Impacts: Sea level rise Waitaha Canterbury & case studies
Conway Flat, North Canterbury image: Sonny Whitelaw
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Background
DISCLAIMER: The two case studies on this page are only to illustrate the complex effects of SLR and how interactive online data mapping tools can be used to help you evaluate risk. See Causes to understand the multiple ways that sea levels rise and fall relative to the land ; this is referred to as relative sea level rise or RSLR. Note the dates in the examples and linked reports. Information and data will likely be superseded when online tools are updated.
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 unpredictable ways. The decisions made by land owners, communities, local and regional councils, rūnanga, and the Government play a crucial role in this rapidly evolving story that’s heading in one direction: sea level rise is irreversible, it’s accelerating, and due to uncertain time frames around the loss of Greenland and Antarctic ice shelves, may rise much faster than existing models anticipate.
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The Waitaha Canterbury coastline is long and varied. Some coasts have been built over millennia by braided rivers delivering gravel from the Southern Alps. Others were created by volcanoes or from layers of peat and mud accumulating in wetlands and lagoons. Many areas such as parts of Kaikoura have been lifted by earthquakes, exposing ancient seabeds and coral reefs. Today, 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 SLR will be felt sooner than areas which are rising (+VLM).
Rising sea levels are exacerbating existing risks 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 (unconsolidated stuff that explodes out volcanoes, not hard lava) erode faster. These ‘soft shore’ coasts include steep cliffs across parts of Pātaka o Rākaihautū Banks Peninsula. Rain percolating into the soil at the tops of these cliffs can trigger their sudden collapse. Sometimes waves and currents carry this eroded sediment to other beaches. This may bring new problems, blocking the mouths of rivers, preventing tuna (eels) from migrating and exacerbating flooding.
How vulnerable are Canterbury’s assets to SLR? (2019)
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 could be more than offset by more powerful storm events including cyclones reaching further south due to a rapidly warming ocean.
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Click on any graph to see the full report from NIWA.
How exposed is Ōtautahi Christchurch and Te Pātaka o Rākaihautū Banks Peninsula Built Environment to hazards?
Figure 2 is from Christchurch City Council: Climate Change Risk Assessment Ōtautahi Christchurch and Te Pātaka o Rākaihautū Banks Peninsula Built Environment 2026. It’s a snapshot of all climate change hazards except fire. While it identifies the number of houses at risk for each type of hazard, the costs refer to Council assets. We recommend reading the full report.
Council isn’t responsible for the cost of managing risk or loss for private homes and businesses but it does manage the infrastructure that services these communities. – p17
Case studies: the physical context
How the Canterbury Plains formed
Millions of years ago the coast of Te Waipounamu 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 (Fig. 3).
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 while Te Pātaka o Rākaihautū Banks Peninsula was a mountain in the middle of the vast alluvial plain.
How past sea level rise shaped the coastline
At the end of the LGM the sea was rising too fast for the rivers to 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, 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.
By 16,000 years ago sea levels were around 100m lower than today. The green area in Figure 4 hides what would have been wide braidplains through which braided 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.
The orange lines in Figure 5 show the approximate coastline 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. 3 & 5] sediments into the western corner of the Ellesmere basin, with a few raised channel distributaries extending into the basin near Lakeside [Fig. 5].- p38 Begg et al 2015
The role of braided rivers
By definition, braided rivers are composed of water channels weaving around temporary low lying shingle islands and wetlands across a braidplain that can be dozens of kilometres wide (Figs. 3 & 5). When the rivers reach the coast, they are confronted by waves and currents that force them to drop their sediment load (Figs. 6 & 7).
On prograding coasts (those that grow seaward), rivers fan out into deltas that keep building new lands (Fig. 3 light yellow areas). 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 channels.
By the mid-twentieth century all of Canterbury’s braided rivers had been confined by stopbanks and levees, preventing them from spreading new sediment across the vast areas they once freely roamed. Now, when the rivers run high in their narrow confines, sand and gravels that don’t feed the gravel barrier are disgorged into the ocean. Some falls into waters too deep to be carried back to shore by gentler beach building waves. Some is also carried alongshore by waves and currents where it’s later deposited on beaches.
Today, rising sea levels are already eroding the alluvial material (sand and gravel) that make up much of the Canterbury coast.
Due to the aspect to waves and currents south of Te Pātaka o Rākaihautū Banks Peninsula, the southern part of the Canterbury Bight is eroding as much as 3.4m annually. While some of the sand and gravel adds to the barrier shown in Figures 6 & 7, some is also carried north by longshore drift to Kaitorete Spit.
Ultimately, along this stretch of the Canterbury Bight coast, if the rivers continue to supply sediment to the coast the hāpuas and their rich biodiversity and sources of mahinga kai will likely maintaining their general (albeit highly dynamic) configuration, however they will also be forced to migrate inland (Fig. 8).
At least until 2120, the City shore sand budget should remain in surplus (and the shore should not begin to erode) except under the worst case RCP8.5 climate change scenario (which couples the effects of changed Waimakariri River sand load, sand losses due to future Future coastal sand budget for Southern Pegasus Bay, future irrigation takes, reduced southward wave-driven sand distribution from the river mouth, a 1.36 m sea-level rise, and sand losses to the ebb-delta at the Avon-Heathcote Inlet) – p65 NIWA for CCC 2018
[at Southshore Spit] Tides are reaching further inland and groundwater is rising, changing the reserve’s land and vegetation. Walking tracks and recreation areas are being damaged, reducing how people can access and enjoy the area. – p1 Christchurch City Council Hazards & Risks | How climate change is affecting northern Otautahi Christchurch 2026
Case studies: Ōtautahi Christchurch
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 (vertical land movement) (Fig. 9). Te Pātaka o Rākaihautū Banks Peninsula, made up of volcanic cliffs and deep narrow bays that flooded the last time that sea levels rose, is sinking faster in some places than others. About halfway around Te Pātaka o Rākaihaut, and into Akaroa, the rate of VLM has slowed. VLM along the 25km-long sand and gravel Kaitorete Spit is barely measurable.
Case study 1: Southshore Spit
The following 4 steps illustrate the process of working out RSLR at Southshore Spit using the year 2080 as an example, and then how to use this to determine three effects and potential impacts: coastal erosion, saltwater intrusion, and flooding. You can use the same steps for any year along any section of the coast.
Step 1: Determine VLM to 2080. The dark red dot in Figure 9 indicates that VLM at Southshore Spit is -7.5mm/year. That is, relative to existing sea levels, it’s dropping. So even without SLR, relative to the year 2020, by 2080 that part of the coast will have dropped 45cm (60 years x 7.5mm). This assumes the rate of -VLM will be constant throughout that length of time and earthquakes will change that in the interim. Remember, this is a projection not a prediction.
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The dots on the map of post-earthquake vertical land movement (VLM) along the Ōtautahi Christchurch coastline. The ocean-side of Southshore Spit is one of the most affected areas, 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. 10), which is based on pre-earthquake data. Click on the image to download the PDF fact sheet explaining VLM and the GNS report.
Step 2: Determine SLR to 2080: Go to the NZ SeaRise tool. Launched in 2022, its VLM figures for Ōtautahi Christchurch (Fig. 10) are based on pre-quake VLM, so do not use the SeaRise VLM figures for greater Ōtautahi Christchurch. SeaRise clearly states that newer local and regional VLM should be used where available. The SeaRise tool will likely be updated soon, so be aware of any changes.
Step2a: While not using its VLM, stay on the SeaRise tool and zoom in to Southshore Spit (Fig. 10 site #4312) and click on the dot. Under the ‘Projection to 2150′ box, select the SSP5-8.5 climate scenario. The pink area on the graph shows the range of potential SLR above ~2005 levels out to the year 2150. The red dotted line in the centre shows the ‘median’ (p50) value. Along the bottom of the graph select the year 2080. A breakout box appears showing that compared to the baseline year of ~2005 the ‘median’ (p50) SLR is projected to be 0.558m while the maximum (p83) is 0.714m SLR. In the example Figure 11, two dates, 2080 and 2130 were selected to show how much sea levels are projected to rise over the next 100 or so years.
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This image is two screengrabs of the SeaRise tool merged into one 04 May 2026 at site #4312 on the ocean side of Southshore Spit. For reasons explained above +VLM is not selected.
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:
Step 3: Add the SLR heights in 2080 to the VLM of 45cm calculated above.
- The SSP5-8.5 (median p50) scenario in Fig. 11 is 55.8cm. Add VLM 45cm = 101cm (1.01m) RSLR by 2080
- The SSP5-8.5 H+ (worst case p83) scenario in Fig. 11 is 71.4cm SLR. Add VLM 45cm = 116.4cm (1.164m) RSLR by 2080
Step 4: Go to the Christchurch City Council’s (CCC) Coastal Hazards online portal (Fig. 12). At the time of writing this case study, the CCC portal uses the earlier ‘2017 Ministry for the Environment (MfE) guidance for local government’ recommended RCP 8.5 climate scenario. This climate scenario was superceded in 2021 by the SSP8.5 scenarios (see the above quote from MfE). Similarly, VLM is not included in their calculations.
The tool is still useful if you use the prior calculations for RSLR worked out above.
The tool is designed to map three effects of SLR during so-called ‘rare events‘ (1-in-100 years, see explainer below):
- Coastal Flooding. 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 the: Kaitorete Spit case study below. Don’t use the date ‘2080’ on the portal. Rather, set the height of sea levels according to the calculations above. That is the (median p50) scenario of 101cm (1.01m) or (worst case p83) scenario of 116.4cm (1.164m) by 2080. The portal then shows you the range and depth of coastal flooding under those circumstances. Note that any erosion or deposition of sediment between 2020 and 2080 will change the effects of coastal flooding.
- Coastal Erosion. Again for the year 2080, don’t use the date ‘2080’ on the portal. Rather, set the height using the above the calculations.
- Groundwater: Don’t use the date ‘2080’ on the portal. Rather, set the height according to the calculations above.
If you wish to look at dates other than the 2080 date used in this example, return to Steps 1 and 2. Similarly, you can use the same steps to look at any other coastal area.
Case study 2: Te Waihora Lake Ellesmere and Kaitorete Spit
This case study illustrates that when evaluating the effects of sea level rise multiple factors must be considered because they compound one another. This case study DOES NOT include the use of the SeaRise tool, as the process of using it is outlined in the Southshore Spit case study and also on the main Sea Level Rise page on this website.
This case study focuses on coastal dynamics and the multiplier effects of flooding.
NOTE: If you use the NZ SeaRise tool to assess future SLR scenarios at Kaitorete Spit, this area was not unduly impacted by the earthquakes (Fig. 14) See the CCC advisory note.
Lake Ellesmere Te Waihora is not a lake. It’s a very large hāpua (Fig. 6) formed by three braided rivers (Fig. 3), enclosed by a sand and gravel barrier. However, unlike the mobile barriers of most hāpua, the north-eastern end of the 25km long Kaitorete is anchored to the volcanic rocks of Te Pātaka o Rākaihautū Banks Peninsula, which acts as a natural groin. By definition, this is why Kaitorete is a spit.
Today, only the Waikiriki River flows into Te Waihora Lake Ellesmere. The Waimakarari River has been banished to the northern side of the Banks Peninsula, held in place by stopbanks and levees to prevent Ōtautahi Christchurch, which is situated on a delta, from flooding. The Rakaia River has also been confined to the south, where it now forms a narrow hāpua (Fig. 7).
As explained in the physical context, these rivers can’t deliver enough sediment to the Canterbury Bight coast to keep pace with rising sea levels. So most of this coast is in a long term state of erosion. As they have done for millennia, waves and currents carry some of this eroded material north-east and deposit it along Kaitorete Spit. Over time, erosion has changed the shape of the coast, pivoting the south-western end of Kaitorete so 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 moved north-east to the middle of Kaitorete Spit. Meanwhile, more sand and gravel continues to pile up at the eastern end of the spit where it’s anchored to Te Pātaka o Rākaihautū Banks Peninsula. This movement of sediment has been tracked over time in the Coastal Change tool (Fig. 15). Note: the National-led Government scaled back the Coastal Change Project.
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The red dots along the coast indicate erosion. The blue dots indicate show that closer to Te Pātaka o Rākaihautū Banks Peninsula, sand and gravel is accreting.
Flood risk from rivers and rainfall
The accumulation of gravels and sediment on the north-eastern end of Kaitorete now prevents floodwaters from Little River and other streams that flow into Te Roto o Wairewa Lake Forsyth from draining into the ocean. A canal has been dug through the northern end of the spit to enable the migration of tuna (eels) from the lake into the ocean. While this canal could also drain away potential floodwaters following heavy rainfall, it must be opened using diggers to clear the ongoing build up of gravel. However, the canal is 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 (Fig. 16).
On the south-western end of the spit, sediment is also being carried by waves and currents from the Rakaia River to the narrow entrance of Te Waihora, often blocking it. This may give a false impression that this part of the coastline is accreting rather than eroding. In fact, it’s part of the dynamic processes that happen when rising sea levels push sediment inland. When the entrance is blocked, water flowing in from the Waikiriki River raises the height of Te Waihora. To prevent low-lying lands surrounding Te Waihora from flooding, the entrance is periodically opened using diggers. However, the risks of flooding are increasing (Figs. 16 & 17).
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Screengrab 2 May 2026. In this example, flooding from rivers, streams, and rainfall is based on current sea levels and present-day temperatures during a 1%AEP rainfall event. It does NOT include the effects of flooding from a coastal storm, which may happen simultaneously (Fig. 17). The dark blue areas are existing water bodies, not flooded areas. The lighter blue shows flooding around Te Roto o Wairewa Lake Forsyth, the township of Little River (1-2m), between the dune swales along Kaitorete Spit, and widespread flooding along the shores of Te Waihora, making the road impassable. 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.
Flood risk from the ocean
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Screengrab 2 May 2026 of the NIWA Coastal Flood Layers based on current sea levels and present-day climate conditions showing the effects of a large storm tide (estimated 1% annual exceedance probability) where the entrance to Te Waihora is breached or parts of the spit are overtopped by the ocean. Click on the map to be taken to the Coastal Flood tool website.
The yellow area that covers Te Roto o Wairewa Lake Forsyth in the valley below Little River does not mean it’s flooded, as it’s already a lake. The modelling indicates the canal between the lake and the coast could be breached. But equally, 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 open.
It’s simply not possible to predict the pattern of extreme erosion/deposition likely to occur during such a storm.
