Rowan Fraser

Swampy, tangled and low-lying, the mangrove is not a particularly flamboyant tree, as far as trees go. Indeed, it’s more of a shrub.  Somehow, it comes across as down-trodden and slightly oppressed.  It lacks the gusto and charm of most other species, and of course can’t compete with the big cats, rhinos or birds for panache in terms of biodiversity.  Nonetheless, over the past few years, the humble mangrove has seen its popularity soar.

The sound of ‘hurray’

In part this is due to the fortuitous confluence, as least for the mangrove, of the conservation, disaster risk reduction and climate change adaptation agendas, all of which find reason to value the mangrove.

For the conservationists, protecting mangroves is important for coastal biodiversity.  For the disaster risk reduction and climate change adaptation practitioners the mangrove effectively reduces the impact of coastal hazards on coastal communities.  In all three camps the mangrove is celebrated, even if conservationists don’t really care about risk, and risk managers don’t really care about biodiversity.  A case of mutual, and rather vigorous back-scratching and ideological convenience.

This has a particular implication for coastal cities, usually adroit at pursuing ruinous policies towards local mangrove forests. Asia, which holds approximately 41 per cent of the world’s mangrove forests, has seen massive declines in stock.  In Southeast Asia, for example, over 50 per cent of forests have been razed over the past 50 years – mainly due to economic, industrial and urban development.  In response to growing focus on mangrove replanting, a few cities in the region have been engaging in mangrove replanting schemes. These schemes tend to focus on the risk management capacity of mangroves, or at least join conservationist goals to risk management goals.


Return of the mangrove in Hai Phong

Vietnam, for example, has ample coastline, numerous costal cities and lots of storms. No surprise then that a number of initiatives have been taking place along the coast.  Hai Phong City is Vietnam’s third largest city (1.9 million people in 2012), and northern Vietnam’s most important seaport, located at the mouth of the Cam River.  The city’s population is growing quickly (approximately 4 per cent per year recently) and its economy contributes around 11 per cent of national GDP (mainly rice farming, fisheries, export, some industry).  The city’s name means ‘coastal defense’ – not the aptest of meanings given the regular typhoons which rack the city’s coast, damaging the 3.9 kilometers of dike designed to protect the city inland.  At least until recently.

Mangrove afforestation along Hai Phong’s coast has been jointly achieved by the public, local authorities and the Red Cross.  An ambitious programme of replanting over the 2000s  has seen the forest cover swell to 450 hectares in 2011.  The forest now stretches in a continuous band the length of the dike, averaging 1 – 1.5 kilometers in width.

This project is part of the International Federation of Red Cross and Red Crescent Societies (IFRC) ‘Community-based Mangrove Reforestation and Disaster Preparedness Programme,’ which has seen afforestation projects in other coastal cities in Vietnam.  The  project has been analyzed, both by the IFRC and by independent specialists.  I caught up with Dr. Anna McIvor, Coastal Research Unit, University of Cambridge, to ask her about her views and findings.

How effective are mangroves at coastal protection?

‘Lucky is perhaps not the best way to put it,’ McIvor notes, ‘but after the replanting project had finished in some areas, a typhoon struck the coast and the Red Cross was able to measure the degree of damage and compare it with earlier typhoons.  Of course, different sections of the coast are never totally comparable but it was possible to get a clear idea of the effectiveness of the mangroves at reducing damage.’

Disaster risk reduction is like airport security: it’s considered effective when nothing out of the ordinary happens.  No one dies, nothing gets damaged.  Measuring effectiveness is therefore somewhat difficult, and the proverbial thorn in the side of risk management practitioners – at least from an advocacy point of view.  Added to this, disasters don’t happen in labs, and being an emergency it’s often hard to get organized and study the effects.  Clear information on effectiveness is therefore sorely needed.

‘Mangroves are more effective at reducing waves, and less effective at reducing storm surges,’ McIvor says.  I ask her what the difference is, exactly.  ‘Waves’ she says, ‘are quick rises in water level caused by wind.  Storm surges, on the other hand, are rises in water level caused by storms and can last for several hours.’

Let’s put some numbers on this. For waves, 500 meters of mangroves can reduce the height of the wave by 50 – 100 per cent, according to McIvor.  But for storm surges, one kilometer of mangroves will only reduce water height by 50 centimeters. Effectiveness of mangroves is therefore dependent on the width of the forest and the type of hazard.

I ask McIvor if the density of the forest also has an impact on effectiveness.  ‘The density of trees affects how rapidly waves are reduced as they pass through forests.  And so dense forests are likely to be most effective at reducing waves and storms surges’ she says.

In the case of Hai Phong city, where 1 – 1.5 kilometers of mangroves have been planted, wave reduction is considerable.  Mangroves are also good at reducing storm surge, just less so that at reducing waves.  But given that storms and waves typcially come together, what is the best way to combat them both?

Concrete and plants for resilience

‘Waves and storms certainly come together,’ McIvor confirms. ‘As a result, we promote the use of hybrid structures for coastal defense.’  Such structures are combinations of hard infrastructure and natural ecosystems.  ‘The mangroves reduce the power of the waves and therefore protect the seawall or dike from damage, but it’s the seawall or dike which actually stops the storm surge.  Without the mangroves, the dike is easily damaged and breached by the waves, and well, then its much less effective at stopping the storm surge,’ she says.

With increasing interest in ecosystems for disaster risk reduction and climate change adaptation, cities increasingly need clear numerical guidance.  ‘Part of our research goes toward providing specific numbers to engineers and policy makers.  If the mangrove forest is 500 meters wide, for example, how high should the wall be, how thick?  At 1 kilometer of forest?  We are starting to get a clearer idea about what needs to happen and what is possible.’

This kind of hybrid model is growing in favor.  Saltmarshes are another ecosystem which have been shown to be effective at reducing the impact of waves (and the current darling on the NYC administration following Hurricane Sandy).  Reforestation of hill slopes around cities and other replanting schemes for soil stability can bring massive benefits in terms of reducing landslide risk.

In this way, disaster risk reduction is providing a very good argument to policy makers for increasing the preservation of ecosystems.  But beyond the ideological convenience of joining concrete and plants for coastal protection is the growing body of research demonstrating the synergies and effectiveness of doing so – and with that comes monetary advantages.  The mangrove forests at Hai Phong in Vietnam, for example, have been demonstrated to save the dike wall (and therefore the local governments) USD 300,000 in damages during various typhoons since the replanting was completed.

Not bad for this humble shrub.


Banner photo by Rianni

Photo 1 by Ceratocentron

Photo 2 by HoangTuanAnh

Photo 3 by TreesForTheFuture



Categories: Infrastructure, Resilience, Safety and Security