Major Coastal Engineering and Management: Wave attenuation by mangroves in Hau Loc District, Thanh Hoa Province

Thereby declare that is the research work by myself under the supervisions of Dr Nguyen Quang Chien and Assoc Prof, Dr Tran Thanh Tung The results and conclusions of the thesis are fidelity, which are not copied from any sources and any forms The reference documents relevant sources, the thesis has cited and recorded as

prescribed, The matter embodied in this thesis has not been submitted by me for the

award of any other degree or diploma.

Hanoi, 15 May 2018 Author

Pham Hoai Thuong

First of all, Ï would like to express my special thanks to my supervisors Dr Nguyen Quang Chien and Assoc Prof Dr Tran Thanh Tung for their patience, enthusiasm and immense knowledge, without them my research study would not have been succeeded, I sincerely thank all the lecturers who taught me in the program especially the lecturers from the Faculty of Marine and Coastal Engineering of Thuyloi University for their useful and interesting lectures,

also would like to acknowledge Prof Dr Marcel Stive and Assoc Prof Henk Jan

Verhagen for their support and inspiration when I was studying in the Netherlands Finally, Lam deeply grateful to my family for their great care and encouragement.

4 Research Objectives and Research Questions

4 Approach and Methodology 'CHAPERI: LITERATURE REVIEW

1.1 The characteristics of mangrove spet

1.1.1 Sonneratia caseolais (Vietnamese name: Cây Ban chua) 1.1.2 Kandelia obovata (Vietnamese name: cây Trang)

1.2 The mechanism of wave attenuation through mangroves forest

13 SWAN-VEG Model

1.4 Some studies about wave attenuation by mangrove forest in Vietnam

CHAPTER 2: STUDY AREA,

21 Location

22 The reality of mangroves forest 23 State of the dyke system

24 Other Conditions 24.1 Topography 242 Wind

243 Wave.

Selection of appropriate types of mangroves and vegetation parameters, Determine bed elevation to plant mangroves

Scenario selection

‘Computational domain and bathymetry.

Hydro-meteorologie condition for normal case 3.5.1 Wave parameters

3.5.2 Wind parameters 3.5.3 Water level

36 Hydro-meteorologic condition for storm case 3.6.1 Design return period

3.7.1 Wave propagation using Swan 2D model 38 3.7.2 Results of wave attenuation by mangrove forest 40 3.8 Comparison with an empirical formula 50 CHAPTER 4: IMPACT OF CLIMATE CHANGE TO MANGROVE FOREST AND WAVE ATTENUATION IN CLIMATE CHANGE SCENARIOS, 53 4.1 Impact of climate change and other factors to the development of mangrove forest 53

4.11 Seallevel rise 33 4.12 Rise in surface temperature s4 4.13 Storm and extreme weather events 35

‘Annex I: SWAN 2D input file 65

Annex 1.1: SWAN 2D input file in Scenario 1 65 Annex 1.2: SWAN 2D input file in Scenario 2 66 Annex 1.3: SWAN 2D input in Scenario 3 6

‘Annex 2: SWAN-VEG inpot file 68 Annex 2.1: Example of SWAN-VEG input file in normal condi mn, SE monsoon 68 ‘Annex 2.2: Example of SWAN-VEG input ile in normal condition, NE monsoon

Annex 2.3: Example of SWAN-VEG input file in Storm condition 70 Annex 3 : Hs, Kt, R in Scenariol,2,3 with 5-year-old tree and 9-year-old tre 71 ‘Annex 3.1: Hs in scenario 1,2,3 with 5-year-old tree and 9-year-old ee 00.71 Annex 3.2: Kt in scenario 1,2,3 with S-year-old tree and 9-yearold tee 73 Annex 3.3: R in seenatio 1,2,3 with 5-year-old tee and 9-year-old tree 16 ‘Annex 4: Hs in scenatios in which density of mangrove forest varies +9 ‘Annex 5: Hs in Scenarios with and without efimate change 83 ‘Annex 6: Some photos from the field tip 85

LIST OF TABLES

Table 0.1 Disuibution and area of mangroves [orests in Vietnam, 3 ‘Table 0.2 Mangroves forest in front of the dyke 4 Table 1.1 Some studies about wave attenuation by mangrove Forest im Vietnam l4 ‘Table 2.1 Characteristic ofthe sea dykes and revetments 20 Table 2.2 Characteristics of tide along Vietnamese coast 2 Table 3.1 Parameters of K, obovata 28 Table 32 Parameters of S.caseolaris 28 ‘Table 3.3 Exposed time in a day in some assumed bed elevation, 30 ‘Table 3.4 Input data in SWAN Simulation 3 Table 35 Statistical offshore wave data a4 Table 3.6 Wind and wave parameters in Van Ly Station 38 Table 3.7 Wave properties at nearshore locations (A and B) 40 ‘Table 3.8 Representative diameters of wave attenuation corresponding with SCN1.2,3

Table 4.1 Sea level rise in location from Hon Dau to Deo Ngang according to the scenarios of climate change and sea level rise 37 Table 4.2 Water level in Scenarios 58 ‘Table 4.3 Wave height behind mangrove forest and wave attenuation coefficient 59

LIST OF FIGURES

Figure 01 Tracks of Global Tropical Cyclones since records began 1958-2015 (Source: seawapa.org)

Figure 02 Map of Vietnamese Provinces (Source: Wikipedia) Figure 0.3 Approach ofthe research

Figure 1.1 Sonneratia caseolaris Figure 1.2 Kandelia obovata

Figure 1.3 Schematic diagram of the mecha wave enersyl2]

Figure 1.4 Schematiz jon of SWAN-VEG model [10] Figure 1.5 Schematization of mangroves is SWAN-VEG Figure 2.1 Map of Hau Loc (source Google Map)

Figure 2.2 Study area (source: Google Earth).

Figure 2.3 Mangroves area in Han Loc District from 1990 to 2015 Figure 2.4 Topography of study area

Figure 2.5 Water level in study area Figure 2.6 Tracks of storms in East Sea Figure 2.7 Monthly average temperature Figure 3.2 Relationship between bed elevation and exposed time in Hau Loc beach 30 Figure 3.3 Cross section in 1D model 33

Figure 3.4 Wave rose from offshore in Thanh Hoa from 2006 to 2017 3 Figure 3.5 Relationship between wave height and wave period 35 Figure 36 Design water level in Hau Loe ~Thanh Hoà 37 Figure 3.7 Distribution of wave height in scenario SCN1 38

Figure 3.8 Distribution of wave height in scenario SCN2 39

Figure 3.9 Distribution of wave height in scenario SCN3 29 Figure 3.10 Wave height in mangrove forest in 3 scenarios dị Figure 3.11 Wave height in mangrove forest in scenario 3 ái Figure 3.12 Wave height in mangrove forest in scenatio 2 2 Figure 3.13 Wave height in mangrove forest in scenario 1 “2 Figure 3.14 Kt in normal condition 44 Figure 3.15 Ktin storm condition 44 Figure 3.16 R in normal condition 44 Figure 3.17 Rin storm condition 44 Figure 3.18 Wave attenuation by mangrove forest corresponding to some survival rates 46 Figure 3.19 Relationship between survival rate and wave attenuation 46 Figure 3.20 Wave attenuation by mangrove forest corresponding to several width of

the forest 47

Figure 3.21Relationship between width of mangrove forest and wave attenuation

Figure 3.23 Relationship between density of mangrove forest and wave attenuation

coefficient s0

Figure 4.1 Four scenarios of generalized mangrove response relative SLR[24] 53 Figure 4.2 Wave height behind mangrove forest in some scenarios with and without

climate change: 38

‘ Problem definition

Vietnam located in the most affected region by storms in the world, Increasing in frequency and intensity of extreme weather phenomena such as floods, storms and tsunamis which is the consequence of global climate change causes a lot of di

and threatens the lives and property of the inhabitants in particularly in coastal regions

Finding out the solutions to mitigate disasters and adapt to climate change is an urgent

problem A lot of recent studi in the world have pointed out that mangrove forests play an important role in riverbanks and coastlines protection and climate regulation, It is considered a multi-objective and sustainable solution for disaster mitigation and climate change adaptation, Fortunately, Vietnam has along coastline and suitable

species, such as Soneratia caseolaris and Kendelia obovarata in the northern part to protect dykes in the coastal areas and river mouths Some fragments of dykes were still the same after a medium storm (Beaufort 6 to 8) In July 1996 when No 2 typhoon (Frankie) with wind velocities from 103 + 117kmíh occurred in Thai Binh, the sea dykes inThai Thuy Commune (in Thai Binh Province) were not damaged due to having the protection of the mangroves fences In contrast, the dyke system in Tien Hai Commune was much damaged as a result of the deforestation of the mangroves to make shrimp ponds In 2000, a No 4 typhoon (Wukong) with wind forces of Beaufort 10 landed on Thách Ha Commune (Ha Tỉnh Province), where the dyke system along Nghen River was still the same because in this location mangroves had been planted in 9 rural communes If mangroves defenses had not existed, the Dong Mon dyke would have broken and Ha Tỉnh ‘Town would have been flooded and the consequences would have been severe The local people in Hau Loc Commune (Thanh Hoa Province) recognized the important role of mangroves in wave attenuation from the experiences of 2typhoons, No 7 in 2005 and No, Sin 2007 The fragments of the dyke which had been protected by mangroves were not destroyed by strong waves; While the fragments of the dyke without the protection of mangroves were broken because waves attach directly to the surface of the dyke [1] From these examples, itis clear that the mangrove forest is an effective solution to maintain and strengthen the dyke system lying behind.

Awareness about the importance of mangroves has risen in recent years The government has been implementing many programs of mangrove forestation in the coastal provinces According to the Ministry of Agriculture and Rural Development, the total area of mangrove forests of Vietnam in 2010 was 209,741 ba The distribution ‘of mangrove forests is shown in Table 0.1

‘Table 0.1 Distribution and area of mangroves forests in Vietnam.

‘Number Region "Tidal marsh Area, Percentage tha) tha) (%)

1 | Quang Ninh and Northern Delta 122,335 37.651 30.7

| North Central Region 30974 1385 608

HH | South Central Region 13,068 2 1530

IV |SowhwestRegion 31484 41,666 51.13

V_ | Cu Long Delta 373301 128,537 34.43

Total | Vietnam 621162 20941 3817

‘Table 0.2 Mangroves foresLin front of the dyke

‘Number Region Length | Dyke with foreshore | Dyke with

ofthe | suitable for mangrove | foreshore not dyke | forest development | suitable for

system [With | Without | mangrove

đem) | mangrove | mangrove | forest (km) (km) | development

1 | Quang - Ninh and 8a 286 301 Northern Delta

1 | North Central Region 338 49 Bs 14

Vv | Cau Long Delta 1.259 780 144 335

From Table 0.1, it can be seen that the proportion of the area of mangroves forests

accounts only more than 30 percent of the area of tidal marshes The proportion of

North Central Region is smallest, about 6 percent Besides, from Table 0.2, itis clear that most of the tidal marshes in front of the dyke system have suitable conditions for planting mangroves but lacking mangroves belts Consequently, strengthening and building mangroves belts in front of dyke system is one of the primary tasks in flood prevention and disaster mitigation The questions are how to plan mangroves to protect dyke system and to calculate the quantitative effect of reduction wave height after having mangroves belts The research not only essential for planning, designing or upgrading dyke systems, but also for the planning of mangrove forestation in the near future, In this thesis, Thanh Hoa which is a coastal province in the northem Central Region of Vietnam is chosen study area,

+ Research Objectives and Research Questions ¥ Research Objectives

[As mentioned above, mangrove belts play an important role in wave attenuation thanks to the distribution and their characteristic Hence, they help to reduce investment for building dykes, maintain the lifetime of dyke system and protect for the residential area inside The effectiveness of wave dissipation mainly depends on the mangrove belt width, species of mangroves, the density and dimension of parts of the

tree Every species of mangroves has the structural feature of trunk, root and canopy ‘with different abilities of wave obstruction,

In Vietnam, two native species of mangroves which are S.caseolaris and K.bovarata are quite popular along the coastline especially in the northern part These are species

‘of mangroves which have been chosen to plant in most of the afforestation projects

recently In a location they normally chose S.caseolaris or Kobovarata or both of them in the different stage of the project However, either the choosing species or calculating parameters of mangrove belt are rough, using the result of previous studies Which are lack of homologous In addition, there are lacking the comparison between these Iypes about wave attenuation to point out which is beter species from

engineering aspect.

Besides, global climate change leads to changing natural features which impact the existence and dimension of mangrove belt The changes in water level, bed elevation, the wide of mangroves belt should be considered when we design mangrove belt with the desire of maintaining affective of the afforestation From the forecasts, schemes of plan added trees, building support structure to create good conditions for maintain and

widening the mangrove belt or upgrade the dyke system adapting new circumstances.

In short, the purpose of the thesis is as follows:

~ Finding out the the relationship between the the parameters of mangroves forest and Wave attenuation with 2 mangrove species of S.caseolaris and Kobovarata; then propose the optimal plan for planting mangroves for study area in Hau Loe District, ‘Thanh Hoa Province

~ Analysis the impact of climate change to the mangrove forest and estimate the wave

height behind mangrove forest when taking into account the change of the sea level se in climate change scenarios.

~ Which is the better species of mangroves in terms of effective wave dissipation?

~ How is the relationship between the parameters of planted mangrove forest with the

wave height behind it?

- How climate ge impact to the mangrove forest?

~ How wave height behind the mangroves change in the context of climate change? ‘ Approach and Methodology

‘Some methods are used in this thesis include:

~ Collection and comprehensive analysis of data about the bathymetry, hydro-meteorological condition of study area; characteristics of mangroves species and parameters of n ingrove corresponding to some age of tree Review existing

documents about wave attenuation by mangroves; impacts of climate change to

mangroves and about modeling,

= Numerical modelling: SWAN-VEG is chosen to calculate wave height behind mangroves bell in my thesis This model has been widely used recently Its calibrated and proved to be a reliable model The model is also more fulfilment thanks to the studies about the drag coefficient of the vegetation parts The current study area has not data about measured wave nearshore and parameters of mangroves so that the validation and calibration of the model need to be based on previous studies Collected data about the mangroves in some forestation projects is necessary to ensure the

practical aspect of the thesis.

- Field survey: measure wave height and parameters of mangroves in the field (if capable)

‘The steps of the approach are schematized as the chart shown in Figure 0.3,

CHAPERI: LITERATURE REVIEW

1.1 The characteristics of mangrove species

‘There are some general principles for choosing species of mangroves to be planted in the tidal zone, Firstly, native species have more priority than other species of mangroves which are suitable for natural conditions ofthe location, This helps them to

survive and adapt to new habitat, The study area lies between the Van Uc river mouth

and the Lach Truong river mouth which deposited by the Red River system According to the research of the Vietnam Academy for Water Resources about the species of mangroves which can plan for each location along the coastline, there are several species of mangroves can plant here They are Soneratia caseolaris (Ban chua), Kendelia obovarata (Trang), Aegiceras corniculatum (Sú), Acanthus ilicifolius (O rô), Avicennia marina (Mắm biên) Besides, Soneratiacaseolaris (Ban chua), Kendeliao bovarata (Trang) are native species here In this thesis Soneratia caseolaris (Ban chua), Kendeliao bovaratea (Trang) are chosen for study because of not only practical aspects but also theoretical ones These species have two specific representative root systems can contribute in different ways in wave energy dissipation.

LLL Sonneratia caseolaris (Vietnamese name: Cây Ban chua)

‘The species lives in subtropical and tropical mangrove forests They have the ability to adapt to the study region, which has a relatively high annual precipitation, an average temperature of 20 ~27 °C, a pH of 6-6.5, and a low salinity Their habitat is often in a river mouth in which the tidal level ris which are 5-'s slowly, They are timber tre

15m high (some of them can have height of 20m) They have a wide leaf canopy, a plain trunk, cone roots with a height of 50-90 em and a diameter of 7 em (Source: htầp/venbaoveoongbih.vnJ)

Figure 1.1 Sonneratia caseolaris 1.1.2 Kandelia obovata (Vietnamese name: cây Trang)

‘This species is found in the downstream estuarine zone in the lower intertidal region This species is easily propagated, and coppices It is considered a hardy species,

although is relatively slow-growing (5 years to grow 1.5 m) This species generally

‘grow up to about 3 meters.

‘They are small timber trees with a height of 5-7m, buttress roots and a plain trunk.

‘They live in the silty sand along the river mouth with varied salinity

1.2 The mechanism of wave attenuation through mangroves forest

Figure 1.3 Schematic diagram of the mechanism by which mangrove forests reduce wave enersy[2]

+ By trunk and root: The water particles in a wave as it moves tum in an orbital motion, When the wave enters ‘mangrove forest where the water is atthe level ‘of the trunks and roots, especially in the case of prop roots, the water particles are met With resistance, or drag, from the trunks and root system, causing the wave energy to ‘gradually dissipate

+ By friction of seabed: In mangrove forests, the seabed is normally a mud bed into which the plants sink their roots, whether they’ are prop roots or buttress roots, as well as through which pneumatophore roots push up into the water body to breathe Because of the massive, complex root systems typical of mangrove forests, a static friction coefficient is created along the mud floor, which is much greater than in coastal areas without such This friction is a significant factor in the dissipation of ‘wave energy that occurs when waves pass through a mangrove forest In addition to

the special root systems, the presence of a fluid mud layer on the seabed also aets to

hold back passing waves and dissipate their energy.

+ By branches and leaves: Normally, as the wind interacts with the water

surface over a certain time and distance, waves will be generated and move towards the shore in fully developed shapes However, in coastal areas where there are mangrove forests, the thick branches and leaves act as a shield against the force of the

Wind, whether it is the wind that has caused the waves or some other wind thai intensifies them, The foliage prevents the wind from perturbing the surface of the water and the waves already penetrating the forest, This mechanism is not directly reducing wave action, Rather, itis a mechanism which causes waves to be less high ‘and ess forceful where there are mangrove forests than in other areas near these forests that are devoid of vegetation Because of these three mechanisms, wave energy is reduced as the wave passes through the mangrove forest, and as a result, the incident Wave height is reduced to the transmitted wave height The reduced height and force of the waves in coastal areas with mangrove forests are of vital importance to the physical processes in play along with the shoreline For example, the movement of sediments is reduced, more sediment is deposited, and coastal erosion is greatly diminished.

1.3 SWAN-VEG Model

‘The research on the dissipation of wave energy, when waves pass through the coastal ‘vegetation including mat roves forests, has only gained attention since the 1990s The research vanis divided in 3 main groups: field studies, laboratory studies and numerical studies.

Numerical studies of wave reduction due (0 coastal vegetation commencedistarted over 30 yes ago [3] Such research may relate to the study of the process of wave energy dissipation or the study of hydrodynamies of the entire water mass These studies are based on a set of basic governing equations consisting of the continuity ‘equation or mass conservation equation, momentum equation and energy equation In the case of the study of wave dissipation processes, the energy of waves propagating through coastal vegetation will be reduced due to the resistance of plant trunks and

roots with a rate of energy dissipation This eneray dissipation rate is then simulated

with a set of mathemati 1 formulas [4].In the case of the study of hydrodynamics of the entire water mass, both waves and currents are taken into account |5] In these

coastal flooding due to long waves, like tsunamis, and the potential of mangrove forests in reducing wave energy is also simulated, such as in the study of Yanagisawa ct al, [7], The relevant equations in those mathematical models could be solved with cither analytical methods or numerical methods.

In the thesis the reduction of wave energy is calculated by applying numerical

modelling “SWAN SWAN is a third-generation wave model for obtaining realistic estimates of wave parameters in coastal areas, lakes and estuaries from given wind, bottom and current conditions, However, SWAN can be used on any scale relevant to ‘wind-generated surface gravity waves The model is based on the wave action balance equation with sources and sinks [8]

‘The SWAN-VEG is a module for wave dissipation by vegetation, based on the SWAN

model in which mangroves are modeled as cylindrical obstacles [9] The original SWAN model itself does not change, only an extra dissipation term is added to the

model Drag force of vegetation causes energy loss, presented by energy dissipation

q—e! Initial | esipaton _„ Resutting L _p

Fr") wave > “ctwove Wave | Hượn

Te | Energy sy Energy | Tre

Figure 1.4 Schematization of SWAN-VEG model [10]

According Kobayashi et (1993) [11] and Mendez & Losada (2004) [4] energy dissipation term determined by the formula

60 = _ x na 2 (11)

Where: Hiswaveheight[m]

Kis wave number +]

Nis density of vegetation [tree/m"] Co is drag coefficient -]

>is stem wiảth [m]

“ah is vegetation height [m]

Figure 1.5 Schematization of mangroves is SWAN-VEG

‘The contribution of each layer is calculated individually and the total energy

<issipation is obtained as the sum of the dissipation in each layer up to the still water level With this implementation of the differences in characteristics of each layer, plants such as mangrove trees may be conveniently input into the SWAN model, The layer-wise segmentation may be implemented during the integration of the energy

issipation over height as suggested below

` ~ (12)

‘The energy dissipation term for a given layeri as follows:

— l0 BuuhuN,2) = FE a¢9,6) (1,3)Saswegs =

Table 1.1 Some studies about wave attenuation by mangrove forest in Vietnam

Number | Author Title of research Year | Species of | Study area

1 | Mazda et | Mangroves as a coastal |1997 | Kendelia |Tong Kin

al protection from waves in candel delta

the Tong Kin delta

Vietnam [12]

2 |Mazda et|Wave reduction in a|2006 | Sonneratia | Vinh

al mangrove forest sp Quang(Tien

dominated by Sonneratia Lang, spHãI Haiphong) 4 |Quanelet|Wave attenuation in|2007 |Kendelia |Do Son,

al coastal mangroves: in the candel Haiphong

Red river delta, Vietnam 4]

3 |Luong | Energy dissipation in non- | 2008 | Avicennia sp | Can Gio Phuoc Vo | uniform mangrove forests and

etal of arbitrary depth [15] Rhizophora

4 |Nguyen | An experimental study on|2009 | Sonneratia | Laboratory

Ba Tuyen | wave reduction efficiency sp and ‘of mangrove forests [16]

Hoang Viet Hung

5 | Tran Effect of mangrove forest | 2011 | Rhizophora | Tien Lang, Quang — | structure on wave mucronata, \Cat Bà, Bao attenuation in coastal Sonneratia | Hoang Tan Vietnam [17] caseolaris, $.| Tien Hai

‘rift, [and Can Gio

Nguyen | Nehién cứu kha năng hip |2012 | Kendetia | Nam Dinh, Thi Kim |thụ năng lượng sóng của and Thả Bình

Cục RNM trồng tại Nam Định Sonneratia ‘Thai Binh

và Thái Bình [18] caseolaris

Vu Duy| Ứng dụng mô hình toán|2015 | Kendelia | Bang La and Ung đánh giá vai trỏ làm giảm candel and | Ngoc - Hai,

độ cao sống của RNM ở Sonneratia | Haiphong vùng ven biển Hải Phòng caseolaris

Nguyen | Modelling the impact ofF|2015 | Rhizophora | Thanh Phú ‘Thi Kim|mangrove vegetation sp

Cueetal [structure on wave dissipation in Ben Tre provine, Vietnam, under different Climate Change

Seenarios [20]

CHAPTER 2: STUDY AREA

2.41 Location

‘Thanh Hoa is a coastal province in the northern Central Region of Vietnam This is ‘one of the locations that are most affected by storms annually Hau Loc is located on a

coastal plain of Thanh Hoa Province, Vietnam It is 25 km from the centre of the city

to the North East, from 19° 56 to 20” 04" N It located between the Len River to the

north, the Lach Truong River and the Cau Sai River to the south and East Sea to the

east The area of the coastal region covers about 5.400 ha and accounts for 40% of the

total natural area of the district, This land was formed by the river mouth alluvia

process from the past Some projects of mangroves reforestation which are being implemented here to protect sea dyke system In this thesis, the study area is the tidal

Figure 2.1 Map of Hau Loc (source Google Map)

Figure 2.2 Study area (source: Google Earth) 2.2 The reality of mangroves forest.

In 1964, there were about 200 hectares of mangroves forest in Hau Loc District However, the area decreased significantly after the war and the mangroves forest

almost disappeared From the 1980s, mangroves started to be planted with the sponsor

of British Children Fund and Red Cross In 1980 the mangroves area was 220 hectares ‘The area has changed dramatically as the result of a lot of natural, social - economic factors especially human factor The change of the mangroves forest in Hau Loc through the years is shown in Figure 2.3.

Mangroves area in Hau Loc

Figure 2.3 Mangroves area in Hau Loc District from 1990 to 2015

‘The mangroves area declined of 70 hectares in 1990, about 150 hectares The main cause of this situation was that it was the first time when mangroves were planted in Mậu Lộc Lacking of experience in forest protection and forestry extension leads to the unpredictability of diseases and harmful aquatic ereatures of mangroves Moreover the inhabitants were not aware of the important role of mangroves On the other hand, the development of aquaculture in the area of mangroves forest was also a primary cause to degradation of forests The area of mangroves forest continuously fell down in 1990, 1995 and 200, at 150 hectares, 120 hectares and 110 hectares respectively ‘The area of mangroves forest decreased nearly a half in the period of 20 years The

reasons why the area still continues declined were unable solving previous problems

and lacking ofpenalties offenders From 2000 to 2015, the mangroves area was significantly recovered and expanded In the petiod of 15 years ending in 2015, the aea grew over 4 times thanks to successful forestry extension activities The awareness of inhabitants about the vital role of mangroves belt had risen since they

win d the effectiveness of mangroves in protection coastline in storms In 2015 it ‘was 468.3 hectares This was the result of implementing successfully a lot of projects

Which were funded by the government and other None GovernmentalOrganisation

such as CARE, Red Cross.

According to the survey about the vegetation in Hau Loe, there are 15 plant species of

13 mangrove species Among them, the dominant species are Avicenniaceae

Rhizophoraceae and Sonneratiaceae, Most areas of mangrove forest belong to S.easeolaris and K obovata In the study area, there was S caseolaris field which was

planted in 2009 with the density of 1,600 tree.ha'!, However, the mangrove field is

«quite sparse and unable to protect the sea dyke 2.3 State of the dyke system.

‘There are 22.2 kilometres of sea dykes and revetments among 102 kilometres of coastline in Thanh Hoa, Some characteristic of the sea dykes and revetments are shown in Table 2.1

‘Table 2.1 Characteristic of the sea dykes and revetments ‘Name/Segment Designed parameters

‘Truong Son and L-Vich Hai

2.4 Other Conditions

“The main factors which impact to survival and development of mangroves include:

~ Climatic elements (temperature, wind, rainfall, sunlight)

~ Hydrological elements (wave, tide, river current, salinity)

~ Bed characteristic

~ Topography

~ Biological elements

In this thesis, applying numerical modelling to calculate wave dissipation though ‘mangroves forest is focused on so that the parameters about topography, wind, wave

and tide are concentrated to analyze in more detail in the next chapter 24.1 Topography

Using hydrographic map from webapp.navionics.com

‘The area has a shallow and quite flat bed, receives alluvial sediment from the river and is party shielded by an island These are also the advantages of planting mangroves 24.2 Wind

‘Wind condition in Thanh Hoa Sea follows the general rules of the wind condition in ‘Tonkin Gulf This location is directly affected by South East Asia monsoon system ‘The prevailing wind directions are from the North, the Northeast and the East from October to March next year, From April to July the prevailing wind direction are from the Southeast and the South Wind change direction in August and September

24.3 Wave

‘The regime of wave in Thanh Hoa have the common characteristics of hydro-meteorological condition but have own distinctions Waves in Thanh Hoa are quite large because of the open sea In winters, the prevailing waves are from the Northeast about 0.8 to 0.9 m, In summers, the prevailing waves are from the Southeast, about 1.2

244 Tide

Tide is a very important factor for distribution and development of mangroves because it directly relates to level and submerged time It also impactstoother factors such as salinity, structure, evaporate of land/ground and other organisms According to research of Pham Nguyen Hong, when we only consider tide aspect, mangroves grow beter in the location having semidiurnal regime ‘This is because both longest submerged time and longest exposed time in a day are short, It means shortening either the time that mangroves cannot “breathe” when they are submerged or the time which fresh water evaporates from the ground especially in hot weather Tidal rage also affectsthe ability of transportation of seedlings and sediment The smaller tidal rage makes the weaker ability of breed source and deposit sediment Mangrove forest will be narrower in locations with smaller tidal range.

~ The tidal regime is complicated and varying along the Vietnamese coast It is governed by tidal regime of the Northwest Pacific Ocean combine with a specific

feature of coast and bank range{20] Tidal regimes at locations along the coast are described in the Table 2.2.

‘Table 2.2 Characteristics of tide along Vietnamese coast

Coastal part Provine Tidal type ‘The Nonhem | Mong Cai —Ninb Bình Fully diurnal

‘Thanh Hoa — Ha Tinh Mixed, mainly diurnal

Ha Tinh ~ Quang Bình Transition from mixed, ‘mainly semi-diumal to fully semi-diumal

‘The Central Coast [Cua Tung -north of Quang] Transition from mixed,

‘mainly semi-diumal to fully

Quang Nam — Binh Thuan Transition from mixed, mainly semidiurnal to mixed, mainly diumal

Binh Thuan ~ South of Centre ‘The diurnal feature declines The Southern

Ba Ria~Ca Mau ‘Mixed, mainly semi-diurnal (Ca Mau — Ha Tien Fully diurnal

- The water level data using in the thesis is taken from Sam Son station (19°45'N, 105°S4°E) which is near the study area These are observed data in 2015

l3 3 š Fe 2 FRE

faa 2 Pe 2 PEP EGG

Figure 2.5 Water level in study area

~ The tidal regime is unequal diurnal, The highest water level is about 350 em, The

lowest water level is about50 em 24.5 Storm

‘The study area is affected directly by storms,

Local storms often happen in August, September and October with heavy rain.

‘Wei béoten Mir bấp nh thánh

tường đ của báo

24.6 Temperature

‘The temperature affects photosynthetic productivity Mangroves stop photosynthesis when the temperature of leaves reaches 40°C.

~ Amplitude of temperature variation is 12-13°C and daily amplitude is 5.5-6°C, ~ Monthly average temperature is 24°C.

Figure 2.7 Monthly average temperature

~ The average temperature is highest in June, about 30°C ~The average temperature is highest in January, about 15°C

~ There are 4 months (from December to March next year) in which the average

temperature is smaller than 20°C.

24.7 Rain fall

~ Mean annual precipitation is 1739mm The rainy season lasts from the beginning of

May to October, and precipitation concentrates from July to October Monthly

precipitation is different between months August and September have a maximum precipitation of about 460mm, January has a minimum precipitation of about 18-22 mm.

249° Humidity

~ Yearly average humidity is 85%-86%,

- The months having a maximum humidity are February, March and April, with a

humidity of about 90%

2.4.10 Salinity

‘The salinity is one of the most important factors with the existence of mangroves The specific characteristic of mangroves is salt tolerance They have the ability of

absorbent, salt balance, and keeping membrane pressure Mangroves discharge excess salt by falling leaves The salinity affects photosynthesis productivity, Raising the

salinity causes đecreaseof photosynthesis productivity Besides, the barnacles will

‘They multiply when the salinity increases Some young barnacles stick on young tre

bend the tree and raise the temperature which causes the death of young tee,

Mangroves can grow well in somewhere have salinity is from 10-25%o, Each species

can adapt to different salinity

In the study area, the salinity of seawater fluctuates a lot depending on the season The salinity of seawater is the highest in the dry season

‘The result of salinity measurements in the nearshore of Hau Loc’s beach: son; 15-26%,

+ Rain season: 6-15%, 24.11 Bed characteristic

- pH from 7.0-7.2 pH is the factor that impacts the ability to take nutrient Most of the vegetation can absorb nutrient at pH from neutrality to inclined alkaline.

- According to the analysis of Institute of Ecology and Works Protection, all mechanical components and nutrient content are suitable for growth of mangroves.

CHAPTER3: WAVE ATTENUATION THROUGH MANGROVES FOREST

3.1 Selection of appropriate types of mangroves and vegetation parameters Mangrove species are chosen to plant inthis area base on some main points as follows:

= History of natural development of mangroves in the area

~The basic standard for growing the mangroves against wave to protect sea dikes ~ Characteristic of each species of mangroves, especially about salt tolerance For

example, Kobovata have a big rage of salt tolerance from 7% to 20% and Scaseolaris have a small rage of salt tolerance from 5°⁄.,to15 *,

~The survey data of bed characteristic

~The status of planted mangrove fields after years in nearby areas

‘To choose the appropriate species and structure of the field to enable high survival rate, growing well and having the highest wave attenuation is a complicated problem Which needs consider a lot of aspects and in-depth studies In this thesis, the process is simplified by choosing 2 native species which are also planted successfully in nearby areas They are K obovata and § caseolaris.

Vegetation parameters need to put in the model include diameter, height, density and

drag coefficient of each part of tree which are root, stem and branch based on the ‘measured parameters from the field and literature review in previous studies The data using in the thes are mainly taken from the research of Vu Doan Thai in Hai Phong and data from the field tip in Hau Loc.

Table 3.1 Parameters of K: obovata Parameters Age | Parts of tree

Height (m ) | Diameter (m) | Number | Drag coefficient

K, obovata is from 2,500 tree ha’ to 10,000 tree ha” [21]. ‘Table 3.2 Parameters of S.caseolaris

dard for growing the mangroves against wave to protect sea \demy for Water Resources (2011) the appropriate density of

Age | Parts of tree

Height (m) | Diameter(m ) | Number | Drag coefficient

‘The appropriate density of planting S caseolaris is from 1.600 tree.ha" to 5.000 tree ha’!

3.2 Determine bed elevation to plant mangroves

Besides the elements of mechanical components and nutrient content of the bed, the primary element which determines the survival of mangrove is the correlation between the position of planting mangroves and tidal level because it relates the ability of submerged bearing of mangroves,

‘The conditions for planting mangroves are mentioned in the "Basie standard for growing the mangroves against wave to protect sea dikes” of Vietnam Academy for Water Resources (2011): Mangroves cannot grow under those circumstances: water

‘depth larger than 3 meters; expose time less than 6 hours per day; the total day of flood

tide less than 5 days per month or more than 29 days per month; extremely eroded beach; proportion of sand in bed over 90%, salinity over 35%.

‘The elevation of bed for planting mangroves can be determined based on the observed data about tidal level from Sam Son station in the year 2015 and the condition about the exposed time in a day and total day of flood tide in a month of mangroves The difference between National Datum Fix and Nautical Datum is 1.9 m [22]

According to the condition of exposed time in a day: the elevation of bed from ~

0.6 m (using National Datum Fix)

so 4 tt ie fe #6 4£ € Cát

Figure 3.1 Illustration for calculation exposed time corresponding to the bed elevation