ORIGINAL PAPER 247

Global Environmental Research ©2013 AIRIES 17/2013: 247-254 printed in Japan

The Protective Role of Mangroves and

Other Coastal Forests against Tsunami Damage: Lesson Learned from Case Studies of Two Tsunamis

Toyohiko MIYAGI1*, Hideaki YANAGISAWA1 and Shigeyuki BABA2

1Tohoku Gakuin University, Sendai, Miyagi 980-8511, Japan

2International 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 with an 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 are parameters 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 interesting observation. 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 and the 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 of varying 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), the 2010 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 come forward with systematic scientific studies on the protec-tive 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 con-served, When we saw standing mangrove trees at such sites, we realized that a better understanding is needed regarding the hydrological dynamics of tsunami flow

forces and protective role of coastal trees. This is why we have continued our research on these two aspects of tsunamis. We know that after the 2004 Indian Ocean Tsunami, international and national organizations, NGOs and local people planted mangroves for coastal protec-tion in case of future recurrence (Chan & Baba, 2009). However, these planting activities were carried out with-out any information on reliable and effective mangrove forest establishment against tsunamis.

It has been reported that areas with coastal trees such as mangroves suffered substantially less damage than areas 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 for tsunamis” and “Coastal forests must be maintained” are common.

Our study on the Indian Ocean tsunami led to more detailed research on the role of mangroves in

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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 reali-ties of tsunami damage and the protective role of coastal forests became clearer.

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 the two tsunamis: the Great East Japan tsunami at Sendai Airport, Sendai Bay and Yamashita of the Sendai Plain in Miyagi Pre-fecture, 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) occurred

on 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 the fourth largest in the world’s, the powerful earthquake triggered gigantic tsunami waves, which devastated the coastal plains of Japan. The earthquake and tsunami resulted in a human death toll of tens of thousands, and caused extensive and severe infrastructural damage to coastal protection structures and buildings, and significant changes to coastal and river morphology. Miyagi et al. (2011) reported that surface seismic fault

ruptures and severe landslides occurred during the earth-quake 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 move-ment over a wide area. An hour later, a devastating tsunami 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 broke through the seaward pine forest, about 100 m inland from the coastline, as the beach ridge was only about three meters above sea level (asl). Subsequent waves reached about ten meters. Although some of the pine trees were more than ten meters tall, most were only 3-5 m and were completely submerged in Fig. 1. Seismic vibrations from the earthquake caused widespread liquefaction or reduc-tion 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 m in height) in at least six locations.

When we visited the site after the tsunami, the ground level 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 to about 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 249

Further inland, the pine trees were taller (~20 m in height), 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 ground water level in addition to severe liquefaction. These factors largely diminished the binding ability of the pine trees’ roots to the soil. When the tsunami struck, the trees were easily uprooted and swept away. It was the momentum of these drift trees with projecting roots swept by the tsunami that destroyed the houses and other property including vehicles.

2.2 Landform, coastal forests and tsunamis

We overlaid the Sendai Plain landform and geological map of Matsumoto (1981) with our information on the tsunami 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 distri-bution of drift trees measured by GPS and added photo-graphs 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 with the present coastal beach ridges (3 m asl) as the highest point. Old beach ridges inland of these were formed over thousands of years. Residential areas were located on small ridges. Freshwater back marshes (0-2 m asl) oc-curring between the present and old beach ridges, were

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 obser-vations 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, we observed many trees with broken trunks and many up-rooted ones.

• Near the banks of the Teizan Canal, where tree heights were about 20 m or more, there were many trees with broken trunks but some had survived.

• Further inland in the back marshes, large numbers of trees had been uprooted and washed away, probably due to ground subsidence, liquefaction and their shal-low 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), and

the 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.

250 T. MIYAGI et al.

2.3 Destructive force of tsunami flow on trees and forests

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 domi-nant 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 of trees. From the figure, it can be inferred that most of the small pine trees were not seriously damaged under less than 3-4 m of water depth, and many trees with 20-30 cm dbh withstood up to eight meters while old trees with 50 cm dbh could not withstand as much as eight meters.

Figure 4 shows tree damage under three tsunami inun-dation depths at Yamashita in the southern part of the Sendai Plain. As the tsunami wave passed through the forest, the inundation depth gradually attenuated and the number of damaged trees decreased. The interpretation of these results is similar to that of Shuto (1992). We realized that we could collect only limited data on the

relationship between tree diameters, tree densities and tsunami flow forces, and that we needed more data such as site conditions, root depth, etc.

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.

Fig. 3 Relationship between the diameter of 50%

survival of trees and tsunami inundation depth in coastal forests along Sendai Bay (after Miyagi, 2012).

The Protective Role of Mangroves and Other Coastal Forests against Tsunami Damage 251

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 a massive 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

(Shofiyati et al., 2005). In areas with flat topography, the width of the corridor reached four km inland. Banda Aceh in Sumatra was one of the worst hit areas. With a massive human toll of more than one million killed or displaced, the economic destruction and environmental damage was extensive. Coral reefs, mangroves, coastal forests, agricultural fields and aquaculture ponds were destroyed. The total area affected was estimated at 120,300 ha, comprising mainly settlements (22%) and agricultural land (28%).

In Thailand, much of the impact was along the Andaman 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 Rhizophora trees ~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 Samoatsunami 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 vil-lages and left tens of thousands homeless.

3.1 Tsunami flow forces and tree damage

We collected data to analyse the relationships be-tween tsunami flow forces and tree damage in Indonesia and Thailand following the Indian Ocean tsunami (Yanagisawa et al., 2010; Miyagi et al., 2009). Through these studies, many observations were made which are summarised in Fig. 5.

3.2 Tsunami inundation depth and survival rates of

mangrove 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 the western side (Fig. 6). Results showed that all trees on the western side had died, while on the eastern side, the sur-vival rate was about 50%, even for smaller trees with 10-15 cm dbh.

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 of broken trunks of Rhizophora species with their locations mapped by GPS. Results from simulations of tsunami flow forces and adaptability were also evaluated to determine tsunami inundation depth distributions at each site. There were clear indications that the smaller the tree and the stronger the maximum bending moment of the tsunami, the lower the number of surviving trees (Fig. 7).

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 of forests, we realized that the mortality pattern of young and sparse Sonneratia and Avicennia forests (~15 cm dbh) was quite different from that of young Rhizophora and Bruguiera forests (~10 cm dbh). The mortality pat-tern of the latter forest type was similar to that of man-grove 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).

Fig. 7 Comparison 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.

M: Bending moment, d: DBH

The Protective Role of Mangroves and Other Coastal Forests against Tsunami Damage 253

occurred on sandbars, formed by the interflow of two tidal channels near the mouth of a large lagoon. Since the substrate 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 man-

grove trees ≥10 cm dbh are not damaged by tsunamis with an inundation depth of about three meters. We as-sume that tsunamis with greater than three meter in-undation depth only happen once in 100 years. When they 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 our

estimation is correct, damage to mangroves and other coastal forests would not be severe. In other words, coastal vegetation does contribute to reducing the im-pacts 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 three meters 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 be considered in future studies. Other factors such as land subsidence, raised ground water level and liquefaction that occurred in the Sendai Plain 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 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 the tsunami, but there were many dead standing trees. E: Not seriously damaged area, and many trees survived.

254 T. MIYAGI et al.

contribute to protecting the coast against tsunami damage but if most of the trees are uprooted, they add to the devastation. We acknowledge that the relationship be-tween tsunami flow forces and the protective role of coastal 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 com-ments.

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Toyohiko MIYAGI Toyohiko MIYAGI is a Professor at the Depart-ment of Regional Management, Tohoku Gakuin University, Sendai. He has carried out a series of studies on topics such as mangrove geoecology, landslide mapping and risk evaluation, and local-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 a series of studies on topics such as the tsunami- mitigating 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 Inter-national Society for Mangrove Ecosystems (ISME). He has planned and conducted many mangrove related projects, such as the World Mangrove Atlas 1997, Global Mangrove Data-

base and Information System (GLOMIS), World Atlas of Mangroves 2010, and more. He has trained not only undergraduate, master’s and doctoral students, but also more than 300 trainees on mangroves through JICA training courses.

(Received 1 June 2012, Accepted 5 October 2012)