How will climate change affect us?
(Image: Sonny Whitelaw)
Rising Sea Levels: Canterbury case studies
Summary
“The most important message from this research is that we really need to get on and get our planning in order because we haven’t got long before we are really feeling the impacts of it.” – Dr Scott Stephens, NIWA
NIWA has assessed most of the Canterbury coastline as being particularly vulnerable to rising sea levels. Impacts include:
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- 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
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.
Case study: Pegasus Bay coastline
The Waimakariri River is one of several Alpine-fed braided rivers that formed the Canterbury Plains. 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 migrated across a wide area. Sometimes it reached the sea south of the Banks Peninsular. At other times to the north, and sometimes where it is today. Christchurch and Kaipoi are built on the river’s low lying delta.
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.
Before being confined, the river once 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. 1).
Now that the river is confined, it can’t add sand and gravel to the land and build up the coastline through natural flooding. Instead, the river carries sediment out into the ocean. Some of this sediment drops into water that’s too deep for beach-building waves to carry ashore. Some sand is still carried onshore, with currents and wave action distributing it north and south of the river mouth. But the dunes and dune plants that once held this sediment in place, protecting the land behind, have largely been removed or replaced with buildings, farmlands, and radiata pine plantations. After 4,500 years of spreading seaward the coast stopped growing around 1992. Since then, the rate of sea level rise has more than doubled.
Protecting Christchurch and Kaipoi from river flooding has, perversely, helped compromise the coast’s natural defences against rising sea levels. Dunes with native vegetation stabilises the sand and create low profiles to storm waves, helping to bleed off some of the energy and protect inland areas from erosion and flooding. Replacing dunes and coastal wetlands with with houses, farms, and pine trees that create steep sloping dunes easily undercut by storm waves, has exacerbated erosion.
As the rate of rising sea levels continues to accelerate, the effect along Pegasus Bay will be different for a multitude of reasons. Dunes and native plants are also being restored along some sections of the bay, recent earthquakes have lifted and also dropped different parts of the coast, and engineering works have and continue to be carried out to protect buildings, critical infrastructure, and public and private property.
Click here for a more detailed report on the vulnerability of the coastline to rising seas between the Ashley and Waimakariri Rivers.
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. 3).
- 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. 4). 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.
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. 5). The other coloured lines are for the remaining South Island Districts.
Explainers
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.
Hapua:
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⋅m−2
- In most graphs, the numbers 2.6, 4.5, 6.0, and 8.5 are W⋅m−2 however ‘W⋅m−2 ‘ 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.
“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
- NIWA: Coastal Erosion and Sediment Systems
- NIWA: Sea level rise Avon-Heathcote Estuary
- 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: (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 Tonkin & Taylor; Coastal Hazard Assessment for Christchurch and Banks Peninsula, prepared for Christchurch City Council
- 2015: Whitelaw; Where will estuaries be allowed to go?
- 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
- 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