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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
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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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