Abstract This study explored the effects of coastalvegetation on tsunami damage based on field obser-

vations carried out after the Indian Ocean tsunami on

26 December 2004. Study locations covered about

250 km (19 locations) on the southern coast of Sri

Lanka and about 200 km (29 locations) on the And-

aman coast of Thailand. The representative vegetation

was classified into six types according to their habitat

and the stand structures of the trees. The impact of

vegetation structure on drag forces was analyzed using

the observed characteristics of the tree species. The

drag coefficient, including the vertical stand structures

of trees, Cd-all, and the vegetation thickness (cumula-

tive trunk diameter of vegetation in the tsunami

direction) per unit area, dNu (d: reference diameter of

trees, Nu: number of trees per unit area), varied greatly

with the species classification. Based on the field survey

and data analysis, Rhizophora apiculata and Rhizo-

phora mucronata (hereafter R. apiculata-type), kinds of

mangroves, and Pandanus odoratissimus, a represen-

tative tree that grows in beach sand, were found to be

especially effective in providing protection from tsu-

nami damage due to their complex aerial root struc-

ture. Two layers of vegetation in the vertical direction

with P. odoratissimus and Casuarina equisetifolia and a

horizontal vegetation structure of small and large

diameter trees were also important for increasing drag

and trapping floating objects, broken branches, houses,

and people. The vertical structure also provided an

effective soft landing for people washed up by the

tsunami or for escaping when the tsunami waves hit,

although its dNu is not large compared with R. apicu-

lata-type and P. odoratissimus. In addition, the creeks

inside mangroves and the gaps inside C. equisetifolia

vegetation are assumed to be effective for retarding

tsunami waves. This information should be considered

in future coastal landscape planning, rehabilitation,

and coastal resource management.

Keywords Rhizophora apiculata � Pandanusodoratissimus � Tsunami protection � Coastalvegetation � Forest structures

Introduction

The Indian Ocean tsunami on 26 December 2004 was a

destructive force of nature that swept away entire vil-

lages and resulted in the deaths of approximately

200,000 people in the countries around the Indian

Ocean. About two-thirds of Sri Lanka (Wijetunge

2005) and the western coast of Thailand were severely

damaged on a scale these countries have never expe-

rienced before. This damage emphasizes the impor-

tance of developing methodologies to minimize the

impact of future tsunamis and other related natural

disasters in order to protect people, the environment,

and the infrastructure. Establishment of a hard infra-

N. Tanaka (&) � Y. Sasaki � K. B. S. N. JinadasaGraduate School of Science and Engineering,Saitama University, 255 Shimo-Okubo, Sakura-ku,Saitama-shi, Saitama-ken 338-8570, Japane-mail: n-tanaka@post.saitama-u.ac.jp

M. I. M. MowjoodFaculty of Agriculture, University of Peradeniya,Peradeniya, Sri Lanka

S. HomchuenFaculty of Science, Khon Kaen University, Khon Kaen,Thailand

Landscape Ecol Eng (2007) 3:33–45

DOI 10.1007/s11355-006-0013-9

123

ORIGINAL PAPER

Coastal vegetation structures and their functions in tsunamiprotection: experience of the recent Indian Ocean tsunami

Norio Tanaka Æ Yasushi Sasaki Æ M. I. M. Mowjood ÆK. B. S. N. Jinadasa Æ Samang Homchuen

Received: 17 January 2006 / Revised: 9 May 2006 / Accepted: 2 August 2006 / Published online: 5 October 2006� International Consortium of Landscape and Ecological Engineering and Springer 2006

structure for tsunami protection is not feasible in many

cases because it would adversely affect the ecology and

aesthetics of the beachfront. Moreover, developing

countries cannot afford such technologically advanced

and capital-intensive solutions.

Coastal vegetation can play a significant role in

reducing the severity of tsunami waves and dissipating

the disastrous amount of energy associated with them

(Shuto 1987; Mazda et al. 1997a; Kandasamy and Na-

rayanasamy 2005; Dahdouh-Guebas et al. 2005). In

addition, restoring green vegetation could enhance the

seaside landscape. Shuto (1987) quantitatively esti-

mated the effectiveness of coastal forests against tsu-

nami by statistically analyzing the physical damage

suffered by pine trees in Japan. In addition, Hamzah

et al. (1999) demonstrated the effects of model man-

grove stands against tsunami attack in experiments

from the viewpoint of hydraulic resistance, and

emphasized that such vegetation provides effective

protection against tsunamis. However, the species they

studied were different from the dominant varieties

found in Sri Lanka (Jayatissa et al. 2002a) and Thai-

land (Sanit et al. 1992). Therefore, the present study

focuses on the role of the coastal vegetation and forest

structures found in these two countries in the minimi-

zation of damage aimed at protecting human lives, the

environment, and the infrastructure in coastal regions

against future tsunami events.

Field survey and analysis

Field survey

The area investigated covered about 250 km (19 loca-

tions) on the southern coast from Negombo to Kal-

munai, Sri Lanka, from 1 to 6 April 2005, and about

200 km (29 locations) on the Andaman coast from

Phuket to Ranong, Thailand, from 27 February to 5

March 2005. Figure 1 shows the site locations. The

representative vegetation was classified into six types

according to their habitat and the stand structures of

the trees, that is, the trunk diameter (d), height (h), and

density of the trees (n) (Fig. 2). The six species are (a)

Casuarina equisetifolia, a representative tree that

grows in beach sand, corresponding to pine trees in

Japan, (b) Anacardium occidentale, a plantation spe-

cies in the coastal zone, (c) Cocos nucifera, a plantation

species in the coastal zone, (d) Avicennia alba or

Avicennia marina, hereafter A. alba-type, representa-

tive mangrove species found in small tidal zones, (e)

Pandanus odoratissimus, a representative tree thatFig. 1 Field survey points: a Sri Lanka (19 locations, southerncoast), b Thailand (29 locations, western coast)

34 Landscape Ecol Eng (2007) 3:33–45

123

grows in beach sand, and (f) Rhizophora apiculata (the

dominant Rhizophora spp. on the western coast of

Thailand) or Rhizophora mucronata (the dominant

Rhizophora spp. on the southern coast of Sri Lanka),

hereafter R. apiculata-type, representative mangrove

species in large tidal zones. Figure 3 shows examples of

the representative tree structures growing near the

coastal region.

While species classification was being carried out,

the forest width in the tsunami direction was also

investigated. Moreover, most mangroves grow in re-

gions of weak waves and currents, so the tsunami

height is usually low compared with that on the sand

beaches. Therefore, this study surveyed mangrove

forests located in island areas facing the Andaman Sea

and Indian Ocean that were attacked directly by the

tsunami.

The tsunami water depth at each site was deter-

mined by the height of broken branches, watermarks

on damaged houses (especially flood marks on broken

roofs), scars on tree trunks or branches, and from the

reports of survivors.

Analysis of drag force coefficient using

representative tree species

Figure 4 shows a schematic of the vegetation charac-

teristics analyzed in this study. To analyze the physical

characteristics of coastal vegetation, we considered the

drag force (Nepf 1999; Mazda et al. 1997b) in the W

(m) tsunami direction (forest width in this direction)

with unit length (cross-stream direction) as:

Dcum¼n�ðdragforceonone treeÞ¼n�Z

1

2Cdiqu

2i dAi

¼n2�ðabCdÞ�qU2� h

d

100

� �

¼12�dn�ab

100�CdqU2h ð1Þ

Fig. 2 Representative coastalvegetation in the observedarea: a Casuarinaequisetifolia, b Anacardiumoccidentale, c Cocos nucifera,d Avicennia alba, e Pandanusodoratissimus, f Rhizophoraapiculata

Landscape Ecol Eng (2007) 3:33–45 35

123

Trunk density (trunks=m2Þ¼ nW�1¼

1

l1� l2¼ 1

l2ð2Þ

ab ¼ 1h

Z h0

aðzÞbðzÞdz ¼ ðbranches effect on CdÞ

� ðleaves effect on CdÞ

¼ 1n

Xni¼1

ai � b ¼1

n

Xni¼1

dAidA1:2

� b ð3Þ

dAi ¼Xm¼i maxm¼1

dAim ð4Þ

where Dcum is the cumulative value of the drag force of

trees along a width of W (m) and a length of vegetation

of 1 m (Fig. 4a) in the tsunami current direction, n is

the number of trees for a vegetation width of W (m)

and length of 1 m [units: trunks/(vegetation width (W)

m · 1 m)] defined by Shuto (1987), Cdi and ui are thedrag coefficient and velocity (m/s) vertically divided by

the area dAi (m2) (Fig. 4c), respectively, q (kg/m3)

is the density of salt water, l1 and l2 (m) are the spac-

ings between the trees in the shore and cross-shore

directions, respectively, l (m) is the average spacing of

the trees, z (m) is the vertical distance from the ground

surface, h (m) is the tsunami depth, d (cm) is the ref-

erence tree trunk diameter 1.2 m above the ground,

where the units of d was taken as cm for comparison

with the regression line between the forest character-

istics (forest width and tree diameter) and the condi-

tion (broken or unbroken) of the trees according to

Shuto (1987), dn [cm/(vegetation width (W) m · 1 m)]is the cumulative tree diameter of the forest in the

tsunami direction, hereafter called the vegetation

thickness according to Shuto (1987), a(z) and ai areadditional coefficients that express the additional

branch area at height z or at the ith layer, respectively,

from the ground surface, normalized by the area,

dA1.2 = 0.5(d/100) [where 0.5 (m) is the divided height

in the vertical direction (Fig. 4)], b(z) and bi areadditional coefficients representing the effect of leaves

at z or the ith layer, respectively, a and b are the depth-averaged values of ai and bi, respectively. Additionaldrag by leaves was taken as the constant b = 1.25 (inleaf-bearing layers) or b = 1 (in leafless layers), refer-ing to previous research (Fukuoka and Fujita 1990). In

this research, the difference in the stream-wise velocity

at each height z, u(z), was not considered, and the

depth-averaged velocity, U (m/s), was used for ui (see

Fig. 4b).

The drag coefficients, including vertical vegetation

structure (Kutija and Hong 1996), Cd-all, the effective

vegetation thickness, dNall [cm/(vegetation width (m)

·1 m)], and the vegetation thickness per unit area, dNu[cm/(unit vegetation area (m2))], were defined from

Eqs. 1, 2, and 3 as:

Cd�all ¼ ab� Cd ð5Þ

dNall ¼ ab� dn ð6Þ

dNu ¼dNall

W¼ dNall

l2n¼ abd

l2ð7Þ

Eqs. 5, 6, and 7 are all related to the drag force in

Eq. 1. Cd-all describes the characteristics of the tree

itself, dNall describes the characteristics including the

effect of the tree structure ab in the W (m) · 1 mvegetation, and dNu describes the characteristics of a

unit vegetation area.

Rhizophora apiculata-type trees have complex aerial

root structures (Sanit et al. 1992; Jayatissa et al. 2002a)

that affect the drag coefficient (Wolanski et al. 1980).

The value of Cdi at each height varied from 1.5 to 1,

according to the spacing of the aerial roots (Furukawa

et al. 1997; Nepf 1999; Tanaka et al. 2005). dAi was the

projected area divided by the height of 0.5 m. Addi-

Fig. 3 Representative trees in coastal regions and their verticalstructures: a C. nucifera, b P. odoratissimus, c R. apiculata, dC. equisetifolia, e Thespesia populnea, f Pongamia pinnata, gA. occidentale, h Terminalia catappa

36 Landscape Ecol Eng (2007) 3:33–45

123

tional areas of the projecting aerial roots or of bran-

ches were calculated by analyzing digital photographs

(Fig. 4c). The values of d, n, W, l1, l2, ai, and h inEqs. 1–7 were obtained in the field survey.

Results

Two representative sites in Sri Lanka and three sites in

Thailand are shown in Figs. 5 and 6, respectively. Re-

lated photographs are shown in Figs. 7 and 8. The

damage to sites in Sri Lanka and Thailand is discussed

in following two sections.

Examples of coastal vegetation effect in Sri Lanka

Kalutara

Figure 5a shows a schematic of the tsunami damage at

Kalutara. The C. nucifera trees growing along the coast

(line A) were 3–7 m in height and 3 m apart, but some

of them were damaged, and houses behind the trees

were damaged up to 100 m from the coast. C. nucifera

was also planted between houses, but these trees

played no role in tsunami protection, even though the

tsunami height was relatively low, reaching 3, 1.3, 1.1,

and 0.9 m at 50, 120, 160, and 220 m, respectively, from

the coast.

Pandanus odoratissimus and C. equisetifolia were

found as natural coastal vegetation at line B, 500 m

south of line A and near the mouth of the Kalu Ganga

river. The vegetation width in the tsunami direction

was 20 and 40 m for P. odoratissimus and C. equiseti-

folia, respectively. At the interface, P. odoratissimus

and C. equisetifolia were mixed for a distance of about

20 m (Fig. 8a). The tsunami height 60 m inland from

the coast was 0.6 m, and the houses located within this

area were not as heavily damaged because the tsunami

height was low compared to its height at line A. It is

supposed that the tsunami current was repelled by the

vegetation and the sand dune because they played a

role in blocking the current.

At line C, mangroves played a role in dissipating the

energy of the tsunami and protecting the bank. The

tsunami was weak by the time it reached the houses

near the mangrove and caused no damage.

Hikkaduwa

A railroad train on the sand dune was hit by the tsu-

nami in this area (Telwatta). P. odoratissimus was

growing along the shoreline, but there were only 1–2

Fig. 4 Definitions of treecharacteristics: n [the numberof trees for vegetation with awidth of W (m) and a lengthof 1 m) and ai (the additionalcoefficient to express theadditional branch area in theith layer from the groundsurface). The height wasdivided into 0.5 m layers inthis study. dAi is the area (m

2)in the ith layer from theground, and dA1.2 is thereference area [referencediameter (d)/100 · height ofthe layer (0.5m)], ai is the areadAi normalized by dA1.2. u(z)is the stream-wise velocity atheight z, and U is the velocityaveraged over depth

Landscape Ecol Eng (2007) 3:33–45 37

123

rows in the tsunami direction. The tsunami height was

more than 5 m at the coast and about 3 m at the nearby

railway track. Behind P. odoratissimus, C. nucifera was

planted about 500 m in the tsunami direction with

large spaces (about 4–40 m) between the trees. The

tsunami passed through the C. nucifera vegetation and

destroyed the houses in the forest, reaching the lagoon

behind the sand dune. The average diameter of C.

nucifera was large enough (0.3 m), but the spaces be-

tween were too large to defend against the tsunami at

this site, similarly to at Kalutara.

Fig. 5 Examples of the effect of vegetation in Sri Lanka: aKalutara (P. odoratissimus), b Medilla (P. odoratissimus andmangrove)

Fig. 6 Examples of the effect of vegetation in Thailand: a PhraThong Island and Kang Islands (R. apiculata), b Ban Thale Nok(C. equisetifolia, A. occidentale), c Laem Son National Park (C.equisetifolia)

38 Landscape Ecol Eng (2007) 3:33–45

123

Medilla

Ten meter-thick clumps of P. odoratissimus were lo-

cated along the shoreline, and C. nucifera was planted

for about 100 m further at this site (Fig. 5b). The P.

odoratissimus was broken, and the tsunami went over

the dune, accelerated down the slope, and destroyed

the resort cottages in C. nucifera vegetation. The tsu-

nami current also broke the mangrove trees down-

stream of the P. odoratissimus–C. nucifera vegetation.

The mangrove forests were damaged inland 45 m (line

I), 64 m (line II), and 84 m (line III) for the P. odo-

ratissimus–C. nucifera, P. odoratissimus, and C. nucif-

era lines, respectively. The tsunami was more than 6 m

and about 4 m high, respectively, at the coast and the

top of the dune. The local scour depth and area in front

of the P. odoratissimus vegetation at line I or line II

were larger than those at line III. The scouring also

occurred inside the vegetation at line III, but not at line

I or II. The species of mangrove trees in the three lines

in the lagoon were not exactly the same, but a band

about 10 m inland of dense P. odoratissimus vegetation

Fig. 7 Damage patterns ofthe representative species: ascoured and dead afterwards(C. equisetifolia at Khao Lak),b scoured and uprooted (C.equisetifolia at Khao Lak), cscoured and uprooted (C.nucifera at Kalutara), dscoured and trunks broken(P. odoratissimus at Medilla),e broken just above the aerialroots (P. odoratissimus atMedilla), f large scour holearound a tree (C. equisetifoliaat Suk Samaran village), gtrunks broken just above theground (C. equisetifolia atLaem Son), h survival despiteadditional drag by debris(R. apiculata at Laem Son)

Landscape Ecol Eng (2007) 3:33–45 39

123

reduced the tsunami velocity, although their main

trunks were completely broken at 1–2 m above the

ground in the front region (2–5 m) of the vegetation.

Yala

The tsunami height was about 5–5.5 m at the coast in the

Yala National Forest Park in Sri Lanka. The Yala Safari

Hotel, which was located in front of an old sand dune at

the beach, was completely destroyed with an enormous

loss of life. Part of the tsunami current was stopped at

the old sand dune about 300 m from the coast, but a part

of the current was turned around by the dune and

formed a new lake behind the sand dune. C. equisetifolia

about 0.15 m in diameter survived the tsunami, but most

Bauhinia spp. and Zizyphus spp., trees about 0.07–0.1 m

diameter, were broken about 200 m in the tsunami

direction. Trees about 0.05 m diameter were completely

washed away. Manikara hexandra, about 0.3 m diame-

ter, withstood the washed up debris of broken houses

and fishing boats, even though the trees received addi-

tional drag force due to the attached objects (Fig. 8d).

Trees with a diameter more than 0.3 m are effective for

trapping debris, but the spacing between trees was too

large in this case. Therefore, the survey indicates the

importance of a lateral distribution of trees with a

diameter of 0.1–0.3 m.

Oluvil

The tsunami water depth at the coast was 2.5 m, which

was low compared to other locations. A maximum of

eight rows of C. nucifera were planted about 54 m in

the direction of the tsunami inundation. The energy

dissipation effect was so low that many houses were

destroyed despite the low tsunami height. However,

the creek behind the vegetation played a role in

decreasing the velocity, and the houses behind the

creek remained intact.

Examples of the effect of coastal vegetation

in Thailand

Phra Thong Island and Kang Islands

The tsunami height was 10 m at Phra Thong Island

(Fig. 6a). The tsunami broke 0.1 m diameter C. equis-

etifolia, passed through 0.5 m diameter C. equisetifolia,

and damaged the town. A. alba was broken only on the

fringe of the vegetation. R. apiculata and R. mucronata

were broken for a distance of about 50 m by an 8 m

tsunami at an upstream island of the Kang Islands.

These species were broken at the trunk, which was

higher than the aerial root system (Figs. 2, 3), similarly

to P. odoratissimus.

Ban Thale Nok

The tsunami height at the coast, where C. equisetifolia

(with a diameter of 0.5–1.0 m) was growing, was about

10 m (Fig. 6b). Behind the C. equisetifolia vegetation, a

wetland (line I) of sparsely spaced C. equisetifolia and

R. apiculata (line II) existed. Some of the trees at line

II were broken, and the broken branches and trees

accumulated in the creek in front of the R. apiculata

vegetation. The R. apiculata behind the creek was not

Fig. 8 Vegetation structures:a two-layer vegetation(C. equisetifolia + P.odoratissimus), b two-layervegetation (C. nucifera +P. odoratissimus), c gap in aC. equisetifolia vegetation,d debris trapped by Manikarahexandra

40 Landscape Ecol Eng (2007) 3:33–45

123

broken even when the tsunami height was presumably

6 m at the inlet of the R. apiculata vegetation. Houses

located 700 m from the coast, at line IV, were de-

stroyed. Cashew nut trees (A. occidentale) were plan-

ted at line III. The branches were broken at the edge

facing the coast, but most of the trees remained. A

house 450 m from the coast and behind the A. occi-

dentale trees was not damaged. Both line III and IV

were cultivated from mangrove to agricultural land,

but the land use affected the magnitude of the damage.

Laem Son National Park

At line A, the tsunami, about 3 m high at the coast,

swept about 1.8 km inland (Fig. 6c). It passed through

200 m of C. equisetifolia vegetation. The trunk diam-

eter of the trees was about 0.5–1 m. At line B, the

tsunami broke young C. equisetifolia, the trunks of

which were about 0.07 m in diameter. Four gaps inside

the vegetation formed following the shape of sand bar

formation (Fig. 8c). The broken branches were accu-

mulated in front of the large C. equisetifolia at the edge

of the gap. At line C, R. apiculata was not damaged by

the 5 m tsunami (Fig. 7h).

The pattern of vegetation and tree damage

in relation to species and tsunami height

Main trunks larger than 0.1 m in diameter were seldom

broken by a 5–10 m tsunami, except for R. apiculata

(Kang Island), which was broken by 8 m tsunami,

R. mucronata (Medilla), by a 6 m tsunami, C. equiseti-

folia (Ban Thale Nok) by 10 m tsunami (Fig. 7g), and

P. odoratissimus (Hikkaduwa, Medilla) by 5–6 m tsu-

nami (Fig. 7d, e). The pattern of uprooting was related

to the strength of the substrate and was usually

observed at the front line of the vegetation (Fig. 7b, d).

C. nucifera had a root zone of more than 10 m growing to

a depth of 0.3–0.4 m. The roots were undercut by erosion

and strong drag forces by a tsunami of more than 5 m

(Fig. 7c). At the front line of a C. equisetifolia vegetation

(d > 0.3 m), C. equisetifolia itself was not broken, but

large amounts of erosion occurred (Fig. 7a, f).

Pandanus odoratissimus and R. apiculata-type trees

have many aerial roots, and the moment of the drag

force can be shared by the aerial roots. Thus, they were

able to withstand a less than 5 m tsunami, even with

debris attached to the aerial roots and additional force

applied (Fig. 7h). However, if the drag moment ex-

ceeded the threshold for the breaking moment when

the tsunami water was high, the trunks were broken

just above the aerial roots (Fig. 7e).

Most of the broken or uprooted trees, e.g., C.

equisetifolia, R. apiculata, C. nucifera, Terminalia cat-

appa, and Borassus flabellifer, were not washed away

but remained in place. Inside the forest, erosion of the

soil seldom occurred except for just around tree trunks.

The local scour around the tree trunk was presumably

caused by three-dimensional turbulent eddies that are

similar to the horseshoe vortices (Baker 1980) gener-

ated around an obstacle on a flat bed (Tamai et al.

1987), i.e., a bridge pier in a river bed (Melville and

Sutherland 1988). The local scour depth reached a

maximum of 0.3–0.4 m for C. nucifera and 0.7–0.9 m

for C. equisetifolia. The local scour depth was less than

the tree diameter and the depth of the root zone. Thus,

the local scouring itself did not uproot the trees.

The characteristics of representative coastal

vegetation in both countries

Figure 9a shows the relationship between the trunk

diameter (d) and the average space between trees (l).

The distance becomes larger with increasing trunk

diameter. This figure shows that a larger tree requires a

larger spacing (lower tree density) to grow. Therefore,

the effect of d and n cannot be discussed indepen-

dently. The relationship varied greatly with the species.

Figure 9b shows the vertical distribution of a(z)b(z).The value of a(z)b(z) becomes increased larger fortrees with larger amounts of branching, including aerial

roots, and larger leaf area density. a(z)b(z) did notvary with tsunami height for C. nucifera, but differed

greatly for R. apiculata and P. odoratissimus in shallow

water. A. occidentale and A. alba had large a(z)b(z)values when the tsunami water was higher than 2 m.

Figure 10a shows the variation of Cd-all calculated

with the average diameter for the different tsunami

heights. The value of Cd-all was close to the value ob-

tained for mangrove trees (Wolanski et al. 1980). The

trend of Cd-all with the tsunami height is similar to that

shown in Fig. 9b, but the value in Fig. 10a does not

include the effect of spacing, whose importance is

indicated in Fig. 9a. Figure 10b shows the variation of

dNu calculated with the average diameter for the dif-

ferent tsunami heights. If we expect a forest to provide

protection against a tsunami higher than 10 m, young

C. equisetifolia (e.g., d = 0.15) is effective because it

grew densely and was not broken by the tsunami.

R. apiculata and P. odoratissimus were also effective,

but they had a tendency to break when they were in the

front line of the vegetation for a tsunami of this height.

If the tsunami height is less than 10 m, all three

species would be effective. The value of dNu for large-

diameter C. equisetifolia is quite small, almost the same

Landscape Ecol Eng (2007) 3:33–45 41

123

as C. nucifera. Age has a quite important effect in the

case of C. equisetifolia. The cashew tree (A. occiden-

tale) and A. alba have slightly larger values than C.

nucifera. Although the value of dNu for A. occidentale

is small, it has another important effect in allowing

people to escape. C. nucifera had little effect from any

point of view.

Discussion

Changes in coastal vegetation and sand dunes

for human needs and their effects on tsunami

protection

The representative landform on the coast of Sri Lanka

has three components: (1) new sand dunes, (2) lagoons

with mangrove trees (Jayatissa et al. 2002a), and (3)

old sand dunes (10–30 m height). Most of the roads

and railway tracks were located on new sand dunes,

and many resort hotels and new houses were con-

structed on sand dunes close to the shoreline. Many

previous studies have investigated the decrease in and

changes of mangrove forests in Sri Lanka (Jayatissa

et al. 2002b), and the importance of monitoring man-

grove forests has been determined (Verheyden et al.

2002). Not only the coastal vegetation but also the sand

dunes have been changed by infrastructure develop-

ments (Argam Bay, Yala in Sri Lanka). The acceler-

ated tsunami flew over the coastal sand dune and

reached the lagoon or old sand dunes. Sparsely planted

C. nucifera vegetation (Hikkaduwa), Bauhinia spp.–

Zizyphus spp. vegetation (Yala), or Syzygium vegeta-

tion (Ban Thale Nok) did not reduce the velocity of the

tsunami current.

Vegetation was destroyed about 200 m inland at

Yala, but Excoecaria agallocha and R. mucronata of

similar diameters were destroyed only 50 m inland at

Medilla. The diameter (d = 0.1–0.15 m) and the tsu-

nami height (4–5 m) were similar at both sites. This

indicates the effectiveness of very dense tree structures

near the ground.

The effect of tree structures on vegetation

thickness, dn, and the drag force

Figure 11a and b shows the relationship between tsu-

nami height and vegetation thickness, dn and dNall,

respectively. Shuto (1987) classified damage to trees

and their surroundings as follows:

Fig. 9 Characteristics of six representative species: a the rela-tionship between trunk diameter and the average space betweeneach tree, b the vertical distribution of a(z)b(z)

Fig. 10 The physical characteristics in relation to the tsunami: athe relationship between Cd-all and the tsunami height, b therelationship between dNu and the tsunami height

42 Landscape Ecol Eng (2007) 3:33–45

123

(A) Main trunks are not broken and the trees can trap

debris, but they have no effect on water velocity or

distance of inundation from the shoreline.

(B-1) Trees are tilted or bent over, but they may be

expected to trap debris.

(B-2) Trees are cut down, and no effect is expected.

(C-1) Both trees and undergrowth are undamaged. Soil

in a forest may remain intact without scouring.

(C-2) Some trees on weak soil or at the fringe of the

forest may be damaged and the soil around the trees

may be scoured, but the damage does not affect the

whole forest.

(D-1) Neither the trees nor soil is damaged.

(D-2) Surface soil may be scoured and damaged to

some extent, but the current velocity and inundation

depth are reduced and the degree of damage is

reduced.

Casuarina equisetifolia almost satisfied the criteria of

Shuto (1987) using vegetation thickness, dn. The dn of

A. occidentale and A. alba were inadequate, but their

values of dNall satisfied the boundary line between the

B-2 and C-2 regions. This is due to the effect of their

branch structures. Thus, dNall is useful for distinguishing

the effect of tree species. R. apiculata was effective with

a narrower vegetation width inland than the boundary

line between the B-2 and C-2 regions in case of tsunamis

lower than 5 m; it was not broken even in the B-2 region.

However, it was broken in the C-2 region when the

tsunami height was 8 m. This is because about 30 or

more aerial roots can share the moment from the drag

force that acts on upper branches in a 5 m tsunami, but

the moment applied to the trunk is higher than that

applied to the aerial roots and exceeds the threshold for

breaking when the tsunami height was taller than the

Fig. 11 The relationshipbetween the tsunami heightand vegetation thickness: adn, b dNall

Landscape Ecol Eng (2007) 3:33–45 43

123

tree height. A similar trend was also observed for P.

odoratissimus. This indicates the effectiveness of the

two species in a 5 m class tsunami.

The effect of horizontal and vertical forest

structures

A horizontal forest structure of small- and large-

diameter trees was also assumed to be effective be-

cause a dense population of small-diameter trees

(d > 0.1 m) could reduce the velocity of the tsunami

current whereas the large-diameter trees (d > 0.3 m)

could trap the broken branches and man-made debris

when the tsunami height was less than 5 m.

The effect of a creek parallel to the coast can be

found at three sites (Oluvil, Ban Thale Nok, and Laem

Son). A mangrove usually has many creeks inside, so

its effect is not restricted to the vegetation effect

(Wolanski et al. 1980).

Two layers of vegetation in the vertical direction

with P. odoratissimus and C. equisetifolia exhibited a

strong potential to decrease the damage behind the

vegetation cover (Kalutara), but a combination of P.

odoratissimus and C. nucifera had little additional ef-

fect (Hikkaduwa, Habaraduwa) because they have a

wide gap in the vertical direction (Fig. 8b).

Disaster prevention functions of coastal vegetation

Schematics of the possible functions that coastal veg-

etation can perform are shown in Fig. 12. For coastal

vegetation management or land-use planning, at least

the functions below should be considered in future.

The soft-landing effect

Many people were washed away by the tsunami. One

survivor was carried about 200 m and landed on tree

branches. To provide a soft landing, a tree species that

has dense branches and leaves, e.g., R. apiculata-type,

is effective.

Trapping effect

Much man-made debris (cars or destroyed houses) was

also washed away and injured people. However, large-

diameter trees trapped the debris at many sites. In

particular, if a mangrove forest is located behind a sand

dune, it could trap most of the debris and prevent the

buildings behind the forest from being damaged.

Escaping effect

Some people survived by climbing trees from the

ground or from the second floor of a building. C.

nucifera and C. equisetifolia are not useful from this

point of view because they have no branches low

enough for people to climb. The effective trees were

Thespesia populnea, Pongamia pinnata, and Termi-

nalia catappa.

Constructing a green belt requires time and is

sometimes prevented by the current land use. How-

ever, the interval between tsunamis is considered to be

longer than the period required for forest develop-

ment, and disasters will possibly occur in the future.

Therefore, more consideration of the planting and

management of coastal vegetation from the point of

view of landscape and urban planning is necessary.

Summary

This study investigated the role of coastal vegetation in

tsunami protection based on field observations carried

out after the 26 December 2004 tsunami. The trunk

Fig. 12 The functions ofcoastal vegetation duringtsunami inundation

44 Landscape Ecol Eng (2007) 3:33–45

123

diameter and the space between trees have a high

correlation with each other, and these characteristics of

vegetation need to be discussed as a combined effect.

Drag forces that affected the flow were analyzed using

the observed characteristics of vegetation. The vertical

distribution of the drag coefficient varied identifiably

with species classification. The field survey found that

mangroves, especially R. apiculata types and P. odo-

ratissimus, were effective in providing protection from

tsunami damage due to their complex aerial root

structure. This phenomenon was noted in cases where

the tsunami wave height was lower than the threshold

height for trunk breakage. A. occidentale and A. alba

were also effective because they have large diameter

branches close to the ground. C. equisetifolia grows at

high density when the trunk diameter is small

(d < 0.07 m), but at this size it can be broken by a 5 m

high tsunami. When the diameter of C. equisetifolia

was larger than 0.1 m, the trunks were not broken by

the tsunami and were effective at that height, but it is

presumed that they had little effect in reducing the

surface velocity when their diameter is large

(d > 0.5 m) with large spaces between trunks (7–

30 m). C. nucifera also had little effect because it has a

simple stand structure and wide spacing. Two layers of

vegetation in the vertical direction with P. odoratissi-

mus and C. equisetifolia exhibited a strong potential to

decrease the damage behind the vegetation cover. A

horizontal forest structure with small- and large-diam-

eter trees is also assumed to be effective because the

densely populated small-diameter trees (d > 0.1) could

reduce the velocity of the tsunami current, while the

large-diameter trees (d > 0.3) could trap the broken

branches and man-made debris. The vertical structure

also provides an effective soft landing for people wa-

shed away by the tsunami or for climbing when the

tsunami waves hit. In addition, creeks inside mangroves

and gap structures inside the C. equisetifolia vegetation

are assumed to be effective for trapping broken bran-

ches and reducing the water velocity.

These observations and data analysis indicate the

importance of preserving the horizontal and vertical

forest structures to serve as a barrier to tsunamis and to

allow people to escape. These data are also important

when selecting appropriate species for the construction

of vegetation strips for tsunami protection.

Acknowledgments Dr. D.R.I.B. Werellagama, University ofPeradeniya, and Dr. Nimal Wijerathne, Ruhuna University, areacknowledged for their useful suggestions during field investi-gations in Sri Lanka. For the field survey in Thailand, Prof. A.Sanit, Kasetsart University, and Dr. S. Havanond, Departmentof Marine and Coastal Resources of Thailand, are acknowledged

for useful comments. The authors would like to thank Mr.Harsha, Mr. Yutani, Mr. Taengtong, and Ms. Wongsorn for theirhelp in field measurements.

References

Baker CJ (1980) The turbulent horseshoe vortex. J Wind EngInd Aerodyn 6:9–23

Dahdouh-Guebas F, Jayatissa LP, Di Nitto D, Bosire JO, LoSeen D, Koedam N (2005) How effective were mangroves asa defence against the recent tsunami? Curr Biol 15(12):443–447

Fukuoka S, Fujita K (1990) Hydraulic effects of luxuriantvegetations on flood flow (in Japanese). Rep PWRI180(3):64

Furukawa K, Wolanski E, Mueller H (1997) Currents andsediment transport in mangrove forests. Estuar Coast ShelfSci 44:301–310

Hamzah L, Harada K, Imamura F (1999) Experimental andnumerical study on the effect of mangrove to reducetsunami. Tohoku J Nat Disaster Sci 35:127–132

Jayatissa LP, Dahdouh-Guebas F, Koedam N (2002a) A reviewof the floral composition and distribution of mangroves inSri Lanka. Bot J Linn Soc 138:29–43

Jayatissa LP, Guero MC, Hettiarachchi S, Koedam N (2002b)Changes in vegetation cover and socio-economic transitionsin a coastal lagoon (Kalametiya, Sri Lanka), as observed byteledetection and ground truthing, can be attributed to anupstream irrigation scheme. Env Dev Sustain 4:167–183

Kandasamy K, Narayanasamy R (2005) Coastal mangrove forestsmitigated tsunami. Estuar Coast Shelf Sci 65(3):601–606

Kutija V, Hong HTM (1996) A numerical model for assessingthe additional resistance to flow introduced by flexiblevegetation. J Hydraul Res 34(1):99–114

Mazda Y, Magi M, Kogo M, Hong PN (1997a) Mangrove oncoastal protection from waves in the Tong King Delta,Vietnam. Mangroves Salt Marshes 1:127–135

Mazda Y, Wolanski E, King B, Sase A, Ohtsuka D, Magi M(1997b) Drag force due to vegetation in mangrove swamps.Mangroves Salt Marshes 1:193–199

Melville BW, Sutherland AJ (1988) Design method for localscour at bridge piers. J Hydrau Eng ASCE 114:1210–1226

Nepf HM (1999) Drag, turbulence, and diffusion in flow throughemergent vegetation. Water Resour Res 35(2):479–489

Sanit A, Gordon SM, Sonjai H, Somchai P (1992) Plants inmangroves. 120 pp, Bangkok

Shuto N (1987) The effectiveness and limit of tsunami controlforests. Coast Eng Jpn 30(1):143–153

Tamai N, Asaeda T, Tanaka N (1987) Vortex structures around ahemispheric hump Bound-Layer Meteorol 39:301–314

Tanaka N, Takemura T, Shirono S (2005) Flow structures andsand deposition behind a colony-type grass on a gravel bar.Proc. XXXI IAHR, pp 2094–2103

Verheyden A, Dahdouh-Guebas F, Thomaes K, De Genst W,Hettiarachchi S, Koedam N (2002) High-resolution vegeta-tion data for mangrove research as obtained from aerialphotography. Env Dev Sustain 4:113–133

Wijetunge J (2005) Future directions for post-tsunami coastalzone management in Sri Lanka. Proc. XXXI IAHR, pp4344–4352

Wolanski E, Jones M, Bunt SJ (1980) Hydrodynamics of a tidalcreek-mangrove swamp system. Aust J Mar Freshwater Res31:431–450

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Coastal vegetation structures and their functions in tsunami protection: experience of the recent Indian Ocean tsunamiAbstractIntroductionField survey and analysisField surveyAnalysis of drag force coefficient using representative tree species

ResultsExamples of coastal vegetation effect in Sri LankaKalutaraHikkaduwaMedillaYalaOluvil

Examples of the effect of coastal vegetation�in ThailandPhra Thong Island and Kang IslandsBan Thale NokLaem Son National Park

The pattern of vegetation and tree damage�in relation to species and tsunami heightThe characteristics of representative coastal vegetation in both countries

DiscussionChanges in coastal vegetation and sand dunes�for human needs and their effects on tsunami protectionThe effect of tree structures on vegetation thickness, dn, and the drag forceThe effect of horizontal and vertical forest structuresDisaster prevention functions of coastal vegetationThe soft-landing effectTrapping effectEscaping effect

SummaryAcknowledgmentsReferences

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