DSpace at VNU: Modelling the Impacts of Mangrove Vegetation Structure on Wave Dissipation in Ben Tre Province, Vietnam, under Different Climate Change Scenarios

Modelling the Impacts of Mangrove Vegetation Structure onWave Dissipation in Ben Tre Province, Vietnam, under Different Climate Change Scenarios Nguyen Thi Kim Cuc†‡, Tomohiro Suzuki§††‡

Modelling the Impacts of Mangrove Vegetation Structure on

Wave Dissipation in Ben Tre Province, Vietnam, under

Different Climate Change Scenarios

Nguyen Thi Kim Cuc†‡, Tomohiro Suzuki§††‡‡, Erik D de Ruyter van Steveninck§§,

Hoang Hai†††

†

Department of Natural Resources

Management

Faculty of Water Resources Engineering

Water Resources University

175 Tay Son

Dong Da, Hanoi, Vietnam

nguyencuc@wru.edu.vn

‡

Mangrove Ecosystem Research Division Centre for Natural Resources and Environmental Studies Vietnam National University Hanoi, Vietnam

§

Flanders Hydraulics Research Berchemlei 115

B-2140 Antwerp, Belgium

††Department of Civil Engineering

Ghent University

Technologiepark 904

B-9052 Ghent, Belgium

‡‡Environmental Fluid Mechanics Section Faculty of Civil Engineering and Geosciences Delft University of Technology

P.O Box 2600 GA Delft The Netherlands

§§Water Science and Engineering Department

UNESCO-IHE Institute for Water Education, Delft, The Netherlands

†††Faculty of Environment

Da Nang University of Technology,

Da Nang, Vietnam

Abstract

Cuc, N.T.K.; Suzuki, T.; Ruyter van Steveninck, E.D de, and Hai, H., 0000 Modelling the impacts of mangrove vegetation structure on wave dissipation in Ben Tre Province, Vietnam, under different climate change scenarios Journal of Coastal Research, 00(0), 000–000 Coconut Creek (Florida), ISSN 0749-0208

Mangroves are widely distributed along the coastline of Vietnam, where they provide protection against sea waves caused by extreme weather Impacts of climate change, together with population growth and economic development, are expected to exert pressure on these vulnerable systems In this study the numerical wave-propagation model SWAN-VEG (Simulating Waves Nearshore–Vegetation) was used to simulate the possible impacts of climate change on the wave-dissipation capacity of different types of mangrove vegetation Vegetation characteristics were assessed in planted plots (Rhizophora apiculata and a mix of R mucronata, Sonneratia caseolaris, Avicennia alba, and Nypa fructicans) and

in natural regenerated areas (A alba and S caseolaris) in Thanh Phu Natural Reserve, Mekong Delta, Vietnam; these assessments were used as model input Different sea levels and mangrove vegetation characteristics were used to simulate the potential impacts of climate change Planted plots with a cover of 70% reduced the height of incoming waves

by 60%, compared with 40% for natural regenerated forest Reducing the vegetation cover in planted plots from 70% to 50%, 35%, and 0% resulted in wave-height reductions of 51%, 42%, and4%, respectively A sea level rise (SLR) up to 0.96 m did not change the wave-dissipation potential of R apiculata planted in the plots However, an assumed decline in the width of vegetation from 1.5 km to 0.5 km, e.g as a consequence of coastal erosion, reduced the height of incoming waves 21% (no SLR) and 29% (0.96 m SLR), as compared to 60% and 59%, respectively, without erosion

ADDITIONAL INDEX WORDS: Mangrove, wave, climate change, SWAN-VEG, Thanh Phu

INTRODUCTION

Tropical coastlines are under great pressure due to a rapid

increase in population, changes in land use, and

infrastruc-tural developments Large-scale mismanagement of these

coastlines and the inability to cope with events such as cyclones can have devastating long- and short-term effects, especially in developing countries Densely vegetated mangrove forests contribute to the protection of coasts against severe sea waves caused by storms or tsunamis, which can affect local people living in tropical coastal areas (Alongi, 2008; Kathiresan and Rajendran, 2005) Thus many efforts to plant mangroves have been made based on qualitative observations (Hong and San, 1993) As argued by Lewis and Streever (2000), however, the function of the ecosystem and the hydrology at individual sites

DOI: 10.2112/JCOASTRES-D-12-00271.1 received 27 December 2012;

accepted in revision 16 April 2013; corrected proofs received 10 June

2013

Published Pre-print online 15 July 2013

Ó Coastal Education & Research Foundation 2013

need to be understood quantitatively in order to determine

what is needed in terms of vegetation density and width to

provide the required level of protection

Unfortunately, very little knowledge has been accumulated

in this regard Studies on the wave-dissipation capacity of

mangrove vegetation are limited compared to those on salt

marshes (e.g Knutson, 1988) or sea grass beds (e.g Fonseca

and Cahalan, 1992) Based on field observations, Mazda,

Wolanski, et al (1997) have shown the quantitative effects of

two mangrove species, Rhizophora stylosa and Kandelia

candel, on reduction of the impact of sea waves Massel,

Furukawa, and Brinkman (1999) have discussed the

wave-dissipation capacity of Rhizophora spp and Sonneratia spp

based on a mathematical model A predictive model of wave

propagation through a nonuniform forest in water of changing

depth in the mangrove forest of Can Gio, Vietnam, has been

developed by Vo-Luong and Massel (2008) Recently, Mendez

and Losada (2004) and Suzuki et al (2011) have incorporated a

full frequency-direction wave spectrum in the numerical wave

model SWAN (Simulating Waves Nearshore) and additionally

included a layer-wise implementation of vegetation

character-istics Their results, however, cannot be applied directly to

other regions or species, as each mangrove species has a unique

configuration of trunks, prop roots, and pneumatophores that

produces a different drag force (Wolanski et al., 2001) and

therefore results in a different reduction rate of sea waves For

example, with regard to Bruguiera spp., Sonneratia spp.,

Avicennia spp., and Nypa fruticans, no information on their

wave-dissipation capacity exists, either on their quantitative

hydrological functions or on their physical impact Accordingly,

for useful and effective mangrove planting, there is a need for

quantitative knowledge of the physical impact of individual

mangrove species in relation to their wave-dissipation

capac-ity

Vietnam is located in the tropical region of Asia With its long

coastlines and high concentration of population and economic

activities in coastal areas, its coastal ecosystems and

commu-nities could potentially be dramatically impacted by a rise in

sea level (CCFSC, 2001) According to Bates et al (2008),

changes in the hydrological cycle are expected to be the most

significant aspect of climate change to affect the Mekong Delta

These include changes in sea level, precipitation patterns, and

monsoon cycles, both frequency and magnitude of these

climatic events Climate change could also increase the

frequency and severity of typhoons (Byrnes et al., 2011)

Coastal erosion and accretion (sedimentation) are both part of

the natural, dynamic interaction between shorelines and the

sea Erosion along the East Sea coast of the Mekong Delta

appears to be mainly structural erosion caused by waves and

longshore sediment transport, which strips sediment from the

coast and deposits it at the confluence of the East sea current

and the circular pattern of water flow from upper streams

Changes in sediment inflow to the coastal zone from rivers in

the Mekong Delta could also be involved (Fabrice and Claudia,

2012) Any rise in sea level or increase in the frequency and

intensity of coastal storms is likely to accelerate the rate of

coastal erosion and could lead to changes in its spatial pattern

Sections of the coastline that are now eroding could become

areas of accretion; conversely, sections that are now accreting

may become areas of erosion in the future Coastlines of the Mekong Delta and Thanh Phu Natural Reserve are facing serious erosion and accretion in different areas

Mangroves have been a key element in reducing the rate of erosion and provide a protective barrier along the shoreline Mangroves alone cannot provide complete protection against erosion in all situations, as illustrated by the current situation

on the Thanh Phu coastline, where there are reasonably well-developed mangrove stands Arguably, however, the rate of erosion would have been much faster without them (Clough and Phan, 2010) Mangroves are widely distributed in coastal areas in Vietnam, particularly in the south (Maurand, 1943) However, deforestation due to war activities and conversion to agricultural land and (more recently) shrimp ponds (Hong, 1999) has resulted in a decline from 408,500 ha in 1943 (Maurand, 1943) to 290,000 ha in 1962 (Rollet, 1981), 252,000

ha in 1982 (FIPI, 1982), 156,608 ha in 2001 (FIPI, 2001), 209,741 ha in 2006 (FIPI, 2007), and 139,955 ha in 2010 (MARD, 2011)

In this paper, the SWAN model has been used to analyze the impact of different mangrove species and densities on wave-dissipation capacity and wave-energy wave-dissipation characteris-tics in Thanh Phu Natural Reserve, Mekong Delta The SWAN model is a third-generation wave model developed at Delft University of Technology, the Netherlands (TU Delft, 2011) The SWAN model computes the processes of wave generation

by wind, quadruplet wave-wave interactions, white-capping, bottom friction, depth-induced breaking interactions, and triad wave-wave interactions; it is thus capable of estimating wave propagation from offshore to nearshore In addition to the fundamental features of waves, the effects of vegetation have been recently incorporated into the model Using data about the present mangrove vegetation in Thanh Phu Natural Reserve, a comparison has been made between a planted area dominated by Rhizophora sp and a naturally regenerated area dominated by Avicennia sp To analyse the impacts of tree density, sea level rise (SLR) associated with climate change, and coastal erosion, only data about the planted forest have been used

MATERIALS AND METHODS Study Area

The study was carried out in Thanh Phu Natural Reserve, Ben Tre Province, Vietnam (Figure 1) Ben Tre is a coastal province in the Mekong Delta The area is characterised by a tropical monsoon climate (Vu, 1994); average air temperature

is about 268C288C In the channels inside Thanh Phu Natural Reserve, the average water temperature is about 288C Normally the rainy season extends from May to November, with maximum monthly rainfall from June to August Monthly maximum river flows occur from August to October About 9%

of the delta area is occupied by channels 20–30 m long The tidal regime is semidiurnal with a tidal amplitude of 1.5–3.75

m Tides can propagate 50–100 km inland with velocities of 0.75–1.80 m s1 Salinity in the estuaries varies from 0%–12% (June–November) to 12%–30% (November–May) (Sub-FIPI II, 2003)

Mangroves concentrated in the districts of Binh Dai, Ba Tri,

and Thanh Phu covered 13,153 ha, or 33%, of the province in

1994 (Sub-FIPI II, 1995) In 2000 and 2007, coverage was

estimated at 3413 and 3520 ha, respectively (Ben Tre DARD,

2010; Ngo, 2004)

The coastal landscape at Thanh Phu consists of sandy belts,

tidal mudflats, saline tidal swamps, and toxic acid-sulphate

swamps Thanh Phu Natural Reserve contains a narrow strip

(about 0.8–5.0 km) of mangroves along the coastline between

two of the mouths of the Mekong River: the Co Chien and Ham

Luong estuaries (Figure 1) As is the case with other sites on

the eastern coastline of the Mekong Delta, Thanh Phu Natural

Reserve is strongly affected by both erosion as well as accretion

(Sub-FIPI II, 1998, 2003)

In 2009, Thanh Phu had a population of 127,574 inhabitants

The district covers an area of 44,350 ha, of which 17,300 ha is

used for aquaculture and 15,000 ha as paddy fields Another

2000–3000 ha is used for industrial crops, fruit farming, etc

(Ben Tre Statistics Office, 2010)

Mangrove Vegetation Structure

In the study area, planted Rhizophora apiculata is the predominant species, representing more than 80% of the mangrove vegetation Three transects perpendicular to the coastline were laid out with a length of 1–2 km, depending on width of the vegetation In each transect three 10 3 10 m plots were established—one close to the shore, one in the middle, and one at the end of the transect—according to accessibility In each plot all trees were counted, while a separate 1 3 1 m subplot was established within each plot to count seedlings (less than 1.0 m in height) and saplings (1.0–4 m in height) Trees taller than 4 m were identified to the species level, and for these the following parameters were measured as described by English et al (1994):

(1) Tree and still root densities (2) Root diameter and height (3) Tree diameter at a height of 1.3 m (4) Height from stratum to height of the first branch (h branch)

Figure 1 Study area in Thanh Phu Natural Reserve

(5) Height from stratum to height of the first leaf (h leaf)

(6) Height from stratum to the top of the tree (h top)

One similar transect with three plots was set up within the

section of natural regenerated vegetation in the study area and

was analysed in the same way

For the model input, possible zonation patterns were

neglected, and the mean values of the measured parameters

per vegetation type, i.e planted and natural regenerated, were

used

SWAN-VEG Model

The SWAN-VEG model consists of the original SWAN model

with a vegetation module added This module consists of a

variable for energy dissipation due to vegetation The

dissipat-ed energy is subtractdissipat-ed from the incoming wave energy, which

results in less wave energy behind the vegetation field and thus

a lower wave height Vegetation is modelled as cylindrical

obstacles; vegetation characteristics such as height, width,

density, and drag coefficient are used to determine the

magnitude of the dissipation term In addition, wave

charac-teristics like significant wave height and peak period influence

the energy-dissipation term (de Oude et al., 2010) The

SWAN-VEG module for wave dissipation by vegetation (Suzuki et al.,

2011) is based on Mendez and Losada (2004) and de Oude et al

(2010) The model assumes a group of cylinders as

represen-tation of vegerepresen-tation It includes vertical-layer schematization,

making it possible to calculate multilayer structures such as

mangroves The energy dissipation term in SWAN-VEG is

described by Suzuki et al (2011):

Sds;veg¼Xn

i¼1

where Sds,vegis the total energy dissipation due to vegetation, n

is the number of vegetation layers, i is the layer under

consideration, and Sds,veg, iis the energy dissipation for layer i

The energy dissipation term for a given layer i therefore becomes

Sds;veg;i¼ 

ffiffiffi

2

p

r

g2CDbvNv

¯ k 2r

3ðsinh3ka¯ ih sinh3ka¯ i1hÞ þ 3ðsinh3ka¯ ih sinh3ka¯ i1hÞ

3 ¯k cosh3kh¯

3 ffiffiffiffiffiffiffiffi

Etot

p

where g is gravitational acceleration, CD is the bulk drag

coefficient, bvthe cylinder diameter, Nvis the cylinder density,

aih is the plant height, and E(r,h) is the wave-energy density

The bulk drag coefficient value 1.0 is used for simplicity in this

study as per de Oude et al (2010) and Narayan et al (2010) As

far as we are concerned, the bulk drag coefficient value of

mangrove forest has not been very clear up to now due to the

complexity of the vortex effect around cylindrical structures (e.g

stems and roots) under wave motion For instance, different

cylinder densities and arrays, which generate different vortex

patterns, give different bulk drag coefficient values under

waves Furthermore, even drag coefficient values of a single

cylinder range from 0.5 to 2.5 (Sarpkaya and Michael, 1981) with

changes in Keulegan–Carpenter (KC) value and beta value The

mean frequency ¯r, the mean wave number ¯k, and the total wave energy Etotare defined as follows (Wamdi Group, 1988):

¯r¼ E1 tot

Z 2P 0

Z ‘ 0

1

rEðr; hÞdrdh

ð3Þ

¯

k¼ E1 tot

Z 2P 0

Z ‘ 0

1 ffiffiffi k

p Eðr; hÞdrdh

ð4Þ

E1tot¼

Z 2P 0

Z ‘ 0

The wave-dissipation term associated with vegetation,

Sds,veg, is implemented in the source term Stot, as shown in Equation (6):

Stot¼ Sinþ Snl3þ Snl4þ Sds;bþ Sds;wcþ Sds;br ð6Þ where Sds,bis bottom friction, Sds,wcis white-capping, Sds,bris depth-induced breaking, Sinis wave growth due to wind input,

Snl4and (Snl3) are energy transfer within the spectrum due to nonlinear wave-wave interactions such as quadruplets (Snl4) and triads (Snl3) These six processes contribute to the source term Stot The evolution of the wave spectrum is described by the spectral action balance equation, which for Cartesian coordinates is given by (e.g Hasselmann et al., 1973)

d

dtNþd

dxcxNþ d

dycyNþ d

drcrNþ d

dhchN¼Stot

where the first term represents the local rate of change in N over time and the second and third terms represent propaga-tion along the x and y direcpropaga-tions with velocities cxand cy Thus the model is capable of calculating wave dissipation associated with vegetation along with other source terms As shown in Equation (7), not only one-dimensional but also two-dimen-sional calculations are feasible by distributing the vegetation factors in space

Model Inputs

Table 1 summarises the parameters used as input data in the SWAN-VEG model to analyse the impacts of (1) vegetation type and coverage, (2) water level as a consequence of SLR, and (3) the width of vegetation on the wave-dissipation capacity of mangroves

Water Level, Wave Height and Wave Peak Period

According to the Centre for Meteorology Forecast in Ben Tre and the Vietnam Institute of Meteorology, Hydrology, and Environment, the maximum recorded value of significant wave height on the coast of Ben Tre during the past 100 years was 0.9

m at a point 2.5 km from the shoreline with a depth of1 m The shoreline refers to the line behind the 1.5 km wide mangrove vegetation (see Figure 2) The peak periods range from 7 s to 10

s This value occurred when the water level was at 4.1 m (total depth at1 m is 5.1 m) This value was used as the extreme-event condition in the simulation

Vegetation Type and Density

The local forest management system allows households to harvest or convert up to 30% of mangrove areas that are managed by them to water surface for aquaculture activities

Thus coverage of 70% of the actual measured figures was used

to compare the impact of planted forests with naturally

regenerated forests (codes A1 and A3, respectively; see Table

1) The impact of vegetation density was analysed by reducing

the coverage in the planted forest from 70% (A1) to 50% (A5),

35% (A6), and 0% (A4) (Table 1)

Sea Level Rise

Sea level rise scenarios for Vietnam have been developed

based on different emission scenarios: low (B1), medium (B2),

and high (A1F1) (MONRE, 2009) MONRE (2010) recalculated

the expected water levels for several specific coastal locations in

Vietnam This resulted in predicted increases in water levels of

0.39 m (scenario B1), 0.65 m (B2), and 0.96 m (A1F1) in Ham

Luong (Ben Tre) in the year 2050, 2075, and 2100, respectively

In this study, these values have been added on top of the

extreme water level of 4.1 m (comparisons A1, B1, B2, and B3;

see Table 1)

Coastal Erosion

The expected SLR will affect the hydrodynamic regime of the

Vietnam East Sea through general circulation, tidal amplitude,

and tidal coastal flooding In addition, factors affecting

hydrodynamic regimes such as wind, storms, and air

temper-ature will be impacted by climate change Changes in these

parameters are expected to lead to changes in erosion and

accretion in the region (Ben Tre DARD, 2010) Therefore, in

this study, future erosion scenarios simulation a reduction in

the width of mangrove forests to 0.5 and 1.0 km have been

included (comparison of A1 with C1 and C2; comparison of B3

with C3 and C4; see Table 1) Although a simplification, it was

assumed that erosion did not change the shore topography, and

thus a narrower mangrove forest resulted in the ocean floor

rising less steeply

RESULTS Mangrove Vegetation Structure

Vegetation in the planted site consisted of Rhizophora

apiculata (.90%) and a mix of R mucronata, Sonneratia

caseolaris, Avicennia alba and Nypa fruticans Sonneratia

caseolaris and A alba were mainly found on the alluvial coastal and river mud flats, while N fruticans was planted in the inland brackish aquaculture ponds In the planted area, the tree density was 0.16 trees m2, with a mean tree height of 12.7

Table 1 Summary of data used as input for the SWAN-VEG model to analyse the impact of mangrove type (planted vs natural regenerated) and coverage, SLR, and coastal erosion with and without SLR.

Code

Vegetation

Type % Coverage

Abbreviations: Width ¼ width of mangrove forest, SLR ¼ possible sea level–rise scenarios based on MONRE (2010), WL ¼ water level, Hs ¼ significant wave height, Tp ¼ wave peak period, P ¼ planted mangrove type, N ¼ natural regenerated mangrove type The simulation codes are also used in Figures 3–6 Note: mangroves coverage(0–70% cover); sea level rise (0.39-0.96 m) and coastal erosion (from 1.5-0.5 km width) with/without sea level rise (0.96 m).

Figure 2 Spatial variation of significant wave height for simulations A1, A3, A4, A5, and A6: vegetation type and density (see Table 1 footnote for explanation of simulation codes) Wave: Hs denotes significant wave height; Tp denotes wave peak period; WL denotes water level The dotted arrow shows an SLR of 0.96 m (scenario 3); the dashed arrow shows an SLR

of 0.65 m (senario 2); the dotted and dashed arrow shows an SLR of 0.39 m (senario 1).

m and a diameter (measured at a height of 1.3 m) of 0.12 m

(Table 2)

Vegetation in the naturally regenerated site consisted

mainly of A alba and S caseolaris Avicennia alba grows

naturally on mud flats with average tidal levels of 1.0 m to 1.5

m and is directly exposed to waves and wind from the sea In

some areas, A officinalis appears landwards from the A alba

zone, protected from the direct impact of waves and wind

Sonneratia caseolaris develops on soft mud at depths of 1.0–1.5

m, affected by tides, waves and wind, and a mixture of seawater

and freshwater from rivers Although tree density in these

plots was higher than in the planted plots (0.24 trees m2), their

average size was smaller (3.9 m high and a diameter of 0.11 m)

Besides these differences, another major difference between

the planted and naturally regenerated sites that could affect

wave-dissipation capacity is caused by the different root

structures for the various tree species, i.e stilt roots for

Rhizophora and pneumatophores for Avicennia (Figure 3)

Based on the characteristics of the planted Rhizophora trees

(h top, 12.73 m; h leaf, 7.20 m; h branch, 6.67 m) and the

maximum water level (5.06 m), it was assumed that the stems

and roots of the trees will play a major role in wave dissipation;

therefore the influence of the canopy was not considered

Model Analyses

Vegetation Type and Density

The wave-simulation model started 2.5 km seaward from the

shoreline with a wave of 0.9 m high After passing the 1.5 km

wide mangrove forest, the wave height was reduced by 60% in

the planted forest dominated by R apiculata with stilt roots

(70% coverage, A1) compared to 40% by the natural

regener-ated forest (A3), which is characterised by A alba and S

caseolaris and their associated numerous, but small (10–15

cm), pneumatophores (Figure 2, Table 3) Decreasing the

mangrove cover in the planted Rhizophora forest to 50% and

35% resulted in a reduction of wave height of 51% and 42%,

respectively (A5 and A6; see Figure 2, Table 3) When the

vegetation was removed completely (A4), the wave height even

slightly increased after passing the 1.5 km of no forest (now

cleared) (Figure 2, Table 3)

Sea Level Rise

Figure 4 shows the transmitted wave heights in the planted

forest for three different sea levels (B1, 4.49 m; B2, 4.75 m; and

B3, 5.06 m) Although the water levels increased, the impact of

the mangrove forest on the incoming wave height was the

same, i.e a 59% reduction, which is comparable to the 60% observed in case A1 (Table 3)

Coastal Erosion

The impacts of coastal erosion, in combination with a more gradually increasing ocean floor (C1 and C2) and an SLR of 0.96

m (C3 and C4), are illustrated in Figure 5 The impact of wave dissipation by vegetation (planted forest with 70% cover) shifts landwards when the width of the mangrove forest is dimin-ished to 1.0 km and 0.5 km This resulted in a clear impact on the reduction of wave height: 46% and 21% reduction by the 1.0

km and 0.5 km forests, respectively (compared to 60% by the 1.5 km forest with the sea level at 4.1 m); and 46% and 29% reduction by the 1.0 km and 0.5 km forests, respectively (compared to 59% by the 1.5 km forest with the sea level at 5.06 m; see Table 3)

Table 2 Mean values (significant difference) of mangrove vegetation

characteristics in both planted Rhizophora (n ¼ 9) and natural

regenerated (n ¼ 3) plots in Thanh Phu Natural Reserve.

Parameter Planted Rhizophora Natural Regenerated

Tree density (m 2 ) 0.16 (0.07) 0.24 (0.10)

Diameter at 1.3 m (m) 0.12 (0.04) 0.11 (0.08)

Stilt roots

Figure 3 Root structures for the different tree species: stilt roots for Rhizophora sp and pneumatophores for Avicennia sp and Sonneratia sp (Hong and San, 1993).

Table 3 Summarised output of the SWAN-VEG model showing percent reduction of the incoming wave height after passage through the mangrove vegetation (see Table 1 footnote for explanation of simulation codes).

Vegetation type

Density

SLR

Erosion

Erosion and SLR

DISCUSSION AND CONCLUSIONS

An analysis of the output of the SWAN-VEG model confirms

the potential role of mangroves in dissipating incoming wave

energy A simulated reduction in mangrove vegetation in

planted R apiculata–dominated forests from 70% to 0%

resulted in an increase in wave height from about 0.4 m to

more than 0.9 m Similar conclusions were made by Mazda,

Magi, et al (1997), Mazda, Wolanski, et al (1997), and Quartel

et al (2007) in the field when measuring surface wave

propagation in mangrove forests of Kandelia candel (in the

Tong King Delta, Vietnam) and Bruguiera gymnorrhiza (in

Nakama-Gawa in Iriomote Island, Japan) and R stylosa and K

candel (in the Red River Delta, Vietnam), respectively

Comparable results were also described by Massel, Furukawa,

and Brinkman (1999) in a study combining theoretical analysis

and a field experiment with Rhizophora sp

Rhizophora apiculata is characterized by very dense

above-ground stilt roots This probably explains the difference in

wave dissipation capacity between the planted Rhizophora

forest and the naturally regenerated plot (0.4 and 0.5 m,

respectively) The latter consists mainly of Avicennia sp and

Sonneratia sp., both species with pneumatophores A similar

difference in wave-energy reduction between species has also

been observed by Mazda, Magi, et al (1997) and Mazda et al

(2006) when comparing vegetation dominated by Sonneratia

sp and K candel, respectively

Besides reducing the height of incoming waves, mangrove

forests probably also reduce wind-driven and tidal currents due

to their dense network of stems, branches, and aboveground

roots (Quartel et al., 2007) The mangrove trees in the study

area rise quite high above the water level (12.7 m on average)

and might efficiently reduce wind energy Further studies on

this role of mangroves should be carried out, especially in the

Mekong Delta area, where the windy season has a strong

influence on the coastline Increasing the sea level in the model

up to 0.96 m, thus anticipating a possible SLR due to climate

change, did not reduce the wave-energy dissipation potential of

the planted Rhizophora forest The resulting wave height of almost 0.4 m is not different in the four tested water depths This probably could be explained by the high density of stems and aboveground roots distributed throughout the whole water depth, even with a rise in sea level

The width of the vegetation, however, seems to have more impact on the wave-dissipation capacity of the planted mangrove forest Although a reduction from 1.5 km to 1.0 km resulted in a slight increase in wave height from 0.4 m to 0.5 m, further reducing the forest width to 0.5 km resulted in a wave height of 0.7 m The actual sea level (present level and an increase of 0.96 m) did not affect these results In addition to vegetation density, the width of the area to be planted is an important factor in wave attenuation for protecting tropical coastlines

Quartel et al (2007) found that wave heights were depth limited Small water depths corresponded with small wave heights and large water depths with higher incident waves With the expected SLR as a consequence of climate change, coastal areas will face the impact of higher wave height The existence of mangrove forests in coastal areas in general, and in the study area in particular, will thus be very important in terms of protecting the coastline from the impacts of sea waves Further investigations are needed to quantify the function of mangroves in wind-driven and tidal currents in protecting the coastal zone

ACKNOWLEDGMENTS

We thank the Department of Agriculture and Rural Development of Ben Tre Province and the Management Board of Thanh Phu Natural Reserve for their unstinting support for data collection and field survey The work reported here was undertaken as part of the research programme ‘‘PRoACC—Postdoctoral Research Programme

on Climate Change Adaptation in the Mekong River Basin’’ The project is funded by the Netherlands Ministry of Development Cooperation (DGIS) through the UNESCO-IHE Partnership Research Fund This research project is a

Figure 4 Spatial variation of significant wave height for simulations B1, B2,

and B3 according to SLR (see Table 1 footnote for explanation of simulation

codes).

Figure 5 Spatial variation of significant wave height in erosion simulations C1/C3 and C2/C4 (see Table 1 footnote for explanation of simulation codes).

joint initiative of UNESCO-IHE Institute for Water

Educa-tion and many partner instituEduca-tions in the Lower Mekong

countries and China

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