Protecting mangroves, to deal with cyclones

Mangroves of the Sundarbans. (Source: Nature Environment & Wildlife Society - NEWS)

Coastal flooding is rising in India and recent evidence shows that as high as 36 million Indians will be at the risk of chronic flooding by 2050. The Indian coastline extends over 7,500 kmts across nine states, two Union territories and two island territories — Andaman & Nicobar and Lakshadweep.

The east coast has historically been more vulnerable to cyclones than the west coast. According to the Indian Meteorological Department, the Bay of Bengal has had 520 cyclones between 1891 and 2018, compared with 126 in the Arabian Sea.

Indeed, the list of cyclones that India has experienced is long with intense cyclones from 1999 to 2020 including the very recent Amphan and others like Kyarr, Maha, Vayu, Fani, Gaja, Titli, Okhi, Varada, Hudhud, Phailin, Helen, Neelam, Phyan and the Odisha cyclone that left a trail of destruction along the coastal states in India.

While factors such as rapid coastal development, population growth, climate change and habitat loss are the main reasons for coastal flooding, an increasing need has been identified to adopt flood mitigation and adaptation strategies to reduce the socio economic and health impacts of coastal flooding. Evidence shows that mangroves can serve as the first line of defense against flooding and erosion in many tropical and subtropical regions and help by reducing waves and storm surges.

What are mangroves

Mangroves can be trees, shrubs, ferns and palms that occupy the boundary between the land and the sea. They mainly grow in or adjacent to areas between the high tide and the low tide. They get regularly covered or immersed in water at high tide and exposed to air at low tide.The roots of mangroves are regularly exposed to saline water. At times, they are also exposed to freshwater surface runoffs and flooding. Mangroves get their nutrition from these tidal saline and freshwater resources and coastal soils and silt that get deposited from the surrounding land after an erosion.

Mangroves cope with coastal flooding

The paper The global flood protection benefits of mangroves published in the journal Nature, Scientific Reports informs that mangroves help in coping with floods by acting as barriers through factors such as bottom friction, the cross-shore width of forests, tree density and shape, which can help in reducing the force of flood waves as they pass through the mangrove forests.

The aerial roots of mangrove forests retain sediments and stabilise the soil in the areas between high tide and low tide (intertidal areas) by reducing erosion during storms and floods. The roots, trunk and canopy of the mangroves can dissipate storm surges and waves. Studies show that mangroves can reduce up to 66 percent of wave energy in the first 100 m of forest width. Mangroves can also cope with sea level rise through gradual vertical growth.

Economic value of mangroves

Threats to mangroves are many. Mangroves world over are declining from 139,777 km2 in 2000 to 131,931 km2 in 2014 because of conversion for aquaculture or agriculture and coastal development. Destruction of mangroves can thus greatly increase coastal risk to infrastructure, livelihoods and lives.

However, the economic value of mangroves for services such as flood protection, is not included within national budgets and wealth accounts in contrast to other services such as timber production. Also, most assessments of the value of mangroves use a benefit transfer (i.e. estimate economic values for ecosystem services by applying available information from studies already completed in one location and/or context to another) or replacement cost method (estimating the costs of replacing mangrove forests by constructing physical barriers to perform the same services) instead of process-based methods that take into consideration local variations in characteristics of storms, mangrove habitat, topography and understanding of the variations in water bodies such as oceans, rivers and lakes.

Understanding and quantifying the contribution of mangroves is crucial for encouraging their conservation and restoration for the benefit of nature and people. This is because the capacity of mangroves to act as natural defenses can vary considerably depending on environmental factors ranging from the sources of flooding in the ocean to mangrove characteristics, coastal topography and also the inland receptors of damage.

The paper presents the findings of a study that aimed at assessing the total expected annual benefits of mangroves considering both cyclonic (tropical cyclones) and non-cyclonic (regular) conditions. Global mangrove benefits were quantified by estimating the difference in flood damages between two scenarios that included  damages with mangroves and without mangroves.

The study found that:

Mangroves are best at fighting tropical cyclones globally

Approximately 90 percent of total benefits of mangroves are for protection from tropical cyclones, while 10 percent are from protection from regular (non-cyclonic) conditions.

For example, mangroves can reduce annual expected flood damages from tropical cyclones by $US 60 billion and protect 14 million people globally. The benefits from mangroves increase as the time between cyclonic events increases and become even more significant during the more intense flood events which can cause significant damage.

If mangroves were not there, property losses produced by 1-in-100-year flood events would increase by 37 million people and US$ 270 billion. Mangrove benefits for tropical cyclones increase sharply after reaching a storm intensity associated to the 1-in-50-year return period events.

Mangrove benefits vary by region

The study finds that flood protection benefits of mangroves vary significantly across regions and countries due to differences in flood characteristics, mangroves expanse and the degree of exposure. Mangroves provide the greatest benefits in the Western Pacific and Caribbean islands.

The countries that receive the greatest annual economic benefits in terms of high value and protection of coastal assets include United States, China, India and Mexico while Vietnam, India and Bangladesh benefit the most from mangroves in terms of people protected due to the high density of coastal populations in these countries.

The national importance of mangroves for flood protection varies considerably when calculated as a percentage of national GDP. Mangroves provide critical flood protection benefits in countries with lower GDPs  like in Mozambique and Bangladesh, which receive over $US 1 billion in benefits annually from mangroves due to the high number of assets being concentrated in exposed and vulnerable coastlines.

Mangroves help at the local level by protecting cities

Mangroves protect several coastal cities and a considerable number of people from flooding annually.

Mangroves protect more than 150,000 people from flooding every year in Abidjan and Lagos in West Africa, Mumbai and Karachi in South Asia, Wenzhou in East Asia, and Cebu and Denpasar in South-east Asia.

In some cities like Miami in the U.S.A and Cancun in Mexico, mangroves provide more than $US 500 million in avoided property damages every year. Mangrove benefits extend to less populated coastal floodplains as well.

The study provides important insights on identifying areas where restoration efforts need to be prioritised. For example, while mangroves provide benefits throughout the Philippines, these values are higher in the central and northern regions of the country, as they are the areas that receive the greatest annual impact from typhoons. Mangroves also provide benefits in densely populated lowland areas, such as in the Ganges-Brahmaputra delta in India and Bangladesh, in the Mekong delta in Vietnam or in the Amazon delta in northern Brazil. These regions are highly sensitive to climate hazards and therefore need specific risk reduction strategies. 

While mangroves continue to disappear at rapid rates around the world, the study demonstrates the urgent need to protect and conserve mangroves where they still exist, by quantifying their value in terms of economic benefits to people and property globally. With climate change, the intensity and frequency of the events like cyclones, floods are predicted to increase thus highlighting the important role of mangroves in averting damage to lives and livelihoods in the future.

A copy of the paper can be accessed from here