Sustainable protection of rapidly subsiding coastlines with mangroves

Along the coastlines of Asia, many rural communities are experiencing alarming rates of sea level rise due to land subsidence of up to 10 cm per year. This causes tremendous challenges on how to live there and protect these coasts. A Dutch-Indonesian team of scientists has investigated the potential of mangrove restoration as a cost-effective and sustainable solution for coastal protection in rapidly subsiding areas. They have published their findings in Nature Sustainability.

Vulnerable coastlines

Unfortunately, precisely in these rural densely populated Asian regions, mangroves have in the past been cleared to free up land for other uses such as aquaculture. This has made these coasts vulnerable to rapid erosion. Restoring mangroves seems a logical solution to reverse this process and protect these densely populated coastlines. However, this requires understanding if mangroves can cope with extreme rates of relative sea-level rise, as experienced in these subsiding areas.

Local land subsidence causes high relative sea level rise

“To be able to answer if mangroves can help, we started with obtaining reliable measurements of the subsidence rate. This is extremely complicated if you work so remotely,” says Celine van Bijsterveldt, NIOZ researcher and lead author of the study.

“Typically, measuring subsidence requires expensive, complicated equipment. As such instruments are lacking in these remote areas, we developed two novel and low-cost methods to approximate relative sea-level rise. In the mangroves we measured the sea level rise by simple pressure gages normally used to measure tides. And in the village we analyzed how often people increased the height of the floor and the roof of their houses. We demonstrate how 20 km of rural coastline and its vegetated foreshore, neighbouring a rapidly subsiding city, were experiencing alarming rate of sea level rise.” Surprisingly, villages experienced much higher rates of sea level rise than mangrove forests, with dramatic consequences for local communities.

The real challenge lies in our human systems and their interaction with the natural environment, particularly in how we pursue development

Two ways local communities ‘cope’ with high relative sea level rise

“Through interviews, we learned that local communities may respond in two ways to the experienced sea level rise: fight (keep the water out by raising the house) or flight (move further land inward or elsewhere)” says Van Bijsterveldt. Flood-prone coastal communities may not always be able to move to higher grounds. Financial and/or social limitations, such as landownership and income source (for instance fisheries) may bind families to the coast.

Nature-based solutions make mangrove restoration possible

Mangroves offer coastal protection by preventing coastal erosion and attenuating waves moving to the coast. Such protection will, however, only work if there is enough mangrove forest to do the job. The researchers discovered that mature mangroves showcased an extremely high tolerance to subsidence and the resulting ‘experienced’ rapid sea level rise.

But an adequate supply of sediment needs to be available along the shore. “Unfortunately, the latter is not the case near Semarang,” says Professor Helmi from Diponegoro University. “But this finding is very promising for many less fast subsiding rural areas along muddy coast, as we can find in Indonesia and many other places worldwide”.

Outlook for strongly subsiding areas

Overall, the study highlights the urgency of addressing land subsidence as a critical factor influencing coastal vulnerability. It also shows that nature, such as coastal mangroves, can be incredibly resilient in the face of climate change and contribute to shoreline stabilization. “The real challenge lies in our human systems and their interaction with the natural environment, particularly in how we pursue development,” explains Annisa Triyanti, assistant professor at the Copernicus Institute of Sustainable Development, Utrecht University. “In the context of Demak, Northern Java's coastal area, and many other coastal rural communities without coastal protection, halting land subsidence by regulating subsurface fluid extraction, especially in adjacent urban areas, is essential for coastal resilience.

Only by bringing together ecologists, coastal physicists and sociologists from Indonesia and the Netherlands was it possible to get an integrated perspective on the whole coast.

“This study provides a glimpse into the future for poor rural areas on the coast struggling with experiencing accelerated sea level rise due to subsidence,” says Professor Helmi from Diponegoro University. “By showing the intricate dynamics between mangroves and their environment, our findings contribute to developing effective strategies for mitigating the impacts of these pressing issues.”

International and interdisciplinary collaboration

“This glimpse into the future has been possible because of a unique international and interdisciplinary collaboration,” says Professor Tjeerd Bouma from NIOZ and Utrecht University. “Only by bringing together ecologists, coastal physicists and sociologists from Indonesia and the Netherlands was it possible to get an integrated perspective on the whole coast. We are also grateful for the active support of NGOs and companies willing to invest in developing critical knowledge for climate-proofing our coasts.”

This research project is part of a collaboration of the Royal Netherlands Institute for Sea Research (NIOZ), Utrecht University, Wageningen University, Deltares, TU Delft, Wetlands International and the Indonesian Diponegoro University. The research was financed by NWO-domain Applied and Engineering Sciences (project# 14753), and co-financed by Boskalis Dredging and Marine experts, Van Oord Dredging and Marine Contractors bv, Deltares, Witteveen & Bos and Wetlands International.