Wave attenuation by mangroves in hau loc district, thanh hoa province major coastal engineering and management code 62 58 02 0

50 CHAPTER 4: IMPACT OF CLIMATE CHANGE TO MANGROVE FOREST AND WAVE ATTENUATION IN CLIMATE CHANGE SCENARIOS .... 55 4.2 Wave attenuation through mangrove forest in Climate change scenario

I hereby 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

ACKNOWLEDGEMENTS

First of all, I 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

I 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, I am deeply grateful to my family for their great care and encouragement

CONTENTS

LIST OF TABLES vii

LIST OF FIGURES ix

INTRODUCTION 1

 Problem definition 1

 Research Objectives and Research Questions 4

 Approach and Methodology 6

CHAPER1: LITERATURE REVIEW 8

1.1 The characteristics of mangrove species 8

1.1.1 Sonneratia caseolaris (Vietnamese name: Cây Bần chua) 8

1.1.2 Kandelia obovata (Vietnamese name: cây Trang) 9

1.2 The mechanism of wave attenuation through mangroves forest 10

1.3 SWAN-VEG Model 11

1.4 Some studies about wave attenuation by mangrove forest in Vietnam 13

CHAPTER 2: STUDY AREA 16

2.1 Location 16

2.2 The reality of mangroves forest 17

2.3 State of the dyke system 19

2.4 Other Conditions 21

2.4.1 Topography 21

2.4.2 Wind 22

2.4.3 Wave 22

2.4.4 Tide 22

2.4.5 Storm 24

2.4.6 Temperature 25

2.4.7 Rain fall 25

2.4.8 Sunny hour 25

2.4.9 Humidity 26

2.4.10 Salinity 26

2.4.11 Bed characteristic 26

CHAPTER 3: WAVE ATTENUATION THROUGH MANGROVES FOREST 27

3.1 Selection of appropriate types of mangroves and vegetation parameters 27

3.2 Determine bed elevation to plant mangroves 29

3.3 Scenario selection 31

3.4 Computational domain and bathymetry 32

3.5 Hydro-meteorologic condition for normal case 33

3.5.1 Wave parameters 33

3.5.2 Wind parameters 35

3.5.3 Water level 36

3.6 Hydro-meteorologic condition for storm case 36

3.6.1 Design return period 36

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 4.1.1 Sea level rise 53

4.1.2 Rise in surface temperature 54

4.1.3 Storm and extreme weather events 55

4.1.4 Other factors 55

4.2 Wave attenuation through mangrove forest in Climate change scenarios 56

4.2.1 Input data 56

4.2.2 Results 58

CONCLUSIONS AND RECOMMENDATIONS 60

 Conclusions 60

 Recommendations 61

ANNEX 65

Annex 1: 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 67

Annex 2: SWAN-VEG input file 68

Annex 2.1: Example of SWAN-VEG input file in normal condition, SE monsoon 68

Annex 2.2: Example of SWAN-VEG input file in normal condition, NE monsoon 69

Annex 2.3: Example of SWAN-VEG input file in Storm condition 70

Annex 3 : Hs, Kt, R in Scenario1,2,3 with 5-year-old tree and 9-year-old tree 71

Annex 3.1: Hs in scenario 1,2,3 with 5-year-old tree and 9-year-old tree 71

Annex 3.2: Kt in scenario 1,2,3 with 5-year-old tree and 9-year-old tree 73

Annex 3.3: R in scenario 1,2,3 with 5-year-old tree and 9-year-old tree 76

Annex 4: Hs in scenarios in which density of mangrove forest varies 79

Annex 5: Hs in Scenarios with and without climate change 83

Annex 6: Some photos from the field trip 85

LIST OF TABLES

Table 0.1 Distribution and area of mangroves forests 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 in Vietnam 14

Table 2.1 Characteristic of the sea dykes and revetments 20

Table 2.2 Characteristics of tide along Vietnamese coast 23

Table 3.1 Parameters of K obovata 28

Table 3.2 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 32

Table 3.5 Statistical offshore wave data 34

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 45

Table 3.9 Representative diameters of wave attenuation corresponding with survival rate 46

Table 3.10 Representive diameters of wave attenuation corresponding with width of mangrove forest 47

Table 3.11 Representative diameters of wave attenuation corresponding with width of mangrove forest 49

Table 3.12 Comparison results of wave height behind mangrove to Bao‟study 51

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 57Table 4.2 Water level in Scenarios 58Table 4.3 Wave height behind mangrove forest and wave attenuation coefficient 59

LIST OF FIGURES

Figure 0.1 Tracks of Global Tropical Cyclones since records began 1958-2015

(Source: seawapa.org) 1

Figure 0.2 Map of Vietnamese Provinces (Source: Wikipedia) 3

Figure 0.3 Approach of the research 7

Figure 1.1 Sonneratia caseolaris 9

Figure 1.2 Kandelia obovata 9

Figure 1.3 Schematic diagram of the mechanism by which mangrove forests reduce wave energy[2] 10

Figure 1.4 Schematization of SWAN-VEG model [10] 12

Figure 1.5 Schematization of mangroves is SWAN-VEG 13

Figure 2.1 Map of Hau Loc (source Google Map) 16

Figure 2.2 Study area (source: Google Earth) 17

Figure 2.3 Mangroves area in Hau Loc District from 1990 to 2015 18

Figure 2.4 Topography of study area 21

Figure 2.5 Water level in study area 24

Figure 2.6 Tracks of storms in East Sea 24

Figure 2.7 Monthly average temperature 25

Figure 3.1 Illustration for calculation exposed time corresponding to the bed elevation 29

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 33

Figure 3.5 Relationship between wave height and wave period 35

Figure 3.6 Design water level in Hau Loc –Thanh Hoa 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 39

Figure 3.10 Wave height in mangrove forest in 3 scenarios 41

Figure 3.11 Wave height in mangrove forest in scenario 3 41

Figure 3.12 Wave height in mangrove forest in scenario 2 42

Figure 3.13 Wave height in mangrove forest in scenario 1 42

Figure 3.14 Kt in normal condition 44

Figure 3.15 Kt in storm condition 44

Figure 3.16 R in normal condition 44

Figure 3.17 R in 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 50Figure 4.1 Four scenarios of generalized mangrove response relative SLR[24] 53Figure 4.2 Wave height behind mangrove forest in some scenarios with and without climate change 58

 Problem definition

Vietnam is 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 difficulties and threatens the lives and property of the inhabitants in particularly in coastal regions

in recent years

Figure 0.1 Tracks of Global Tropical Cyclones since records began 1958-2015

(Source: seawapa.org) Finding out the solutions to mitigate disasters and adapt to climate change is an urgent problem A lot of recent studies 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 ÷ 117km/h 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 4th typhoon (Wukong) with wind forces of Beaufort 10 landed on Thach Ha Commune (Ha Tinh 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 Tinh 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 5in 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, it is 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 ha The distribution

of mangrove forests is shown in Table 0.1

Table 0.1 Distribution and area of mangroves forests in Vietnam

(ha)

Area (ha)

Percentage (%)

I Quang Ninh and Northern Delta 122,335 37,651 30.77

IV

V

Table 0.2 Mangroves forest in front of the dyke

of the dyke system (km)

Dyke with foreshore suitable for mangrove forest development

Dyke with foreshore not suitable for mangrove forest development (km)

With mangrove (km)

Without mangrove (km)

I Quang Ninh and

Northern Delta

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, it is 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 is 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 northern 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 K.obovarata 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 types about wave attenuation to point out which is better 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 K.obovarata; then

propose the optimal plan for planting mangroves for study area in Hau Loc 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 rise 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 change 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, meteorological condition of study area; characteristics of mangroves species and parameters of mangrove corresponding to some age of tree Review existing documents about wave attenuation by mangroves; impacts of climate change to mangroves and about modeling

hydro Numerical modelling: SWANhydro VEG is chosen to calculate wave height behind mangroves belt in my thesis This model has been widely used recently It is 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

SWAN 2D

1 CHAPER1: 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 of the 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 (Bần chua), Kendelia obovarata (Trang), Aegiceras corniculatum (Sú), Acanthus ilicifolius (Ô rô), Avicennia marina (Mắm biển) Besides, Soneratiacaseolaris (Bần chua), Kendeliao bovarata (Trang) are native species here In this thesis Soneratia caseolaris (Bần 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

1.1.1 Sonneratia caseolaris (Vietnamese name: Cây Bần 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 oC, a pH of 6-6.5, and a low salinity Their habitat is often in a river mouth in which the tidal level rises slowly They are timber trees, which are 5-15m 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 cm and a diameter of 7 cm (Source: http://vienbaovecongtrinh.vn/)

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 energy[2]

+ By trunk and root: The water particles in a wave as it moves turn in an

orbital motion When the wave enters a mangrove forest where the water is at the 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 acts 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 that 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, it is a mechanism which causes waves to be less high and less 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 mangroves 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 to coastal vegetation commenced/started over 30 years ago [3] Such research may relate to the study of the process of wave energy dissipation or the study of hydrodynamics 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 energy dissipation rate is then simulated with a set of mathematical 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

et al [7] The relevant equations in those mathematical models could be solved with either 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 term

Figure 1.4 Schematization of SWAN-VEG model [10]

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

∑ (1.2) The energy dissipation term for a given layeri as follows:

√ ̃ ( ̃̃) ( ̃ ̃ ) ( ̃ ̃ )

̃ ̃ √ ( ) (1.3)

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

Number Author Title of research Year Species of

1997 Kendelia

candel

Tong Kin delta

4 Quartel et

al

Wave attenuation in coastal mangroves in the Red river delta, Vietnam [14]

2011 Rhizophora

mucronata, Sonneratia caseolaris, S

Tien Lang,

Hoang Tan ,Tien Hai

griffithii, Aegiceras corniculatum, Avicennia marina, Kandelia candel

and Can Gio

và Thái Bình [18]

2012 Kendelia

candel, Sonneratia caseolaris

and

Aegiceras corniculatum

Nam Dinh, Thai Binh Thai Bình

Ung

Ứng dụng mô hình toán đánh giá vai trò làm giảm

độ cao sóng của RNM ở vùng ven biển Hải Phòng [19]

2015 Kendelia

candel and

Sonneratia caseolaris

Bang La and Ngoc Hai, Haiphong

2015 Rhizophora

sp

Thanh Phu

2 CHAPTER 2: STUDY AREA

2.1 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 190 56‟ to 200 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 marsh in front of the dyke system in Hung Loc Commune, Hau Loc District, Thanh Hoa Province

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

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 Hậu Lộc Lacking of experience in forest protection and forestry extension leads to the unpredictability of diseases and harmful aquatic creatures 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 period of 15 years ending in 2015, the area 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 witnessed 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 Loc, 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.caseolaris 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-1 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 I-Vich Hai

Loc

From K0 to K2+850

Crest elevation Zđ = 4.3 † 4.6 m Ztg = 5.2 ÷ 5.5m

Toe armouring with concrete structures and large rocks

I-Vich and Ninh Phu

From K2+850 to K6+808

Zđ = 4.8 m Ztg = 5.5 m

B = 6 m Toe armouring with concrete structures and large rocks

Ninh Phu I

From K6+808 to K7+600

Zđ = 4.8 m Ztg = 5.5 m

B = 2.65 m Toe armouring with concrete structures and large rocks

Seaward berm Z= 3.5m , B = 6 m Ninh Phu I

From K7+600 to K9+795

Zđ = 4.5m

No seawall presented

B = 6m Toe armouring with concrete structures and large rocks

Zđ = 4 m

B = 3 m Toe armouring with concrete structures and large rocks

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)

2.4.1 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 partly shielded by an island These are also the advantages of planting mangroves

2.4.2 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

2.4.3 Wave

The regime of wave in Thanh Hoa have the common characteristics of 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

hydro-m

2.4.4 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 better 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

The Northern

Coast

Mong Cai – Ninh Binh Fully diurnal Thanh Hoa – Ha Tinh Mixed, mainly diurnal

Ha Tinh – Quang Binh Transition from mixed,

mainly semi-diurnal to fully semi-diurnal

The Central Coast Cua Tung –north of Quang

Nam

Transition from mixed, mainly semi-diurnal to fully semi-diurnal

Quang Nam – Binh Thuan Transition from mixed,

mainly semidiurnal to mixed, mainly diurnal

Binh Thuan – South of Centre The diurnal feature declines The Southern

Coast

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°54‟E) which is near the study area These are observed data in 2015

Figure 2.5 Water level in study area

- The tidal regime is unequal diurnal The highest water level is about 350 cm The lowest water level is about50 cm

2.4.5 Storm

The study area is affected directly by storms

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

Figure 2.6 Tracks of storms in East Sea

2.4.6 Temperature

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

- Amplitude of temperature variation is 12-130C and daily amplitude is 5.5-60C

- Monthly average temperature is 240C

Figure 2.7 Monthly average temperature

- The average temperature is highest in June, about 300C

-The average temperature is highest in January, about 150C

- There are 4 months (from December to March next year) in which the average temperature is smaller than 200C

2.4.7 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

2.4.9 Humidity

- Yearly average humidity is 85%-86%

- The months having a maximum humidity are February, March and April, with a humidity of about 90%

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:

3 CHAPTER3: WAVE ATTENUATION THROUGH MANGROVES

FOREST

3.1 Selection of appropriate types of mangroves and vegetation parameters

Mangrove species are chosen to plant in this 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, K.obovata have a big rage of salt tolerance from 7o/oo to 20o/oo and

S.caseolaris have a small rage of salt tolerance from 5o/oo to15 o/oo.

- 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 S 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 thesis are mainly taken from the research of Vu Doan Thai in Hai Phong and data from the field trip in Hau Loc

Table 3.1 Parameters of K obovata

Age Parts of tree

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

planting K obovata is from 2.500 tree ha-1 to 10.000 tree ha-1 [21]

Table 3.2 Parameters of S.caseolaris

Age Parts of tree

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

The appropriate density of planting S caseolaris is from 1.600 tree.ha-1 to 5.000 tree.ha-1

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 "Basic 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)

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