A horizontal plywood platform, representing a beach model of height of ca. ... 33.97 m from the wave generator and 2.0 m behind the end of the beach slope, ...
ATTENUATION OF SOLITARY WAVE BY PARAMETERIZED FLEXIBLE MANGROVE MODELS Agnieszka Strusińska-Correia1, Semeidi Husrin2 and Hocine Oumeraci1 The systematic laboratory investigation on tsunami attenuation by flexible mangrove models was performed in order to improve the knowledge on tsunami-coastal forest interaction. A sophisticated parameterization method, based on structural and bio-mechanical properties of a mature mangrove (Rhizophora sp.), was developed for the construction of the mangrove models under assumption of stiff and flexible structure. The forest model examined in the laboratory experiments consisted of the selected flexible mangrove models, arranged in different configurations, which was impacted by a tsunami-like solitary wave of varying height, propagating in different water depths. Based on the envelopes of max. wave height and wave forces induced on single tree models, wave evolution modes were determined to identify the source of wave attenuation. The results indicate the dependence of wave transmission on the observed wave evolution modes and relative forest width: the highest transmission coefficient is attributed to nonbreaking waves (ca. 0.78 and 0.55 for forest width of 0.75 and 3.0 m, respectively), while the lowest transmission coefficient corresponds to wave breaking in front of/in the forest model (ca. 0.5 and 0.3 for forest width of 0.75 and 3.0 m, respectively). eywords: mangroves; parameterization method; solitary wave; wave attenuation; laboratory experiments
INTRODUCTION
The positive role of coastal vegetation in the preservation of coastal ecosystems and in the prevention of the inland from flooding due to storms and cyclones is undisputed – coastal forests have been employed to enhance stabilization of the coastal zone (particularly dunes), to protect the adjacent fields from salt spray and wind, to support fisheries and to dissipate energy of short-period waves (e.g. Badola and Hussain 2005, Wolanski 2007, Mazda et al. 1997b). The recent tsunami events in 2004 and 2011 have attracted public attention to another aspect of the protective role of a coastal forest, in particularly mangroves, namely tsunami mitigation. In many of post-tsunami field survey reports (e.g. Kathiresan and Rajendran 2005, EJF 2006, Yanagisawa et al. 2009), lower casualties and property losses were claimed due to the presence of a dense mangrove forest. However, a detailed insight into the local tsunami hydrodynamics, local topography-bathymetry features and pre-tsunami forest conditions was very often ignored in these reports, what might have led to overestimation of the performance of mangrove forests during the 2004 Indian Ocean Tsunami. Definitely, performance of a coastal forest is conditioned by several factors such as vegetation characteristics (vegetation type, age, health state, density, width of the green belt), tsunami parameters (height, period, angle of impact), local morphology and soil properties, and is limited by the magnitude of the tsunami event and the tree resistance to wave impact. Although some vegetation zones were found intact after the recent tsunami events, most of them, particularly those facing directly an open sea, were moderately to heavily damaged by tilting, trunk/branches breakage and uprooting (e.g. Shuto 1987, Latief and Hadi 2006, Yanagisawa et al. 2009). The influence of the above-mentioned parameters on the effectiveness of a mangrove forest in tsunami mitigation has not been addressed in previous experimental investigations. Particularly the lack of a comprehensive parameterization method for the development of a tree model under consideration of structural and bio-mechanical tree properties is the main disadvantage of these studies, enabling comparison of the experimental results. The geometry of mangrove trees was either idealized by using single cylinders representing tree trunk (and thus neglecting the roots and the canopy) or modeled by means of material of very different properties than the prototype (dimensions, stiffness, etc.). Fig. 1 shows selected mangrove models, examined in previous studies. The parameterization approach applied to the development of a tree model determines the hydrodynamic performance of the forest model considered and the processes accompanying wave-tree interaction. Inappropriate assumptions on structural and bio-mechanical properties of the tree as well as too strong simplification of tree structure may lead to gaining incorrect experimental data and wrong assessment of the damping characteristics of the forest.
1
2
Hydromechanics and Coastal Engineering, Technical University Braunschweig, Beethovenstr. 51a, Braunschweig, Lower Saxony, 38106, Germany Research Institute of Coastal Resources and Vulnerability, Ministry of Marine and Fisheries Affairs, Padang-Painan Km.16, Padang, West Sumatera, 25245, Indonesia
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Figure 1. Examples of parameterized mangrove models from previous studies: a) cylinder-shaped model by Massel et al. 1999, b) porous model by Harada and Imamura 2000, c) wire-made model by Istiyanto et al. 2003, d) model by Kongko 2004 with cylinders representing roots, trunk and canopy (after Husrin 2013).
In this paper, the effectiveness of a parameterized mangrove forest in attenuation of a solitary-like tsunami wave is discussed, based on laboratory experiments performed for different forest configurations, water depths and incident wave conditions. The approach proposed for the parameterization of mangrove Rhizophora sp., accounting for physical tree properties, represents the core of this study. DEVELOPMENT OF APPROACH FOR PARAMETERIZATION OF MANGROVES
Considering the wave damping properties of a mangrove in respect to its geometry and biomechanical properties, Rhizophora sp. with a very dense, wide root system and a well-developed canopy represents mangroves that are most suitable for the purposes of this study (see Fig. 2a). Tree resistance to flow depends largely on tree growth stage, during which tree dimensions, root/canopy density and trunk elasticity change greatly. In order to ensure maximum tree resistance to flow, a mature Rhizophora sp. was considered in the parameterization method. Development of a generic parameterization approach was conditioned by the necessity of use of simple mangrove models in the experiments and the reduction of the effort spent on the preparation of the experimental set-up. Unlike the previous studies, the simplification of mangrove geometry, particularly the complex 3D root system and the canopy, was based on both structural and biomechanical properties of a mangrove, under condition of same hydraulic losses in the prototype and the model. The tree characteristics (i.e. dimensions, trunk elasticity, root and canopy density) was defined using data available from previous studies, extended by own field measurements performed on a young, mid age and mature Rhizophora apiculata by Semeidi Husrin in Angke Kapuk Protected Forest Region on Java Island in Indonesia (see Husrin 2013 for more details). When determining a typical geometry of a mature Rhizophora sp., shown in Fig. 2b, other available field data by e.g. Mazda et al. 1997a, Istiyanto et al. 2003, Harada and Kawata 2005 were additionally used. In the literature, values of Young modulus, defining the elasticity of the trunk, range from 8.27 × 109 N/m2 for 17% of moisture content for Rhizophora sp. (Hawa 2005) to 20.03 × 109 N/m2 for mangroves (Vallam et al. 2011). Young modulus of 13.6 × 109 N/m2 (for moisture content of ca. 15%) was obtained from deflection tests of wooden probes, performed in the framework of own field surveys. Concept of a submerged root ratio Vm/V, introduced by Mazda et al. (1997a), was adopted to define the density of the root system as a function of water depth (where Vm represents the volume of submerged roots and V the water control volume). This method required counting of the roots at selected water levels, which in case of the field work by Mazda et al. (1997a) were limited to the tidal level of 0.5 m and were extended up to ca. 1.5 m in own field surveys. Canopy density was specified using Leaf Area Index (LAI), which represents a ratio of a total onesided leaves area to the downward projected area of a canopy. By estimating the number of leaves on mangrove main branches, leaves area and number of the branches, LAI of 1.3, 3.3 and 2.9 was determined for a young, mid-age and mature tree, respectively, in own field measurements. In the literature, LAI of ca. 7 was reported for mangroves by Green and Clark (2000), while Clough et al. (2000) underlined its dependency on tree age (LAI > 5.0 for mangroves younger than 5 years, LAI ca. 1.8 for 36 year old mangroves). LAI of 4.5 was considered in this study.
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COASTAL ENGINEERING 2014 3.75 m
canopy
b)
http://1.bp.blogspot.com/‐8Sml4yn3onI/TtBlaTrxu‐ I/AAAAAAAAQcA/qng3m23ZmCA/s400/Rhizophora‐stylosa‐1.jpg
trunk
0.15 m
MHWL
roots
0.5, 1.0, 1.5, 2.5 m 2.0 m
10.0 m
5.5 m
a)
0.125 m 3.75 m
Figure 2. Mature mangrove Rhizophora sp. in natural habitat (a) and its geometry (b).
Based on the defined structural and bio-mechanical properties of a mature Rhizophora sp., two parameterization approaches were developed in order to determine the contribution of each structural tree element (i.e. the root system, the trunk and the canopy) to wave attenuation, depending on tsunami flow depth: Stiff structure assumption, in which the tree model consisted of a parameterized root system and a stiff trunk. The stiff mangrove model was submerged maximally up to the top of the trunk, what corresponded to tsunami of a moderate magnitude (see Strusińska-Correia et al. 2013). Flexible structure assumption, in which the tree model consisted of a parameterized root system (identical to the one in the stiff structure assumption), a flexible trunk and a canopy. In this case, stronger tsunami flooding conditions were reproduced in the laboratory by using higher water levels, reaching the top of the mangrove canopy. Stiff structure assumption
In order to compare the properties of a mangrove prototype and a mangrove model, three real-like models of a different root density (model group A with low density, model group B with medium density and model group C with high density) were constructed at a scale of 1:20 (Fig. 3). The trunk and the roots were modeled using a modeling clay, hardened in an oven. The relationship between the submerged root volume ratio and root submergence, plotted in Fig. 4a, agrees very well with the pattern determined by Mazda et al. (1997a). Geometry of these models is provided in Tables 1 and 2. In the next step, three parameterized models for each real model were introduced, accounting for a varying frontal root area Af (i.e. the area perpendicular to flow direction) with changing root submergence (see Fig. 3). The nature-like-shaped trunk and the roots were replaced in these models by steel and plastic cylinders of varying number, diameter and height (see Tables 1 and 2). Based on results of experiments performed at a scale of 1:20 (Froude similitude law) under quasisteady flow conditions, which aimed at a comparison of the properties of the real and the parameterized stiff mangrove models in terms of drag coefficient, reduction of flow velocity and hydraulic gradient, model A2 was selected as the best representation of the mangrove prototype. Detailed information on these experiments is provided in Husrin (2013). Flexible structure assumption
The stiff mangrove model A2 was further modified into the flexible model by introducing a trunk and a canopy. Similarly to the stiff structure parameterization procedure, a real flexible mangrove model was constructed first to provide a reference for the parameterized flexible models (Fig. 4b and Table 3). The canopy and trunk in this model was made of plastic branches with leaves, resulting in a Leaf Area Index of 4.5. The parameterization of the mangrove prototype under flexible structure assumption was performed in two stages in order to determine the representative properties of the canopy and the trunk. In the first step, the five tree models were equipped with a stiff trunk, while the
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canopy properties (density and frontal area) varied by controlling the volume of the fibrous material used (see Fig. 4b and Table 3). The models were examined under quasi-steady flow conditions in terms of current-induced forces (see Husrin 2013 for more details). Based on the comparison of the experimental data, canopy model M2FS was selected for the construction of the entirely flexible mangrove models (see Fig. 4b and Table 3). The trunk in the three flexible models was made of Polytetrafluoroethylene (PTFE), which is the only feasible material with elasticity (0.5 × 109 N/m2 at model scale 1:25) fitting to the aforementioned range of the Young modulus for mangrove trunks. Density of this material (ca. 2000 kg/m3) is however double as compared to mangrove trunk density (ca. 1000 kg/m3). Comparative analysis of the deflection pattern of the flexible models (see Husrin 2013) indicated that model M2FF is the best representation on the mangrove prototype. a) Side view
b) Plan view
“Real” model
Increasing root density (Vm/V) A1
A1
Parameterized model
B1
C1
C1
Decreasing frontal area (Af)
B1
A2
B2
C2
A3
B3
C3
A4
B4
C4
Figure 3. Developed stiff mangrove models: a) side view, b) plan view. The root model applied to the flexible mangrove models is marked in red frame.
b) Flexible mangrove models
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Model group A
14
Model group B
Stiff trunk
Model group C
M1FS
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Nakama-Gawa (Japan), Mazda et al. ((1997) 1997) Coral Creek (Australia), Mazda et al. ((1997) 1997)
8 6 4 2 0 0.00
0.10
0.20
0.30
0.40
Parameterized model
Water depth h [cm]
12
“Real” model
a) Root density
Same density and frontal area
M2FS
30% less dense, same frontal area
M3FS
M4FS
M5FS
50% less dense, same frontal area
30% less dense, 50% less dense, 30% less frontal 50% less frontal area area
Flexible trunk M1FF
M2FF
M3FF
Submerged root volume ratio Vm/V [-] Same density, 8% less frontal area
30% less dense, 8% less frontal area
50% less dense, 8% less frontal area
Figure 4. Relationship between mangrove root density and root submergence (a) and constructed flexible mangrove models (b). The selected flexible model is marked in red frame.
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COASTAL ENGINEERING 2014 Table 1. Properties of stiff mangrove models at model scale of 1:20 (height and diameter in cm). Structural part Trunk
Root type I Root type II Root type III Root type IV Root type IV
Model group A A1 A2 1 1 15 15 1 1 9 12 12.5 0.6 0.5 26 0 10 0.4 0.5 20 20 7.5 0.35 0.5 25 28 5.0 0.3 0.5 74 2.5 0.5
No. Height Diameter No. Height Diameter No. Height Diameter No. Height Diameter No. Height Diameter No. Height Diameter
A3 1 15 1 4 12.5 1.0 0 10 1.0 4 7.5 1.0 8 5.0 1.0 18 2.5 1.0
A4 1 15 1 2 12.5 1.5 0 10 1.5 2 7.5 1.5 4 5.0 1.5 4 2.5 1.5
Model group B B1 B2 1 1 15 15 1 1 12 20 12.5 0.6 0.5 40 12 10 0.4 0.5 40 28 7.5 0.35 0.5 63 80 5.0 0.3 0.5 46 2.5 0.5
B3 1 15 1 8 12.5 1.0 0 10 1.0 8 7.5 1.0 8 5.0 1.0 20 2.5 1.0
Model group C C1 C2 1 1 15 15 1 1 16 96 12.5 0.6 0.5 40 0 10 0.4 0.5 40 44 7.5 0.35 0.5 150 52 5.0 0.3 0.5 474 2.5 0.5
B4 1 15 1 4 12.5 1.5 0 10 1.5 0 7.5 1.5 8 5.0 1.5 10 2.5 1.5
C3 1 15 1 24 12.5 1.0 0 10 1.0 12 7.5 1.0 12 5.0 1.0 66 2.5 1.0
C4 1 15 1 12 12.5 1.5 0 10 1.5 0 7.5 1.5 12 5.0 1.5 20 2.5 1.5
Table 2. Frontal area of stiff mangrove models for relative root submergence h/hmdl = 0.05-0.15 at model scale of 1:20 (hmdl denotes total mangrove model height). A1: 30.6-61.8 cm2 B1: 34.7-70.1 cm2 C1: 56.9-115.1 cm2
A2: 50.1-86.6 cm2 B2: 65.1-119.1 cm2 C2: 72.6-142.9 cm2
A3: 47.5-75.0 cm2 B3: 47.5-125.0 cm2 C3: 74.3-148.2 cm2
A4: 41.3-66.3 cm2 B4: 45.0-110.0 cm2 C4: 84.0-171.1 cm2
Table 3. Characteristics of flexible mangrove models (model scale of 1:25). Model ReMS M1FS M2FS M3FS M4FS M5FS 116.16 87.10 Total submerged 165.20 174.20 116.16 87.10 volume [cm3] Canopy width [m] 0.150 0.135 0.135 0.135 0.095 0.068 Canopy frontal 0.0291 0.0297 0.0297 0.0297 0.0208 0.0148 area [m2]
M1FF 174.20
M2FF 116.16
M3FF 87.10
0.124 0.0273
0.124 0.0273
0.124 0.0273
EXPERIMENTAL SET-UP Experimental facility
The laboratory investigation on solitary wave attenuation by the parameterized mangrove forest was conducted in a 2 m – wide wave flume at the Leichtweiss-Institute for Hydromechanics and Coastal Engineering at the Technical University of Braunschweig in Germany. The flume is approximately 90 m long and 1.25 m deep and is equipped with a piston type wave maker (see Fig. 5a). The model was scaled according to Froude similitude law, using a scale of 1:25. a)
WG1-WG4
WAVE MAKER
SWL
SWL
0.42m
1.08m
WG9-WG15
WG5-WG8
9.19m
0.76m
0.42m
0.5m
1.08m
0.76m
0.5m 0.5m
h=0.815m 31.97m
10.0m
b) B=3.0 m
13.71m
8.33m
c)
0.42m 0.33m
1.1m
FOREST
1.08m
ADV2 h r=0.415m
BEACH MODEL 2.0m
0.42m
0.76m
0.51m
ADV1
h=0.595m
WG16-WG19
B
d)
Figure 5. Experimental set-up: a) arrangement of measuring devices, b) forest model of width of B=3.0 m, c) force transducer connected to mangrove model, d) installation of force transducer in forest model.
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Mangrove forest model and beach model
In order to investigate the conditions most favorable for wave attenuation, maximum possible forest density was considered in this test series, corresponding to ca. 44 trees/m2. This density was achieved by arranging the tree models in shifted rows, consisting of 12 and 13 mangrove models (see Figs. 5b and 6). 62 and 250 mangrove tree models were used to construct forest model of width of B = 0.75 m and B = 3.0 m, respectively. A proper modelling of a bathymetry/topography as well as applied wave force measurement technique (described in more details in the following section) required placement of the forest model on an elevated ground. A horizontal plywood platform, representing a beach model of height of ca. 0.41 m and a seaside slope of 1:20, was constructed for this purpose. The toe of the beach model was placed at a distance of ca. 23.7 m from the wave maker, while the front of the forest model was 33.97 m from the wave generator and 2.0 m behind the end of the beach slope, as shown in Fig. 5a. a) B=0.75 m B=0.75m T13 T1
T38 T39
T15
T40
T16 T4
T41
T17
T42
T6 FTS1 T7 FTS2
T19 FTS3
T56 T44
FTS6
T69
T57 FTS7
FTS8
T45
T9
T46
T10
T47
T23 T11
T48
T12
T73
T49
T25
T50
T87
T148
T123 T136 T137
T100
T150
T125
T234 T247 T235
T198
T223 T211
T248 T236
T199
T175
T246
T222 T210
T187
T162
T245 T233
T221 T209
T186 T174
T244 T232 FTS11
T220 T208
T197
T173 T161
T149
T124
T195
T185
T160
T219 T207
T196
T172
T243 T231
T194
T184
T159
T242 T230
T218 T206
T183 T171
T147 T135
T112
T170 T158
T134
T111 T99
T182
T146
T122
T98
T75
T133
T110
T157 T145
T120
T217 T205
T193
T181 T169 FTS10
T241 T229
T192
T156 T144
T121 T109
T86 T74
T62
T37
T132
T97 T85
T61
T36 T24
T119 T107 FTS9 T108
T84
T60
T35
T131
T216 T204
T180 T168
T240 T228
T191
T167 T155
T143
T106
T96
T72
T130 T118
T215 T203
T179
T239 T227
T190
T166
T238 T226
T214 T202
T178
T154 T142
T117
T95 T83
T59
T34 T22
T94 T82
T71
T129
T105 T93
T70
T58
T33 T21
T92
T81
T141
T116 T104
T80 T68
T32 FTS5
T20
T8
T67
T43 T31 FTS4
T91
T165 T153
T213 T201
T189 T177
T152
T128
T188 T176
T164
T140
T115
B=0.75m
T163 T151
T139 T127
T103
T79
T55
T30 T18
T90
T66
T126 T114
T102
T78
T54
T29
T5
T65
T138
T113 T101
T89 T77
T53
T28
B=0.75m
T88 T76
T64 T52
T27
T3
T63 T51
T26 T14
T2
B=0.75m
T224 T212
T249 T237
T200
T225
T250
b) B=3.0 m B=0.75m T13 T1
T38
T14
T39
T16
T41
T17 T5
T42
T18 T6 FTS1 T7 FTS2
T19
T43 T31 FTS3
T44
T20
T8
T45
T21
T10 T23
T48
T24 T12
T49
T25
T74 T62
T50
T98
T75
T100
T149
T124 T137
T112 T125
T150
T247 T235
T223
T199
T248 T236
T224 T212
T200
T246 T234
T222
T211
T187 T175
T221
T198
T174
T245 T233
T210
T186
T162
T244 T232 FTS11
T209 T197
T173
T243 T231 FTS10
T219 T220
T196
T185
T161
T218
T208
T184 T172
T148 T136
T195
T171
T160
T206 FTS9
T242 T230
T207
T183
T159 T147
T123
T99 T87
T146
T135
T111
T170 T158
T134 T122
T194
T182
T157
T217
T193 FTS8
T241 T229
T205
T181 T169
T216
T192
T168 T156 FTS7
T145
T121
T110
T86
T132 T133
T109 T97
T73 T61
T37
T96
T85
T144
T120 T108
T84 T72
T60
T36
T95
T71
T131
T119 T107 FTS6
T240 T228
T204
T180
T239 T227
T215
T191
T167 T155
T143
T118 T106
T94
T83
T59 T47
T35
T11
T70
T46 T34
T22
T93 T81 FTS5
T130
T214
T203
T179
T238 T226
T202 T190
T166
T142
T117
T189
T178
T154
T213 T201
T177 T165
T153
T129
T105
T82
T58
T33
T9
T69
T57
T32
T92
T164
T141
T116 T104
T80 T68
T56 FTS4
T128
T188 T176
T152 T140
T115
T91
T67
T127
B=0.75m
T163 T151
T139
T114
T103
T79
T55
T30
T126
T102 T90
T66 T54
T29
T89
T78
T138
T113 T101
T77 T65
T53
T28
T4
T64
T40
B=0.75m
T88 T76
T52
T27 T15
T3
T63 T51
T26
T2
B=0.75m
T249 T237
T225
T250
Figure 6. Arrangement of mangrove tree models (green squares marked as T1 - T250) and force transducers (orange squares marked as FT1 - FT11) in forest model of width of: a) 0.75 m, b) 3.0 m.
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Measuring technique
The pattern of wave propagation in front of, in, and behind the mangrove forest model was recorded by means of 19 wire-type wave gauges (WG), which arrangement is presented in Fig. 5a. Three wave gauges were placed within the forest model: wave gauge WG13 at the front, WG14 in the middle and WG15 at the rear tree row. In addition to the wave profile analysis, the pattern of wave forces induced on single tree models was examined by means of force transducers (FT), developed at the LWI for the purposes of this investigation (Figs. 5c and d). The force transducers are capable of measuring positive and negative wave forces up to 60 N, exerted in the direction of wave propagation. In each forest model configuration, 11 force transducers were employed within the forest, and in case of the shorter forest model (B = 0.75 m), also behind the forest as depicted in Fig. 6. Two Acoustic-Doppler velocimeters (ADV), installed at the beginning and at the end of the forest model, measured horizontal flow velocity in the direction of wave propagation (see Fig. 5a). Experimental programme
For each of the considered configurations of the mangrove forest model (i.e. forest width of 0.75 m and 3.0 m), varying water depth conditions and solitary wave height were examined, what resulted in a total number of 50 tests. In order to investigate the influence of the submergence of the mangrove tree model on solitary wave attenuation, total water depth in front of the beach model h was varied from 0.595 m to 0.815 m (with an increment of 0.055 m), which corresponded to the tree model submergence range of dr = 0.18 - 0.40 m. As shown in Fig. 7, the tree submergence covered the entire height of tree canopy, with the lowest submergence reaching the canopy bottom and the highest one reaching canopy top. For each water level, solitary waves of five different nominal incident heights (Hi,nom = 0.04, 0.08, 0.012, 0.016 and 0.20 m) were generated. 0.124 m
h=0.815 m (dr=0.400 m) 0.055 m
0.22 m
0.055 m
0.40 m
0.055 m
(other root height: 0.02, 0.04, 0.06 m)
0.10 m
0.08 m
0.055 m
h=0.760 m (dr=0.345 m)
h=0.705 m (dr=0.290 m)
h=0.650 m (dr=0.235 m)
h=0.595 m (dr=0.180 m)
0.008 m
0.005 m
upper surface of beach model 0.15 m × 0.15 m
Figure 7. Examined water depth conditions in respect to the geometry of flexible mangrove model.
ANALYSIS OF EXPERIMENTAL RESULTS Classification of wave evolution modes
Due to the fact that two sources of wave energy attenuation were observed in the conducted tests, namely the wave-forest interaction and wave breaking process, introduction of wave evolution modes into the data analysis was very crucial for the proper assessment of the forest damping performance. In case of tests with nonbreaking solitary waves, which dominated in the performed test series, the reduction of wave energy was solely due to wave propagation in the forest model. In contrast, in the experiments under broken wave conditions, both the forest model and the bathymetry/topography contributed to wave attenuation. The classification of the wave evolution modes was based both on the generation of wave breaking and the location of the breaking point in respect to the geometry of the beach-forest model. The following wave evolution modes (EM) were distinguished:
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COASTAL ENGINEERING 2014
1.
Nonbreaking solitary waves disintegrating into solitons: Nonbreaking waves (EM1): a solitary wave train, consisting of waves of decreasing height (solitons), was generated as a result of the wave fission process due to the change of the water depth over the beach model (see Fig. 8a). 2. Breaking solitary waves disintegrating into solitons: Waves breaking between the end of the beach slope and the beginning of the forest model (region 2) and disintegrating into solitons (EM3), as illustrated in Fig. 8b. Waves breaking in the forest model (region 3) and disintegrating into solitons (EM4), shown in Fig. 8c. Waves breaking behind the forest model (in region 4) and disintegrating into solitons (EM5), depicted in Fig. 8d. Evolution mode EM2, in which wave broke over the beach slope (in region 1), was not observed in the performed test series. As shown in Fig. 9, the wave evolution modes were governed predominantly by the incident solitary wave height and the water depth, and partially by the width of the forest model (to distinguish between wave breaking induced in and behind the forest model).
region 3
region 2
Water free surface elevation η [m]
0.14
a) Nonbreaking waves (EM1) region 1
region 4
dr h
hr
forest model
wave shoaling wave fission wave fission 8.33m
2.0m
0.12 0.10
development of wave fission
0.06 0.04 0.02 0.00
86
90
88
Water free surface elevation η [m]
region 3
region 4
dr h
hr wave shoaling 8.33m
wave breaking 2.0m
forest model broken wave
region 4
forest model
wave shoaling 2.0m
8.33m
wave breaking B
broken wave
d) Waves breaking behind forest model (EM5) region 1
region 2
region 3
region 4
dr h
forest model
hr wave shoaling 8.33m
2.0m
B
wave breaking
Figure 8. Classification of wave evolution modes.
wave fission
dr=0.18m hr=0.415m
broken wave
0.20 0.16
development of wave fission
0.12
Hi,nom=0.20m B=3.0m h=0.595m dr=0.18m
0.08 0.04 0.00
95
94
93
92
96
97
98
99
100
101
0.28
WG14 WG11 WG17 WG9 WG13 WG19 WG15
generation of wave breaking
0.24 0.20 0.16
broken wave
Hi,nom=0.20m B=3.0m h=0.76m dr=0.345m
B=3.0m
h=0.76m
dr=0.345m hr=0.415m
development of wave fission
0.12 0.08 0.04 0.00 -0.04
wave fission
94
96
95
98
97
102
101
100
99
Time t [s]
Water free surface elevation η [m]
hr
WG18
B=3.00m
Time t [s]
Water free surface elevation η [m]
region 3
WG10 WG13 WG9 WG11 WG14
h=0.595m
-0.04 91
wave fission
dr h
102
0.24
0.32
region 2
100
98
96
generation of wave breaking
0.28
B
c) Waves breaking in forest model (EM4) region 1
94
92
Time t [s]
0.32
region 2
WG19
dr=0.18m hr=0.415m
B
b) Waves breaking in front of forest model (EM3) region 1
WG15
B=3.00m
h=0.595m
0.08
-0.02 84
wave fission
WG10 WG13
generation of wave fission
Hi,nom=0.08m B=3.0m h=0.595m dr=0.18m
0.28 0.24 0.20 0.16
WG14 WG16 WG11 WG15 WG19 WG9 WG13
broken wave
generation of wave breaking
dr=0.40m
Hi,nom=0.20m B=3.00m h=0.815m dr=0.40m
h=0.815m
B=3.00m
hr=0.415m
development of wave fission
0.12 0.08 0.04 0.00 -0.04 76
77
78
79
80
81
Time t [s]
82
83
84
85
9
0.24
EM1 EM3 EM4 EM5
a) B=0.75 m 0.20 0.16 0.12 0.08 0.04 0.00 0.540
0.595
0.650
0.705
0.760
0.815
0.870
Nominal solitary wave height Hi,nom [m]
Nominal solitary wave height Hi,nom [m]
COASTAL ENGINEERING 2014 0.24
EM1 EM3 EM4 EM5
b) B=3.0 m 0.20 0.16 0.12 0.08 0.04 0.00 0.540
0.595
Water depth h [m]
0.650
0.705
0.760
0.815
0.870
Water depth h [m]
Figure 9. Observed wave evolution modes as a function of water depth conditions and solitary wave height in case of forest width of: a) B = 0.75 m, b) B = 3.0 m.
Evolution of wave height
Effectiveness of the forest model in wave attenuation was analyzed first in terms of solitary wave height reduction, which was governed forest width, water level and incident wave height. For this purpose, maximum solitary wave height was determined for each performed experiment. Exemplary results of this analysis, plotted in Fig. 10 for both forest model configurations and extreme water level conditions examined, clearly indicate four regions of different wave behavior. Solitary wave height remained constant over the horizontal part of the flume and increased as a result of shoaling process over the slope of the beach model. Depending on the evolution mode, the wave height decreased either in front of the forest model due to wave breaking process (wave evolution mode EM3), or in the forest model (EM1 and EM4), where particularly significant wave height reduction was observed for the wider forest model. The wave height in front of the forest model yielded: ca. 0.05 m for Hi,nom = 0.04 m, 0.1 m for Hi,nom = 0.08 m, 0.15 - 0.20 m for Hi,nom = 0.12 m, 0.20 - 0.25 m for Hi,nom = 0.16 m and 0.23 - 0.27 m for Hi,nom = 0.20 m for all examined water depths. These wave heights were attenuated as follows at the end of the forest model of width of 0.75 m: ca. 0.02 - 0.04 m for Hi,nom = 0.04 m, 0.07 m for Hi,nom = 0.08 m, 0.11 m for Hi,nom = 0.12 m, 0.12 - 0.16 m for Hi,nom = 0.16 m and 0.14 - 0.20 m for Hi,nom = 0.20 m for all examined water depths. As a result of a longer propagation distance in case of the forest model of width of 3.0 m, further wave height reduction was observed at the rear tree row: ca. 0.03 m for Hi,nom = 0.04 m, 0.05 m for Hi,nom = 0.08 m, 0.06 - 0.10 m for Hi,nom = 0.12 m, 0.07 - 0.16 m for Hi,nom = 0.16 m and 0.08 - 0.24 m for Hi,nom = 0.20 m for all examined water depths. The pattern of solitary wave height behind the forest model was generally more irregular for higher water levels and forest model of B = 0.75 m due to the more intensive process of solitary wave fission and triggering of wave breaking behind the forest (EM5). Evolution of wave forces induced on single mangrove models
Pattern of maximum wave forces exerted on the single mangrove models was analyzed in a similar way to the envelope of the maximum solitary wave height. Fig. 11 presents exemplarily the development of the forces in the forest for representative experiments, in which point 0.0 m on the horizontal axis represents the beginning of the forest model. The magnitude of the wave forces was governed by the water depth and wave height conditions and the resulting evolution mode – the forces decreased with the increasing submergence depth of the tree models and on the other hand they became larger for greater wave heights, e.g. ca. 1 N for Hi,nom = 0.04 m, 5 - 6.5 N for Hi,nom = 0.08 m, 7 - 15 N for Hi,nom = 0.12 m, 10 - 24 N for Hi,nom = 0.16 m and 13 - 27 N for Hi,nom = 0.20 m for all examined water depths. The highest forces were exerted at the frontal tree row at the direct impact of the wave. A general trend of the decrease of the wave forces with wave propagation in the forest model was observed (see FT1 - 7 for B = 0.75 m and FT1 - 11 for B = 3.0 m in Fig. 11). A smaller force reduction was attributed to the narrow forest model due to the shorter wave propagation distance and thus weaker interaction with the tree models. The forces recorded at the end of this forest model corresponded well to that measured by force transducers FT4 in forest of width of 3.0 m and yielded: ca. 0.7 - 1 N for Hi,nom = 0.04 m, 3 - 4 N for Hi,nom = 0.08 m, 6 - 9 N for Hi,nom = 0.12 m, 9 - 15 N for Hi,nom = 0.16 m and 10 - 15 N for Hi,nom = 0.20 m for all examined water depths. Further reduction of the forces (between FT4 and FT10/11) was observed within the wide forest model.
10
COASTAL ENGINEERING 2014 0.35 Hi,nom=0.04m (EM1)
Max. height of solitary wave Hs,max [m]
a) B=0.75m, h=0.595m
Hi,nom=0.08m (EM1)
0.30
Hi,nom=0.12m (EM5)
B=0.75m 0.25
Hi,nom=0.16m (EM4) Hi,nom=0.20m (EM3)
0.20 0.15 0.10 0.05 WG 5,7+8
WG 2‐4
0.00 10
15
20
WG 9‐19
25
30
35
40
45
40
45
Horizontal distance from wave maker x [m] 0.35 Hi,nom=0.04m (EM1)
Max. heiht of solitary wave Hs,max [m]
b) B=0.75m, h=0.815m
Hi,nom=0.08m (EM1)
0.30
Hi,nom=0.12m (EM1)
B=0.75m
Hi,nom=0.16m (EM1)
0.25
Hi,nom=0.20m (EM1)
0.20
0.15
0.10
0.05 WG 2‐4
0.00 10
WG 5,7+8
15
20
WG 9‐19
25
30
35
Horizontal distance from wave maker x [m] 0.35 B=3.0m
Max. heiht of solitary wave Hs,max [m]
c) B=3.0m, h=0.595m
Hi,nom=0.04m (EM1) Hi,nom=0.08m (EM1)
0.30
Hi,nom=0.12m (EM1) Hi,nom=0.16m (EM4) Hi,nom=0.20m (EM3)
0.25 0.20 0.15 0.10 0.05 WG 5,7+8
WG 2‐4
WG 9‐19
0.00 10
15
20
25
30
35
40
45
Horizontal distance from wave maker x [m] 0.35 B=3.0m
Max. heiht of solitary wave Hs,max [m]
c) B=3.0m, h=0.595m
Hi,nom=0.04m (EM1) Hi,nom=0.08m (EM1)
0.30
Hi,nom=0.12m (EM1) Hi,nom=0.16m (EM4) Hi,nom=0.20m (EM3)
0.25 0.20 0.15 0.10 0.05 WG 5,7+8
WG 2‐4
WG 9‐19
0.00 10
15
20
25
30
35
40
45
Horizontal distance from wave maker x [m]
Figure 10. Envelopes of maximum solitary wave height for: a) B = 0.75 m and h = 0.595 m, b) B = 0.75 m and h = 0.815 m, c) B = 3.0 m and h = 0.595 m, d) B = 3.0 m and h = 0.815 m.
11
COASTAL ENGINEERING 2014 35 Hi,nom=0.04m (EM1)
Max. force exerted on tree model Fmax [N]
a) B=0.75m, h=0.595m
Hi,nom=0.08m (EM1)
30
Hi,nom=0.12m (EM5)
FT1,2 FT3
25
Hi,nom=0.16m (EM4)
End of forest model
Hi,nom=0.20m (EM3)
FT4
20
FT5
FT6,7 FT10
FT8
15
FT9 FT11
10
5
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Horizontal distance from beginning of forest model [m]
Max. force exerted on tree model Fmax [N]
16
FT1,2
Hi,nom=0.04m (EM1)
b) B=0.75m, h=0.815m
Hi,nom=0.08m (EM1)
14
Hi,nom=0.12m (EM1)
FT3
End of forest model
12
FT6,7
FT4
Hi,nom=0.16m (EM1)
FT10
Hi,nom=0.20m (EM1)
FT9 FT8
10
FT11
8 6 4 2 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Horizontal distance from beginning of forest model x' [m] 35
Max. force on tree model Fmax [N]
FT1,2
c) B=3.0m, h=0.595m
Hi,nom=0.04m (EM1) Hi,nom=0.08m (EM1)
30
Hi,nom=0.12m (EM1)
FT4
Hi,nom=0.16m (EM4)
FT3
25
Hi,nom=0.20m (EM3)
20
End of forest model FT5
15
10 FT6
5
FT7
FT8
FT9 FT10,11
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Horizontal distance from beginning of forest model x' [m] 16 FT1,2
Hi,nom=0.04m (EM1)
d) B=3.0m, h=0.815m
Hi,nom=0.08m (EM1)
Max. force on tree model Fmax [N]
14
Hi,nom=0.12m (EM1) Hi,nom=0.16m (EM1)
12
FT3
Hi,nom=0.20m (EM5)
10
FT4 FT6
8
End of forest model
FT8 FT9
FT7
FT10,11
6 4 2 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Horizontal distance from beginning of forest model x' [m]
Figure 11. Envelopes of maximum wave forces exerted on single mangrove models for: a) B = 0.75 m and h = 0.595 m, b) B = 0.75 m and h = 0.815 m, c) B = 3.0 m and h = 0.595 m, d) B = 3.0 m and h = 0.815 m.
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COASTAL ENGINEERING 2014
The range of the wave forces at the end of this forest model was as follows: ca. 0.4 - 0.8 N for Hi,nom = 0.04 m, 1.2 - 2.2 N for Hi,nom = 0.08 m, 2 - 4 N for Hi,nom = 0.12 m, 2.4 - 6 N for Hi,nom = 0.16 m and 2.8 - 7.5 N for Hi,nom = 0.20 m for all examined water depths. The additional contribution of the breaking process to wave attenuation in the forest can be clearly seen in Figs. 11a and c, what resulted in a greater reduction of the wave forces as compared to the nonbreaking waves. In case of the narrow forest model, the data measured by the force transducers placed behind the forest model (FT8 - 11) indicate a slight increase and then decrease of the forces to the value behind the forest model, particularly for larger wave heights. Due to the limited number of the constructed force transducers, analysis of this force pattern behind forest model of B = 3.0 m was not possible and requires further investigation. Wave transmission
Due to the disintegration of the incident solitary wave into a soliton train, wave transmission coefficient was calculated as a function of wave energy as postulated by Liu and Cheng (2001). Wave energy was considered as a sum of kinetic and potential wave energy (Longuet-Higgins and Fenton 1974):
Etot Ek E p ,
(1)
determined at wave gauge WG13 as total energy of incident wave and at WG15 as total energy of transmitted wave. The energy components are expressed as follows:
Ek 0 .5
u dzdx, 2
(2)
h
E p 0.5 g 2 dx.
(3)
Approach by Al-Banaa und Liu (2007) was used to transform the energy components from spatial into temporal domain: t2
Ek 0.5 c u 2 dzdt,
(4)
t1 h
t2
E p 0.5 g c 2 dt,
(5)
t1
in which dx = c·dt. The horizontal particle velocity can be defined after Munk (1949) as:
uc
h
,
(6)
with solitary wave speed calculated as:
c g h H .
(7)
In Eqs. 1-7, c denotes the wave celerity [m/s], Ek the kinetic wave energy [J/m], Ep the potential wave energy [J/m], g the gravitational acceleration [m/s2], h the water depth [m], H the wave height [m], u the horizontal particle velocity [m/s] and η the water free surface elevation [m]. Figure 12 shows the relationship between computed wave transmission coefficient Kt and relative forest width B/L with corresponding wave evolution modes. Two very clear data clouds can be distinguished in this graph: one for forest width of B = 0.75 m with the transmission coefficient ranging from ca. 0.48 to 0.78 and another for forest width of B = 3.0 m with Kt = 0.29 - 0.56. The lowest wave transmission was attributed to waves breaking in front of and in the forest model (EM3 and EM4, respectively) due to the combined wave energy dissipation through the wave-tree model
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COASTAL ENGINEERING 2014
Transmission coefficient in terms of wave energy Kt [‐]
interaction and the turbulence at breaking. Nonbreaking waves (EM1) were dominant in the performed test series, resulting in much higher transmission coefficient as compared to the broken waves. A better performance of the forest model can be also observed for smaller water depths, particularly when comparing the data for a constant wave height. h
1.0
Kt
0.9 0.8
EM1
E tot,WP15 WG15 E tot,WP13 WG13
0.6 0.5
[-]
0.04 m
0.08 m
0.16 m
0.20 m
0.705 m 0.760 m
0.4
0.815 m
EM1 EM1
EM1
EM5 EM1 EM1
EM1
EM4
EM4 EM1
EM1 EM1
EM1
EM1
EM4
EM1
0.3
EM1
B = 0.75 m
0.2
0.12 m
0.650 m
EM1
EM1 EM1 EM1 EM1 EM1 EM1 EM4 EM1 EM4 EM1 EM1 EM1 EM1 EM1 EM1 EM1 EM1 EM1 EM4 EM4 EM1 EM3 EM1 EM5 EM4
0.7
Hnom
0.595 m
EM1
EM4 EM4
EM3
EM4
B = 3.0 m
L2
0.1
2.12h H/h
0.0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Relative forest width [m] Relative forest widthB/L B/L gen[-]
Figure 12. Wave transmission coefficient as a function of relative forest width.
CONCLUSIONS
Consideration of the most important physical properties of the selected mangrove species as well their dependency on the tree age makes the parameterization approach presented in this paper more reliable than those proposed in the previous studies. Although it was developed for a specific tree species, the assumptions made and the systematic parameterization procedure itself can be applied to other types of coastal vegetation. However, there are two shortcomings of the parameterization procedure to be mentioned: firstly, experimental modelling of a tree failure (e.g. trunk breakage or uprooting) under wave impact is not possible and secondly, the trunk model, despite same elasticity as mangrove wood, has density that is double density of the prototype. The idealized forest conditions assumed in these experiments, namely the maximum forest density, healthy mangrove trees of idealized resistance to wave attack, result in overestimation of the attenuation performance of the forest model. As indicated by the results, wave energy reduction by 70% was attributed to the widest forest of B = 3.0 m (75 m in prototype) in case of the weakest flow depth conditions examined. In case of larger flow depths, reaching the top of the canopy, wave transmission is reduced to ca. 40 - 50%. Despite such a large forest width, relatively poor performance of the forest model was observed, which is, in fact, even poorer when considering the horizontal scale of a real tsunami and laboratory limitations in generation of long waves. Moreover, execution of such large vegetation zones at the coast would be very difficult in many regions due to the limited availability of the land. The effectiveness of the narrower forest model of B = 0.75 m (18.75 m in prototype) is too low (between 20 and 50%) in order to consider the mangrove forest as a defense barrier against tsunami. Despite the low performance in attenuation of extreme tsunami, planning and maintenance of coastal green belts is highly recommended due to the coast stabilization functions and protection from flooding, mentioned before in the introductory part of this paper, as well as due to the life-saving role and debris-stoppage reported from the recent tsunami events. ACKNOWLEDGMENTS
The TAPFOR project was performed in the framework of the project "Tracing Tsunami impacts on- and offshore in the Andaman Sea Region (TRIAS)", funded by the Deutsche Forschungsgemeinschaft (DFG) and the Office of the Research Council of Thailand (NRCT) and partly by the DFG within the Graduate College of TU Braunschweig "Risk Management of Natural and Civilization Hazards on Buildings and Infrastructure" (GRK 802).
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COASTAL ENGINEERING 2014
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