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Ecosystems: Coastal dunes & wetlands

Ecosystems: Coastal dune systems

Tuhaitara Coastal Park: Image Cody Whitelaw

Coastal dunes & wetlands

Summary

We love living close to the sea. We’ve covered the coastline in houses and roads, filled in the wetlands, and planted grass for agriculture and pine forests for forestry over what was once (and to some still is) regarded as ‘unproductive’ dune lands.

Since humans arrived on Aoteara’s shores, 80.5% of our dune ecosystems have been lost, destroying our most important natural defense against rising sea levels.

We’ve compounded the problem by forcing our braided rivers into ever-narrowing channels, so that instead of dropping sediment along the coast, building up the height of the land, it’s now carried out into deep water, where beach-building waves can’t reach it to carry it back to shore to help defend our coasts against sea level rise.

“For 100 years, we’ve tried to engineer the coast to hold back the sea. Now, it’s time to make space for nature… At $30-$50 per square metre, that’s about one-hundredth the cost of building a sea wall. But it won’t hold back climate change hazards—as the sea pushes inland, the beach buffer will follow. The only real solution is changing mindsets and retreating inland.

“If there’s still a job for engineering, it’s the top two inches of the human skull—where we have to re-engineer our expectations and realign those with how natural coasts behave… We locked ourselves into a terrible circle really, of building too close, and then the assets that we’d put too close were being threatened, and then we used enormous amounts of ratepayers’ money to build humungous sea walls to destroy the very asset that drew us to the place in the first place. That cycle is a lot harder to escape from than we realise.”  – Coastal scientist Jim Dahm

Tuhaitara Coastal Park: Image Cody Whitelaw

Coastal dunes & wetlands

Summary

We love living close to the sea. We’ve covered the coastline in houses and roads, filled in the wetlands, and planted grass for agriculture and pine forests for forestry over what was once (and to some still is) regarded as ‘unproductive’ dune lands.

Since humans arrived on Aoteara’s shores, 80.5% of our dune ecosystems have been lost, destroying our most important natural defense against rising sea levels.

We’ve compounded the problem by forcing our braided rivers into ever-narrowing channels, so that instead of dropping sediment along the coast, building up the height of the land, it’s now carried out into deep water, where beach-building waves can’t reach it to carry it back to shore to help defend our coasts against sea level rise.

“For 100 years, we’ve tried to engineer the coast to hold back the sea. Now, it’s time to make space for nature… At $30-$50 per square metre, that’s about one-hundredth the cost of building a sea wall. But it won’t hold back climate change hazards—as the sea pushes inland, the beach buffer will follow. The only real solution is changing mindsets and retreating inland.

“If there’s still a job for engineering, it’s the top two inches of the human skull—where we have to re-engineer our expectations and realign those with how natural coasts behave… We locked ourselves into a terrible circle really, of building too close, and then the assets that we’d put too close were being threatened, and then we used enormous amounts of ratepayers’ money to build humungous sea walls to destroy the very asset that drew us to the place in the first place. That cycle is a lot harder to escape from than we realise.”  – Coastal scientist Jim Dahm

Fig 1: Healthy coastal dune systems are complex and dynamic, adapting and responding to waves and storms, protecting the land behind. Native grasses like pingoa and spinifex bind loose grains of sand, creating low-profile dunes that sap much of the energy from storm waves. The shape of shrubs and small native trees helps the wind lift up and over the dunes, dropping more sand and holding it in place. Wetlands and estuaries behind the dunes are filled with native plants, many salt-tolerant, creating peaty soils that capture and sequester carbon dioxide while supporting native fish and invertebrates such as mussels and clams. Together these species clean nitrates and other pollutants from streams and rivers that flow into them. Behind the wetlands, native shrubs and forests capture and store more carbon and provide habitat for native species.
Fig 1: Healthy coastal dune systems are complex and dynamic, adapting and responding to waves and storms, protecting the land behind. Native grasses like pingoa and spinifex bind loose grains of sand, creating low-profile dunes that sap much of the energy from storm waves. The shape of shrubs and small native trees helps the wind lift up and over the dunes, dropping more sand and holding it in place. Wetlands and estuaries behind the dunes are filled with native plants, many salt-tolerant, creating peaty soils that capture and sequester carbon dioxide while supporting native fish and invertebrates such as mussels and clams. Together these species clean nitrates and other pollutants from streams and rivers that flow into them. Behind the wetlands, native shrubs and forests capture and store more carbon and provide habitat for native species.
Fig 2: Degraded coastal dune system. Wetlands and estuaries have been filled in and converted to agriculture and covered in buildings and roads. Radiata pine plantations create artificially steep dunes (Fig. 3). Storm waves slam up against steep dunes, tearing out their bases and undercutting them. The waves carry the eroded sand out into deep water.
Fig 2: Degraded coastal dune system. Wetlands and estuaries have been filled in and converted to agriculture and covered in buildings and roads. Radiata pine plantations create artificially steep dunes (Fig. 3). Storm waves slam up against steep dunes, tearing out their bases and undercutting them. The waves carry the eroded sand out into deep water.
Fig. 3: Dunes planted in radiata pine and marram grass create incredibly steep slopes and dunes up to 8m high. There steep slopes make them vulnerable to undercutting by storm waves. Eventually, in spite of their height, storm waves will destroy them as easily as the largest sandcastles always succumb to high tides. (Image: S. Whitelaw)
Fig. 3: Dunes planted in radiata pine and marram grass create incredibly steep slopes and dunes up to 8m high. There steep slopes make them vulnerable to undercutting by storm waves. Eventually, in spite of their height, storm waves will destroy them as easily as the largest sandcastles always succumb to high tides. (Image: S. Whitelaw)
Fig. 4: While some eroded sand is carried along shore by wind and currents, storm waves carry most of it into deep water, where gentler beach-building waves can't reach it. The next storm eats away more land. As sea levels rise and the dunes are washed away, the storm waves will reach far further inland. As sea levels rise, the problem is compounded. (Image: S. Whitelaw)
Fig. 4: While some eroded sand is carried along shore by wind and currents, storm waves carry most of it into deep water, where gentler beach-building waves can’t reach it. The next storm eats away more land. As sea levels rise and the dunes are washed away, the storm waves will reach far further inland. As sea levels rise, the problem is compounded. (Image: S. Whitelaw)

The problems

Coastal squeeze: Healthy dune systems vegetated with native plants (Fig. 1) are our first defense against rising sea levels by acting as a buffer, giving us time to move critical infrastructure and private assets inland from low-lying soft-shore coasts. In spite of knowing their importance, our dunes systems remain under pressure from development, weeds and animal pests, and vehicles and people destroying sand binding plants that help protect dunes from being eroded by waves and wind. Many dune systems can’t be replaced as they have already been built on or converted to agriculture (Fig. 2) As sea levels rise, some existing ones may be protected, but others will be lost to incredibly expensive ‘hard’ engineering strategies to hold back the ocean.

When sea levels rose in the past, these ecosystems migrate inland. The dunes became mobilised, blowing inland and overtopping wetlands, which also migrated inland ahead of the, as water table rose and previously dry areas were unable to drain. Today, that’s going to threaten public and private property:

“The most probable response to sea level rise will be to protect assets and infrastructure by erecting new hard barriers to prevent erosion, planting sand dunes to stabilise them, and infilling encroaching wetlands and installing new drainage. This scenario (often termed ‘coastal squeeze’ in the international literature,) means that rising sea levels will probably remove large areas of the rich biological habitat.’” – Nottage et al, 2010

Where healthy dune systems protect property, they may be supported…until sea levels rise too high. The problem is then one of choosing between saving property or saving ecosystems. The conflicting priorities are self-evident in the 2010 Coastal Policy Statement, Policy 3: Precautionary Approach 2, which states that steps should be taken:

2(a), to ensure that, avoidable social and economic loss and harm to communities does not occur

2(b), natural adjustments for coastal processes, natural defences, ecosystems, habitat and species are allowed to occur

So which is it? Allow dune systems and coastal wetlands to migrate inland and cause social and economic losses? Or stop them moving inland, leaving them to be inundated by rising seas, losing their ecosystem services, and even faster social and economic losses and critical infrastructure? To a certain extent, the choice exists in regional policies, as rising sea levels are classed as a natural hazard (see the Canterbury Regional Policy Statement below).

However, the responsibility for engaging with communities to enable this has been passed to local authorities, that is, district councils, and the wording allows plenty of wriggle room to avoid doing so. ‘Should‘ is very different to ‘Will’. Moreover, no time frames have been set and no consequences are in place to prosecute local authorities that fail to act.

The solutions

1.  Clarify legislation and update coastal policies to enable better decision-making and stricter enforcement of policies.

2. Protect existing dune systems by preventing all motor vehicle access except for emergency services, limiting foot traffic to beach access paths, fencing to keep out livestock, and trapping/killing pest animals such as rabbits and hares.

3. Restore degraded dune systems with native plants including saltwater tolerate plants. Accommodating for the influx of saltwater in wetlands can create blue carbon sinks. Failing to do so is likely to result in rotting forestry and pasturelands becoming methane and carbon dioxide emitters. (See videos 1 and 2 and Tuhaitara Coastal Park.)

4. Encourage and faciliate low-lying marshes to migrate inland as the seas rise. When properly managed, new marshands drawdown and sequester larger quantities of carbon dioxide than established marshlands. This will help to maintain a buffer against storms, may present opportunities for carbon farming, and will continue to provide habitats for native species, which will migrate inland with them.

5. Engage with local coastal communities so they can understand and participate in protecting and restoring their coastal ecosystems, and to have access to realisitic options for retreating, because sea levels will continue to rise for centuries.

Video 1:  “Dunes are a very effective way of keeping the sea where we want it.”

Video 1:  “Dunes are a very effective way of keeping the sea where we want it.”

More information

Pīngao grass: image Wikipedia Creative Commons