘BP’), the shoreline was much further out to sea. Between 9,500-4,500 years ago the coast was drowned as eustatic sea levels rose. By 4,500 year ago, the climate and with it global sea levels were stable. Subsequently, sediment, mostly from the Waimakariri River, built the coastline outwards (mustard-coloured area of ‘progradation’). (Image: Whitelaw).
Fig. 4: A ‘bathtub’ snapshot (ie, assuming no coastal erosion) of the effect of rising sea levels and flooding along a section of Pegasus Bay north of Ōtautahi Christchurch. TbeSea levels are expected to reach0.6m (left image) to 1.36m this century. The speed and impacts will depend on how soon we stop emitting greenhouse gasses, how quickly ice caps melt, if we can drawdown the excess already in the atmosphere, and how much we can restore coastal ecosystems that once acted as a buffer to rising seas. Note: this ‘bathtub’ image of rising sea levels doesn’t factor in what might happen to sand on steep and unstable dunes. If the sediment in them is eroded by wind and waves and deposited inland, it will temporarily raise the height of low-lying wetlands behind. If they’re not eroded (unlikely as they’re highly unstable), then the dunes will become barrier islands as per the images above (Image: Waimakariri District Plan Review-Natural Hazards p 60). Also note that these figures DO NOT take into consideration that the coastal area is dropping almost as fast as sea levels are rising, meaning the effects of SLR will be felt much sooner than anticipated in this report. See the link to the October 2023 GNS study below.
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The Waimakariri River is one of several Alpine-fed braided rivers that formed the Canterbury Plains by depositing material eroded from the Southern Alps. Today, the river enters the ocean to the north of the low-lying city of Ōtautahi Christchurch, just south of the equally low lying town of Kaiapoi. However this was not always the case. Over the past several thousand years the river has migrated across a wide area. Sometimes it reached the sea south of the Banks Peninsula through Te Waihora Lake Ellesmere, what is today a very large hāpua. At other times the river has reached the sea where it is today, albeit through a much wider outlet. When it flooded, the river spread sand and shingle across the coastal delta. This prograded (built the beach outwards) the shoreline over the past 4,500 years when eustatic (global) sea levels were relatively stable (Fig. 3).
To protect Ōtautahi Christchurch and Kaiapoi from floods—or from a path being torn through multiple towns and farms if the Waimakariri tries to once again migrate south of the Peninsula—the river has been confined by engineering so that it now flows through a vastly restricted corridor. By the time it reaches the ocean it’s confined to just one channel with an outlet to Brooklands lagoon. Confined, sand and gravel that once build up the coastline through natural flooding, is now carried out into the ocean. Some falls into water that’s too deep for beach-building waves to carry ashore. Some is still carried onshore by currents and waves that distribute it along the bay. But the dunes and dune plants that once held this sediment in place have largely been removed or replaced with buildings, farmlands, and radiata pine plantations.
After 4,500 years of extending seaward, the coast stopped growing in the 1990s. Since then, the rate of sea level rise has more than doubled.
As sea level rise accelerates, the effect along different parts of Pegasus Bay will differ for reasons explained here (see also Fig. 4).
See how biodiversity is being restored along the coastline at Tuhaitara Coastal Park between the Rakahuri Ashley River and Waimakariri River, to help mitigate and adapt to the impacts of climate change.
Fig. 3: The changing coastline of Pegasus Bay: 9,500 years ago (‘before present’ or ‘BP’), the shoreline was much further out to sea. Between 9,500-4,500 years ago the coast was drowned as eustatic sea levels rose. By 4,500 year ago, the climate and with it global sea levels were stable. Subsequently, sediment, mostly from the Waimakariri River, built the coastline outwards (mustard-coloured area of ‘progradation’). (Image: Whitelaw).
Fig. 4: A ‘bathtub’ snapshot (ie, assuming no coastal erosion) of the effect of rising sea levels and flooding along a section of Pegasus Bay north of Ōtautahi Christchurch. Sea levels are expected to reach at from 0.6m (left image) to 1.36m this century. The speed and impacts will depend on how soon we stop emitting greenhouse gasses, how quickly ice caps melt, if we can drawdown the excess already in the atmosphere, and how much we can restore coastal ecosystems that once acted as a buffer to rising seas. Note: this ‘bathtub’ image of rising sea levels doesn’t factor in what might happen to sand on steep and unstable dunes. If the sediment in them is eroded by wind and waves and deposited inland, it will temporarily raise the height of low-lying wetlands behind. If they’re not eroded (unlikely as they’re highly unstable), then the dunes will become barrier islands as per the images above (Image: Waimakariri District Plan Review-Natural Hazards p 60). Also note that these figures DO NOT take into consideration that the coastal area is dropping almost as fast as sea levels are rising, meaning the effects of SLR will be felt much sooner than anticipated in this report. See the link to the October 2023 GNS study below.
More information
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RCP or Representative Concentration Pathway, is a way of showing the path our climate would take based on the concentration of greenhouse gases in the atmosphere. Four credible pathways, RCP2.6, RCP4.5, RCP6, and RCP8.5 represent four possible ranges of radiative forcing values: 2.6, 4.5, 6, and 8.5 Watts/m2, respectively. The numbers refer to the effect of heat retained in the atmosphere due to actual concentrations of greenhouse gases, not the amount of emissions that put them there, because the natural carbon cycle absorbs a percentage of carbon emissions.The term RCP was adopted by the IPCC for climate modeling and research for the fifth assessment report in 2014.The researchers who developed the RCP 8.5 scenario describe the pathway in detail here.The IPCC sixth assessment report (2021) used instead the term Shared Socioeconomic Pathways (SSPs). These are scenarios of projected socio-economic global changes up to 2100. That is, how different policies around greenhouse gas emissions will result in different outcomes. The following explanation is from the 2019: FINAL DRAFT Chapter 1 Supplementary Material IPCC SR Ocean and Cryosphere, Chapter 1: Framing and Context of the Report Supplementary Material (page 7):
Five SSP narratives describe alternative pathways for future society (Figure SM1.1). Each SSP looks at how the different RCPs could be achieved within the context of the underlying socioeconomic characteristics and shared policy assumptions of that world. The SSPs five alternative socio-economic futures compromise: sustainable development (SSP1), middle-of-the-road development (SSP2), regional rivalry (SSP3), inequality (SSP4), and fossil-fuelled development (SSP5) (Kriegler et al., 2016; Riahi et al., 2017). Across these five SSP narratives there are a total of 23 ‘Marker’ SSP scenarios. Appendix 1.A, Figure 2 shows some specific SSP Markers compared with the RCPs, according to (O’Neill et al., 2016). SSP5-8.5 represents the high end of the range of future pathways, corresponding to RCP8.5. SSP3-7.0 lies between RCP6.0 and RCP8.5, and represents the medium to high end of the range of future forcing pathways. SSP4-6.0 corresponds to RCP6.0, fills in the range of medium forcing pathways. SSP2-4.5 represents the medium part of the range of future forcing pathways and updates RCP4.5. SSP5-3.4 (Overshoot) fills a gap in existing climate simulations by investigating the implications of a substantial 21st century overshoot in radiative forcing relative to a longer-term target. SSP4-3.4 fills in the range of low forcing pathways, and there is substantial mitigation policy interest in this scenario that reaches 3.4 W m–2 by \] 2100. SSP1-2.6 is similar to RCP2.6. It is anticipated that it will produce a multi-model mean of less than 2°C warming by 2100. –
SSP1: Sustainability: The world shifts gradually but consitently toward a more sustainable path, with net zero emissions by 2050
SSP2: Low emissions: Emissions decline to net zero by 2050
SSP3: Regional rivalry: resurgent nationalism, conflicts, less investment in education and technological development, consumption is material-intensive, and inequalities persist or worsen over time. Emission stay at current levels and fail to reach net zero by 2100
SSP4: Inequality: increasing conflict and decreasing global co-operation. Social cohesion degrades and conflict and unrest become increasingly common. Investments in both carbon-intensive fuels like coal and unconventional oil, but also low-carbon energy sources. Environmental policies focus on local issues around middle and high income areas. ie, the wealthy are protected. Emissions double by 2100
SSP5: Fossil-Fueled Development: increasing faith technological ‘fixes’ to manage social and ecological systems, including by geo-engineering if necessary. Emissions triple by 2075.Both SSP 1 and 2 are unlikely. Our current emissions pathway (2026) is between SSP3-7.0 and SSP5-8.5. This may change over time.
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The term ‘critical infrastructure’ is used to describe built
structures. Defined by the National Emergency Management Agency (links to PDF), it includes:- Power (electricity, fuel, gas)
- Transport (roads, rail, bridges, airports)
- Communications (cell towers etc.)
- Three waters (supply of safe freshwater, wastewater treatment, and stormwater removal)
The term is mechanistic insofar as all parts are interlocking, interdependent, and can be rationally explained. However, this refers to critical built infrastructure, which supports modern society but not human existence. Ecosystem services are also mechanistic, providing natural critical infrastructure:In the real world, critical natural infrastructure is a higher order priority because it provides the life-supporting services we need to exist. This includes:
- clean healthy water
- oxygen to breath
- nutrient recycling
- food to eat
- carbon capture and storage
- stable climate
In short, the entire toolkit of essential infrastructure services without which humans cannot exist.When natural critical infrastructure is damaged or destroyed, it disables or removes the capability of built infrastructure. Hence, natural critical infrastructure is a first order essential that supersedes and underpins built critical infrastructure.Reference: 2022: Wagner, Formally designate blue-green infrastructure for climate adaptation, Nature article 26 July, 2022
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- 2026: 2nd National Climate Change Risk Assessment, Aotearoa Climate Change Commission
- 2026: Lentz et al, Challenges and Opportunities for National-Scale Projections of Future Coastal Landscape Change AGU Earth’s Future 31 March (open access)
- Plain English article: How to Study Coastal Evolution EOS
- 2025; Harrison et al; Societal vulnerability to flooding: Little River/Wairewa case study. Prepared for MBIE programme: Increasing flood resilience across Aotearoa, Earth Sciences New Zealand
- 2025; Measures & Dudley, Causes of saline intrusion in the Kaiapoi River Analysis of monitoring data Prepared for Environment Canterbury December 2025 Earth Sciences New Zealand
- 2025: Kearse et al; InSAR measurement of vertical land motion in New Zealand cities, and implications for sea-level rise projections, New Zealand Journal of Geology and Geophysics 06 April (Open access)
- 2025: Cox et al; Empirical Models of Shallow Groundwater and Multi-Hazard Flood Forecasts as Sea-Levels Rise AGU/Earth’s Future 08 February (Open access; study undertaken in Dunedin.)
- Plain English: Flooding from Below: The Unseen Risks of Sea Level Rise
- 2024: MfE; Coastal hazards and climate change guidance. Ministry for the Environment
- 2023: Levy et al; Melting ice and rising seas – connecting projected change in Antarctica’s ice sheets to communities in Aotearoa New Zealand, Journal of the Royal Society of New Zealand 54 | 4
- 2023: Hamling & Kearse; Inter-seismic, co-seismic and post-seismic rates of vertical land movement in the Christchurch district and implications for future changes in sea level ,GNS Science Consultancy Report 2023/81 (October)
- 2023: Paulik et al; National Assessment of extreme sea-level driven inundation under rising sea levels, Frontiers in Environmental Science (Open access)
- 2023:Ministry for the Environment Community-led retreat and adaptation funding: Issues and options (15 August)
- 2022:National adaptation plan; Ministry for the Environment
- 2022: Albuquerque et al; On the projected changes in New Zealand’s wave climate and its main drivers, New Zealand Journal of Marine and Freshwater Research
- NIWA: Coastal Erosion and Sediment Systems
- NIWA: Sea level rise Avon-Heathcote Estuary
- Resilient Shorelines : Canterbury & Marlborough research projects
- 2022: Braddock et al; Relative sea-level data preclude major late Holocene ice-mass change in Pine Island Bay Nature Geoscience 15 pp568-572
- 2022: Wagner, Formally designate blue-green infrastructure for climate adaptation, Nature article 26 July, 2022
- 2021: LDRP097 Multi-Hazard Baseline Modelling Joint Risks of Pluvial and Tidal Flooding, GHD for Christchurch City Council
- 2021: Tonkin & Taylor; Coastal Hazard Assessment for Christchurch and Banks Peninsula
- 2020: Canterbury’s Greenpark Huts residents vow to fight after iwi tells them to demolish homes and move, Newshub 02 September
- 2020: Ngāi Tahu orders removal of historic lakeside baches near Christchurch, Stuff 29 August
- 2020: Orchard et al: Coastal tectonics and habitat squeeze: response of a tidal lagoon to co-seismic sea-level change, Natural Hazards
- 2020: Local Government NZ: Community engagement on climate change adaptation
- Deep South Science Challenge (NZ): Will your property become uninsurable?
- Deep South Science Challenge (NZ): Planning for coastal adaptation
- Deep South Science Challenge (NZ): Extreme weather, climate change & the EQC
- Deep South Science Challenge (NZ): How should the risks be shared?
- 2020: Measures et al; Processes controlling river-mouth lagoon dynamics on high-energy mixed sand and gravel coasts Marine Geology 420/106082
- 2020: Stephens et al; Spatial and temporal analysis of extreme storm-tide and skew-surge events around the coastline of New Zealand Natural Hazards Earth Systems Science 20 pp783–796
- 2019 NIWA: Coastal Flooding Exposure Under Future Sea-level Rise for New Zealand; prepared for Deep South Challenge
- 2019: Hicks (NIWA): Rising sea-level impacts on braided river mouths (hapua). Braided Rivers 2019 Seminar
- 2018 NIWA: Coastal-Sediment-budget-for-Southern-Pegasus-Bay-stage-A
- 2018: Cope; A coastal hazard screening assessment for Selwyn District, Environment Canterbury
- 2018: (IPCC) Church et al; Sea Level Change. In: Climate Change 2013: The Physical Science Basis. Contribution of
Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change - 2017: Ministry for the Environment: Preparing for coastal change: summary of coastal hazards and climate change guidance for local government
- 2017 Tonkin & Taylor; Coastal Hazard Assessment for Christchurch and Banks Peninsula, prepared for Christchurch City Council
- 2015: Whitelaw; Where will estuaries be allowed to go?
- 2015; Begg et al; Geology and geomorphology of urban Christchurch and eastern Canterbury, GNS Science
- 2014 IPCC Climate Change (AR5): Impacts, Adaptation, and Vulnerability
- 2013 IPCC Climate Change (AR5): The Physical Science Basis
- 2011: Whitelaw; The Vulnerability of Tuhaitara Coastal Park to Rising Sea-levels
- 2010: New Zealand Coastal Policy Statement
- 2010: Goring & Henry, Short period (1–4 h) sea level fluctuations on the Canterbury coast, New Zealand Journal of Marine and Freshwater Research 32 | 1 pp119-134
- 2010: Hicks & Enright; Shoreline and Beach volume changes along the Canterbury Bight, 1991-2010, NIWA
- 2010: Renwick et al; 2010, Climate Change impacts on Lake Ellesmere (Te Waihora), NIWA
- 2008: Forsyth et al; Geology of the Christchurch Area GNS Science, Lower Hutt. NZ
- 2002: Pescini; The transition between sand and mixed sand and gravel beaches in Northern Pegasus Bay. Unpublished M.Sc. thesis, Department of Geography, University of Canterbury.
- 1974: Campell; Processes of littoral and nearshore sedimentation in Pegasus Bay. Unpublished M.A. thesis, Department of Geography, University of Canterbury