Understanding Coastal Risks, With And Without Mangroves

• A new study computes the ability of mangroves in Pichavaram and Muthupet in Tamil Nadu, to mitigate coastal exposure to environmental hazards such as storms, cyclones and floods.
• The researchers also conducted inundation modelling studies for three different sea level rise scenarios — low, medium and high— to calculate resulting changes in mangrove covers.
• Monitoring the impacts of human activities on mangrove deforestation, analysing the effectiveness of current policies and governance, including financial mechanisms for conservation, and focusing on genetic diversity is important, say experts.

Mangroves in Pichavaram and Muthupet in Tamil Nadu significantly reduce the magnitude and occurrence of extreme coastal hazards such as tsunamis, cyclones, coastal floods etc., finds a recent study by researchers from the School of Public Policy, Indian Institute of Technology – Delhi (IIT-Delhi), along with researchers from The University of Tokyo and Institute for Global Environmental Strategies in Japan. The study analyses the impact of these mangroves in the southern state in reducing coastal exposure while also warning of significant loss of these habitats by the end of the century due to increase in sea levels.

Mangroves, globally, play a critical role in climate resilience. They are capable of extracting five times more carbon than forests on land. Halophytic or salt-tolerant plants, that can grow in the intertidal zones, mangroves are the first line of defence for coastal regions, protecting them from various natural disasters.

The presence of the mangroves in Pichavaram reduces the region’s coastal exposure value from 3.47 to 2.80 and Muthupet’s exposure value from 4.78 to 2.10, notes the study published in Progress in Disaster Science. The values are expressed on a scale of one to five, where 5 indicates ‘very high’ exposure and 1 indicates ‘very low’ exposure.

The study addresses two key gaps in the existing literature — evaluating the role of mangroves in reducing coastal vulnerabilities and the impact of sea level increases on future ecosystem availability.

Mitigation impacts of mangroves

Located between the Vellar and Coleroon estuaries in the Cuddalore district of Tamil Nadu, Pichavaram contains the second-largest mangrove forest in the world. It became an exemplar of how mangroves protect people and places from the devastating consequences of natural disasters following the Indian Ocean tsunami of 2004. In the immediate aftermath of the calamity, researchers at the Centre of Advanced Study in Marine Biology, Annamalai University, conducted a study, examining the impacts on 18 hamlets occupying a 25-km coastline in Parangipettai, Tamil Nadu. The lowest death toll was observed in hamlets that were situated behind mangrove forests or in elevated places with steep shorelines. The per capita loss of wealth in these hamlets also showed a similar trend, underscoring the critical importance of mangrove vegetation in mitigating the impacts of the tsunami.

Similar reports also emerged from Muthupet, which is located in the state’s Tiruvarur district, in the southernmost region of the river Cauvery’s delta. Despite facing significant damage, Muthupet mangroves also saved lives in the region during the 2018 cyclone, Gaja.

Both Pichavaram and Muthupet house several mangrove species, with 75% of the Muthupet area being home to diverse mangrove flora and Pichavaram providing a thriving environment for 12 exclusive species. The most dominant mangrove species in Pichavaram include Avicennia marina, Avicennia officinalis, Rhizophora apiculata and Rhizophora mucronata. Avicennia marina is the most common in Muthupet too. Other mangrove species in Muthupet include Exoecaria agallocha, Acanthus ilicifolius and Aegiceros corniculatum. These regions are also home to several other floral, aquatic, avian and terrestrial species.

Rajarshi Dasgupta, Faculty at the School of Public Policy, IIT-Delhi and one of the authors of the study, shares that the research is a result of ongoing collaborations with the Tamil Nadu Forest Department supported by the Japan International Cooperation Agency (JICA). “With the availability of required geospatial data on the Pichavaram and Muthupet mangroves and considering their active role in mitigating the impacts of the tsunami, the team felt that modelling studies from these regions will aid in devising specific intervention strategies,” shares Dasgupta.

The team used the open-source InVEST (Integrated Valuation of Ecosystem Services and Tradeoffs) software models to compute the shoreline’s exposure to various physical hazards. These tools have been developed by Stanford University’s Natural Capital Project and encourages collaborative efforts to better understand the value of natural resources.

“The InVEST project provides tools to measure ecosystem services; simply put, benefits that we get from mother nature. One way to look at it is to add a price tag for these benefits. For example, mangroves purify the air around them (by filtering and trapping pollutants), and these purification services reduce the costs of environmental damage by a certain measure. However, it is not always possible to add a value to these services and certain aspects of these ecosystems, such as nutrient cycling or protection of communities against natural disasters, are priceless. So, we decided to focus on biophysical and geophysical quantification, where we try to understand how a coast with and without mangroves differs in terms of risk and potential damage,” Dasgupta explains.

Using data such as topographic features, landmass, depth of water, wave forecasting, geomorphology, natural habitats, population data, administrative boundaries, seabed-related information and sea level rise, the team divided the coastline into 100-metre patches, examining the mitigation impacts of mangroves on each patch.

The results show that in the presence of mangroves, the coastal exposure of Pichavaram mainly ranges from ‘very low’ to ‘moderate’ exposure. However, without the habitat, the entire coastal stretch falls under ‘high’ exposure conditions. While Muthupet mainly exhibits ‘high’ exposure conditions with the mangroves, the lack of habitat catapults more than 60% of the coastline to ‘very high’ exposure conditions.

Further, the team also conducted inundation modelling studies for three different sea level rise scenarios — low, medium and high— to calculate resulting changes in mangrove covers. In the worst-case scenario (1.27-metre sea level rise), the study reveals that about 58% of Pichavaram and 48% of Muthupet will be inundated, leading to a corresponding loss of 0.39 and 2.28 Tg of carbon storage from these mangrove ecosystems, respectively.

“By creating a 100-metre resolution coastline map that demarcates areas where the potential damage risk is very high, we can establish no-go zones as an immediate means of protection. Also, the state government is taking significant steps to plant new mangroves and these maps can pave the way for strategic planting to mitigate future risks,” adds Dasgupta.

A roadmap for future research

While the study provides some much-needed insights into the role of mangroves in mitigating natural disasters and the threats faced by the ecosystem, it does look at the coastline in a static manner, which serves as a drawback, explains Dasgupta. “Coastlines are built through erosions and accretions, but for the sake of this study we have considered a snapshot of the way things exist at the moment,” he adds.

Dasgupta and Shizuka Hashimoto, Professor at the Graduate School of Agricultural and Life Sciences, Department of Ecosystem Studies, University of Tokyo and a co-author of the study, share that further research should factor in changes to the coastline and its dynamic nature for a more nuanced understanding.

“Key areas where further research is needed include continued monitoring and analysis of the impacts of human activities on mangrove deforestation for purposes such as aquaculture and urbanisation, and analysis of the effectiveness and limitations of current policies and governance, including financial mechanisms for conservation,” shares Hashimoto.

Kathiresan, Honorary Professor, Annamalai University, and Member, IUCN Mangrove Specialist Group who is not a part of the study, shares that the research comes at a crucial time when the mangroves of south India need our immediate attention. In May 2024, the International Union for Conservation of Nature (IUCN) published the first global assessment of an ecosystem functional group, the Global Red List of Mangrove Ecosystems. The report shares that 50% of the world’s mangroves are at a risk of collapse and designates south India’s mangroves as critically endangered.

“Mangroves require some amount of freshwater influx for survival and most of the mangroves in south India are located in the Cauvery delta. Due to the poor influx of freshwater in this region, the salt content of the soil is increasing, and the high salinity is a significant factor contributing to the degradation of these mangroves,” explains Kathiresan, who co-authored the Red List Assessment of Mangroves of South India and Sri Lanka, and Maldives.

He further adds that the Tamil Nadu Forest Department, along with the local communities, is using artificially made canals to flush water and reduce the salinity thereby addressing this concern. “Additionally, along with JICA’s support, mangrove restoration projects are also being carried out,” shares Kathiresan, who is also the Mangrove Ecosystems Expert for the Tamil Nadu Biodiversity Conservation & Greening Project for Climate Change Responses (TBGPCCR).

Apart from the ongoing measures, Kathiresan emphasises that the key to mangrove conservation for climate resilience is to focus on biodiversity. “Mangrove plantation initiatives often focus on a few species like Rhizophora and Avicennia as they grow rapidly. This results in a loss of biodiversity. Mangrove conservation must factor in biodiversity and genetic diversity so we can achieve coastal resilience for various future challenges,” he says, suggesting other hardy species such as Bruguiera gymnorhiza, Avicennia officinalis, Excoecaria agallocha and Agecieras corniculatum for Tamil Nadu. “Additionally, rare species such as Sonneratia apetala, Xylocarpus granatum, Kandelia candel and Lumnitzera racemosa can also be considered, and the Forest Department is making significant efforts in this regard (to protect rare species),” he adds.

(Published under Creative Commons from Mongabay India. Read the original article here)