Mangroves for coastal defence
Guidelines for coastal managers & policy makers

© 2014 Wetlands International and The Nature Conservancy

Published by
Wetlands International and The Nature Conservancy

Suggested Citation
Spalding M, McIvor A, Tonneijck FH, Tol S and van Eijk P (2014) Mangroves for coastal defence. Guidelines for coastal managers & policy makers.
Published by Wetlands International and The Nature Conservancy. 42 p

About The Nature Conservancy
The mission of The Nature Conservancy is to conserve the lands and waters upon which all life depends. For general information, visit: www.nature.org.
For more information about the Mapping Ocean Wealth project, visit: www.oceanwealth.org.

About Wetlands International
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organisation, active in around 100 countries, which works through a network of many partners and experts to achieve its goals. For more information, visit
www.wetlands.org.

Acknowledgements
We gratefully acknowledge Mike Beck, Jan van Dalfsen, Joanna Ellison, Filippo Ferrario, Dolf de Groot, Evamaria Koch, Catherine Lovelock, Karen McKee, David
McKinnie, Denise Reed, Pam Rubinoff, I. Nyoman Suryadiputra, Norio Tanaka, Trevor Tolhurst, Mai Sỹ Tuấn, Ty Wamsley, Bregje van Wesenbeeck, Jo Wilson, Han
Winterwerp, Eric Wolanski and Colin Woodroffe for their input into the early stages of this research, which they gave either through correspondence, or at the
Mangroves as Coastal Protection workshop held in Bogor, Indonesia, 19-22 January 2012, or the Natural Coastal Protection workshop help in Cambridge, UK,
27-29 March 2012.

Credits
This publication was made in the framework of the project Mangrove Capital: `Capturing Mangrove Values in Land Use Planning and Production Systems´,with support from the Ministry of Foreign Affairs the Netherlands/ DGIS.
This work forms part of the Nature Conservancy’s Mapping Ocean Wealth project, a collaborative work to quantify the value of coastal and marine
ecosystem services at global to local scales – www.nature.org/oceanwealth. This work is supported through a lead gift from the Lyda Hill Foundation.
All drawings were developed by Joost Fluitsma / Jam Visual Design.
Photo front cover: Calm water amidst mangrove roots, Berau, Indonesia, by Mark Spalding
Phot back cover: People and property facing coastal hazards in Indonesia, by Stefan VerschureMangroves for coastal defence
Guidelines for coastal managers & policy makers
Waves

Storm Surges

Tsunamis

ErosionSea level rise

Authors: Mark Spalding, Anna McIvor, Femke Tonneijck, Susanna Tol and Pieter van Eijk

Contents
Preface5

Executive summary

6

Section 1. Is my shore at risk?

8

Section 2. The role of mangroves in coastal risk reduction

13

2.1 Mangroves reduce wave damage	

14

2.2 Mangroves reduce damage from large storms 	

16

2.3 Mangroves can help to reduce tsunami damage	

18

2.4 Mangroves reduce erosion and bind soils together	

20

2.5 Mangroves may keep up with sea level rise 	

22Section 3. Managing mangroves for coastal defence24

3.1 Integrating mangroves into coastal defence strategies	24

3.2 Mangroves as part of coastal zone management 	28

3.3 Bringing the mangroves back	

31

Section 4. Recognising the multiple values of mangroves34

Asmat people entirely dependant on mangroves,
Indonesia, by Wetlands International

Destruction after typhoon Hayan in the Philippines, by Cordaid

Preface
The role of mangroves in protecting our coasts against natural hazards such as storms, tsunamis
and coastal erosion has been widely acknowledged. Even so, the level of protection provided bymangroves remains subject to debate. Numerous mangrove restoration projects were instigatedafter the 2004 East Asian tsunami in the belief that replacing lost mangroves would reduce futurerisk, but others raised concerns that not all of these projects were well conceived, and that somemight create greater risks by inducing a false sense of security. Can mangroves reduce waves
and storm surges? How will they influence the forces of a tsunami? Do they actually contributeto stabilizing coasts and build-up of soils? Can they keep up with sea level rise? A rich scientific
literature exists describing many of these processes, but careful scrutiny is needed to determine
what is known or unknown, and what remains uncertain.
The Nature Conservancy and Wetlands International together with the University of Cambridge
set out to map the current state of knowledge about the role of mangroves in coastal defence
and put the different findings and views in perspective. An extensive review process yielded
three technical reports that describe the extent to which mangroves reduce wind and swell
waves, storm surges and erosion and how they build up soils in response to rising sea levels. The
conclusion is that mangroves can indeed reduce risk from a large number of hazards.
This practical guidebook summarises the findings of the reviews and provides practicalmanagement recommendations to coastal zone managers and policymakers. It helps the reader
to assess the risk context in a target area, to define hazard-specific mangrove management
interventions and to incorporate these in risk reduction strategies, climate change adaptation
protocols and broader coastal development planning. Case studies provide practical examples ofmangrove management approaches and references to background information, practical tools forrisk assessment and mangrove management are provided throughout the book.

restoration
tsunamibuild and bind soils

storm surges

risk reduction aquaculture
vulnerability
sediment waves coast subsidenceplanning

fresh water flowmangrovesea level riseconservation wind Building with Naturegreenbelt community management
erosion resilience roots

Preface

Page 7

Executive summary
The role of mangroves in protecting our coasts against natural hazards such as storms, tsunamis and coastal erosion has been widely promoted. But the supposed
coastal protection services of mangroves have also been subject to debate. The Nature Conservancy and Wetlands International together with the University ofCambridge set out to map the current state of knowledge about the role of mangroves in coastal defence and put the different findings and views in perspective. The
conclusion is that mangroves can indeed reduce the risk from a large number of hazards. This practical guidebook summarises the findings of the reviews and provides
practical management recommendations to coastal zone managers and policymakers. These are the key messages:
Section 1. Is my shore at risk?
• Coastal managers need to understand risk in terms of hazard, exposure and vulnerability prior to determining what role mangroves can play
• The importance of mangroves in coastal defence and disaster risk reduction will depend both on the site characteristics and the local hazard context.
Section 2. The role of mangroves in coastal risk reduction• Wind and swell waves are rapidly reduced as they pass through mangroves, lessening wave damage during storms.
• Wide mangrove belts, ideally thousands of meters across, can be effective in reducing the flooding impacts of storm surges occurring during major storms (alsocalled cyclones, typhoons or hurricanes). This can significantly reduce flood extent in low lying areas. Narrower mangrove belts, hundreds of meters wide, will
still be able to reduce wind speed, the impact of waves on top of the surge and flooding impact to some degree.
• Wide areas of mangroves can reduce tsunami heights, helping to reduce loss of life and damage to property in areas behind mangroves.
• The dense roots of mangroves help to bind and build soils. The above-ground roots slow down water flows, encourage deposition of sediments and reduce erosion.
• Over time mangroves can actively build up soils, increasing the thickness of the mangrove soil, which may be critical as sea level rise accelerates.
Section 3. Managing mangroves for coastal defence• Mangroves don’t always provide a stand-alone solution; they may need to be combined with other risk reduction measures to achieve a desired level ofprotection. If they are integrated appropriately, mangroves can contribute to risk reduction in almost every coastal setting, ranging from rural to urban and fromnatural to heavily degraded landscapes.
• For mangroves to optimally contribute to risk reduction, their conservation needs to be incorporated into broader coastal zone management planning: they need
to be protected and restored, allowing wise use where possible.
• Mangroves, and their coastal risk reduction function, can recover in most places where appropriate ecological and social conditions are present or restored.
Section 4. Recognizing the multiple values of mangroves• Mangroves are among the most valuable ecosystems in the world. Decision makers, and the public, need to take full account of the many benefits that mangroves
provide, and consider the true costs that may incur from mangrove loss.

Page 8

Executive Summary

Natural mangrove in Senegal, by Wetlands International

Section 1. Is my shore at risk?
To understand whether mangroves can help to reduce risk on a particular stretch of coast, it is first necessary
to assess the risk context in the target area.

Risk is a combination of the occurrence of given hazards (frequency, intensity)and the exposure and vulnerability of people to those hazards. Coastal
lands are subject to multiple natural hazards such as erosion, wave damage,tsunamis, storms and flooding, and over the long-term, rising sea levels.
Such hazards may threaten lives, livelihoods, property, health and economic
development.
Exposure refers to the extent to which people and their property are situatedin hazard zones and therefore face potential losses from extreme events.
Vulnerability can be defined as the characteristics of a community, system orasset that make it susceptible to the damaging effects of a hazard. Disaster
Risk Reduction aims to reduce the damage caused by natural hazards likeearthquakes, floods, droughts and cyclones, by taking preventive actions.
Figure adapted from the National Institute of Water and AtmosphericResearch New Zealand www.niwa.co.nz
How can I tell which hazards affect my area?
To understand the local threats from the sea it is important to consider the
everyday conditions that may cause gradual “slow-onset” events, as well as therare high impact events that cause massive damage. As every coastline is different,
this assessment needs to be undertaken on a case by case basis. Often local
communities have good insights into the local risk profile and any changes thathave occurred over time. Other sources of information include coastal engineering
reports, government records of past events and their effects, and historical media
reports or other historical accounts. Various risk assessment tools may help toorganize this assessment process in a structured manner (examples included under‘more information’).

Page 10

Section 1. Is my shore at risk?

Sources of information for the main coastal hazards covered in this guide include:

Wind and swell waves
The shape of the coastline and thedepth of near shore waters bothexert a strong influence over thepatterns of waves reaching theshore. Some mangrove coasts are
relatively exposed to wind andswell waves, but in more shelteredareas in estuaries and behindbarrier islands such waves may
only occur during major storms.
Alongside local knowledge, useful
information on the local patterns
and behaviour of wind and swell
waves near the shore can be gained
from meteorological records ofwind speeds and topographicalmaps showing the slope of theseabed.

Storm surges
Global and regional storm trackrecords can both help improveunderstanding of the potential
likelihood and intensity of storms,
while local knowledge and expert
assessment can help build up an
understanding of local conditions
during storms, including natural
amplification or amelioration by thelandscape. It should be noted thatstorms are rare, high impact, eventsand a historic lack of storms is still
no guarantee of safety especially inview of climate change.

Tsunamis
These are rare events andlocal knowledge or memory
may be misleading. Data on
seismic activity and proximity toseismically active regions, alongwith long-term historical data andgeomorphological data (seabed,
coastal profile), can help to assess
the risk and potential intensity oftsunami waves.

Erosion
An understanding of recent changes
along a coastline will give an
indication of the rate of erosion.
Older residents may well remember
a time when the coastline wasdifferent. Comparing historicalmaps and satellite images tocurrent ones will give an indicationof erosion rates. In areas where
erosion is a problem, there may bedead mangrove trees at the seaward
edge or a steep drop between the
mangrove surface and adjacentmudflat. Experts may judge thedynamics of the coastline on thebasis of a number of parameters
including size and frequency ofwaves, tidal range, long-shore
currents and sediment supply.

Local sea level rise
Global and regional models of sea
level rise provide a crude indicationof risk from future sea level rise, but
local changes in the land level areoften also important. In some places
land is naturally sinking or rising which
will exacerbate or reduce the relative
impact of sea level rise. Local tide gauge
information can help to understandrecent local change which may also givean indication of the future threats. In
considering sea level rise it is importantto also consider the adjacent land area
which could be inundated and otherrelated changes such as intrusion ofsaltwater. Particularly vulnerable are
low-lying coastal plains or coralline
platforms. Among the most vulnerableof all are low-lying areas of peat lands
and other wetlands that have been
drained – such areas will subside
dramatically as their soils dry out andare broken down: this could create
large areas of coastal lowlands that are
actually below sea level.

Page 11Assessing exposure and vulnerability
Disasters, involving great damage or loss of life, are often linked to natural hazards.
The size or scale of a disaster arising from any hazard is related both to the exposureand the vulnerability of humans and infrastructure. Exposure is a measure of where
people and property are located – low-lying areas with no sea defences are clearly
more exposed than those higher up or further from the shore. Vulnerability is
distinct from exposure and is a measure of the ability of people and infrastructure todeal with a hazard, etiher during or after the event. For example, if an early warning
system is in place and people are trained in using it, then the vulnerability is lower.
Likewise if the community have effective resources and finances to support recovery,
clean-up and rebuilding following storm surge flooding, they can be consideredless vulnerable. The tools mentioned below help to identify a broad variety ofvulnerabilities, including those related to resources, capacities and behaviours.
Hazards, vulnerabilities and exposure need to be assessed at multiple scales: withincommunities, at the level of multiple communities and infrastructure networks andacross the wider landscape. These dimensions interact, and changes at each level
might contribute to either an increase or a decrease in risk. Construction of a seawall
for example may in some case offer increased protection. In other cases it may
increase vulnerability in other locations, e.g. when it triggers erosion further down the

Page 12coast and makes that area more susceptible to storms. Likewise the degradation ofcoastal ecosystems may exacerbate hazards such as flooding, thus increasing people´sexposure. Understanding these interrelationships helps to assess the risk context anddesign truly integrated approaches to risk reduction and coastal zone management.
How do mangroves fit in?
Mangroves are found on the front line in terms of their position relative to many
coastal hazards. The fact that they thrive in many coastal settings give someindication of their ability to cope with such hazards, or at least to recover frommajor impacts. Mangroves often modify coastlines through their ability to attenuate
waves, capture sediments and build soils and it is these same properties whichmake them important in terms of reducing hazard intensity on landward margins
and of decreasing exposure. They also provide many associated benefits that
can help reduce the vulnerability of coastal communities and support recovery
following hazard impacts. Such benefits are not universal, and every coastline andmangrove forest has unique properties and so it is critical to understand both thelocal mangroves and the many environmental, social and economic settings of theindividual locations where mangroves are found.

Guidance for coastal managers• Coastal managers need to understand risk in terms of hazard, exposure andvulnerability prior to determining what role mangroves can play• The importance of mangroves in coastal defence and disaster risk reduction will
depend both on the site characteristics and the local hazard context.

More information• CARE International (2009) Climate Vulnerability and Capacity Analysis (CVCA) Handbook.
• IISD (2012) CRiSTAL – Community-based Risk Screening Tool. The International Institute for
Sustainable Development, Winnipeg, Canada.
• UNISDR (2009) Terminology: Basic terms of disaster risk reduction.
• Wetlands International (2013) Integrating ecosystems in resilience practice. Criteria for EcosystemSmart Disaster Risk Reduction and Climate Change Adaptation.

Flooding and erosion impact people and property in Indonesia,by Stefan VerschureNatural mangrove forest in the Philipines, by Wetlands International

Section 2. The role of mangroves in coastal risk reduction
This section summarizes our current understanding of how and to what extent mangroves reduce coastal hazards that put us at risk. It provides
practical guidance for coastal zone managers in terms of the mangrove characteristics that are needed to optimize protection against thesehazards. A mangrove greenbelt should always be part of a wider risk reduction approach (see section 3).

Mangrove forest properties

Hazard

Width

Waves

Storm surges

Hundreds of meters needed
to significantly reduce waves(wave height is reduced by 1366% per 100m of mangroves)Hundreds of meters needed tosignificantly reduce wind andwaves on top of surge
Thousands of meters needed
to reduce flooding impact(storm surge height is reduced
5 - 50 cm/km)

Structure

Tree Size

TsunamiHundreds of meters needed to reducetsunami flood depth by 5 to 30%
Mangroves do not provide a secure
defence (nor do many engineered
defences)

ErosionSea level rise

Sufficient mangrove forest width needs to bepresent to maintain sediment balance. This canhelp to prevent erosion and may encourage active
soil build-up.

The more obstacles the better:dense aerial root systems andbranches help attenuate waves

Open channels and lagoons allow free passage, while dense aerial rootsystems and canopies obstruct flow

Complex aerial root systems help slow water flows,
allowing sediment to settle and causing sedimentto accrete rather than erode.

Young & small mangroves canalready be effective

Smaller trees and shrubs may be overtopped by tsunamis and the verylargest storm surges

Young trees already enable soils to build up. The
more biomass input into the soil the better.

Link to otherecosystems

Sand dunes, barrier islands, saltmarshes, seagrasses and coral reefs can all play an additional role in reducing waves

Allow room for landward
retreat of the mangrove

Underpinning
factors

Healthy mangroves are a prerequisite for all aspects of coastal protection. Healthy mangroves require: sufficient sediment and fresh water supply and connections
with other ecosystems. Conversely, pollution, subsidence (due to deep groundwater/oil extraction or oxidation upon conversion) and unsustainable use jeopardizes
mangroves.

Section 2. The role of mangroves in coastal risk reduction

Page 15

2.1 Mangroves reduce wave damage
Wind and swell waves are rapidly reduced as they passthrough mangroves, lessening wave damage during storms.

Wind and swell waves are commonly seen at the sea shore,as they break on the beach or smash against rocks. They
are created by wind, sometimes far out to sea, sometimes
close to the shore. They vary in height, from almost
imperceptible ripples to large waves, reaching several
metres in height, which can pound the shore relentlessly
during storms. During flooding, waves can reach further
inland, causing damage to houses and infrastructure.

Page 16

2.1 Mangroves reduce wave damage

The role of mangroves
Mangroves reduce the height and energy of wind and swell
waves passing through them, reducing their ability to erode
sediments and to cause damage to structures such as dikes andsea walls. During rising tides, as the sea comes in, waves enter
the mangrove forests. They lose energy as they pass throughthe tangled above-ground roots and branches and their heightis rapidly diminished, by between 13 and 66% over 100 m ofmangroves. As this happens, waves lose their ability to scour
the sea bed and carry away sediments. Mangroves also reduce
winds across the surface of the water and this prevents the
propagation or re-formation of waves.

Waves are most rapidly reduced when they pass through agreater density of obstacles. This means that mangroves withaerial roots attenuate waves in shallow water more rapidly
than those without. Tightly packed arching prop roots andlow branches clearly present a more solid obstacle to waves
than sparse areas with few or no aerial roots. When the watergets deeper, waves may pass above aerial roots, but then thelower branches or the dense fronds of the Asian Nypa palm canperform a similar function in obstructing waves. Mangroves witha complex structure of dense aerial roots and low branches,with various species of different age and size, are most likely tobe effective at reducing wave heights.

Guidance for coastal managers• Maintain wide mangrove belts. To significantly reduce everyday waves you need a belt of
hundreds of meters wide – the more the better. A few trees won’t help very much.
• The more obstacles the better, so mangroves of various species of different age and size are mostvaluable in reducing wave heights. In a dense mangrove forest packed with aerial roots and low
branches, a wave may be reduced to half its height after 100m passage through the mangroves,
while in a more open forest, it might take 500m of mangroves to reduce such a wave.
• Even relatively young or small mangrove forests can reduce the height of common wind andswell waves. Hence, reforested areas become effective at reducing waves in just a few years.
• Protection of adjacent ecosystems is important as these can also help attenuate waves. They
include sandbanks, seagrasses and coral reefs, as well as landward dunes and saltmarshes.

More information• McIvor AL, Möller I, Spencer T & Spalding M (2012a) Reduction of wind and swell waves
by mangroves. Natural Coastal Protection Series: Report 1. Published by The Nature
Conservancy and Wetlands International.

Mangrove roots dissipating waves, by Mark Spalding2.2 Mangroves reduce damage from large storms
Wide areas of mangroves can reduce storm surge flood levels, while narrower belts can still reduce the impacts of large
waves and high wind speeds occurring during major storms (also called cyclones, typhoons or hurricanes).
The role of mangroves
Mangroves may contribute to reducing loss of life and damage toproperty from storms and cyclones as they reduce the impacts ofwaves, storm surges and high winds.
Waves: Mangroves can rapidly diminish the height of wind and swell
waves as discussed in Section 2.1. Even during relatively large storm
surges the leaves and branches of the forest canopy will help toreduce wave energy providing the trees are tall enough.

Tropical storms, the largest of which are termed cyclones,
hurricanes or typhoons, are widespread in many tropicaland subtropical areas. Large waves, raised sea levels (thestorm surge), strong winds and torrential rainfall, cancause widespread flooding, extensive damage and deaths.
These storms form during the warmest parts of the year.
The surges are caused by high winds and low atmosphericpressure allowing sea water to pile up as it approaches
land. Such storms take several
hours or longer to pass over any place. Their surges can
carry a lot of debris, which can cause further damage. The
largest storms typically affect any particular area every
few years or decades.

Page 18

2.2 Mangroves reduce damage from large storms

Storm surges: Where mangroves are extensive they are able toreduce storm surge water depths as the surge flows inland. Whilestorm surge depths may only be reduced by 5-50 cm per kilometre
width of mangroves, nevertheless a small reduction in water level
can already greatly reduce the extent of flooding in low lying areas
behind the mangroves. Debris movement can also be reduced bymangroves: the complex network of roots and branches can serve totrap even large moving objects.
High wind speeds: The dense mangrove forest canopies also
reduce wind speeds locally. This prevents further development ofwind and swell waves in and immediately behind the mangroves,potentially reducing damage to nearby infrastructure. In the most
severe storms, mangroves may themselves be damaged or torn up

by the high winds and waves, although breakage or uprooting ofmangrove trees is relatively rare.
Other determining factors
Mangrove forest structure is important in determining potential
risk reduction by mangroves. Open channels and lagoons allow
surges to travel freely landwards, and this should be taken into
account in spatial planning. Dense forest vegetation helps toreduce surge depths and also reduces the height of wind waves
riding on the surface. Sparse or low-canopy shrub mangroves may
be less effective at reducing both surge and wave height. Dense
mangroves are also a highly effective debris trap, protecting
people and infrastructure from the potential damage that might
be caused by the physical pounding of solid debris carried bystorm waters. In the aftermath of a storm, mangroves enhancerecovery by providing food, fuel wood and construction wood.
The capacity of mangroves to reduce storm surge peak waterlevels also depends on the size and forward speed of the storm,
as well as the characteristics of the coast. Storms can be 100
km or more across, and larger storms tend to generate higher
water levels and greater flooded areas. Mangroves are more
effective at reducing surge levels if storms pass over relatively
quickly. Extensive areas of shallow water near the coast produce
larger surges than steep off-shore slopes. Adjacent ecosystems– including offshore sandbanks, sea grasses and coral reefs, anddunes and saltmarshes on landward margins – can also help
attenuate waves and reduce storm impacts.

Guidance for coastal managers• Maintain wide mangrove belts, ideally thousands of meters across, as these can be effective
in reducing the flooding impacts of storm surges, typically reducing water heights by 5 to 50
centimetres per kilometre of mangroves. This can significantly reduce flood extent in low lying
areas. Narrower mangrove belts, hundreds of meters wide, will be able to reduce the impacts ofwind waves during storms.
• Ensure structural diversity, the more obstacles the better. Mangrove trunks, branches and leaves
all help reduce wave energy in times of very high seas, so diverse mangroves with various speciesof different age and size are most valuable.
• Open channels and lagoons allow free passage of floods, which should be taken into account in spatialplanning.
• Since storm surges reach high up into the forest, trees need to be tall to act as an obstacle to theflow. Hence, mature and tall forests will perform better than young and small forests.

Destruction after typhoon Philippines by Cordaid

More information• McIvor AL, Spencer T, Möller I & Spalding M (2012b) Storm surge reduction by mangroves. Natural
Coastal Protection Series: Report 2. The Nature Conservancy and Wetlands International.

19

2.3 Mangroves can help to reduce tsunami damage
Wide areas of mangroves can reduce tsunami heights, helping to reduce loss of lifeand damage to property in areas behind mangroves.

Tsunamis are caused by earthquakes andlandslides disturbing large masses of water.
The resulting tsunami waves can travel rapidly
over very long distances across the ocean. Whentsunami waves approach land, they increase inheight, sometimes becoming several metreshigh at the shore. Tsunamis can result in massive
destruction and loss of life, as seen during the2004 Asian tsunami. While the water depths inboth storm surges and tsunamis can be similar,
tsunamis arrive more quickly as one or a seriesof waves that may appear and flood an area in amatter of seconds or minutes.

Page 20

2.3 Mangroves can help reduce tsunami damage

The role of mangroves
There is growing evidence, particularly from the devastating Indian
Ocean tsunami of 2004 that mangroves reduced tsunami impacts by
reducing the destructive energy of water flowing inland. Mangrove
belts several hundred meters wide have been shown to reducetsunami height by between 5 and 30%. Wider mangrove forests are
more effective at reducing tsunami height, as well as speed of thewater and the area flooded by the tsunami. Dense forest vegetation
also helps to reduce tsunami depth and area of flooding. However,large tsunamis (more than 4 m in depth) can damage mangroves,
eventually destroying them and making them less effective at
reducing tsunami flows. The highest tsunami waves may be taller
than mangrove trees, so that the incoming water flows over them. Of
course the same is true for engineered structures, which are rarely
built to the height of mangroves.

Despite these limitations, mangroves can still be an important
asset in many settings. Even a modest reduction in the total area ofinundation can already reduce loss of lives and economic damage.
Equally importantly, mangroves trap floating debris, and provide asafety net for human victims caught up in the incoming flood or the
returning waters. Mangroves also enhance recovery by providing
food, fuel wood and construction wood directly after the event, whenthese resources may be scarce.
Clearly the size of the tsunami is a major control on the ability ofmangroves to protect communities living behind the mangrove. A
very large tsunami, arriving with little or no warning, will be highly
destructive. As with storm surges, extensive areas of shallow-water near
the coast produce larger tsunami waves than steep off-shore slopes.

Guidance for coastal managers

Mangrove restoration by Wetlands International• Maintain wide mangrove belts, ideally several hundred
meters wide, as these will be more effective than narrow
strips in reducing impacts from tsunamis.
• Mangrove trunks and branches are expected to reduce theenergy and force of incoming tsunami waves. Taller trees areless easily overtopped by a tsunami than small ones. Mature
forests with thicker trees can withstand greater wave energy.
• Open channels and lagoons allow free passage to floods,
which needs to be taken into account in spatial planning.

BOX 1. Mangrove restoration
post-tsunami
In 2004 a massive tsunami hit the coastlinesof South East Asia. It killed 230,000 people
and caused enormous damage. In response tothis tragedy, Wetlands International, together
with IUCN, WWF and Both Ends initiated theGreen Coast project (2005-2008). Fundedby Oxfam Novib, this project supported
disaster-recovery in 6 countries by restoring
mangroves and developing sustainable
livelihoods. In Indonesia alone 70 community
groups restored no less than 2000 hectaresof mangroves. In return they received support
to develop new livelihoods such as crab
farming, small-scale fisheries and sustainableaquaculture. Currently these communities
live in mangrove landscapes, with patches ofmangroves alternating with agriculture andaquaculture. These resilient landscapes, alongwith a well-educated local population androbust infrastructure ensure that the people
are much better prepared for and equipped
to deal with natural hazards that may occur inyears to come.

2.4 Mangroves reduce erosion and bind soils together
The dense roots of mangroves help to bind and build soils. The above-ground roots slow
down water flows, encourage deposition of sediments and reduce erosion.

The role of mangroves
On every coastline waves and currents create change, sometimes bringing
sediments to the coast, but sometimes causing erosion and the loss ofland. Where mangroves occur they generally reduce erosion and enhance
sedimentation. The mangrove vegetation reduces wave energy and slows
the flow of water over the soil surface, reducing the water’s capacity todislodge sediments and carry them out of the mangrove area. At the sametime the slower water flows can allow already suspended sediments tosettle out from the water, resulting in increased deposition of sediment.

Erosion refers to the removal of
sediments from the shore, resulting
in the loss of land and a retreating
shoreline. Erosion can be caused bothby the small waves that lap against theshore on a daily basis, and by infrequent
large waves, storm surges or tsunamis.
Erosion can occur from the sedimentsurface, resulting in a lowering of thesurface (leading to more frequent
flooding by waves and tides). It can also
occur at the mangrove edge, e.g. throughthe formation of an erosive cliff (a stepchange in surface height) at the seaward
edge of the mangroves.

Page 22

Typical mangrove soils are rich in organic matter produced by the
mangroves themselves, including living roots but also dead leaves andwoody materials. The dense network of fine roots helps to armour the soil
from erosive forces and to trap and bind soil particles together. Because
mangrove soils are often waterlogged and so have very low oxygen content(anaerobic conditions), much of this organic matter accumulates, forming alayer of peat that increases in thickness over time.
When mangroves are lost, the patterns of sediment movements can change
dramatically – where mud and soil were stable or even gradually building
up, they may begin to erode so that land disappears into the sea. Further,drainage and conversion of mangrove areas to other uses leads to the rapid
breakdown of organic matter as oxygen becomes available in the soil, which
in turn causes subsidence. This is very clearly seen in a number of coastal
areas where mangroves have been converted to aquaculture or agriculture,such as along the coasts of the Gulf of Thailand, in northern Java, and in
Guyana. These formerly stable or even advancing coasts are now retreating
at rates of many meters every year.

2.4 Mangroves reduce erosion and bind soils together

Other determining factors
Besides mangrove deforestation or degradation, other drivers of land loss
include subsidence caused by extraction of deep groundwater or oil, sealevel rise and the construction of hard infrastructure (see also box 2, page25). Adequate sediment supplies are vital to allow mangroves to build up
soils. Sediment supplies can be affected when infrastructure along thecoast alters long shore currents or when upstream dams block sedimentsupplies to the rivers which then bring them to the coast. Coastal
engineering works should be planned to maintain sediment flows, sothat mangroves receive a healthy sediment supply. In some cases active
interventions may be needed to trap sediments and restore sedimentsupplies, by placing permeable dams to trap sediment or through mud
nourishment or agitation dredging.

Guidance for coastal managers• Maintain wide mangrove belts. Wider mangrove belts are better able to maintain sedimentflows, reducing sediment losses and erosion. Conversion of even parts of the mangrove belt
can lead to reductions in sediment flows, reductions of deposition and increasing erosion.
• Ensure healthy mangrove growth. In most places healthy natural mangroves are likely tobe relatively stable habitats with little erosion: complex root systems help slow waterflows, allowing sediment to settle and causing sediment to accrete rather than erode; while
productive root growth and leaf litter supply are critical to build and bind soils.
• Maintain or restore sediment supply and avoid sediment starvation caused by certain coastal
and inland engineering works that block the flow of sediments from rivers or along the coast.
• Processes that lead to subsidence, such as oxidation caused by drainage and deforestation or
extraction of deep groundwater and oil should be prevented.

More information• McIvor AL, Spencer T, Möller I & Spalding M (2013) The response of mangrove soil surface
elevation to sea level rise. Natural Coastal Protection Series: Report 3. Cambridge Coastal
Research Unit Working Paper 42. Published by The Nature Conservancy and Wetlands
International.

Mangrove roots protecting the upper banks of the
Daly Estuary in Australia, by E. Wolanski

2.5 Mangroves may keep up with sea level riseOver time mangroves can actively build up soils, increasing the height of the mangrove soil
surface, which may be critical as sea level rise accelerates.

Globally, sea levels are rising as a resultof the melting of ice sheets and glaciers,
and the expansion of the oceans as theywarm with climate change. At more local
scales the height of the land itself may
be slowly rising or sinking as a result ofgeological changes, the compaction of thesoil or extraction of water or oil from deep
within the ground. Recent sea level rise
has averaged 3.2mm per year. Sea level
rise rates are accelerating, but they are alsovariable across the oceans due to patterns
in ocean currents and differences in ocean
temperatures. Wherever sea levels arerising or the land is subsiding, the impactsof waves, storm surges and erosion willreach further inland.

Page 24

The role of mangroves
Mangrove soils are actively growing in many places. Mangroves contributeto an increase in soil volume by capturing riverine or coastal sedimentsthat pass through, as well as adding their own organic matter in the formof roots, leaves and woody material. The fine mangrove roots also help
to trap and bind the particles. Due to a lack of oxygen in the waterlogged
soil, organic matter is not broken down by soil organisms. This allows theorganic matter to build up over time, producing the deep peaty soils that
underlie mangroves in some areas. Mangrove root growth also pushes thesoil upward, resulting in a higher soil level.
These processes can allow mangroves to keep pace with rising sea levels.
Some mangroves sit on top of deep layers of mangrove peat that may be
6 meters deep or more, that were built up over thousands of years as sea
levels rose. These mangrove soils grew vertically at rates of up to 10 mm
per year in sites from Australia to Belize, suggesting that mangroves may
be able to keep up with similar rates of sea level rise into the future, where
local conditions allow. While not all mangroves may be able to fully “keepup” with rising seas, even a small increase in soil surface height over time
may help to reduce the impact of sea level rise on coastal areas. Mangroves
may be able to colonise landward areas if space is available, and thus
continue to provide coastal defence services against waves and storms.
Other determining factors
Mangroves are more able to keep up with sea level rise in areas withhigh rates of sediment input. Supplies of sediments can be rapidly
reduced through inappropriate coastal engineering projects or thedamming of rivers. Hence, shore structures should be designed such thatthey allow long shore sediment transport, and dams on rivers should
be carefully planned to reduce impacts on sediment flows to coastalmangrove areas. In cases of sediment starvation it may be necessary
to artificially enhance sediment retention and supply through theplacement of permeable dams to trap sediment, or the use of mud
nourishment or agitation dredging.

2.5 Mangroves may keep up with sea level rise

Further, mangroves thrive in brackish water, a mix of salty sea water andfresh water. If fresh water supplies are disturbed due to dams upstream,
conditions may become too saline and mangrove growth rates anddiversity will be reduced, which may affect their production of organic
matter and ability to bind and build soils.
Subsidence counteracts the ability of mangroves to keep up with sea
level rise. Processes that lead to subsidence such as oxidation caused bydrainage and deforestation and extraction of deep groundwater and oilshould be prevented.

Guidance for coastal managers• Ensure healthy mangrove growth. Healthy root growth and leaf litter
supply are critical to build and bind soils. If mangroves are to keep up
with sea level rise, activities that decrease organic matter input into thesoil (such as harvesting) should be minimised. It is important to maintainor restore fresh water supply to keep mangroves healthy.
• Maintain or restore sediment supply and avoid sediment starvation
caused by certain coastal and inland engineering works that block theflow of sediments from rivers or along the coast.
• Allowing mangroves space to retreat landwards will help to ensure theirsurvival so that they can continue to act as a first line of defence against
waves and surges.
• Avoid drainage and conversion of mangrove areas as subsequentoxidation causes a severe loss in soil surface height. Extraction of deep
ground-water and fossil fuels can similarly lead to subsidence leaving
mangroves and adjacent lands prone to flooding as sea levels riseMore information• McIvor AL, Spencer T, Möller I & Spalding M (2013). The response of mangrove soil surface elevation to sea level rise. Natural Coastal Protection Series: Report 3. Cambridge Coastal Research Unit Working
Paper 42. Published by The Nature Conservancy and Wetlands International.
• Ellison JC (2012) Climate Change Vulnerability Assessment and Adaptation Planning for Mangrove Systems. WWF, Washington D.C.

Page 25

Mangrove flower, by Wetlands International

Section 3. Managing mangroves
for coastal defence
Mangroves can contribute to risk reduction in many coastal settings. In someplaces this role of mangroves can be enhanced by combining such benefits withother risk reduction measures, making them part of a wider coastal defence anddisaster risk reduction strategy (section 3.1). Mangroves also contribute to societal resilience. They generate valuable fish, timber, fuel and fibre which help
recovery after disasters. To ensure maintenance of these valuable services, it iscritical to engage the many sectors and stakeholders involved in coastal areas
through integrated management (section 3.2). Mangroves even play a role inseverely degraded settings, where their restoration can bring back their many
valuable functions (section 3.3).

3.1 Integrating mangroves into coastal defence strategies

Mangroves don’t always provide a stand-alone solution;
they may need to be combined with other risk reduction
measures to achieve a desired level of protection. If they areintegrated appropriately, mangroves can contribute to risk
reduction in almost every coastal setting, ranging from rural
to urban and from natural to heavily degraded landscapes.
Disaster risk reduction is the practice of reducing disaster risks throughsystematic analysis and management of the causal factors of disasters. This
includes reducing exposure to hazards, lessening vulnerability of people andproperty, improving preparedness for adverse events and wise managementof land and the environment. Healthy and well managed mangroves can
reduce disaster risk.

Page 26

3.1 Integrating mangroves into coastal defence strategies

As demonstrated in the previous section, mangroves
can reduce the risks associated with waves, storms
and erosion and over the long term can evencontribute to maintaining land as sea levels rise.
Every coastline around the world is different though,with a unique combination of hazards and associatedexposure and vulnerability. Mangroves too are highlyvariable from place to place. It is critical to consider
these different factors to understand if, how and towhat extent mangrove management can play a rolein risk reduction planning. Modelling, measuring andmonitoring is needed to ensure a predefined level ofrisk reduction is achieved.
Mangroves and coastal engineering
Mangroves as a stand-alone solution may not always
offer full protection against large magnitude hazards
like tsunamis or cyclones. The same also holds for hard
defence structures. In addition, hard infrastructure
might have various adverse impacts, for example
interrupting sediment flows and exacerbating

erosion (see box 2, opposite). In recent years, somecoastal zone managers have begun to approachthese challenges by combining green (nature-based)with grey (engineered) solutions. This ‘Building
with Nature’ approach has shown promising results,
representing a remarkable shift in paradigm: fromfighting against nature, to working with and alongsidenatural processes. The more coastal defence relies
on mangroves, the more important it is that this isintegrated within coastal engineering approaches(Section 2). Where there is a lot of space, mangroves
can play a larger role than where there is little space.
Mangroves and risk reduction
Engineered and ecosystem-based coastal defence
measures also need to be implemented alongsidea suite of other risk reduction measures. This
includes avoidance of human activities in highrisk zones, improved building standards, increased
preparedness, early warning systems and evacuation
procedures.

Risk reduction measures diagram. Adapted from diagram presented by Ty Wamsley, US Army Corps of Engineers

BOX 2. Why hard-engineering structures can fail in
muddy coasts

In a healthy mangrove ecosystem, waves take sediment away and the tides bringsediment in. The mangroves’ aerial root systems help to dissipate the waves
and to capture and stabilize the sediment. By contrast, hard structures, such as
breakwaters and also the earthen or concrete dykes surrounding aquaculture
ponds, only protect against wave impacts in the short term, and may needrebuilding frequently as they are undercut by erosion. This occurs because
waves can get 2 – 4 times bigger when they reflect off a hard structure. Thesebigger waves increase erosion in front of the structure, and can eventually leadto its collapse. Such collapsed sea walls are then useless in reducing waves or
preventing erosion. Moreover, hard structures disturb the balance of incoming
and outgoing sediment because they block the flow of water and sediment into
areas that were previously flooded by the tides.

Failed coastal defence structure in Guyana,by Han Winterwerp

Section 3. Managing mangroves for coastal defence

Page 27

Mangroves contribute to risk reduction in different settings
In natural settings, where the mangrove belt is still very wide, mangroves are capable of loweringstorm surge height and of dampening the waves on top of the surge within the first few hundredsof meters of mangrove. Wise use of mangroves, in rural settings, ensures that a mangrove beltof hundreds of meters wide is maintained and continues to play a critical role in reducing windor swell waves, thus reducing erosion. Restoration remains an important option in areas where
mangroves have been lost – interventions to enhance sediment supplies, or using permeablebarriers to reduce erosion, may help mangroves to become re-established. In all settings,
mangroves may be able to keep up with sea level rise, provided that the sediment and fresh waterbalance enable mangroves to thrive. Therefore they are an adaptive solution that may help tocope with climate change impacts.

Page 28

Depending on the hazard, vulnerability and mangrove condition, combinations of green and grey
solutions may be needed to provide optimal risk reduction. In urban settings, where space is limited,even small areas of mangroves offer some protection from waves, reducing construction andmaintenance costs for hard infrastructure behind them that in those settings are the primary defence.
In all these settings, mangroves add additional value in the form of enhanced fisheries, carbon storage,
forest products and recreation. Mangroves thus support food security, climate change mitigation andhuman wellbeing and can enhance recovery after major hazards.

Guidance for coastal managers• Mangroves can play a role in reducing risk in natural, rural, urban and even degraded settings.
• Mangroves are rarely a stand-alone solution and need to be appropriately combined with other risk
reduction measures. This includes integrating mangroves in the design of coastal infrastructure.
• Modelling, measuring and monitoring the role of mangroves in coastal defence is needed to inform how toachieve a desired level of risk reduction.
• Mangroves should be embedded in coastal defence and disaster risk reduction strategies and in multisector integrated management planning for large coastal areas.
• Long-term provisions for coastal realignment may be needed to give space for the landward migration ofmangroves for the eventuality that sea level rise might lead to losses of mangroves at the seaward margins.

More information• Wetlands International (2013) Integrating ecosystems in resilience practice. Criteria for Ecosystem-Smart Disaster
Risk Reduction and Climate Change Adaptation.
• Partnership for Environment and Disaster Risk Reduction (PEDRR) website.
• De Vriend HJ & Van Koningsveld M (2012) Building with Nature: Thinking, acting and interacting differently.
EcoShape, Building with Nature, Dordrecht, the Netherlands.

3.2 Mangroves as part of coastal zone management
For mangroves to optimally contribute to risk reduction, their conservation needs to be incorporated into broader
coastal zone management planning: they need to be protected and restored, allowing wise use where possible.
Sustainable development is the management and conservation of the natural resource base to ensure the attainment and continued satisfactionof human needs for present and future generations. Such sustainable development conserves land, water, plant and animal genetic resources, isenvironmentally non-degrading, technically appropriate, economically viable and socially acceptable (FAO 1989).

Managing mangroves in the landscapeMangroves are under threat from many development pressures.
This includes overharvesting, conversion for alternative uses (urban,
agriculture, aquaculture) and on- and off-site development of
infrastructure. Integrated coastal management and development plans
are required to safeguard the risk reduction values of mangroves.
These plans should prevent further degradation of existing mangrove
forests and support rehabilitation of mangroves where possible, andmost importantly should consider mangroves as being a fundamental
part of the local economy. One vision for the integration of mangroves
into coastal landscapes and seascapes would be the promotion oftiered protection, with strictly protected mangrove areas alternatingwith wise use zones as well as more intensively used production
systems.
Since mangroves can be impacted by measures taken quite far fromthem, and since coastal systems tend to include a great variety
of sectors and stakeholders, it is important to consider mangrove
management from a broad geographic perspective, and to place
use and development needs in a wider framework of planningand management. Such integrated coastal zone management will
help address the root causes of coastal vulnerability. This may
include ensuring sustainable use of adjacent land, rivers and marine
ecosystems to avoid impacts to soil surface height, freshwater,
nutrient and sediment supplies and to ensure the ecological andphysical conditions remain in place for mangroves to flourish.
Key to this process is the involvement of local communities andother stakeholder groups in both the design and implementation of
spatial plans and management plans. Further, successful mangrove
management requires an effective system of ownership, use andPage 30

3.2 Mangroves as part of coastal zone managment

access. Some countries have framework legislation which establishes
who owns mangroves, or who has the right to use mangroves. Clear
ownership, by state, local communities or individuals, provides
a setting for wise use and management. There is also a need tounderstand existing legal and policy systems that may have relevancefor mangroves. Many countries have unwittingly encouraged
mangrove degradation or loss through policies that provide incentives
for aquaculture or coastal development. Others have ensured longterm coastal protection by legally requiring mangrove belts of acertain width to be left in front of developments.
Sustainable financing mechanisms can help to ensure the long-term
maintenance of appropriate management interventions. Whenwisely used, mangroves can support aquaculture and fisheries while
simultaneously reducing coastal risk. Map-based tools are becoming
increasingly available, and this may help with deciding on bestpractices for coastal land-uses to maximize benefits and to minimize
costs (box 3, page 30).
Mangrove management options
Protected areas cover some 36% of the world’s remaining mangrove
areas. Defined as areas set aside primarily for conservation purposes,
protected areas such as national parks and nature reserves typicallyforbid many extractive activities such as timber harvesting. In reality,
however, most mangrove protected areas still allow some levels ofsustainable fishing, harvesting of non-timber forest products andtourism.
Sustainable silviculture has been developed in many mangrove
forests. In both Matang forest in Malaysia and in the Bangladesh
Sundarbans, the mangroves have been harvested for over 120 years.

Such long-term management is so well developed that it gives
valuable security in terms of production, income and job-creation.
Standards for sustainable forestry can help determine managementthat sustains social, ecological and economic benefits over the
longer term. Care should be taken to ensure that harvesting does notjeopardize the coastal protection function.
Sustainable aquaculture in some cases needs to be developed in thevicinity of mangrove areas to address food security concerns and to
redirect one of the most important drivers of mangrove conversion.
It is essential to avoid siting of new aquaculture ponds in mangrove
areas. Mangroves play a role in coastal risk reduction, water quality
improvement and fisheries enhancement which can be of critical valueeven to the aquaculture sector itself. Even where ponds are alreadylocated in mangrove areas, further mangrove loss should be minimised
and wide margins of mangroves should be restored where possible
in front of ponds. Mangroves can also be planted within aquaculture
ponds and on the pond margins where they can help to maintainwater quality, reduce erosion and support nutrition, but this will haveminimal benefits in terms of coastal defence. Standards for sustainableaquaculture increasingly take mangroves into account, prohibiting
siting in mangrove areas and requiring mangrove rehabilitation.
Community based management can be highly effective. Local
communities can be stewards for mangrove management, and theirsupport for any management is likely to increase a project’s chancesof success. Such communities can help in projects for restoration; they
can help to regulate use and access, and are the direct beneficiaries ofsustainable management.

Mangroves at the coast of Panama City,by Wetlands International

Keeping mangroves healthy and avoiding threats
Healthy mangroves are a prerequisite for all aspects of coastal protection. Robust
growth should be encouraged by avoiding degradation through pollution, waste
dumping, drainage or unmanaged timber extraction. Mangrove health can be
impacted by measures taken quite far from the mangroves. The building of dams
for hydropower, and the use of freshwater for crop irrigation has led to profoundchanges in many estuaries and deltas, reducing freshwater supplies and changing
sediment loads in rivers. Increased salinities in deltas and estuaries can greatly affectsurvival of some mangrove trees, while reduced sediments can lead to erosion.

Natural mangrove regeneration in South Africa,
by Wetlands International

Climate change could also have implications for mangroves. Typically mangroves
are opportunistic, growing well in many conditions. Thus far it would appear thatno species are negatively affected by heat, but some may be able to expand theirranges as warm conditions expand around the world. Indeed warming temperatures
are probably already contributing to the colonization of mangroves in salt-marsh
communities in Australia, New Zealand and the USA. Alongside warming of course,
there are likely to be significant changes in patterns of rainfall, and possibly
storms, which may influence some areas in the longer term. Sea level rise has been
implicated as a threat to mangroves. In reality current rates of change are still
relatively low and unlikely to be driving any large-scale losses just yet. Indeed there
is a chance that some mangrove areas may be able to mitigate, or at least reduce
the rate of change as they capture and build sediments to raise the elevation of themangrove soils.

Guidance for coastal managers

BOX 3. Managing Complex Trade-offs along Belize’s
Coastal Zone

• Prevent mangrove conversion, and in all cases maintain wide seaward greenbelts. Allow wiseuse (forestry, fisheries) to the extent that it does not jeopardize the coastal risk reduction
function or mangrove health in general.

Belize’s barrier reefs, mangrove forests, and wetlands all provide buffers against
erosion and storm surges, thus helping to protect 40% of Belizeans that livein the coastal zone. Stakeholders in Belize recently formulated an Integrated
Coastal Zone Management Plan that designates areas for preservation, restoration,
development, and other uses. This plan was based on mapping and valuationof ecosystem services, including their value for coastal protection, but also
other benefits such as tourism and fisheries. These were modelled both forpresent conditions and under alternative zoning schemes in the future, enabling
stakeholders and planners to take nature’s values into account when taking
management decisions. More information: http://www.naturalcapitalproject.org/

Page 32• Healthy mangroves are a prerequisite for coastal protection. Encourage robust growth by
avoiding degradation through pollution, waste dumping, drainage or unmanaged timber
extraction and by maintaining or restoring fresh water supplies.
• Protected areas, sustainable silviculture, sustainable aquaculture and community based
management can be effective mangrove management measures.
• Integrated coastal zone management that places such measures into a wider framework ofplanning and management and involves multiple stakeholders is needed to ensure longterm security and broad support.

More information• UNEP (2014) The Importance of Mangroves to People: A Call to Action. Cambridge, UK,
United Nations Environment Programme World Conservation Monitoring Centre.and
Wetlands International.

3.3 Bringing the mangroves back
Mangroves, and their coastal risk reduction function, can recover in most places
where appropriate ecological and social conditions are present or restored.
Restoration can be defined rather broadly as ‘a return from a disturbed or totally altered condition to a previously existing natural or altered condition by some action of humans’. Rehabilitation issimilar to restoration but the goal is not to return to some previous condition as defined by measurable reference criteria, but instead the conversion of an altered wetland to some beneficial use
as defined by locally agreed upon goals and criteria.

Mangrove recovery
Mangroves are remarkably robust andopportunistic species, which means that, while
mangroves have been lost or degraded inmany areas, they can also recover, and indeed
such recovery can be enhanced with a variety
of interventions. Generally speaking it is still
more cost-effective to prevent mangrove loss
than to allow loss and then have to invest in
rehabilitation or restoration, but this is notalways an option. Around the world hundreds ofthousands of hectares of mangroves have been
actively planted in mangrove restoration projects
from Bangladesh to Cuba, and from Florida toAustralia. Many of these have highlighted therole of mangroves in coastal defence as a key
motivating factor. Not all restoration projects
have been successful, but then again many areasof mangroves have re-established themselves
without active planting or management. Clearly,
recovering and restored mangroves begin tobring back many of the ecosystem services thatwere lost or degraded when the mangroves
were absent, but this will not be instant. Before
any efforts are made towards restoration it iscritical to understand two things: firstly, why the
mangroves are gone, and secondly, who owns
and/or uses the place where they were.

Principles for mangrove restoration
Ensure ecological conditions are appropriate for
mangrove recovery. Mangroves may have been
lost through local changes in hydrology, changes
in freshwater supply, or loss of sediments. Thesein turn might be linked to local infrastructuredevelopments, building of aquaculture ponds,
coastal engineering works preventing thelong-shore transport of sediments, or to dams
on rivers far inland. There is no point investingmoney in mangrove restoration if they will
simply be lost again.

restoration efforts have focused on the easiest
species to plant, such as Rhizophora species with
their large seedling-like propagules, but thesewill not always grow on exposed outer margins,
and such efforts can easily fail. All mangroves
need to be in shallow intertidal areas: planting
them in deeper water areas may seem an easierproposition if such areas are state owned, butmangroves cannot grow out of their depth.
Furthermore planting of mangroves in the wrong
places may destroy valuable ecosystems such as
seagrass meadows and mudflats.

Mangrove recovery may require more active
intervention. Where there is small scale erosion
it may be possible to reduce erosive impacts
through protecting individual seedlings until
they become more established. In disturbed
areas such as former aquaculture ponds active
levelling and the restoration of hydrologicalflows may be needed. The restoration of
sediment supplies may also require the alterationof coastal engineering structures or even waterflows from inland dams, to allow sufficient
sediment to flow back into coastal areas.

Ensure that social conditions will allow
mangrove recovery. If mangroves have been
removed by local people this could easily happen
again. The ownership and use rights of potential
restoration areas need to be clearly established,
and there must be both a desire for recovery anda possibility for management. Many restoration
projects have failed because local people did notsupport or understand the need for mangroves.

Mimic nature. Natural mangrove forests are
not monocultures and new forests will be more
resilient and provide greater benefits if they canreplicate the density, structural complexity, andnatural restocking of natural mangroves, using
multiple native species with natural zonation.

Building with Nature. On rapidly eroding muddy
coasts in Indonesia and Vietnam, new techniques
are being developed to stabilise or enhance
sediments to allow mangrove recovery, using
permeable structures to trap sediments, orsediment-agitation dredging and nourishment toenhance sediment supply (see bpx 4, page 33).

Follow nature. Some of the worst failures inmangrove restoration occurred because people
attempted to plant the wrong species, often
in the wrong places. Mangroves are intertidal
species which grow best above mid-tide levels.
Different species have more specific needs: many

In many places it is not necessary to plantmangroves at all. Restoring the natural patternsof water flow, preventing human disturbance andperhaps removing choking weed species may
be enough to allow natural regeneration. This isalready a widespread practise in many mangrove“plantations” (production mangrove forests).

Afforestation may be appropriate in someplaces. Planting mangroves in places where
they have not occurred before is known asafforestation, rather than restoration. This may
be an appropriate intervention in places where
there has been major coastal engineering,
and where mangrove appropriate intertidal
substrates have been created.

3.3 Bringing the mangroves backPage 33

Community mangrove planting in Indonesia,
by Wetlands International

Guidance for coastal managers• Mangroves can recover in most areas, as long as ecological and social conditions areappropriate for mangrove recovery.
• Follow and mimic nature: mangroves are intertidal species which grow best above midtide levels. Natural mangrove forests are diverse in terms of species, structure and age.
• In many places it is not necessary to plant mangroves at all and mangroves may naturally
recover.
• In areas with erosion, mangrove restoration may require more active intervention such asprotecting individual seedlings or restoring the sediment balance and hydrology.
• Ensure that mangroves can be restored, following termination or abandonment ofaquaculture or aquaculture development. Trust funds, built up through payments byoriginal developers, could be established to support this.

More information• Lewis III R. & Brown B (2014). Ecological Mangrove Rehabilitation - a field manual forpractitioners. Mangrove Action Project, USA.
• Primavera JH, Savaris JP, Bajoyo BE, Coching JD, Curnick DJ, Golbeque RL, Guzman AT, Henderin
JQ, Joven, RV, Loma RA & Koldewey HJ (2012) Manual on community-based mangrove
rehabilitation. Mangrove Manual Series no. 1 London, UK: ZSL viii +240p.
• Primavera JH, Yap WG, Savaris JP, Loma RA, Moscoso ADE, Coching JD, Montilijao CL, Poignan
RP & Tayo ID (2013) Manual on Mangrove Reversion of Abandoned and Illegal Brackishwater
Fishponds – Mangrove Manual Series No. 2. London, UK: ZSL. xii + 108 p.
• Green Coast (2009) Best Practice Guidelines on Restoration of Mangroves in Tsunami
Affected Areas.

Building with nature in Indonesia,
by Nanang Sujana

BOX 4. Restoring severely eroding coastlines through Building with Nature
Northern Java’s coasts suffer from severe erosion. In one decade, some coastal areas have retreated by threekilometres. Development of coastal infrastructure and aquaculture went hand in hand with the removal ofprotective mangrove forests and the disturbance of sediment flows towards the coast and floodplains. Previous
replanting efforts have not been successful, as the sediment balance is currently too disturbed to enable
mangroves to survive. To counter this destructive erosion, a “Building with Nature” approach was tested in
Timbul Sloko village in Central Java. The approach entails the placement of permeable dams that break thewaves and trap sediment thus reclaiming land. Once the land is back, mangroves can recolonize the area andhelp protect the coastline against erosion. The waves are clearly much lower inside the grid of permeable dams
than outside. In some cases, pioneering mangrove trees are already becoming established. In a direct responseto these initial tests, the village signed a decree in 2014, demarcating 100 hectares of the recently lost land asprotected area, ensuring that, upon recovery, it will not be damaged or destroyed again.

Harvesting from the mangroves in Indonesia,
by Nanang Sujana

Section 4. Recognising themultiple values of mangroves
Mangroves are among the most valuable ecosystems in the world. Decision
makers, and the public, need to take full account of the many benefits
that mangroves provide, and consider the true costs that may incur frommangrove loss.

Ecosystem services are the benefits people obtain from ecosystems. Ecosystem services canbe grouped in four broad categories: provisioning, such as the production of food and water;
regulating, such as coastal risk reduction and control of disease; supporting, such as nutrient
cycling and water purification; and cultural, such as spiritual and recreational benefits. To help
inform decision-makers, many ecosystem services are being assigned economic values.

In many parts of the world mangroves have been lostas people have cleared them to build aquaculture
ponds or to make space for agriculture or urban
developments. Although rates of loss have slowed
in many areas, elsewhere they are continuing, with
rapid expansion of aquaculture in many parts ofIndonesia and South America. Mangrove removal isfrequently justified on economic grounds, but thisis often based on an incomplete analysis. Typicallythe initial value of the mangrove is under-estimated,
as fisheries, employment, carbon, water purificationand other benefits are ignored. Meanwhile profits
from conversion are typically short-term, with initial
profits from aquaculture or agriculture rapidly falling

Page 36

Section 4. Recognising the multiple values of mangrovesaway as ponds become polluted or degraded, orsoils become acidified or saline. Few will consider
the heightened risk of erosion or flooding.
Communities, managers and planners need to takefull account of all the services and functions ofmangroves. Alongside protecting coasts from wave
damage and erosion and building up sediments,
mangroves are breeding grounds for fish, and richsources of timber and fuel wood. They can help tofilter pollution from estuaries, and in some places
generate rich revenues from tourism and recreation.
They are valuable to coastal villagers, for whomthey supply jobs, income, a stable food supply

and support their recovery after coastal disasters,
providing food, fuel wood and construction
materials. They are also valuable beyond the local
villages, supplying luxury foods such as crabs andshrimp to national and international markets. At
the global level mangroves are some of the richest
stores of carbon. In taking decisions about managing
coastal resources it is important that communities,
managers and planners take full account of all thesevalues, and consider the true costs that may incurfrom mangrove loss by conducting a socio-economic
cost benefit analysis.

Forest products. Mangrove trees
produce dense, valuable wood, whichis resistant to decay and to the impact
of termites. Mangrove timber andpoles have been used for centuries inhouse-building, while thinner “polewood” is also valued for the buildingof fish-traps in mangrove waters. The
same wood is important as fuel and for
production of charcoal. The mangrovepalm, or Nypa, is another highly valued
species. Its enormous frond-leaves arewidely used for making roofs, while
this species also produces considerable
volumes of sugary sap, used to makesugar, and alcohol. Fruits of someother species are also harvested insome countries. Mangroves havebeen sustainably harvested for over
120 years in parts of Malaysia and
Bangladesh, but any harvestingneeds to be managed to ensure themangroves are not degraded or lost.
Commercial-scale mangrove forestry
requires extensive areas of mangrove,but can be sustainable and provide
essential jobs and income.

Fisheries. Mangroves are among theworld’s most productive fishing grounds,
yielding vast numbers of fish, crabs,
shrimps and molluscs. Oysters cling tomangrove roots, crabs and cockles arecollected from the mangrove mud, while
many other species move with the tides.
Fish and shrimp forage among the trees
when the tide is high, but congregate in
the channels as the water falls. It can be
hard to put a value on such fisheries asmuch goes straight to local communities,
and does not pass through conventional
markets. Tens of millions of people rely onsuch fisheries, and without them would
lose their primary source of protein,work or income. In a few cases the valueof mangroves also converts directly tomacro-economic outputs. This can include
rich fisheries located quite far from themangroves. Many fish and shrimp species
use mangroves for just a short part of theirlife-history – adult fish may come into themangroves to breed, and many larvae or
young fish use mangroves as their homein early life. As they mature, such fish andshrimp move out into surrounding waters,
to seagrasses, coral reefs and the wider
expanses of offshore waters. The large
and highly profitable commercial shrimp
fishers from as far afield as Guyana,eastern Malaysia or northern Australia all
depend on shrimps that in turn depend on
mangroves.

Carbon. Vast quantities of carbon
are tied up both in the living treesof the mangrove forests and in theirrich soils. Unlike many other tropicalforests, they sequester carbon andstore it away in the soils where it canremain for millennia, as the saline,
waterlogged conditions prevent the
breakdown of organic material by fungior bacteria. Mangroves are thus often
seen as critical for long-term storage
and sequestration of carbon – removing
carbon dioxide from the atmosphereand storing it in their biomass and soil.
Conversely, mangrove destruction canlead to massive releases of carbon
dioxide, driven not only by the loss oftrees, but by the breakdown of the soil
carbon as well.

Tourism and recreation. Recreational
fishing in mangrove areas can be
big business – certain species suchas bonefish, tarpon and snapper are
considered top “game” fish. These
recreational fisheries are thought tocontribute US$1 billion each year to
Florida’s economy, and $57 million
and $141 million respectively to Belize
and the Bahamas. Such benefits areenjoyed by relatively few fishers, but
if access can be facilitated, mangroves
can be popular for a much wider groupof visitors to explore, by boat or on
foot. Boat trips take visitors to seebirds, monkeys and other wildlife, andin a few places visitors are encouraged
to explore alone or in guided canoe
trips. Visitors on foot are encouragedby trails along board-walks that wind
through mangroves and enable accessat low and high tides. The success ofsuch ventures probably still depends
on accessibility and proximity to othertourism attractions, but numbers insome locations are now very highindeed, with tens of thousands ofvisitors each year to many sites, from
Venezuela to China. Tourism andrecreation in mangroves requires upfront investment, but can yield higheconomic returns to local communities.

Water purification. Mangroves,and associated biota such as algae,
bacteria, and filter-feeding animals,
play an important role in removing
excess nutrients from the water. Someof these are bound into the soils;
others may be broken down and, inthe case of nitrogenous waste, may
be released into the water as inert
nitrogen. Mangroves can also help
to trap sediment passing throughtheir complex aerial root structure.
Such services are not limitless, andexcessive nutrients can remain andthreaten marine life, but in manycases the role in removing toxins andnutrients is highly important. Thismay be particularly important where
mangroves are found near coral reefs,
which are highly sensitive both toexcess sediments and to nutrient
pollution. Water purification canalso be a service of importance foraquaculture.

Page 37

Using mangrove leaves,by Wetlands International

Unfortunately many of these values are overlooked, for three reasons:
1. The value may be high in social terms, but low in conventional economic terms – many mangrove areas are
critical for food supply or jobs, but generate little income.
2. Values from mangroves may be generated elsewhere. Nowhere is this clearer than with offshore shrimp fisheries
where the mangroves provide critical nursery areas to commercially valuable species.
3. Mangroves provide long-term stable income or intermittent high value benefits, which are typically overlookedat the short-term time-scales of many planning bodies. Hence rapid profits made from conversion to agriculture
or aquaculture seem more appealing, although these often have a limited lifetime beyond which they provide
few additional benefits. Over decadal timeframes mangroves would outperform these short-term investments.
Mangrove value can be expressed in terms of monetary value, jobs and security. The growing awareness of thevalue of mangroves is leading to a positive trend. Mangrove planting, as well as natural recovery, has led to
growing mangrove extents in many communities. Such trends, although insufficient to counter losses in otherareas, look set to rise in the future.

Guidance for coastal managers:• Get a full picture of mangrove values and the true costs of mangrove conversion over decadal time scales – using a socio-economic cost benefit analysis - in order to inform coastal
planning and development.
• Note that some values are often overlooked, not only by policy-makers but even by the direct beneficiaries. Notable examples are offshore prawn fisheries and coral reef fisheries,
which often benefit from mangroves.
• Many of the mangrove values need to be considered not only for their direct monetary benefits, but also in terms of providing jobs and food security in places where there may be
few alternatives.

More information• TEEB website: The Economics of Ecosystems and Biodiversity (TEEB) is a global initiative
focused on drawing attention to the economic benefits of biodiversity including the growing
cost of biodiversity loss and ecosystem degradation. TEEB presents an approach that can help
decision-makers recognize, demonstrate and capture the values of ecosystem services andbiodiversity.
• Ecosystem Services Partnership: a worldwide network to enhance the science and practicalapplication of ecosystem services assessment.
• Marine Ecosystem Services Partnership: a virtual centre for information and communication on
the human uses of marine ecosystems around the world.

Page 38• TESSA toolkit: providing practical guidance on how to identify which services may be significant at
a site of interest, what data are needed to measure them, what methods or sources can be used toobtain the data and how to communicate the results.
• INVEST tools: InVEST (Integrated Valuation of Environmental Services and Tradeoffs) is a suiteof software models used to map and value the goods and services from nature that sustain andfulfill human life.
• Hutchison J, Spalding M & zu Ermgassen P (2014) The Role of Mangroves in Fisheries
Enhancement. The Nature Conservancy and Wetlands International. 56 pages.
• UNEP (2014) The Importance of Mangroves to People: A Call to Action. Cambridge, UK, United
Nations Environment Programme World Conservation Monitoring Centre.

References
These guidelines are mainly based on 3 reviews, together summarizing hundreds ofscientific papers on the role of mangroves in coastal risk reduction:• McIvor, AL., Möller I., Spencer, T. and Spalding, M. 2012a. Reduction of wind and swell
waves by mangroves. Natural Coastal Protection Series: Report 1. Cambridge Coastal
Research Unit Working Paper 40. Published by The Nature Conservancy and Wetlands
International.
• McIvor AL., Spencer, T., Möller, I. and Spalding, M. 2012b. Storm surge reduction
by mangroves. Natural Coastal Protection Series: Report 2. Cambridge Coastal
Research Unit Working Paper 41. Published by The Nature Conservancy and Wetlands
International.
• McIvor, AL., Spencer, T., Möller I, and Spalding, M. 2013. The response of mangrove
soil surface elevation to sea level rise. Natural Coastal Protection Series: Report
3. Cambridge Coastal Research Unit Working Paper 42. Published by The Nature
Conservancy and Wetlands International.

Join our efforts
Join us and support our on-going initiatives by advocating
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conservation and restoration, and funding our activities onmangroves and coastal resilience.

Stay in touch
Wetlands International
Femke Tonneijck, program manager
Tel: +31 318660937
Email: femke.tonneijck@wetlands.org
www.wetlands.org
The Nature Conservancy
Mark Spalding, Senior Scientist
Tel: +44 1223334459
Email: mspalding@tnc.org
www.nature.org

Wetlands International@WetlandsInt
Wetlands International