Managed realignment (intertidal)

Tidal mudflats, Hobson Bay, Auckland. Photo: FreeWalks NZ

When adressing sea level rise and coastal flooding, modern methods have often relied on a hard line of defence such as a seawall. Managed realignment refers to the process of realigning that defensive line inland, to create an intertidal habitat between the old and new lines that can attenuate wave energy, reduce offshore sediment transport and therefore minimise coastal erosion (UN CTCN, n.d). See also coastal setbacks.

The intertidal zone is where the ocean meets the land between high and low tides; the area above water level at low tide and underwater at high tide. If the intertidal zone consists of a natural habitat, like a mudflat, it can absorb and dissipate water more easily. 

Managed realignment and creation of intertidal habitats is a strategy that has been used around the world in areas where coastlines are subject to ongoing erosion and inundation. In Europe, managed realignment is carried out to create saltmarshes, where studies show they are capable of attenuating up to 97% of incoming wave energy depending on the width of the marsh (Doody, 2008).

The restoration or creation of mudflats as an intertidal habitat is an effective way to manage flood inundation while restoring and supporting natural ecosystems and biodiversity both on land and in the water. This is increasingly relevant in the low-lying Pacific Islands, where sea level rise is a significant challenge, and will continue to have devastating effects on communities, reefs and ecosystems. 

As well as effectively managing sea level rise and coastal inundation, tidal mudflats are critical for supporting ecosystems and habitats. They act as nursery grounds for many aquatic species and are habitats for fish, crabs and molluscs. Mudflats filter water, and absorb sediment and pollutants (White, 2018). 

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Name of NbS

Managed realignment (intertidal)

Type of NbS

  • Ecosystem restoration
  • Ecosystem protection

Location

low-lying coastal areas

Relationship to Indigenous knowledge

Pacific Island nations are on the front lines of climate change, seriously at risk of sea level rise, coastal inundation, increase in extreme weather events, and ocean acidification. Increasingly, external aid is coming into the islands from foreign countries, where strategies, as well as measures of success, are categorised by Western knowledge and belief systems, however, peoples of the Pacific have intimate knowledge of the dynamic island ecosystems they live within and in some cases are the forefront of innovative adaptation techniques (Mcleod et al., 2019).

Donner & Webber’s (2014) study of seawalls in Kiribati shows that hard infrastructure methods like sea walls are not effective because they do not take into account local knowledge, beliefs and culture. Small Island nations have the challenge of implementing climate change adaptation strategies that will have long-term success, in a way that can be implemented, maintained and understood on a local level. 

The people of Oceania are resilient and have been adapting to climate change for milenia with nature-based solutions. Implementation of managed realignment, and the introduction and support of mudflats to protect island nations against coastal inundation, as opposed to the hard infrastructure of seawalls, may better ensure that local communities are more invested in projects and understand how to adapt to climate change.

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Climate change benefits
  • Coastal erosion
  • Coastal inundation and storm surge
  • Salt water intrusion into aquifers
  • Ocean acidification
  • Sea level rise

Rising sea levels and increased intensity of storm surges have significant effects on the degradation of coastal regions in the Pacific (McCulloch, 2021). Coral reefs that form a natural barrier around low-lying islands are increasingly at risk, and coastal erosion threatens coastal infrastructure, intertidal habitats, and water quality. 

Intertidal habitats like mudflats and wetlands function like sponges and buffers as the water rises and inundates the fringes of our oceans, rivers, and estuaries (White, 2018). They support ecosystems, contribute to flood defence and create habitats for birds and fish (Clifton-Dey et al., 2016). 

Coastal communities – both human and non-human can be protected from storm surges, flooding, and sea level rise somewhat with managed realignment and the creation of intertidal habitats. As well as creating climate change resilience, intertidal habitats mitigate carbon dioxide and methane emissions and promote recreation and ecotourism (UN CTCN, n.d).

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Societal / socio-cultural benefits
  • disaster risk reduction and resilience

Low-lying Pacific nations are vulnerable to increased natural disaster events like tsunamis, tropical cyclones, and floods. Natural disasters are four times more likely to affect people in Asia and the Pacific than those in Africa, and 25 times more likely than those in Europe (ADB, 2019). These events are devastating, on economic and social levels, and countries with fewer resources and protections are less able to prepare for and recover from such events.

The intertidal habitat reintroduces a reciprocal relationship between people, the land and the sea that aligns with traditional indigenous knowledge systems. 

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Ecological and biodiversity benefits
  • Climate regulation
  • Disturbance prevention
  • Habitat provision
  • Species maintenance

Sea level rise, damage to coral reefs and changes in ocean patterns and temperatures affect ecologies and the biodiversity of both the land and the sea. The intertidal habitat is unique in that it supports life both in the ocean and on land. 

Over time, swathes of mudflats, wetlands, and other intertidal landscapes have been lost to land development, and their unique ecosystems have been destroyed. In Aotearoa New Zealand for example, wetlands support the greatest concentration of wildlife out of any other habitat, yet 90% of the country’s wetlands have been lost (Corlett, 2021). 

Managed realignment is an opportunity to create a nature-based solution with both the ability to create resilience to climate change and to reintroduce habitats that support healthy ecosystems and native species.

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An explanation of managed realignment (in this case a salt marsh) and its efficacy as opposed to a large seawall. Image: Estuarine Ecohydrology
Managed realignment and creation of intertidal habitats. Image: Xianli Zhu

Technical requirements

While there are no specific technical requirements, implementing managed realignment and the creation of intertidal habitats requires a deep knowledge of the environment in which it is being implemented and ocean characteristics. The area should be carefully designed and monitored and managed.

Issues and Barriers

One key barrier to managed realignment in Te Moananui Oceania is the availability of land. The relocation of any infrastructure present in the realigned area requires land elsewhere. In densely populated coastal areas, this will be a challenge (UN CTCN, n.d).

Opportunities

Managed realignment can be implemented in Pacific regions where low-lying coastal areas are suffering from coastal inundation, storm surge and flooding. Many Island nations are already considering new ways of managing and maintaining their shorelines, and this strategy can be implemented to create resilience and habitat provision.

Financial case

There is a strong financial case for the implementation of ‘soft’ coastal infrastructure, as opposed to ‘hard’ infrastructure solutions such as seawalls.

Implementation of managed realignment is a relatively simple and very cost-effective way of increasing resilience to sea level rise, flooding and storm surge. 

Beyond the initial cost of implementation, managed realignment and coastal habitats are not costly. The intertidal habitat like mudflats can be managed by local communities, using local knowledge of climate, the land and the natural vegetation. 

References