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Impacts: rising sea levels case studies

Impacts: rising sea levels case studies

NIWA has assessed most of the Canterbury coastline as being particularly vulnerable to rising sea levels. Impacts include:

    • Erosion
    • Flooding
    • Inundation (permanently drowned coastlines)
    • Saltwater intrudes into freshwater & aquifers
    • Changing coastal ecosystems and mahinga kai
    • Existing and increasing risks to critical infrastructure 
    • Existing and increasing risks to public and private property
    • Existing and increasing risks to mahinga kai
    • Existing and increasing risks to biodiversity
Fig. 1: Coastal erosion North Canterbury (Image: Whitelaw)
Fig. 1: Coastal erosion North Canterbury (Image: Whitelaw)

Case study: braided river mouths

The Canterbury Plains have been build over time by deposition of sediments carried from the Southern Alps by braided rivers. While tens of metres thick in places, this relatively soft alluvium is easily eroded. Rising sea levels will accelerate existing coastal erosion and speed up the retreat of the coastline.

Rivers that originate in the mountains are much larger than those that originate in the foothills. As the climate changes, these larger rivers are predicted to flood more often as rainfall is predicted to increase in their catchment areas. The volume of water in rivers that originate in the lowlands is predicted to decline as rainfall here declines and the risk of drought increases.

In spite of—or in fact because of—their highly variable flow regimes, one of the unique attributes of all braided rivers, no matter where they originate, is the complex formation of channels and hapua where they meet the sea.

By definition, this dynamic configuration naturally changes with floods and storm waves, so that from year-to-year, the hapua and the gravel bars that contain them may migrate north or south of the main river channel.

While rising sea levels are exacerbating the erosion of the coastline, even if rivers originating in the foothills flood less often, or even dry up during droughts, some of this eroded material from cliffs (made from alluvial deposits) either side, should continue to provide an ongoing supply of sand and gravels to river mouths. Here, hapua, with their rich biodiversity and sources of mahinga kai, will most likely migrate inland while maintaining their general (albeit highly dynamic) configuration (Fig. 2: eg Ashburton River mouth at Ashton beach).

Other hapua that form at the mouth of low lying rivers such as the Rakaia River, may become saltwater estuaries instead. See here for other examples and a fuller explanation.

Fig. 2: This slide is from a presentation to the 2019 Braided Rivers seminar to outline the most likely changes in the planform (shape, size, and location in two dimensions) of the Ashburton River hapua as sea levels rise (SLR) 1m. The estimated 100-125m retreat of the coastline is over a time frame of 100 years based on 2013 IPCC scientific findings (see footnotes this page). Research and observations since 2013 indicate that sea levels could reach this height much sooner.

Case study: Pegasus Bay coastline

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 Christchurch, just south of the equally low lying town of Kaipoi. 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 Peninsular. At other times where it is today. 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 Christchurch and Kaipoi from floods—or from a path being torn through multiple towns and farms if the Waimakariri tries to once again migrate south of the Peninsular—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, where currents and waves 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 Ashley and Waimakariri Rivers, 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. Sediment, mostly from the Waimakariri River, built the coastline outwards (mustard coloured area of ‘progradation’). (Image: Whitelaw).
Fig. 4: A ‘bathtub’ snapshot of rising sea levels along a section of Pegasus Bay north of Christchurch. Sea levels are expected to reach these heights this century. The speed and impacts will depend on how soon we stop emitting greenhouse gasses, how quickly 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).

Cast study: Christchurch

“In this paradigm [Christchurch earthquakes], risk reduction decisions are highlighted as key influences on outcomes. As applied to natural environments, decisions are required to prevent the reaching of tipping points that result in loss of natural features and resources. This study illustrates the potential for rapid sea-level changes to exceed such tipping points with deleterious effects that result largely from anthropogenic influences.”  Orchard et al (2020)

The effects of rising sea levels on Christchurch are complicated by the city’s highly varied coastal topography. The main city and surrounding suburbs sit on a low lying delta and vulnerable ‘soft’ coastline at the southern end of Pegasus Bay, more than half the length of which is wetlands, estuaries, and a low sand spit. The greatest length of the coastline, the Banks Peninsular, is made up of basalt cliffs, deep narrow bays. The southernmost section is a 25km long sand and gravel spit fronting a lake that was once the mouth of the Waimakariri River.

Sections of this coastline were changed during the 2011-2012 Canterbury earthquake sequence. Some were pushed up; others down. These physical changes and the human response is a case study for how we go about addressing rising sea levels in the future. The impetus to reduce the risks associated with rising sea levels can itself exacerbate these risks by destroying natural habitats that act as natural defences:

  • The most recent report on the predicted impacts of rising sea levels to Christchurch is from Tonkin & Taylor: Coastal Hazard Assessment for Christchurch and Banks Peninsula (2017).
  • The Local Government Community engagement on climate change adaptation (2020) report includes a case study on Southshore and South New Brighton (Fig. 5).
  • Christchurch City Council also have an interactive map on their website where you can enter a street address and determine the level of risk and possible time frames based on the scenarios in the 2013 IPCC Report (click on Fig. 6). The names of the scenarios: ‘RCP 2.6’ etc. are explained in at the bottom of this page. When making these calculations please see this footnote as current (2020) research and observations imply that the worst case (RCP8.5) may be reached sooner than anticipated.
Fig.5: 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.
Fig. 6: Click in the map to be taken to the Christchurch City Council coastal hazards zone property search.

Case study: Greenpark Huts

Lake Ellesemere Te Waihora, in the Selwyn District between Ashburton and Christchurch, is a large coastal lagoon and as such, the ‘lake’ is subject to the impacts of rising sea levels. Greenpark Huts is a lakeside settlement on land that belongs to Ngai Tahu and is leased by residents. When the leases expire 30 June 2024 they will not be renewed and residents will be required to remove the built structures before then.

“In a letter to owners, Ngāi Tahu said the limited availability of acceptable quality drinking water, non-compliant wastewater systems and the inevitable impact of sea level rise were behind its decision.  Stuff, 29 August 2020

This highlights the complexity of impacts from rising sea levels. As sea levels rise so too will groundwater —currently just 20cm below the surface—posing significant environmental problems on multiple levels. For example, some toilets in the Huts are ‘long-drop’ toilets that cannot prevent seepage into the groundwater and concurrent impacts to the health of the surround environment including mahinga kai.

”The Greenpark Huts sit on a site of immense cultural and mahinga kai significance to Ngāi Tahu.” – Ngāi Tahu general manager te o tūroa Trudy Heath, Stuff, 29 August 2020

Current and increasing coastal flooding exposure to Canterbury

Canterbury faces the greatest exposure to the effects of rising sea levels; represented by the teal-coloured uppermost lines in all graphs below (Fig. 6). The other coloured lines are for the remaining South Island Districts.

Fig. 6:  Click on any graph to see the full report from NIWA.


Critical infrastructure:

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 Canterbury Plains:

Were formed over millions of years by several braided rivers that deposited silt and gravel eroded from the Southern Alps. In effect, the entire Canterbury Plains is one large coalesced braidplain.

Braided Rivers:

Globally rare ecosystems. Unlike ‘normal’ rivers, they consist of a network of river channels separated by small often temporary islands. The dynamic nature of braided rivers is to change, primarily laterally and over time—what has been referred to as a ‘fourth dimension’. A defining feature of braided rivers is that during high water flows their multiple channels often join into one single channel that fills the entire braidplain. When the water recedes, new channels may have migrated to different locations within the braidplain.


This is 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 is significantly affected by longshore drift. They are commonly seen along the Canterbury coastline.

In the IPCC Fifth Assessment Report 2013/2014, Representative Concentration Pathways (RCPs):

These represent the concentration of greenhouse gasses in the atmosphere based on how these gasses retain heat.

  • Heat is measured in watts per metre squared, written as W⋅m2
  • In most graphs, the numbers 2.6, 4.5, 6.0, and 8.5 are W⋅m2 however ‘W⋅m2 ‘ is implied, and the four units are written instead as four scenarios: RCP2.6 etc. (for example, Fig. 6).
  • In some graphs, this is written without a decimal place: RCP26, RCP45 etc.
Fig. 6: It’s important to note that the 8.5 ‘worst case scenario’ (dark orange) doesn’t include any effects from collapsing Antarctica and/or Greenland ice sheets, because at that time, there wasn’t enough data to work out the probability:

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 2013

References and further reading