Innovative solutions are needed to address the ongoing loss of ecosystem services due to unsustainable land use.
In tropical coastal areas, large tracts of mangrove forest have been cleared to make room for aquaculture ponds, urban settlements and other land use. The resulting coastal erosion, saltwater intrusion and increased vulnerability to flooding from storm surges increase the exposure of people and their livelihoods to natural and man-made hazards. In order to address this problem, coastal managers tend to turn to ‘hard’ engineered structures-such as dykes and breakwaters. However, such structures are often expensive and inflexible, and may fail to provide adequate protection to people and property. Sometimes these structures become counterproductive, exacerbating the problem they were supposed to solve. Hybrid engineering is an innovative concept that seeks to work with nature rather than against it. It combines engineering techniques with natural processes and resources, resulting in dynamic solutions that are better able to adapt to changing circumstances. Some hybrid structures may literally grow on their own and/or maintain themselves, for example where they facilitate the establishment of living plants or bivalves. Such structures may get stronger over time, as mangrove trees grow and oysters settle on top of one another; by contrast, man-made structures generally become less effective over time and have a limited lifespan. Moreover, hybrid structures can provide a variety of ecosystem services in addition to coastal protection, like the provision of food and climate regulation. Hybrid engineering approaches can be applied in a variety of situations and ecosystems. Several examples exist- some are hundreds of years old and reflect local and/or traditional knowledge regarding ecosystem management.
Since 2008, the Building with Nature programme has been implementing hybrid engineering approaches in different contexts. Intact mangroves forests protect mud coasts by attenuating the height and strength of sea waves and by reducing the impacts of storm surges. In the long term, they provide protection by vertically building up the soils through the storage of organic matter and sediment. In addition, healthy mangrove forests provide a variety of ecosystem goods and services, such as fish, shellfish, fuelwood, fibres, water filtration and carbon storage. They are also an important nursery for commercially exploited offshore fish species. Healthy mangrove mud coasts are in a dynamic equilibrium, with sediment naturally eroding and accreting as a result of wave and tidal action. However, in most areas, the net effect of erosion and accretion is more or less stable. Nowadays, many tropical mud coasts face dramatic erosion. The conversion of mangroves into fish or shrimp ponds has led to a loss of their coastal protection function (Vermaat, J.E and Thampanya, U. (2006).
Mangroves reduce coastal erosion. IVM working paper. IVM Amsterdam). In some areas, the coastline has receded between 100 and 200 metres, jeopardising people’s homes and livelihoods. Aquaculture ponds are lost to the sea, and crucial infrastructure is damaged. Other ecosystem goods and services provided by mangroves are also destroyed. These problems are exacerbated by sea level rise and soil subsidence, caused by drainage, peat oxidation or water extraction from deep wells. In a healthy mangrove ecosystem, waves take sediment away and the tides bring sediment in.
The mangroves’ root system helps to capture and stabilize the sediment. The tidal flat is convex up, with a gentle slope and shallow water at the seaward edge of the mangrove forest. Hard structures, such as aquaculture pond bunds and breakwaters, disturb the balance of incoming and outgoing sediment. Waves reflect on the structure, becoming bigger and taking even more sediment away. The tide cannot bring enough sediment in, as it is blocked by the hard structure.
The tidal flat becomes concave up, with steep slopes, and deep water at the seaward edge of the mangrove forest. In order to stop the erosion process and regain a stable coastline, the first necessary step is to reverse the loss of sediment. More sediment needs to be deposited on the coast than the amount that washes away. The best way to do this is by working with nature, using smart engineering techniques-giving nature a little help, but letting it do the hard work for the people. Hybrid engineering combines these permeable structures (to break the waves and capture more sediment) with engineering techniques such as agitation dredging, which increase the amount of sediment suspended in the water.
Once the erosion process has stopped and the shoreline starts accreting, mangrove restoration can take place. The mangrove saplings are no longer washed away by the currents and a new mangrove belt can further break the waves and capture sediment in the long term. The hybrid engineering technique described above is applied in grids, to slowly but steadily reclaim land from the sea. This technique has been applied successfully in salt marshes in the Netherlands for centuries.
Hybrid engineering is being increasingly applied in vulnerable coastal areas across the world, replacing hard structures in a cost-effective manner. Mangrove management needs to maximize wave attenuation, for instance by opting for species with aerial roots and by aiming for a mix of trees of different age and size in order to continuously sustain and enhance coastal protection. Once mangroves are restored, they can offer multiple benefits besides coastal protection. Such benefits include carbon sequestration and storage, enhancing offshore fisheries, and enhancing aquaculture (on the landward edge).