Tuhaitara Coastal Park

Tuhaitara Coastal Park

Image: Cody Whitelaw

Tūhaitara Coastal Park

Official website

Summary

  • Located just north of Christchurch on the east coast of New Zealand’s South Island
  • Restoring the natural ecosystems and biodiversity of this once degraded part of the coastline is restoring essential life supporting ecosystem services including its ability to sequester more carbon dioxide, return to its role as an important source of mahinga kai, and act as buffer to rising sea levels.

Context

During the past 4,500 years when the world enjoyed a stable climate, the Pegasus Bay coastline was made up of rich interlinked delta ecosystems. Older inland dunes were covered in native bush and forest, salt and freshwater marshes supported a myriad of plants and animals, and newer, younger dunes at the beach face were held in place by low lying native grasses, helping to protect the lands behind from storms.

Tūhaitara Coastal Park is part of the braided Waimakariri River flood plain. Periodic floods from the river delivered vast quantities of sediment to the coast, ensuring the delta continued to build upwards (accretion) as well as seaward (progradation) ~1m/year for 4,500 years (Fig. 1).

Unusuallyand only in retrospectit’s easy pinpoint when the coastline stopped growing. It was the early 1990s. By then, Europeans had altered the entire coastline (and indeed the entire Canterbury Plains all the way to the Southern Alps). Vast interconnecting freshwater springs and salt water marshes, ecosystems that had once provided an endless source of mahinga kai that welcomed the periodic floods from the Waimakariri River to enrich them, had long since been filled in and levelled to become poorly drained paddocks.

Fig. 1: 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 (global) sea levels rose due to melting ice caps at the end of the last glacial. By 4,500 year ago, the climate—and with it global sea levels—stabilised. Sediment, mostly from the Waimakariri River, built the coastline outwards (mustard coloured area of ‘progradation’). (Image: Whitelaw; click for a larger image).

The dunes had been thoroughly remodelled by exotic tree-lupin, marram grass, and plantation radiata pine (Fig. 2) making them exceptionally vulnerable to erosion. And the once mighty Waimakariri River had been blockaded by levees and embankments, preventing its floodwaters from adding layers of more sediment that once built the coastal lands seaward. Now, floods carry sediment out to sea, where much of falls into waters too deep to be picked up and carried back to shore by beach-building waves.

All this was happening as the climate was changing, causing sea levels to rise ~16cm by the 1990s. Since then, sea levels have risen another 8cm. And the pace of sea level rise is accelerating

Fig. 2: Radiata pine creates artificially steep dunes prone to undercutting and rapid erosion during storm surges. (Image: Whitelaw)

The Park

“Tūhaitara Coastal Park is located from the mouth of the Waimakariri in North Canterbury, and extends north along the coast for 10½ kilometres to Waikuku Beach (Fig. 3). The park totals approximately 700 hectares and contains Tūtaepatu, a 49 hectare spring fed lagoon and important mahinga kai site. The lagoon is the source of the coastal freshwater system that runs the length of the park connecting the Waimakariri and Ashley Rakahuri rivers. The park also includes a significant wetland at The Pines Beach, fore dune and back dune habitat, coastal protection and commercial pine forestry and over 100 hectares of farmland reverting to open wetland.” – Te Kōhaka o Tūhaitara Trust | 2015-2025

Fig. 3: Tuhaitara Coastal Park (Image: Te Kōhaka o Tūhaitara Trust)

The Park was established by the Te Kōhaka o Tūhaitara Trust in 1998 as one of the outcomes of the Ngai Tahu settlement with the Crown under the Treaty of Waitangi. Under the terms of the settlement, the Tutaepatu Lagoon and Wetlands Draft Restoration Plan was formulated to restore the lagoon and wetlands ‘for the benefit of future generations’. Tutaepatu Lagoon and wetlands have significant importance to the Ngäi Tahu as mahinga kai, urupa, and potential kainga nohoanga, and are a habitat for the endangered Canterbury mudfish (Neochanna burrowsius) (Video 1) regarded as taonga by iwi.

The original restoration plan included a vision that the park’s ecosystems would remain in their approximate location for the next 200 years. The plan was drafted some years before the impacts of climate change and rising sea levels was taken into consideration by planning authorities.

In 2012, the Trustees received a report that outlined its vulnerability to rising sea levels. They responded by modifying their original plan, to place additional emphasis on restoring dunes with native spinifex and pingao grasses. These grasses lower the profile of the dunes, holding the sand in place and allowing the energy of storm waves to wash over them rather than being undercut by trying to deflect waves (listen to the podcast on Radio New Zealand about these amazing grasses). They are also replacing the radiata pine forests with natives.

A subsequent technical report in 2018 shows the degree to which the low-lying lands will be subject to inundation as sea levels rise (Fig.4).

Restoring this once degraded part of the coastline means restoring its essential life supporting ecosystem services including its ability to sequester carbon dioxide, return to its role as an important source of mahinga kai, and act as buffer to rising sea levels.

Even if we reach net carbon zero in the coming decades, because of climate feedback effects and dangerous tipping points, sea levels will continue to rise for several centuries.

Video 1: Park manager Greg Byrnes explains how the park’s wetlands have grown naturally, ‘…just by digging a hole and helping it a bit’. (Credit: Seven Rivers Walking documentary film)

Some dunes will eventually drown or be swept away in storms, the ocean will find pathways into the freshwater lagoons and wetlands, and the mouths of the Waimkariri River to the south and the Ashley Rakahuri River to the north will retreat inland so that existing coastal areas not washed away or inundated will become sand barrier islands (Fig. 4).

The Park managers recognise that climate changes are underway, so they have developed an adaptive management plant that includes actions such as planting ‘nodes’ of salt-tolerant species that are now naturally spreading and thriving. Recognising changes are inevitable enables a smoother ecological transition from the existing freshwater ecosystems to hapuas, saltwater marshes and estuaries, and a natural retreat of forests from the coastline. Fostering this change from terrestrial ‘green’ carbon sequestration to coastal and oceanic ‘blue carbon‘ sequestration will ensure crucial ecosystem services will continue, albeit in different ways. Restoring and creating saltwater marshes, for example, has the potential to sequester even larger volumes of carbon dioxide than some terrestrial ecosystems (conditional on farm nitrate pollution in waterways being rapidly reduced), helping to mitigate the impacts of climate change not just for the Park over the next 200 years, but for all of us.

Fig. 4: A ‘bathtub’ snapshot of rising sea levels along the Waimakariri District coastline. 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), then the dunes will become barrier islands as per the images above (Image: Waimakariri District Plan Review-Natural Hazards p 60).

How this helps mitigate and adapt to the impacts of climate change

  • Native plants sequester more carbon dioxide (drawdown) than pasture grass.
  • Restoring coastal wetlands with salt-tolerent species will enable a more stable transition between fresh and saltwater ecosystems as sea levels continue to rise over the coming centuries.
  • Restoring biodiversity also restores the co-benefits of life-supporting ecosystem services:
    • Reduces erosion by restoring native dune plants. This is particularly important as a natural buffer to mitigate the impact of rising sea levels.
    • Increases habitats for endemic taonga species including insects and birds that pollinate plants and help the soil absorb carbon dioxide.
  • Provides a new source of seeds at nodal points from which native plants can spread along the coast.
  • Raises public awareness of climate change and rising sea levels.
  • Healthy ecosystems are sources of mahinga kai, helping us become more food resilient as the climate changes.
Video 2: Student Volunteer Army shows how it’s done!

Key actions

  • Managed by Te Kohaka o Tūhaitara Trust as one of the outcomes of the Ngai Tahu settlement with the Crown under the Treaty of Waitangi.
  • Supported by the Waimakarairi District Council and Environment Canterbury
  • Created long term restoration and management plans that consider the impacts of climate change, particularly rising sea levels. This includes but is not limited to:
    • Plan to plant 10,000 podocarp forest; first planting of 2000 with volunteers organised by the Student Volunteer Army and trees from Trees that Count (Video 2) coincided with New Zealand and 174 other countries signing the Paris Agreement on Climate Change the previous night.
    • Joint research project with the Coastal Restoration Trust of New Zealand to replace logged pines and marginal pasture with foredunes of sandbinders, mid-zone coastal shrublands, wetlands and lagoons, and a landward coastal podocarp forest.
    • Establishing ‘biota nodes’ along the 10.5km length of the Trust’s lands at approximately 250m centres (Fig. 5)
    • Education: each node is to be adopted and maintained by a school, class or community group; developed lesson plans linking to levels 1 – 3 of the Living World within the New Zealand Schools science curriculum.
    • Predator control: protects regenerating plants and native species including natural seed carriers and pollinators, which in turn build foster the expansion of nodes
    • Community engagement: biota nodes fostered and supported by volunteers; management runs community planting days
Fig. 5. Click to be taken to the full list of projects on the website.

Resources and contacts

Explainers

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 timewhat 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.


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.


Plantation radiata pine

While radiata pine absorbs carbon dioxide quickly, it creates artificially steep dunes subject to undercutting and rapid erosion during storm surges (Fig. 2). See here for more of the many problems associated with plating pine forests (this website).


Floods carried sediment out to sea: the sediment budget for beach-building processes

More sediment is carried in floodwaters of rivers whose widths have been narrowed, such as the Waimakariri River, because water flows faster in a confined space.  When sediment in floodwaters is carried into ocean water deeper than 20-30 metres, the sediment can no longer be picked up by constructive (beach-building) swell waves. So it’s no longer available to help rebuild eroded beaches. As sea levels rise, by definition the water gets deeper, so even less of this mobilised sediment can be picked up and taken back to the shore.

The problem will be compounded as storms are predicted to become bigger and more frequent. Storm waves carry eroded beach/dune sediment into increasingly deeper waters, and this will also become unavailable to shallow, beach building waves. See Wright et al and Wright & Short.

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.


The end of the last glacial

This is more commonly but incorrectly referred to as ‘the end of the last ice age’:


Video 2: What is blue carbon?

Salt water marshes sequestering carbon dioxide (blue carbon)

Wetland soils are globally important carbon stores, and natural wetlands provide a sink for atmospheric carbon dioxide (CO2) through ongoing carbon accumulation. Recognition of coastal wetlands as a significant contributor to carbon storage (blue carbon) has generated interest into the climate change mitigation benefits of restoring or recreating saltmarsh habitat (subject to risks from coastal squeeze). See for example the recent research in Christchurch post-quakes.

References and further reading