Engineers Use Mangroves to Tackle the Effects of Sea Level Rise

Imagine if we had a machine that could protect coastlines from fierce storms and coastal erosion. One that could create land and raise it so that it always remains higher than the rising sea level. What if our machine could act as a carbon sink, removing four times as much carbon from the atmosphere as a tropical rainforest, locking it safely away? Such a machine exists and costs nothing to run.

It is the mangrove.

Today, a growing number of engineers are learning about the prowess of mangroves in preventing coastal erosion and how they can work alongside traditional engineering techniques. It wasn’t always like this. During the 1970s and ‘80s, mangroves were seen as a blight on tropical shorelines. These coastal forests stood in the way of progress, and the demand for agriculture, aquaculture, and urbanization saw the deforestation of around 20% of the world’s mangroves. In subsequent years, as the benefits of mangroves became understood, the rate of deforestation slowed significantly. According to Global Mangrove Watch, the annual loss over the last decade was just 66 km2 or 0.04%. Moves are now afoot to protect the remaining 147,000 km2 of mangrove forest and regenerate more.

An Engineer’s Dream

Mangroves create a natural barrier to coastal erosion. Their dense biomass and tight root systems are remarkably resilient against wave action, even cyclones, and they can thrive in harsh coastal environments where there is little oxygen in the soil. Their ability to trap sediment from rivers and oceans is both their strength and weakness. As the sediments settle around the roots, they contribute to the gradual build-up of land, but if that flow of sediment is cut off, the fragility of the mangrove is exposed. Vietnam’s Mekong Delta has suffered an 80-90% reduction in sediment due to dams further upriver. Although necessary for freshwater supply and improving living standards, the flipside of this demand for water poses a threat to this unique ecosystem.

Elsewhere in Vietnam, the Can Gio Mangrove Biosphere Reserve has seen 20,000 ha of planted and 7,000 ha of regenerated mangrove. This UNESCO-listed site acts as a buffer, shielding nearby Ho Chi Minh City and other communities from the impacts of rising sea levels and storm surges. This nature-based coastal protection area is now a thriving center for eco-tourism, illustrating the value of mangroves as important habitats for wildlife.

An Engineering Mix

Protecting coastal areas by naturally regenerating mangroves is not always an option, particularly where land has already been reclaimed. Cost considerations and practicalities are paramount. However, nature-based solutions can work alongside more traditional forms of sea defenses. For example, in an urbanized area where space is limited, it is often more practical to use hard structures, while further along the coast where land is more plentiful, mangroves are a viable option.

The two can work in tandem, but it requires expertise and careful planning. For example, if the strip of mangroves before the sea wall is less than 50–100 m deep, there is a risk it will be eroded from behind. Also, without constant water flow that delivers sediment, nutrients, and freshwater, mangroves cannot thrive.

Wherever possible, natural regeneration is preferable to planting. Furthermore, planting is not always necessary, as natural regeneration will occur in the right conditions. Mangroves are relatively quick to regenerate. Juvenile red mangroves (a Rhizophora species) can grow up to 5 feet in a single year.

In Singapore, regeneration has worked well by restoring the natural hydrology and letting the mangroves grow naturally. Between 1958 and 2014, Singapore lost 80% of the land occupied by mangroves due to land reclamation and damming. Now, the Sungei Buloh Nature Park Network is core to the island state’s plan to protect its northern coast from the effects of climate change while enhancing biodiversity, and with 17 km of trails, it provides citizens with extra green spaces.

The ability of mangroves to dampen hydrodynamic flows and dissipate hydrodynamic energy is well known. A healthy mangrove can absorb more than 75% of the kinetic energy of incoming waves. Less well-known by engineers is how to incorporate nature-based solutions into an engineering project. It requires a mind shift. By considering the natural world first, we make nature-based solutions a central part of the engineer's toolkit and not an experimental afterthought.

Our understanding of mangrove dynamics, their thresholds, and the optimal conditions they require for flood risk management needs to be codified and mapped. Information needs to be made available in the form of functional guidelines for engineers and decision-makers to work with. Using this, they can make assumptions taking into account the local environment and other factors.

Stakeholder management is also critical. Collaboration with local businesses, local and national authorities, conservation groups, and local communities is key. When done properly, it fosters a sense of ownership and stewardship.

We can only do this from a position of understanding. By appreciating and harnessing the potential of these coastal ecosystems, engineers can contribute to more resilient and sustainable flood mitigation strategies. The proven positive impacts on biodiversity, eco-tourism, and local communities are equally profound.

Petra Dankers is the Leading Professional for Nature-based Solutions at Royal HaskoningDHV, the international engineering consultancy. She has a wealth of experience in coastal zone management projects that use nature-based solutions. She has a degree in physical geography from Utrecht University and a Ph.D. from the Delft University of Technology, where she studied hydraulic engineering and fluid mechanics.

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