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ORIGINAL PAPER

The Protective Role of Mangroves andOther Coastal Forests against Tsunami Damage:Lesson Learned from Case Studies of Two Tsunamis
Toyohiko MIYAGI1*, Hideaki YANAGISAWA1 and Shigeyuki BABA2
1

Tohoku Gakuin University, Sendai, Miyagi 980-8511, Japan
2
International Society for Mangrove Ecosystems (ISME),
Nishihara, Okinawa 903-0129, Japan*e-mail: miyagi@izcc.tohoku-gakuin.ac.jp

Abstract
In this paper, some findings on tsunami damage and the protective role of mangroves and other coastal
forests are presented based on case studies of two tsunamis: the Great East Japan tsunami at Sendai Airport,
Sendai Bay and Yamashita of the Sendai Plain in Miyagi Prefecture, Japan; and the Indian Ocean tsunami at
Banda Aceh (Sumatra, Indonesia), and at Khao Lak and Nam Kem (Southern Thailand). From our studies at
Banda Aceh in Indonesia, we found that mangrove trees with ≥10 cm dbh are not damaged by tsunamis withan inundation depth of ~3 m. More than 50% of mangrove trees with ≥10 and ≥30 cm dbh would survive
tsunamis with inundation depths of 4 and 6 m, respectively. Mangroves and other coastal forests do serve to
reduce the impacts of tsunamis by trapping debris and mitigating flow force effects. Mangrove trees growing
on sandbars at Nam Kem in Thailand were an exception as they suffered severe damage during the Indian
Ocean tsunami at 3-4 m inundation depth, suggesting that soil types and possibly root anchorage areparameters to be considered in future studies. Other factors such as land subsidence, raised ground water level
and liquefaction, like that which occurred in the Sendai Plain of Miyagi Prefecture in Japan during the East
Japan earthquake, are noteworthy as they weaken the protective function of coastal forests against tsunami
damage. The destruction of houses and other property by drift trees with projecting roots is another interestingobservation. It appears that mangroves and other coastal forests do contribute to protection against tsunami
damage but if they give way, they add to the devastation. The relationship between tsunami flow forces andthe protective role of coastal vegetation is complex and warrants further investigation.
Key words: coastal forests, earthquake, Great East Japan tsunami, Indian Ocean tsunami, mangroves

1. Introduction
Over the last decade, several tsunamis have occurred
throughout the world. Generated by earthquakes ofvarying magnitudes (M), they included the 2004 Indian
Ocean tsunami (M 9.1), the 2007 Solomon Island
tsunami (M 8.0), the 2009 Samoa tsunami (M 8.1), the2010 Chile tsunami (M.8.8) and the 2011 Great East
Japan tsunami (M 9.0). Some of these tsunamis heavily
impacted coastal vegetation. Despite the serious damage
caused by these tsunamis, too few researchers have comeforward with systematic scientific studies on the protective role of coastal vegetation.
During our visits to the sites damaged by tsunamis,
we wondered if the damage would have been reduced if
mangroves and other coastal vegetation had been conserved, When we saw standing mangrove trees at suchsites, we realized that a better understanding is needed
regarding the hydrological dynamics of tsunami flow
Global Environmental Research
17/2013: 247-254
printed in Japan

forces and protective role of coastal trees. This is why wehave continued our research on these two aspects oftsunamis. We know that after the 2004 Indian Ocean
Tsunami, international and national organizations, NGOs
and local people planted mangroves for coastal protection in case of future recurrence (Chan & Baba, 2009).
However, these planting activities were carried out without any information on reliable and effective mangrove
forest establishment against tsunamis.
It has been reported that areas with coastal trees suchas mangroves suffered substantially less damage thanareas without them (Kathiresan & Rajendran, 2005;
Danielsen et al., 2005). Comments such as, “Because
there were mangrove forests, damage to settlements was
reduced,” “We should plant mangroves in preparation fortsunamis” and “Coastal forests must be maintained” arecommon.
Our study on the Indian Ocean tsunami led tomore detailed research on the role of mangroves in©2013 AIRIES

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disaster prevention and mitigation of tsunami damage(Yanagisawa et al., 2010). Following the Great East
Japan tsunami, which struck the Pacific coast of the
Tohoku region, Miyagi Prefecture in particular, the realities of tsunami damage and the protective role of coastal
forests became clearer.
In this paper, some findings on tsunami damage andthe protective role of mangroves and other coastal forests
are presented based on case studies of the two tsunamis:
the Great East Japan tsunami at Sendai Airport, Sendai
Bay and Yamashita of the Sendai Plain in Miyagi Prefecture, Japan; and the Indian Ocean tsunami at Banda
Aceh (Sumatra, Indonesia), and at Khao Lak and Nam
Kem (southern Thailand).

2. The Great East Japan Tsunami
The powerful East Japan earthquake (M 9.0) occurredon March 11, 2011, with its typical plate boundary
located 38°1’N, 142°9’E (Japan Meteorological Agency,2013). Ranked the largest in Japan’s history and thefourth largest in the world’s, the powerful earthquake
triggered gigantic tsunami waves, which devastated thecoastal plains of Japan. The earthquake and tsunami
resulted in a human death toll of tens of thousands, andcaused extensive and severe infrastructural damage tocoastal protection structures and buildings, andsignificant changes to coastal and river morphology.
Miyagi et al. (2011) reported that surface seismic fault

ruptures and severe landslides occurred during the earthquake and the aftershocks of the earthquake.
2.1 Devastation of the Sendai Plain
The earthquake lasted more than three minutes, from
14:46 to 14:49, and had a continuous seismic intensity of
5-6 on the Shindo scale of the Japan Meteorological
Agency. These seismic vibrations caused crustal movement over a wide area. An hour later, a devastatingtsunami occurred.
A photo taken at 15:55 from a helicopter leaving the
Sendai Airport when the tsunami struck is shown in
Fig. 1. The first wave (8-10 m in height) easily brokethrough the seaward pine forest, about 100 m inland fromthe coastline, as the beach ridge was only about threemeters above sea level (asl). Subsequent waves reachedabout ten meters. Although some of the pine trees weremore than ten meters tall, most were only 3-5 m and werecompletely submerged in Fig. 1. Seismic vibrations fromthe earthquake caused widespread liquefaction or reduction of soil strength and stiffness. Liquefaction occurred
as grey coloured areas. Even an hour after the earthquake,spouting water was ejected upwards like fountains (~1 min height) in at least six locations.
When we visited the site after the tsunami, the groundlevel had subsided by 40 cm and shifted two meters
eastward. The ground water level had risen by 40 cm.
The ground level about 300 m from the coastline sank toabout about one meter asl, resembling a back marsh.

Fig. 1 The first tsunami wave easily broke through the seaward pine forest, inflicting catastrophic damage to landward settlements near
Sendai Airport (photo provided by Kyodo News).

The Protective Role of Mangroves and Other Coastal Forests against Tsunami Damage

Further inland, the pine trees were taller (~20 m inheight), but some had been uprooted and swept away by
the tsunami flow force, destroying human settlements.
The tsunami’s velocity was 5-6 m/s.
We postulate that the strong earthquake tremors led to
ground subsidence over wide areas and the rise in groundwater level in addition to severe liquefaction. These
factors largely diminished the binding ability of the pinetrees’ roots to the soil. When the tsunami struck, thetrees were easily uprooted and swept away. It was themomentum of these drift trees with projecting roots
swept by the tsunami that destroyed the houses and otherproperty including vehicles.
2.2 Landform, coastal forests and tsunamis
We overlaid the Sendai Plain landform and geologicalmap of Matsumoto (1981) with our information on thetsunami inundation depth, topography and coastal forest
destruction at the same location recorded two days after
the disaster (Fig. 2). In the map, we indicated the distribution of drift trees measured by GPS and added photographs taken in the field with some explanations.
The Sendai Plain is characterized by a low-lying
coastal terrain of 0-3 m asl and about ten km width withthe present coastal beach ridges (3 m asl) as the highest
point. Old beach ridges inland of these were formed overthousands of years. Residential areas were located on
small ridges. Freshwater back marshes (0-2 m asl) occurring between the present and old beach ridges, were249

mostly used for rice cultivation (Fig. 2). The tsunami
struck the coast with waves up to ten meters depth at an
estimated velocity of 10 m/s. Several hundred meters
inland, inundation was six meters and a region extending
up to 4-5 km inland was inundated. The coastal pine trees
planted on beach ridges 100 m or more inland were
20-30 years old, but some of them along the Teizan Canal,
which was built about 400 years ago, were about
100 years old, and most of pine trees planted for erosion
control were over 50 years old.
Based on analysis of field data, the following observations were made:• On the coast, almost all young trees (10-20 cm dbh)
were toppled but not uprooted.
• Behind the coastal beach ridges, where the ground level
was lower and tree diameters were over 20 cm, weobserved many trees with broken trunks and many uprooted ones.
• Near the banks of the Teizan Canal, where tree heights
were about 20 m or more, there were many trees withbroken trunks but some had survived.
• Further inland in the back marshes, large numbers oftrees had been uprooted and washed away, probably
due to ground subsidence, liquefaction and their shallow root system of only 0.3-1.0 m.
• After the tsunami, numerous holes were observed.
They indicated locations where trees had been uprooted
and swept away.

Fig. 2 Photos taken in the field (top), GIS data of land features and the distribution of drift trees with some field evidence (middle), andthe cross profile landform geology, elevation and tsunami inundation depth (bottom) on the Sendai Plain. Blue spots indicate
locations where trees had been uprooted and washed away. Red spots indicate locations where drift trees accumulated.

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2.3 Destructive force of tsunami flow on trees andforests
When estimating the fluid force of a tsunami,
many parameters such as water depth, water speed,
bottom friction (roughness), etc. need to be considered(Yanagisawa et al. 2009). The protective role of coastal
trees against a tsunami depends on the strength of dominant trees based on their diameters and densities.
After the tsunami, we measured over 100 pine trees in
the Sendai Plain. Figure 3 shows the relationship between
water depths of the tsunami and dbh of 50% survival oftrees. From the figure, it can be inferred that most of thesmall pine trees were not seriously damaged under lessthan 3-4 m of water depth, and many trees with 20-30 cm
dbh withstood up to eight meters while old trees with50 cm dbh could not withstand as much as eight meters.
Figure 4 shows tree damage under three tsunami inundation depths at Yamashita in the southern part of the
Sendai Plain. As the tsunami wave passed through theforest, the inundation depth gradually attenuated and thenumber of damaged trees decreased. The interpretationof these results is similar to that of Shuto (1992). We
realized that we could collect only limited data on therelationship between tree diameters, tree densities andtsunami flow forces, and that we needed more data suchas site conditions, root depth, etc.

Fig. 3 Relationship between the diameter of 50%
survival of trees and tsunami inundation depth
in coastal forests along Sendai Bay (after
Miyagi, 2012).

Fig. 4 Field evidences of the tsunami impact, at Yamashita, southern part of Sendai Coastal Plain.
A: Relationships between the tree diameter, and damaged and survived trees (modified from Miyagi, 2012). B: Aerial photo 40 days after the tsunami.
C: Iso depth of the tsunami inundation. D: Evidence of the tsunami impact. 1: Destructed building by 10 m deep tsunami. 2: Destructed pine forests by
7-8 m deep tsunami at the seaside. 3: Less damaged forest at the landward. 4: House located 550 m from the coast with clear watermark in 3.5 m of water
depth.

The Protective Role of Mangroves and Other Coastal Forests against Tsunami Damage

3. The Indian Ocean Tsunami
On December 26, 2004, the Indian Ocean tsunami
struck 13 countries in Asia with disastrous impacts(Kathiresan & Rajendran, 2005; Chandrasekar & Ramesh,2007; Osti et al., 2008). Tsunami waves generated by amassive earthquake in the ocean bed spread in all
directions, with speeds up to 900 kph. The tsunami killed
more than 200,000 people, made two million people
homeless and resulted in losses of US$ 6 billion.
With the earthquake epicenter less than 40 km from
Sumatra, Indonesia was the worst affected country(Srinivas & Nakagawa, 2008). Extending up to two km
inland, ~600 km of the coastline of Sumatra was damaged

251(Shofiyati et al., 2005). In areas with flat topography, thewidth of the corridor reached four km inland. Banda
Aceh in Sumatra was one of the worst hit areas. With amassive human toll of more than one million killed ordisplaced, the economic destruction and environmental
damage was extensive. Coral reefs, mangroves, coastal
forests, agricultural fields and aquaculture ponds weredestroyed. The total area affected was estimated at120,300 ha, comprising mainly settlements (22%) andagricultural land (28%).
In Thailand, much of the impact was along theAndaman coast, affecting six coastal provinces, namely
Phuket, Phang Nga, Krabi, Ranong, Trang and Satun(Harakunarak & Aksornkoae, 2005). The tsunami killed

Fig. 5 Damage to mangrove forests caused by the Indian Ocean tsunami and the Samoa tsunami.
1. Large Rhizophora trees which showed no discernable physical damage after inundation depths of 3−4 m, but died one year later (Khao Lak, Thailand);2. Young trees bent over by the tsunami hydraulic force (Nam Kem, Thailand); 3. Toppling of Sonneratia trees due to scouring of the soil around their roots(Nam Kem); 4. Remaining mature trees of Rhizophora with broken trunks (Banda Aceh, Indonesia); 5. Broken trunks of mature Rhizophora trees 1 km inland(Banda Aceh); 6. Survival of only a few mature mangrove trees under the brutal hydraulic force in areas where the tsunami height was 7−8 m, (Banda Aceh);
7. All mangrove trees completely washed away in locations where the tsunami reached 10 m in height (Banda Aceh); 8. A remaining cluster of Rhizophoratrees ~500 m from the coast (Banda Aceh); 9. Destruction of a mature Rhizophora forest by the tsunami of 7−8 m in height (Khao Lak); 10. Tree debris
accumulated seaward of an inland mangrove forest brought in by the tsunami with 4 m inundation depth (Ko Phi Phi, Thailand). Photos of the 2010 Samoa
tsunami are included for comparison. 11. Debris trapped by large Bruguiera trees (Upolu Island, Samoa); 12. Tsunami inundation of 3 m or less caused little
damage to large Bruguiera trees and their root systems (Upolu Island).

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more than 5,000 people, affected about 490 fishing villages and left tens of thousands homeless.
3.1 Tsunami flow forces and tree damage
We collected data to analyse the relationships between tsunami flow forces and tree damage in Indonesia
and Thailand following the Indian Ocean tsunami(Yanagisawa et al., 2010; Miyagi et al., 2009). Throughthese studies, many observations were made which aresummarised in Fig. 5.

3.3 Tree damage in Indonesia and Thailand
During our survey in Banda Aceh, Indonesia, and in
Khao Lak, Thailand, we measured the dbh and height ofbroken trunks of Rhizophora species with their locations
mapped by GPS. Results from simulations of tsunami
flow forces and adaptability were also evaluated todetermine tsunami inundation depth distributions at eachsite. There were clear indications that the smaller the tree
and the stronger the maximum bending moment of thetsunami, the lower the number of surviving trees (Fig. 7).

3.2 Tsunami inundation depth and survival rates ofmangrove trees
The relationship between tsunami inundation depth
and survival rates of mangrove trees (mainly Rhizophora
apiculata) was determined for Banda Aceh. Inundation
depth was about four meters on the eastern side, six
meters at two middle sites and over ten meters on thewestern side (Fig. 6). Results showed that all trees on thewestern side had died, while on the eastern side, the survival rate was about 50%, even for smaller trees with
10-15 cm dbh.

3.4 Mangrove forest damage in Thailand
In Nam Kem, southern Thailand, mangrove forests
with relatively young mangrove trees were severely
damaged by the tsunami. After surveying about 33 ha offorests, we realized that the mortality pattern of young
and sparse Sonneratia and Avicennia forests (~15 cmdbh) was quite different from that of young Rhizophoraand Bruguiera forests (~10 cm dbh). The mortality pattern of the latter forest type was similar to that of mangrove forests in Banda Aceh.
Young and sparse Sonneratia and Avicennia forests

Fig. 6

Relationship between tree diameters and survival rates in Banda Aceh,
Indonesia (Yanagisawa et al. 2007).

M: Bending moment, d: DBH
Fig. 7Comparison between maximum bending moment, and diameter and survival of
Rhizophora trees in Banda Aceh and Khao Lak (Yanagisawa et al. 2010). Survival
rates were calculated based on trees with average dbh of 22 cm and tsunami
inundation depth of 6 m.

The Protective Role of Mangroves and Other Coastal Forests against Tsunami Damage

occurred on sandbars, formed by the interflow of two
tidal channels near the mouth of a large lagoon. Since thesubstrate was ill-consolidated sand and the ground level
was lower than the mean high water level, the surface soil
was easily eroded away. The tsunami uprooted and swept
away most of the trees (Fig. 8). We thought that a 3-4 m
tsunami would not seriously damage mangrove trees
based on earlier observations elsewhere. However, in
Nam Kem, mangrove forests on sandbars showed that a
3-4 m tsunami could inflict serious damage.

4. Conclusion
From our studies at Banda Aceh, we found that mangrove trees ≥10 cm dbh are not damaged by tsunamis
with an inundation depth of about three meters. We assume that tsunamis with greater than three meter inundation depth only happen once in 100 years. Whenthey do occur, more than 50% of mangrove trees ≥10 and≥30 cm dbh would survive tsunamis with inundation
depths of four and six meteres, respectively. If our253

estimation is correct, damage to mangroves and othercoastal forests would not be severe. In other words,
coastal vegetation does contribute to reducing the impacts of tsunamis by trapping debris and mitigating flow
force effects. Thus, mangroves and other coastal forests
do provide protection to human lives and properties. If
these coastal trees are well managed, they may serve as
an effective buffer against tsunamis not exceeding threemeters in inundation depth.
The mangroves at Nam Kem presented an interesting
exception to our general findings. Trees growing on
sandbars suffered severe damage during the Indian Ocean
tsunami with 3-4 m inundation depth, suggesting that soil
types and possibly root depth are parameters to beconsidered in future studies. Other factors such as land
subsidence, raised ground water level and liquefactionthat occurred in the Sendai Plain during the East Japan
earthquake are noteworthy as they weaken the protectivefunction of coastal forests against tsunami damage. The
destruction of houses and other property by drift trees
with projecting roots is another interesting observation.
It appears that mangroves and other coastal forests do

Fig. 8 Severely damaged mangrove forests on sandbars in Nam Kem, Thailand.
Two IKONOS images taken just before and the after tsunami. Left: taken in January 2004, Right: taken in January 2005.
A: Direction of the tsunami. B: Strongly eroded area by the tsunami. C: Eroded area by the tsunami. D: Non-eroded area by thetsunami, but there were many dead standing trees. E: Not seriously damaged area, and many trees survived.

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contribute to protecting the coast against tsunami damage
but if most of the trees are uprooted, they add to thedevastation. We acknowledge that the relationship between tsunami flow forces and the protective role ofcoastal vegetation is complex and warrants further
investigation.

Acknowledgements
This study was partly supported by the Strategic
Research Foundation Grant-aided Project for Private
Universities of Japan. The authors are thankful to H.T.
Chan and M. Kainuma for their amendments and comments.
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Toyohiko MIYAGIToyohiko MIYAGI is a Professor at the Department of Regional Management, Tohoku Gakuin
University, Sendai. He has carried out a series ofstudies on topics such as mangrove geoecology,
landslide mapping and risk evaluation, andlocal-scale disaster presention. He has taught
undergraduate, masters and doctoral students,and has served as an instructor for JICA training courses on mangroves.

Hideaki YANAGISAWA
Hideaki YANAGISAWA is a lecturer for the
Department of Regional Management, Tohoku
Gakuin University, Sendai. He has carried out aseries of studies on topics such as the tsunamimitigating effects of mangrove forests, tsunami
fragility functions, and risk evaluation. He has
participated as a lecturer in IISEE tsunami
training courses, where he taught the trainees how to run tsunami
simulation models.

Shigeyuki BABA
Shigeyuki BABA is a Professor at the Tropical
Biosphere Research Center, University of the
Ryukyus, and Executive Director of International Society for Mangrove Ecosystems(ISME). He has planned and conducted many
mangrove related projects, such as the World
Mangrove Atlas 1997, Global Mangrove Database and Information System (GLOMIS), World Atlas of Mangroves2010, and more. He has trained not only undergraduate, master’s anddoctoral students, but also more than 300 trainees on mangroves through
JICA training courses.

(Received 1 June 2012, Accepted 5 October 2012)