Along the Asian coastlines, there are many areas where rural communities experience alarming rates of sea level rise due to land subsidence. Sometimes up to 10 cm per year. This causes tremendous challenges in living there and keeping coasts protected. With that in mind, a team of Indonesian and Dutch scientists jointly investigated the potential and limitations of mangrove restoration as a cost-effective and sustainable solution for coastal protection in cases where coastlines were rapidly subsiding. They have published their findings in Nature Sustainability.
Unfortunately, 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 is a logical solution to reverse this process and protect these densely populated coastlines. However, this requires understanding if mangroves can cope with the extreme rates of relative sea level rise which impacts the coasts of these areas.
I saw how much the people suffered from coastal erosion and frequent inundation. To be able to answer if mangroves can help, I started with obtaining reliable measurements of the subsidence rate. This is extremely complicated if you work remotely.
Local land subsidence causes high relative sea level rise, but more in villages than in mangroves
Typically, measuring subsidence requires expensive complicated equipment. These tools are lacking in these remote areas, so we developed two novel and low-cost methods to approximate relative sea-level rise. In the mangroves, we measured the sea level rise with simple pressure gauges, that are 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. Doing so we demonstrate how 20 km of rural coastline and its vegetated foreshore, neighboring a rapidly subsiding city, were experiencing an alarming rate of sea level rise. Surprisingly, households experienced much higher rates of sea level rise than mangrove forests, with dramatic consequences for local communities.
Two ways local communities ‘cope’ with high relative sea level rise
Via 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)t. Flood-prone coastal communities may not always be able to move to higher ground due to financial or social limitations, such as landownership and income sources (for instance fisheries) which may bind families to the coast.
Nature-based solutions can protect us and mangrove restoration is possible
Mangroves offer coastal protection by preventing coastal erosion and attenuating waves moving to the coast. This protection will, however, only work if there is enough mangrove forest to do the job. My research showed that mature mangroves showcase an extremely high level of tolerance to subsidence and the resulting ‘experienced’ rapid sea level rise.
However, as noted by Professor Helmi from Diponegoro University, an adequate supply of sediment needs to be available along the shore. Unfortunately, the latter is not the case near Semarang. 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 around the world”.
Outlook for strongly subsiding areas
Overall, the present study highlights the urgency of addressing land subsidence as a critical factor influencing coastal vulnerability. In areas where the experienced relative sea level rise caused by subsidence is not compensated by sufficient sediment supply, the mangroves’ ability to stabilize the coastline will deteriorate, resulting in the gradual inland migration of the mangrove forest. Rural communities are left with little choice but to also retreat landward. In this respect, the present study offers a future perspective on the fate of global coastal communities under accelerated global sea level rise.
Helmi agrees that 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. By showing the intricate dynamics between mangroves and their environment, my findings contribute to developing effective strategies for mitigating the impacts of these pressing issues.
Climate-proofing our coasts asks for international and interdisciplinary collaboration
My colleague, Tjeerd Bouma from NIOZ and Utrecht University, notes how important international partnership is to this kind of work and that this glimpse into the future has only been possible because of the unique international and interdisciplinary collaboration. Only by bringing together ecologists, coastal physicists, and sociologists from Indonesia and the Netherlands, it was 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 the development of 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.
Subsidence reveals potential impacts of future sea level rise on inhabited mangrove coasts
Nature Sustainability, August 2023
C.E.J. van Bijsterveldt, Peter M.J. Herman, Bregje K. van Wesenbeeck, Sri Ramadhani, Tom S. Heuts, Corinne van Starrenburg, Silke A.J. Tas, Annisa Triyanti, Muhammad Helmi
Celine van Bijsterveldt, NIOZ Researcher