How mangroves are helping engineers to combat the effects of sea level rise

Today’s engineers are learning more about the remarkable power of mangroves to prevent coastal erosion and how they complement traditional engineering techniques. Sadly, it wasn’t always like this.

During the 1970s and 1980s, mangroves were viewed as a blight on tropical shorelines. These coastal forests obstructed progress. Demand for agriculture, aquaculture, and urbanisation saw the deforestation of about 20% of the world’s mangroves. More recently, as the benefits of mangroves became better understood, deforestation slowed dramatically. According to Global Mangrove Watch, the annual loss over the last decade was just 66km2 or 0.04%. Today, moves are underway to protect the remaining 147,000 km2 of mangrove forest and regenerate more.

Nature’s capable engineer

Mangroves are a natural barrier to coastal erosion. Their dense biomass and tight root systems are remarkably resilient against wave action, even hurricanes, and they can thrive in harsh coastal environments where little oxygen is present in the soil. Their uncanny ability to capture sediment from rivers and oceans is their strength and weakness. As sediments settle around the roots, they help create land, but if that sediment flow 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 upriver. Although necessary for freshwater supply which improves living standards, the flipside of this need for fresh water poses an existential threat to this natural ecosystem.

Elsewhere in Vietnam, the UNESCO-listed Can Gio Mangrove Biosphere Reserve has seen 20,000 ha of planted and 7,000 ha of regenerated mangrove. It acts as a buffer, shielding nearby Ho Chi Minh City and other communities from the effects of storm surges and rising tides. This nature-based coastal protection area is now a hotbed for eco-tourism and illustrates the significance of mangroves as a habitat for wildlife.

Complementing hard coastal defences

Protecting the coast by naturally regenerating mangroves is not always an option. This is especially the case where land has been reclaimed. Nevertheless, nature-based solutions may complement and work perfectly alongside more traditional sea defences. For instance, in an urban environment where space is limited, it will often be more practical to use hard structures, while further along the coast, where land is plentiful, mangroves offer an ideal low-cost solution.

Different solutions can work in tandem but require expertise and careful planning. For instance, if an area of mangroves before the sea wall is less than 50-100 m deep, it could well be eroded from behind. Furthermore, without a constant flow of water that bears sediment, nutrients, and freshwater, the mangroves are unlikely to survive.

Natural regeneration is preferable to planting. Planting is not always necessary as regeneration will occur naturally if the conditions are right. Mangroves can regenerate relatively quickly. Juvenile red mangroves - a Rhizophora species - can grow up to 5 feet in a single year.

In Singapore, mangrove regeneration is working well, mainly due to the restoration of natural hydrology. Between 1958 and 2014, Singapore lost 80% of its mangroves due to damming and land reclamation. Today, the Sungei Buloh Nature Park Network is core to protecting Singapore’s northern coast from the effects of climate change. Furthermore, it enhances biodiversity, and its 17 km of trails provide citizens with extra green spaces.

Our understanding of how mangroves dampen hydrodynamic flows and dissipate hydrodynamic energy is well known. A healthy mangrove can absorb more than 75% of the kinetic energy from incoming waves. Perhaps less well-known by engineers is how to build nature-based solutions into an engineering project. It calls for a mind shift. One where nature-based solutions are at the very centre of the engineer's toolkit and not some experimental afterthought.

Knowledge sharing

A detailed understanding of mangrove dynamics, their thresholds, and the optimal conditions they require for flood risk management, should be codified, and mapped. Functional guidelines for engineers and decision-makers ought to be made available. With this information, informed assumptions can be made whilst accounting for local, environmental, and other factors.

Stakeholder management is critical. Collaboration with businesses, local and national authorities, conservation groups, and local communities is key. It promotes a sense of ownership and stewardship.

The applicability of mangroves for coastal defences must be done from a position of understanding. Only by appreciating and harnessing the potential of these amazing coastal ecosystems, will we be able to best contribute to more resilient and sustainable flood mitigation strategies. Climate resiliency strategies with the added benefits of promoting biodiversity, eco-tourism, and the regeneration of local communities.

This article was originally published in ECO Magazine on 02 November 2023.