Mangroves as Resilient Natural Coastal Defences

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Mangroves as Resilient Natural Coastal Defences

“The Role of Ecosystems in Disaster Risk Reduction” is a book published by the United Nation University press in 2013, supported by Ecosystem for Adaptation and Disaster Risk Reduction (PEDRR), IUCN, UNEP, CEM, UNU-EHS (Institute for Environment and Human security). The book generally acknowledges the importance of ecosystems especially to the local communities who depend on them for protection, economic well-being and recovery and evaluates the consequences of ignoring ecosystems in disaster risk reduction. It also offers a range of well-considered and practical solutions which could be used in many existing regulations, policies and risk reduction activities. The following is a brief review of section 4 of part II of this book written by Carmen Lacambra, Daniel A Friess, Tom Spencer and Iris Miller. It describes mangroves as resilient natural coastal defences.

Mangroves are an important coastal ecosystem that inhabits the dynamic intertidal zone. The ecosystem functions of mangroves vary across a range of time-scales. Any changes on this ecosystem have an impact on the broad socioeconomic and ecosystem services provided by this system. Mangroves provide a wide range of ecosystem services and help support the adjacent ecosystems and local livelihoods. For example, they support offshore fisheries owing to nutrient export across the food chain, they provide nursery grounds for juvenile fish and shellfish, they assimilate pollutants, they are source of wood products and they protect the coast and reduce disaster vulnerability especially to the local population.

In understanding mangroves as a bio-shield, it is important to understand that impacts of natural disturbances are part of an ecosystem’s life history and dynamics, but the ability of a system to move from an equilibrium state to a disturbance and recovery state is of value. These ecosystems have developed strategies that allow a response to disturbances and recovery to an equilibrium state. This is called ecosystem resilience and is essential to the sustained and long-term provision of ecosystem services in complex systems like mangroves.

Mangroves have the ability to cope with extreme events (like tsunamis, storms, hurricanes/ cyclones/ typhoons, lighting strikes and freshwater discharge) impacts and possess characteristics that mitigate the forces of these extreme events on coastal populations. However, such capabilities are limited by the degree of degradation which in turn affects their capacity to cope with multiple stressors. Mangroves play an important role in disaster risk reduction by attenuating the hydrodynamic energy up to a certain water level and wave height threshold. At a single plant scale, mangroves can reduce the hydrodynamic energy by first the bottom friction from the mud surface, secondly by drag forces owing to the complex morphology of the mangrove’s root system and foliage at higher water level and lastly by the eddy viscosity owing to turbulent water motion through the root gaps. It is also important to note that just like in saltmarshes, the effectiveness of hydrodynamic attenuation by mangroves is dependent on the species and on stem density.

There is still much to learn regarding the biophysical performance of mangroves during natural disturbances and other factors that may reduce the vulnerability of coastal population, despite mangroves’ role in disaster reduction, especially during the 2004 Asian tsunami. There is also the need to incorporate other ecosystems with the same ability to buffer the shorelines like wetlands to ensure effective protection of coastal communities.

Incorporation of ecosystems into disaster risk reduction can save lives, aid recovery and help build a more resilient and secure planet for all.

The original version of the book can be obtained online using: http://unu.edu/publications/books/the-role-of-ecosystems-in-disaster-risk-reduction.html#overview

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