Te Brake and Van Huijgevoort 2008 - Hydrological classification of mangrove forests in Can Gio and...

B. te Brake M.H.J. van Huijgevoort January 2008

Hydrological characterization of

mangrove forests in Can Gio and

Ca Mau, Vietnam

MSc Thesis

Hydrology and Quantitative Water Management Group

Hydrological characterization of mangrove forests in Can Gio and

Ca Mau, Vietnam

MSc Thesis Hydrology and Quantitative Water Management HWM-80436

Bram te Brake reg. no.: 840121 116 120

Marjolein van Huijgevoort reg. no.: 840924 379 100

Wageningen University Department of Environmental Sciences Hydrology and Quantitative Water Management Group Supervisors: R. Dijksma Hydrology and Quantitative Water Management Group, Wageningen University M.E.F. van Mensvoort Land Dynamics Group, Wageningen University

Wageningen, January 2008

Photo on cover: mudflat in front of mangrove forest in Ca Mau, at low tide.

Abstract Mangrove rehabilitation projects often fail to achieve their goals, because hydrological characteristics of sites are not taken into account. This is partly because only one tool is available to asses the relationship between hydrology and vegetation in mangrove forests. This tool is a hydrological classification developed by Watson in 1928. In this hydrological classification Watson grouped several mangrove species in five inundation classes based on tidal regime, elevation and flooding frequency. After an exploratory research in Can Gio, Vietnam, Van Loon et al. (2007) proposed an extended hydrological classification. The objective of the current study was verifying this extended classification and testing it for a wider range of hydrological characteristics. Therefore during a measuring campaign from March until May 2007 tidal regimes, elevation profiles, water levels, vegetation, creek flow and groundwater flow were investigated in two study areas in southern Vietnam; Can Gio and Ca Mau. Both study areas have an irregular semi-diurnal tidal regime, but the diurnal component is stronger in Ca Mau. The tidal amplitudes are much larger in Can Gio than in Ca Mau. The elevation profiles showed ridges of a range of magnitudes in several directions and extensive basins, indicating that micro-topography is abundantly present in the mangrove forest. This micro-topography impedes overland flow, so at ebb tide water has to be discharged through slower flow paths like creeks or groundwater. This leads to longer durations of inundations at the sites than expected from their elevation. The calculated hydraulic conductivities indicate that groundwater flow can occur in the mangrove, especially when biopores are present. Net discharge in mangrove creeks could not be calculated, because measurements were not done over complete tidal cycles. Flow velocities showed a tidal asymmetry; ebb velocities were larger than flood velocities. Inundation characteristics of the measurement sites are determined from the measured water levels. These inundation characteristics are used to assign inundation classes to the sites according to the Watson and extended classification. The parameter flooding frequency in the Watson classification leads to unrealistic results in areas with an irregular tidal regime. The extended classification gives better results for areas with an irregular tidal regime and irregular elevation profiles, but still some sites are not classified correctly. During this research mixed zones with both Avicennia alba and Rhizophora apiculata were observed. None of the inundation classes predicts a vegetation pattern with both these species. Therefore a new hydrological classification is proposed with an extra class, 2*. The differences in vegetation and hydrologic conditions in the mixed zone are emphasized by this extra class and sites can be classified more accurately, especially in zones with a lot of variation in vegetation. Since it yields unrealistic results, the parameter flooding frequency is omitted completely. The new classification does not make a distinction between regular or irregular elevation profiles, which makes it easier to use in forest management. For the measurement sites in both Can Gio and Ca Mau the new classification resulted in better predictions of the inundation classes for the measurements sites, although the areas have different tidal regimes and elevation profiles. So, the new classification gives promising results for mangrove rehabilitation projects, because it is suitable for different elevation profiles and tidal regimes and describes vegetation patterns in detail. Keywords: Hydrology; Mangroves; Ecosystems; Hydrological classification; Mangrove restoration; Mangrove rehabilitation; Vietnam.

Preface This report is the result of about 9 months work for our MSc-thesis Hydrology and Water Quality, specialization Hydrology and Quantitative Water Management. A thesis about mangrove forest management could be expected in a study like Forest and Nature Conservation, but it was certainly not the first thing we thought of when we started to look for a subject. A field campaign of 3 months in an ecosystem we only knew from pictures and the practical relevance for mangrove management, were two factors that made us decide to go for it. Our very limited knowledge of mangrove forests could be regarded as a disadvantage, but we think it turned into an advantage since it made our work much more interesting and challenging. Of course it was sometimes confusing as well, but we think we managed quite well to stay focused and keep the work relevant. The water dependency of mangroves and the complexity of hydrological processes that occur, definitely offer very interesting opportunities for hydrologists to study. We encountered this during our work in Can Gio and Ca Mau mangrove areas and the subsequent data analysis back in Wageningen. The enormous amounts of articles, books and reviews about mangroves (and mangrove hydrology) that are published emphasize this. It is difficult to tell which part of the work was hardest for us. “Walking” through tick layers of mud, being far away from home and doing hardly anything else than work, sometimes were unpleasant parts of the field campaigns. But, working for months behind a computer in the rainy Netherlands and bringing all the information together might have been even harder. However, the possibility of working in wonderful areas, the people we have met and all the knowledge we have gained, do definitely counterbalance the side-effects. The fact that one of our main sources of information was a book published in 1928, underlines the economical, ecological and social values mangroves have and had. It also indicates the need for research with up to date techniques, since information from 1928 might not be suitable for current projects. Because of the importance of mangrove forest, we found it very inspiring to work on a tool which can be used in mangrove management and rehabilitation projects. There are a couple of people we want to thank for their contribution in our research. First of all, our supervisors, Roel Dijksma and Tini van Mensvoort. To work with you both was a pleasure for us and the conversations and discussions we had were very motivating. Tini, we might have been the last students you supervised, and that certainly is a pity for all the other students after us. We also like to thank our supervisor in Vietnam, Dr. V.N. Nam of Nong Lam University, Ho Chi Minh City. He was of great help in introducing us to mangroves in general and the study areas in particular, arranging research permissions and discussing some results with us. Furthermore, we thank Anne van Loon for providing a lot of information and data, and her willingness to answer all our questions. Other people who helped us during the fieldwork are the staff from the Can Gio Forestry Service, Can Gio Forestry Park and Mui Ca Mau National Park. Special thanks to A. Kiet for his help on the recognition of mangrove species in Can Gio and to Mr. No, who was very helpful in transporting us and our materials in Ca Mau everyday. The same did Mr. Son in Can Gio, for even a longer time, for which we are very grateful. Finally, but certainly not in order of importance, Bram wants to thank Marjolein for the pleasant cooperation and company in the previous year. Marjolein wants to thank Bram for the nice cooperation and all the good times we had during this research. Of course not everything went very smoothly all of the time, but serious problems were absent, while serious fun was abundantly present! We hope that reading this report will please you, as a reader. But above all, we hope that our work can contribute in management of mangrove forests and in successfully carrying out mangrove rehabilitation and restoration projects. Bram te Brake & Marjolein van Huijgevoort Wageningen, January 2008.

Contents

1 INTRODUCTION........................................................................................................................................ 1

1.1 BACKGROUND........................................................................................................................................ 1 1.2 OBJECTIVE OF THE RESEARCH................................................................................................................ 2 1.3 RESEARCH QUESTIONS........................................................................................................................... 2

1.3.1 Characterization of mangrove hydrology ......................................................................................... 2 1.3.2 Hydrological classification ............................................................................................................... 2

1.4 STRUCTURE OF THE REPORT................................................................................................................... 2 1.5 DEFINITIONS........................................................................................................................................... 3

2 THEORY ...................................................................................................................................................... 5

2.1 INTRODUCTION....................................................................................................................................... 5 2.2 HYDROLOGY .......................................................................................................................................... 5

2.2.1 Tides.................................................................................................................................................. 5 2.2.2 River discharge ................................................................................................................................. 6 2.2.3 Meteorology ...................................................................................................................................... 6 2.2.4 Hydrology of mangrove forests ......................................................................................................... 7

2.2.4.1 Surface water............................................................................................................................................7 2.2.4.2 Groundwater............................................................................................................................................. 7

2.3 MANGROVE ECOLOGY............................................................................................................................ 7 2.3.1 Mangroves......................................................................................................................................... 7 2.3.2 Zonation and succession ................................................................................................................... 9 2.3.3 The relation between hydrology and ecology.................................................................................... 9

2.4 REHABILITATION AND RESTORATION OF MANGROVE ECOSYSTEMS...................................................... 10 2.5 HYDROLOGICAL CLASSIFICATIONS....................................................................................................... 11

2.5.1 Watson hydrological classification ................................................................................................. 12 2.5.2 Disadvantages of the Watson classification.................................................................................... 12 2.5.3 Extended hydrological classification .............................................................................................. 13

3 SITE DESCRIPTION................................................................................................................................ 15

3.1 CAN GIO............................................................................................................................................... 15 3.1.1 History............................................................................................................................................. 15 3.1.2 Can Gio Man-and-the-Biosphere reserve ....................................................................................... 17 3.1.3 Tidal regime .................................................................................................................................... 17 3.1.4 Hydrology........................................................................................................................................ 17 3.1.5 Topography ..................................................................................................................................... 17 3.1.6 Vegetation ....................................................................................................................................... 18

3.2 CA MAU ............................................................................................................................................... 19 3.2.1 History............................................................................................................................................. 19 3.2.2 Mui Ca Mau National Park............................................................................................................. 19 3.2.3 Tidal regime .................................................................................................................................... 20 3.2.4 Hydrology........................................................................................................................................ 20 3.2.5 Topography ..................................................................................................................................... 20 3.2.6 Vegetation ....................................................................................................................................... 21

4 METHODOLOGY..................................................................................................................................... 23

4.1 SITE SELECTION.................................................................................................................................... 23 4.1.1 Selection criteria of the measurement plots .................................................................................... 23 4.1.2 Locations of the measurement plots ................................................................................................ 23

4.2 TIDAL PREDICTIONS ............................................................................................................................. 25 4.3 METEOROLOGICAL DATA ..................................................................................................................... 25 4.4 WATER LEVEL ...................................................................................................................................... 26

4.4.1 Divers.............................................................................................................................................. 26 4.4.2 Analysis ........................................................................................................................................... 27 4.4.3 Piezometer locations ....................................................................................................................... 29

4.5 ELEVATION .......................................................................................................................................... 32 4.5.1 Laser levelling................................................................................................................................. 32 4.5.2 Combination of laser levelling and water level data....................................................................... 34

Contents

4.5.3 Locations......................................................................................................................................... 34 4.6 VEGETATION........................................................................................................................................ 34 4.7 CREEK FLOW........................................................................................................................................ 34

4.7.1 Locations and tidal regime.............................................................................................................. 34 4.7.2 Cross-section................................................................................................................................... 35 4.7.3 Measurements ................................................................................................................................. 36 4.7.4 Calculations .................................................................................................................................... 36

4.8 HYDRAULIC CONDUCTIVITY ................................................................................................................. 36 4.8.1 Description of the tests.................................................................................................................... 37 4.8.2 Equations ........................................................................................................................................ 37 4.8.3 Aquifer thickness ............................................................................................................................. 39

5 RESULTS.................................................................................................................................................... 41

5.1 TIDAL REGIME ...................................................................................................................................... 41 5.1.1 Can Gio ........................................................................................................................................... 41

5.1.1.1 Tidal predictions Vung Tau.................................................................................................................... 41 5.1.1.2 Open water measurements...................................................................................................................... 41

5.1.2 Ca Mau............................................................................................................................................ 43 5.1.2.1 Tidal predictions Ha Tien and Dinh An.................................................................................................. 43 5.1.2.2 Open water measurements at C0 and D0................................................................................................ 45

5.2 METEOROLOGY.................................................................................................................................... 47 5.2.1 Air temperature and pressure ......................................................................................................... 47 5.2.2 Precipitation.................................................................................................................................... 47

5.3 ELEVATION .......................................................................................................................................... 48 5.3.1 Elevation measurement sites ........................................................................................................... 48 5.3.2 Elevation profiles Can Gio.............................................................................................................. 48 5.3.3 Elevation profiles Ca Mau .............................................................................................................. 52

5.4 WATER LEVEL MEASUREMENTS........................................................................................................... 55 5.4.1 Can Gio ........................................................................................................................................... 55

5.4.1.1 Plot A ..................................................................................................................................................... 55 5.4.1.2 Plot B...................................................................................................................................................... 56 5.4.1.3 Inundation characteristics ....................................................................................................................... 57

5.4.2 Ca Mau............................................................................................................................................ 59 5.4.2.1 Plot C...................................................................................................................................................... 59 5.4.2.2 Plot D ..................................................................................................................................................... 60 5.4.2.3 Inundation characteristics ....................................................................................................................... 61

5.5 VEGETATION........................................................................................................................................ 62 5.5.1 Can Gio ........................................................................................................................................... 62

5.5.1.1 Plot A ..................................................................................................................................................... 62 5.5.1.2 Plot B...................................................................................................................................................... 64

5.5.2 Ca Mau............................................................................................................................................ 65 5.5.2.1 Plot C...................................................................................................................................................... 65 5.5.2.2 Plot D ..................................................................................................................................................... 65

5.6 CREEK FLOW........................................................................................................................................ 66 5.6.1 Can Gio ........................................................................................................................................... 66

5.6.1.1 Creek profiles ......................................................................................................................................... 67 5.6.1.2 Flow velocity and water level................................................................................................................. 68 5.6.1.3 Discharge................................................................................................................................................69

5.6.2 Ca Mau............................................................................................................................................ 69 5.6.2.1 Creek profiles ......................................................................................................................................... 70 5.6.2.2 Flow velocity and water level................................................................................................................. 70 5.6.2.3 Discharge................................................................................................................................................73

5.7 HYDRAULIC CONDUCTIVITY ................................................................................................................. 73

6 DISCUSSION ............................................................................................................................................. 75

6.1 TIDAL REGIME ...................................................................................................................................... 75 6.1.1 Amplitude ........................................................................................................................................ 75 6.1.2 Diurnal vs. semi-diurnal ................................................................................................................. 75

6.2 ELEVATION .......................................................................................................................................... 75 6.3 WATER LEVEL MEASUREMENTS........................................................................................................... 76

6.3.1 Time lag between measurement sites .............................................................................................. 76 6.3.2 Inundation characteristics............................................................................................................... 77

Contents

6.4 CREEK FLOW........................................................................................................................................ 78 6.5 HYDRAULIC CONDUCTIVITY ................................................................................................................. 81

7 HYDROLOGICAL CLASSIFICATIONS....................... ........................................................................ 85

7.1 RESULTS EXISTING HYDROLOGICAL CLASSIFICATIONS......................................................................... 85 7.2 ERRORS AND UNCERTAINTIES IN EXISTING CLASSIFICATIONS............................................................... 88 7.3 NEW HYDROLOGICAL CLASSIFICATION................................................................................................. 90 7.4 APPLICATION OF THE NEW HYDROLOGICAL CLASSIFICATION............................................................... 91

8 CONCLUSIONS......................................................................................................................................... 95

8.1 CONCLUSIONS CHARACTERIZATION OF MANGROVE HYDROLOGY........................................................ 95 8.2 CONCLUSIONS HYDROLOGICAL CLASSIFICATION.................................................................................. 95

9 RECOMMENDATIONS........................................................................................................................... 97

10 REFERENCES....................................................................................................................................... 99

APPENDICES APPENDIX A ABSTRACTS EARLIER RESEARCH APPENDIX B COORDINATES OF THE PIEZOMETER LOCATIONS APPENDIX C TIDAL PREDICTIONS APPENDIX D GRAPHS OF THE WATER LEVELS APPENDIX E VEGETATION APPENDIX F LOCATIONS DISCHARGE MEASUREMENTS APPENDIX G GRAPHS OF THE PERMEABILITY TESTS APPENDIX H VERIFICATION OF THE NEW CLASSIFICATION

List of figures 2.1 THE LUNAR PHASE EFFECT, RESULTING IN SPRING AND NEAP TIDE PERIODS . ..................................................... 5 2.2 COMMON PROJECTION OF THE EARTH’S ORBITAL PLANE AROUND THE SUN AND THE MOON’S ORBITAL PLANE

AROUND THE EARTH........................................................................................................................................ 6 2.3 EXAMPLE OF A PROFILE DIAGRAM OF A TIDAL FLAT IN NORTHERN AUSTRALIA. ................................................ 9 2.4 NURSERY OF MANGROVE SEEDLINGS IN THAILAND . ........................................................................................ 11 2.5 TIDAL PREDICTION FOR THE PORT OF VUNG TAU FOR THE PERIOD 28 APRIL TO 3 MAY 2004 WITH THREE

IMAGINARY SURFACE LEVELS. ...................................................................................................................... 13 3.1 LOCATION OF CAN GIO AND CA MAU; ZOOMING IN FROM THE WORLD TO SOUTHEAST ASIA AND TO SOUTHERN

VIETNAM . ..................................................................................................................................................... 16 3.2 MAP OF THE CAN GIO BIOSPHERE RESERVE..................................................................................................... 18 3.3 MAP OF THE SOUTHERN PART OF CA MAU PROVINCE. ..................................................................................... 20 4.1 LOCATIONS OF THE MEASURING PLOTS IN CAN GIO. ........................................................................................ 24 4.2 LOCATIONS OF THE MEASURING PLOTS IN CA MAU. ........................................................................................ 25 4.3 LOCATIONS OF THE TIDAL STATIONS, METEOROLOGICAL STATIONS AND STUDY AREAS. ................................. 26 4.4 A STANDARD DIVER. ........................................................................................................................................ 26 4.5 PIEZOMETER IN THE FOREST AT FLOOD TIDE. ................................................................................................... 27 4.6 DIVER IN A STILLING WELL .............................................................................................................................. 28 4.7 WATER LEVEL MEASUREMENT BY A DIVER. .....................................................................................................29 4.8 LOCATIONS OF THE PIEZOMETERS IN PLOT A. .................................................................................................. 30 4.9 LOCATIONS OF THE PIEZOMETER IN PLOT B. ....................................................................................................31 4.10 LOCATIONS OF THE PIEZOMETERS IN PLOT C................................................................................................. 31 4.11 LOCATIONS OF THE PIEZOMETERS IN PLOT D.................................................................................................. 32 4.12 CONCEPT OF THE LASER LEVELLING METHOD................................................................................................ 33 4.13 PHOTOGRAPH OF THE LASER LEVELLING EQUIPMENT ON A TRIPOD. ............................................................... 33 4.14 CALCULATION OF THE CROSS SECTIONAL AREA A. ........................................................................................ 35 4.15 PHOTOGRAPH OF THE SET UP USED TO MEASURE HYDRAULIC CONDUCTIVITY . .............................................. 37 5.1 PREDICTED WATER LEVELS AT THE PORT OF VUNG TAU; A) 3 TO 24 MARCH 2007, B) 24 MARCH TO 14 APRIL

2007, C) 14 APRIL TO 5 MAY 2007, D) 5 MAY TO 27 MAY 2007 ................................................................... 42 5.2 COMPARISON OF PREDICTED WATER LEVELS FOR VUNG TAU AND MEASURED WATER LEVELS IN DONG TRANH

RIVER FOR THE PERIOD 11 APRIL TO 18 APRIL 2007..................................................................................... 43 5.3 PREDICTED WATER LEVELS AT A) HA TIEN AND B) DINH AN, FROM 21 APRIL TO 20 MAY 2007 ..................... 44 5.4 OPEN WATER MEASUREMENTS AT SITE C0 AND D0 FROM 21 APRIL TO 20 MAY 2007. .................................... 46 5.5 DETAIL OF PREDICTED WATER LEVELS AT HA TIEN AND DINH AN AND MEASURED WATER LEVELS AT SITE D0

FOR THE PERIOD 23 APRIL TO 28 APRIL 2007................................................................................................ 46 5.6 MEASURED PRECIPITATION AT CAN GIO AND NAM CAN WEATHER STATIONS FROM 1 MARCH TO 22 MAY 2007

(AFTER: SOUTHERN REGIONAL HYDROMETEOROLOGICAL CENTER, 2007) AND OBSERVED RAINFALL EVENTS

AT MUI CA MAU. .......................................................................................................................................... 47 5.7 LOCATIONS OF THE LASER LEVELLING TRANSECTS IN PLOT A.......................................................................... 49 5.8 LOCATIONS OF THE LASER LEVELLING TRANSECTS IN PLOT B.......................................................................... 49 5.9 ELEVATION PROFILES IN PLOT A; A) PROFILES PERPENDICULAR TO THE MAIN CHANNEL (TOP), B) PROFILES

PARALLEL TO THE MAIN CHANNEL (BOTTOM).. ............................................................................................. 50 5.10 ELEVATION PROFILES IN PLOT B. ................................................................................................................... 51 5.11 LOCATIONS OF THE LASER LEVELLING TRANSECTS IN PLOT C........................................................................ 53 5.12 LOCATION OF THE LASER LEVELLING TRANSECT IN PLOT D. .......................................................................... 53 5.13 ELEVATION PROFILES IN PLOT C; A) PROFILES PERPENDICULAR TO THE MAIN CHANNEL (TOP), B) PROFILE

PARALLEL TO THE MAIN CHANNEL (BOTTOM).. ............................................................................................. 54 5.14 ELEVATION PROFILE IN PLOT D PERPENDICULAR TO MAIN CHANNEL. ............................................................ 55 5.15 DETAIL OF WATER LEVELS IN PLOT A FROM 21 MARCH 13:00 TO 22 MARCH 13:00, 2007............................. 56 5.16 DETAIL OF WATER LEVELS IN PLOT B AND AT A0 FROM 16 APRIL 21:00 TO 17 APRIL 6:00, 2007.................. 57 5.17 DETAIL OF WATER LEVELS AT SITE C1 AND THE ARTIFICIAL SURFACE LEVEL LINE ON 2 CM +SURFACE. ........ 58 5.18 DETAIL OF WATER LEVEL AND THE APPARENT SOIL SURFACE AT SITE A3...................................................... 59 5.19 DETAIL OF WATER LEVELS IN PLOT C FROM 17 MAY 9:00 TO 18 MAY 9:00, 2007. ........................................ 60 5.20 DETAIL OF WATER LEVELS IN PLOT D FROM 22 APRIL 12:00 TO 23 APRIL 6:00, 2007.................................... 61 5.21 OVERVIEW OF OBSERVED VEGETATION ZONES IN PLOT A.. ............................................................................ 62 5.22 MIXED ZONE WITH SHRUB LAYER AT THE NORTH BORDER OF TRANSECT 6. ................................................... 635.23 A. ALBA AND R. APICULATA DOMINATED MIXED ZONE WITH C. TAGAL. ........................................................ 64 5.24 SOLITARY A. ALBA IN THE DENSE R. APICULATA PLANTATION IN PLOT B...................................................... 64 5.25 CUT R. APICULATA STRIP IN PLOT C. YOUNG TREES CAN BE SEEN AT THE OPEN SPOTS. ................................. 65

List of figures

5.26 VEGETATION IN PLOT D WITH A. ALBA , R. APICULATA AND B. PARVIFLORA. ................................................ 66 5.27 CREEK PROFILES IN CAN GIO: PLOT A (LEFT) AND PLOT B (RIGHT)................................................................ 67 5.28 EXAMPLES OF THE VARIETY OF DISCHARGE MEASUREMENT LOCATIONS; CREEK II, LOCATION 1 (LEFT) AND

CREEK VI, LOCATION 6 (RIGHT). ................................................................................................................... 68 5.29 VELOCITY AND WATER LEVELS AT LOCATION 1 AND WATER LEVELS AT SITE A0; 18 MARCH 2007 (LEFT),

28 MARCH 2007 (RIGHT)............................................................................................................................... 69 5.30 CREEK PROFILES IN CA MAU: PLOT C (LEFT) AND PLOT D (RIGHT). ............................................................... 70 5.31 WATER LEVELS AT SITE C0 ON 30 APRIL AND 1 MAY 2007 (LEFT) AND AT SITE D0 ON 3 MAY 2007 (RIGHT).71 5.32 FLOW VELOCITY AND WATER LEVELS AT LOCATION 1 ON 30 APRIL 2007 AND LOCATION 2 ON 1 MAY 2007. 72 5.33 FLOW VELOCITY AND WATER LEVELS AT LOCATION 3 AND 4 ON 3 MAY 2007. ..............................................72 5.34 DISCHARGE AT LOCATION 1 (30 APRIL), LOCATION 2 (1 MAY), LOCATION 3 (3 MAY) AND LOCATION 4 (3

MAY). ........................................................................................................................................................... 73 6.1 EXAMPLE OF A PARABOLIC VERTICAL VELOCITY PROFILE IN OPEN CHANNELS................................................. 79 6.2 SHIFT IN NEAR BED AND NEAR-SURFACE VELOCITIES AT THE TRANSITION FROM EBB TO FLOOD TIDE.............. 80 6.3 EXAMPLE OF A PARABOLIC HORIZONTAL VELOCITY PROFILE IN OPEN CHANNELS. ........................................... 80 6.4 EXPECTED HYDRAULIC CONDUCTIVITY PROFILE IN MANGROVE SOILS INTERSECTED BY BIOPORES.................. 82 7.1 RELATION BETWEEN INUNDATION CLASS AND GROWING CONDITIONS FOR 4 SPECIES...................................... 89

List of tables 2.1 WATSON’S HYDROLOGICAL CLASSIFICATION................................................................................................... 12 2.2 EXTENDED HYDROLOGICAL CLASSIFICATION................................................................................................... 14 4.1 PIEZOMETER LOCATIONS, DISTANCES TO MAIN CHANNEL AND MEASURING PERIOD......................................... 30 5.1 MEASURED TEMPERATURE DATA IN CAN GIO AND CA MAU. .......................................................................... 47 5.2 ELEVATION OF ALL MEASUREMENT SITES. ....................................................................................................... 48 5.3 AVERAGE TIME LAG AND DISTANCE TO DONG TRANH RIVER FOR PIEZOMETER SITES IN PLOT A. .................... 55 5.4 INUNDATION CHARACTERISTICS FOR PLOT A AND B. ....................................................................................... 58 5.5 AVERAGE TIME LAG AND DISTANCE TO RANG ONG LINH RIVER FOR PIEZOMETER SITES IN PLOT C. ................ 59 5.6 AVERAGE TIME LAG AND DISTANCES TO CUA LON RIVER FOR PIEZOMETER SITES IN PLOT D........................... 60 5.7 INUNDATION CHARACTERISTICS FOR PLOT C EN D........................................................................................... 61 5.8 OVERVIEW OF DISCHARGE MEASUREMENTS IN CAN GIO. ................................................................................ 67 5.9 MAXIMUM FLOW VELOCITIES IN CREEKS IN CAN GIO. ..................................................................................... 69 5.10 OVERVIEW OF DISCHARGE MEASUREMENTS IN CA MAU................................................................................ 70 5.11 CALCULATED K -VALUES FOR ALL PLOTS........................................................................................................ 74 7.1 INUNDATION CLASSES ATTRIBUTED TO THE MEASUREMENT SITES USING THE WATSON CLASSIFICATION........ 86 7.2 INUNDATION CLASSES ATTRIBUTED TO THE MEASUREMENT SITES USING THE EXTENDED CLASSIFICATION OF

VAN LOON ET AL. (2007)............................................................................................................................... 87 7.3 DIFFERENCES BETWEEN EXPECTED AND CALCULATED INUNDATION CLASSES................................................. 88 7.4 NEW HYDROLOGICAL CLASSIFICATION AND THE SOUTHEAST ASIAN MANGROVE SPECIES ATTRIBUTED TO

EACH CLASS. ................................................................................................................................................. 92 7.5 INUNDATION CLASSES ATTRIBUTED TO THE MEASUREMENT SITES USING THE NEW CLASSIFICATION OF

TABLE 7.4. .................................................................................................................................................... 93

1

1 Introduction

1.1 Background Mangrove forests occur in sub-tropical and tropical regions around the world (Alongi, 2002). There are about seventy known mangrove species, which are all tolerant to salt and brackish waters (Field, 1998). The total area occupied by mangroves globally is difficult to determine, but is estimated to be between 181 000 and 198 800 km2 in 1997 by Spalding et al. (in Field, 1998). Mangrove ecosystems are highly productive, but also very vulnerable (Tabuchi, 2003). According to Alongi (2002) “approximately one third of the mangrove forests over the world have been lost in the past 50 years”. However Kairo et al. (2001) report that “less than 50% of the original total cover of mangroves” has remained. The losses of mangroves can be contributed to the fact that they are heavily exploited, since mangroves are highly productive ecosystems. The main threats for mangroves are overexploitation of the natural resources, deforestation, conversion to aquaculture and salt-ponds, mining, pollution and industrial or urban development (Field, 1998, Alongi, 2002). Natural disasters like tropical cyclones (Tri et al., 1998) and the tsunami of 26 December 2004 in Asia, can also devastate mangrove ecosystems (Barbier, 2006, Van Loon et al., 2006). Mangroves are valuable ecosystems that provide a natural barrier against storms, stabilize coastlines and have a high economical value for humans, who depend on their natural resources (Hong and San, 1993). Therefore rehabilitation and restoration projects are carried out all over the world to prevent further degradation and losses of mangrove areas. Rehabilitation is defined by Field (1998) as “partially or fully replacing structural or functional characteristics of an ecosystem”. Field emphasizes that ecological rehabilitation may also hold substitution of the disturbed or degraded state to a situation of alternative characteristics than those originally present, as long as these alternative characteristics have more social, economic or ecological value. Restoration on the other hand is described by Field as “bringing an ecosystem back into its original condition”. Rehabilitation projects in general have three main objectives: conservation of a natural system and landscaping, sustainable production of natural resources and protection of coastal areas (Barbier, 2006, Field, 1998). Unfortunately in mangroves many of these rehabilitation projects fail to achieve their goals or result in mono-specific plantations, which can not be seen as successful ecological restoration (Lewis, 2001). The failure of these projects is often caused by lack of adequate site selection (Ellison, 2000) and no determination of characteristics of the sites (Lewis, 2005). Especially the hydrological characteristics of sites are often not taken into account. For example, in Vietnam Rhizophora apiculata has been planted on mudflats in front of the forest, where this species can not survive partly due to the wet conditions. According to Lewis (2001) “the single most important factor in designing a successful mangrove restoration project is determining the normal hydrology (depth, duration and frequency of tidal flooding) of existing natural mangrove plant communities”. The influence of hydrology on the mangrove ecosystem is also recognized by Hughes et al. (1998), who mention it as a “key determinant” for several processes, and by Field (1998), who states that “hydrology of the site is of great importance”. However, little research is undertaken to quantify the relation between hydrology and vegetation. In 1928, Watson developed a hydrological classification in which he grouped the main mangrove species in five inundation classes based on tidal regime, elevation and flooding frequency. In this way he described the distribution of mangrove species near Port Swettenham at the Malay peninsula (Watson, 1928). This classification is often used in rehabilitation projects (Hong and San, 1993, Lewis, 2005, Van Loon, 2005), because no other general hydrological tool is available. Nam (2007) however stated that the classification is not used anymore, since it does not describe hydrological site characteristics well. After an exploratory hydrological research in Can Gio, Van Loon et al. (2007) also concluded that the Watson classification gave unsatisfactory results in this area (Appendix A). The classification was found to be unsuitable for regions with an irregular elevation profile and/or an irregular semi-diurnal tidal regime. Therefore Van Loon et al. developed an extended hydrological classification for regions with regular as well as irregular elevation profiles and tidal regimes. The main parameter added in the extended classification is duration of inundation. This classification gave better results for the Can Gio area (Van Loon et al., 2007). With this extended classification mangrove rehabilitation projects might be more successful in the future.

1 Introduction

2

1.2 Objective of the research The development of this extended classification gave rise to further research of the interaction between hydrology and vegetation occurrence in mangrove areas, to increase the number of successful rehabilitation and restoration projects. Therefore the main objective of this study is to verify the extended Watson classification, as is proposed by Van Loon et al. (2007), and to test the applicability of this classification for a wider range of hydrological characteristics. Since the extended classification is based on relatively short data series and limited measuring locations, only some sites in Can Gio, this study focuses on extending the existing data series in Can Gio and on testing the classification in a mangrove area with different hydrological characteristics. In this research field campaigns have been carried out in Can Gio and Ca Mau, both located in southern Vietnam. This study is divided in two main sections. First, the general hydrological characteristics of the study areas are investigated. Furthermore the hydrological classification is tested using these characteristics, to determine its suitability for different areas.

1.3 Research questions

1.3.1 Characterization of mangrove hydrology For the first part, investigating general hydrological characteristics in the study areas, the central question is: What are the hydrological characteristics of the research areas, with respect to the

tidal regime and flow patterns and how do these characteristics change in time? To answer this question the following sub-questions are formulated and answered for each area:

- What is the tidal regime in the area and what are the differences within each of the areas? - What is the flow pattern in the area and how does this change over time? - How can the groundwater flow of the mangrove forest be characterized?

1.3.2 Hydrological classification The central question of the part of the research focusing on the applicability of the extended classification is: Is there a consistent relation between hydrological characteristics and mangrove

development and can this lead to an extended Watson classification? Sub-questions formulated with this question are:

- What are the differences with respect to the factors frequency and duration of tidal inundation within a transect and between transects on different locations?

- What are the interactions of the elevation, tidal regime and the groundwater flow with the dynamics of the mangrove vegetation along a transect?

1.4 Structure of the report The different chapters in this report are mentioned and shortly described below. Chapter 2 Theory; gives a description of mangrove ecosystems in general, the vegetation within

mangroves and the role of hydrology, based on literature. The hydrological classifications of both Watson (1928) and Van Loon et al. (2007) are described in this chapter.

Chapter 3 Site description; contains information about the two different study areas. The history, hydrology and vegetation in both areas are treated based on literature research.

Chapter 4 Methodology; presents all the methods of the measurements carried out in the mangrove forest during this research.

1.5 Definitions

3

Chapter 5 Results; describes the results of the measurements carried out during this research which are aimed at determining the hydrological characteristics of the study areas.

Chapter 6 Discussion; explains parts of the results in more detail. It discusses connections between different observations within this research and between these observations and results of other research available from literature.

Chapter 7 Hydrological classifications; discusses the results of the hydrological classifications of Watson and Van Loon et al. obtained from the measured hydrological characteristics. A new hydrological classification is proposed.

Chapter 8 Conclusions; gives the conclusions of both the characterization of mangrove hydrology and the hydrological classifications.

Chapter 9 Recommendations; indicates possibilities for further research. The appendices contain additional data obtained from the measurements during this research, a list of optimum requirements of several mangrove species with regard to soil-type and frequency of inundation, a list of measurement locations and additional information.

1.5 Definitions The word ‘mangrove’ has several definitions in literature. It is both used to refer to “the constituent plants of tropical intertidal forest communities or to the community itself” (Tomlinson, 1986). In this report the word ‘mangrove’ refers to the community, so ‘mangrove forest’ and ‘mangrove’ are used in the same way. The definition of mangrove forest as given by the Joint Group of Experts of the Scientific Aspects of Marine Environmental Protection of IMO/UNESCO/WMO/WHO/IAEA/UN/ UNEP is used in this report. This group defines a mangrove forest as (European environment agency glossary, 2007): “A community of salt-tolerant trees and shrubs, with many other associated

organisms, that grows on some tropical and sub-tropical coasts in a zone roughly coinciding with the intertidal zone.”

The individual species within the mangrove forest are referred to by their scientific name. Abbreviations are used for the most common species in this report. The genus Rhizopora is abbreviated to R., Avicennia to A., Bruguiera to B. and Ceriops to C., the species names are not abbreviated. So for example the species Bruguiera parviflora becomes B. parviflora and Rhizophora apiculata is written as R. apiculata. The annex spp. is used to indicate several species of the same genus together, like Rhizophora spp.. When the different vegetation zones are described the abbreviations Rh and Av refer to the Rhizophora zone and Avicennia zone. For the analysis of the data from the measurements several equations are needed. Together with the definition of the parameters in these equations the dimensions in which the parameters should be expressed are given, instead of units. These dimensions are notated between square brackets. So the dimension length of a parameter is displayed with [L], which can be centimetres, metres or kilometres. The same applies to [T], which is the notation for the dimension time.

1 Introduction

4

5

2 Theory

2.1 Introduction This report discusses a research on the relation between hydrology and ecology of mangrove forest. Therefore, some background information might be needed on the hydrology of mangrove forests, the ecology of mangrove forests or the connection between these two. In this chapter an overview of relevant topics in these subjects is given. Van Loon (2005) incorporated a literature study on mangrove hydrology and ecology in her study on water flow and tidal influence in Can Gio. The paragraphs 2.2, 2.2.2, 2.2.3, 2.2.4, 2.3.3 are mainly based on this literature study. The complete literature study of Van Loon can be found in Van Loon (2005).

2.2 Hydrology According to Hughes et al. (1998) the hydrology is a key determinant in species distribution, wetland productivity and nutrient cycling and availability in mangrove systems. Therefore studying the hydrology in a mangrove-delta system has a high priority. Hydrological research in mangrove systems is done by, among others, Wolanski (1980 to 1992), Mazda (1990 to 2006), Hughes (1998) and Kitheka (1997). In tropical coastal waters the main forcing factors for the coastal hydrology are the tide, river discharge and meteorology (Kitheka, 1997), which interact in different ways.

2.2.1 Tides Tide is the phenomenon of periodic sea level rise and fall. Tidal water movements, both horizontal (flow velocity) and vertical (water level), are caused by a complex interaction of astronomical forces and hydrodynamic effects of the ocean bottom topography and the coastal configuration. The astronomical forces relevant in the generation of tides are mainly the attractive power of the moon and the sun. During the 29.53 day’s cycle of the moon around the earth, the gravitational attraction of moon and sun may variously act along a common line or at different angles (Figure 2.1).

Figure 2.1 The lunar phase effect, resulting in spring and neap tide periods (Center for Operational

Oceanographic Products and Services, 2005). This lunar phase shift results in an alternation of higher (spring tide) and lower (neap tide) than average tidal range over approximately 2 weeks. Next to this, both the moon and the earth revolve in elliptical orbits and consequently the distances between the sun and the earth and the moon and the earth vary (Figure 2.2). Increased tide-raising forces are produced when the moon is at position of perigee, its closest position to the earth (once each month), or the earth is at perihelion, its closest

2 Theory

6

position to the sun (once each year, around 2 January). Figure 2.2 shows the situation of perigee coinciding with perihelion. In this situation tides of increased range are generated. On the other hand considerably reduced tidal ranges occur when apogee, aphelion, and the first- or third-quarter moon coincide at approximately the same time. In general, tidal forces are mainly induced by the moon, since solar tide generating effects are smaller than the lunar effect. (Center for Operational Oceanographic Products and Services, 2005)

Figure 2.2 Common projection of the earth’s orbital plane around the sun and the moon’s orbital plane around

the earth (Center for Operational Oceanographic Products and Services, 2005). The varying interaction of astronomical forces and hydrodynamic effects results in different tidal regimes between different regions. Tidal regimes are characterized by the amplitude and the frequency of the tides. In coastal seas the bathymetry has a large influence on the tidal regime. A coastal sea is usually relatively shallow leading to an increase in tidal wave height (Rijn, 1990, in Van Loon, 2005). Tidal regimes are often classified as diurnal, semi-diurnal and mixed, based on the frequency of high and low water levels. There are however no sharply defined limits separating the groups. In general, the tide is said to be diurnal when both high tide and low tide occur only one time each day during the greater part of the month. The tide is semi-diurnal when two high and two low tides occur each day with approximately the same amplitude. In mixed tidal regimes the diurnal and semi-diurnal components are both important factors and the tide is characterized by large variations in high and/or low water levels. There will usually be two high and two low waters each day, but occasionally the tide will become diurnal. Therefore these tidal regimes are called irregular semi-diurnal. (Voigt, 1998)

2.2.2 River discharge In delta areas the tidal wave from the coastal sea enters the creek system. In this region the tides experience the influence of the discharge of river water through the creeks, generally in the direction of the sea, although different flow routes may be determined by the magnitude of the discharge, the tidal regime, the configuration of the creeks, and possible hydrological obstructions.

2.2.3 Meteorology River discharge depends on precipitation and losses of water, mainly evapotranspiration, in the river basin. Major rainfall events, which occur in tropical regions during only one season, are highly significant for flow patterns, but have only a short-term effect on water levels (Hughes et al., 1998). Furthermore, wind has an influence on the water movement in a delta region. Wind influences the tidal regime through possible dampening and amplifying effects, and the direction of the wind can affect the water distribution in the creek system.

2.3 Mangrove ecology

7

2.2.4 Hydrology of mangrove forests

2.2.4.1 Surface water According to Mazda et al. (1997) “reports on mangrove hydrodynamics are largely restricted to the tidal creeks, and measurements in the swamp itself are sparse”. Water that reaches a tidal flat occupied by mangrove forest by overland flow, behaves differently than creek water due to the presence of vegetation and the limited water depth. Due to bottom friction large vertical shear exists, which might result in low flow velocities or even stagnant water. Mazda et al. (1997) carried out hydrodynamic measurements in a mangrove swamp and presented observations of the drag force due to vegetation on tidal currents through mangrove swamps. Tidal regime and elevation are not the only factors determining the frequency, duration and height of inundation of a tidal flat. Topography and vegetation at the tidal flat have a significant influence on the duration (Van Loon et al., 2007) and frequency of inundation. In the wet season higher precipitation and river discharge can cause the water at high tide to flood a larger area than in the dry season (Thom et al., 1975). With strong tidal currents, as during spring tide, the tidal influence is the dominant water transporting process in the mangrove swamp and groundwater flow does not contribute much to hydrodynamics (Wolanski, 1992). However, during neap tide the groundwater movements can become an important factor in the mangrove hydrology.

2.2.4.2 Groundwater As the topography of a delta region is flat and a large area of land is frequently flooded, groundwater levels are usually very high. Groundwater behaviour is controlled by a combination of effects of tidal processes, precipitation and evapotranspiration and possibly regional groundwater flow (Hughes et al., 1998). Depending on the location in the delta, the distance to open water and the period of the year, the tidal regime or the meteorological variables are the most important. According to Hughes et al. (1998) the tidal forcing is the dominant mechanism for pore water movement in the saturated and intertidal zone of a delta. Close to the creek water table movement is directly coupled to fluctuations in water level of the creek and thus of the tidal movements. With increasing distance from the creek the fluctuations in groundwater level rapidly decline. At a distance of 5 to 10 m from the creek the water table movement is negligible. Consequently, at the inland parts of the mangrove swamp evapotranspiration is the only way groundwater levels can be lowered. In the wet season fluctuations in groundwater level are considerable due to the irregular character of the rainfall if the area is not flooded for a long period. During the dry season the water table will drop gradually due to the increasing evapotranspiration in case of no replenishment. The latter situation results in a high groundwater salinity. At some inland locations mangrove trees are not able to survive and a salt marsh with specific salt-tolerant vegetation will develop (Hughes et al., 1998).


2.3.1 Mangroves Mangroves are forests consisting of a group of salt-tolerant trees and shrubs that can develop along sub-tropical and tropical coasts. They develop best along sheltered coastlines and in delta regions where waves are broken. In sheltered estuaries and lagoons mangroves are usually extensive and may stretch up to several kilometres inland, with a gradual transition to terrestrial vegetation (Tomlinson, 1986). Mangroves grow along rivers and creeks as long as there is tidal movement and the water is salt or brackish (Poorter and Bongers, 1993, in Van Loon, 2005). The distribution of mangroves is divided in two groups by several authors (Chapman, 1976, Duke et al., 1998, Tomlinson, 1986), which are the Eastern and Western mangroves. Here the names of the groups indicate the hemisphere on which the species are found, but other names are also mentioned in literature. The total number of true mangrove

2 Theory

8

species1 in the Eastern group, including East Africa, India, southeast Asia, Australia and the Western Pacific, is 40. In the Western group only eight true mangroves species are found. (Tomlinson, 1986) Composition of the groups not only differs in number, but also in species; no species is present in both groups. Mangroves in Vietnam are part of the Eastern group. The southeast Asian sub-region is recognised as the biogeographical province supporting the most diverse mangroves in the world. The highest diversity of mangrove plant species has been recorded in this sub-region (Tri et al. 2000). Mangrove forests are frequently inundated by tides, which is a primary existence factor for many of its species. Mangrove trees however perform best under fresh water conditions, but they loose competition with other species in fresh water environments. Due to specific physiological adaptations in their tissue, mangrove trees can survive in saline and brackish water environments and under anaerobic conditions which occur during moments of inundation. These adaptations make mangrove families an unique group of trees and plants that are able to survive along coastlines, which form an inaccessible habitat for other species. Adaptations to tidal inundations and saline water are not the only characteristic features of mangrove species. Mangroves have to cope with variable water levels, unstable soils, salinity of the water leading to physiological dryness, lack of oxygen, water flow etc. (Van Loon, 2005). Tomlinson (1986) reports a list of major features that are typical for all or most of the mangroves species:

1. Complete restriction to the mangrove environment; they occur only in mangrove forest and do not extend into terrestrial communities.

2. A major role in the structure of the community and the ability to form pure stands. 3. Morphological specialization that adapts mangroves to their environment; the most obvious

being aerial roots and vivipary of the seed. 4. Some physiological mechanism for salt excretion which enables mangroves to grow in saline

water. 5. Mangrove species are separated from their relatives at least at the generic level and often at

the subfamily or family level. Especially criterion 1 and the vivipary of seed are very distinctive. Many mangrove species have special roots, called pneumatophores. These roots enable the trees to get some air, during shallow inundations and in water saturated soils. Aerial roots are often regarded as the main feature of mangrove species, but many other forest swamp plants develop aerial roots as well (Tomlinson, 1986). Mangrove seeds develop on the parent tree and grow out into propagules, which are viviparous. When released from the parent tree they might be transported by water movements or settle near the parent tree. Most mangrove species have propagules that float on water. The establishment of propagules depends on the number of days propagules remain buoyant and viable, the strength of surface currents, the water conditions, and the availability of suitable sites (Duke et al., 1998). Suitability of sites is determined by the depth of inundation, the presence of other mangrove trees and the salinity of the water (Van Loon, 2005). Vanspeybroeck (1992) found that mangrove seedlings in Kenya are restricted to sites where their parent trees are found, even when parental trees have been felled. This can be due to a poor dispersal of propagules or the presence of suitable environmental conditions (Ashton and Macintosh, 2002). Clarke and Kerrigan (2000) and Matthijs et al. (1999) also report hypotheses and observations of propagule and seedling distributions following parental zonation. Other studies show that mangrove seedlings establish on a different site than their parent trees due to changes in site conditions. For example, A. germinans propagules can establish in zones where they are not usually found (Patterson et al., 1997) and in general it applies that mature mangrove trees can survive on sites with environmental conditions that are sub-optimal for seedling establishment (Watson, 1928).

1 True mangrove species consist of plants which are absolutely confined to salt or brackish water, while mangrove associates are plants which belong to more inland vegetation but can frequently be found with true mangrove species (Hong and San, 1993).


9

2.3.2 Zonation and succession The existence of vegetation zones, often monospecific, along environmental gradients is called zonation. Zonation is often very evident in mangrove forests (Tomlinson, 1986). Profile diagrams, as often used to describe zonation, may give the impression that zonation is a regular series of vegetation bands parallel to the coastline. However, according to Tomlinson (1986) “any regular zonation is modified by local topography, which determines tidal and fresh-water runoff, and by sediment composition and stability”. An example of a schematic and generalized profile is shown in Figure 2.3.

Figure 2.3 Example of a profile diagram of a tidal flat in northern Australia. HWS indicates high water level at

spring tide. (Adapted from: Tomlinson, 1986) A common assumption is that the zones of species along a transect represent their succession in time (Chapman, 1976, Thom et al., 1975, Tomlinson, 1986). Pioneer species establish on newly exposed mudflat and as environmental conditions change, more climax species can enter the region and displace the pioneer species. The gradient in conditions perpendicular from the coast is thought to be the main factor controlling zonation (Thom et al., 1975). Ellison et al. (2000) state that ordering of groups of species, at a given location with respect to elevation is predictable, with the upper limit of one group marking the lower limit of a second. Numerous authors have given environmental factors determining zonation. According to Chapman (1976) tidal factors, salinity, drainage, currents and soil composition are the most important factors. Rabinowitz (1978) states that factors related to the once mentioned by Chapman, like length of the submersion period, daily and seasonal fluctuations in salinity, soil consistency or texture, availability of fresh water, competitive ability and water-logging, “are thought to occur in gradients from the front to the back of the swamp or along channels”. Mangrove species respond physiologically to these factors such that each species has a preferred area within the forest. Tomlinson (1986) also mentions physiological responses to gradients as one of the factor influencing zonation, but he also discusses other, both biotic and abiotic, factors. These are geomorphology, inundation classes, propagules sorting and, competition. Inundation classes to describe zonation are extensively discussed by Watson (1928) (chapter 2.5.1) and propagules sorting by Rabinowitz (1978). According to Lewis (2005) zonation is based upon the nature of the tide that inundates an area rather than the number of times or total period of inundation; few have ever quantified it.

2.3.3 The relation between hydrology and ecology All the above mentioned ecological factors are assumed to influence mangrove distribution in an area. Apparently, not one set of environmental factors is causing mangrove zonation (Matthijs et al., 1999). However, the main determining factor for the ecology in a mangrove system is water.

2 Theory

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Species distribution along a gradient is strongly dependent on hydrological factors. Hogarth (1999, in Van Loon, 2005) pointed out the importance of the hydrological variables, frequency and duration of inundation. In their study Stumpf and Haines (1998) mention the importance of the elevation relative to Mean Sea Level (MSL) or Mean High Water (MHW). From an ecological perspective the highest tide each day (MHHW) should be the most relevant in determining species distribution, but MSL is a good estimate. According to Vanspeybroeck (1992) however, the elevation above Mean Low Water (MLW) is the predominant factor affecting the distribution of mangrove trees. Of course all these factors are strongly interrelated. The tidal inundation frequency is found to be the most common variable to illustrate species zonation patterns (Ellison et al., 2000). Additionally the intensity of water flow dynamics and waves influence the establishment of mangrove propagules and the development of the seedlings (Matthijs et al., 1999, Vanspeybroeck, 1992). For example Rhizophora seedlings will generally not survive or even settle if exposed to direct sea action, while A. alba is a species often found at the coastline under the influence of tidal currents and wave action. The influence of the water on mangrove ecology is not only direct. Inundation influences a number of other environmental variables. Soil factors may be altered by flooding (Matthijs et al., 1999) and it may prevent salt accumulation in mangrove soils (Susilo and Ridd, 2005).

2.4 Rehabilitation and restoration of mangrove ecosystems As mentioned before (chapter 1.1), mangrove ecosystems are very vulnerable and large areas are lost in recent periods. However, the importance of the mangroves for coastal protection and the unique values of these ecosystems are becoming more widely recognized and therefore rehabilitation and restoration projects are carried out globally. The main objectives of these projects are: conservation of a natural system and landscaping, sustainable production of natural resources and protection of coastal areas (Barbier, 2006, Field, 1998). However, according to Ellison (2000) “the majority of projects, especially those in southeast Asia, continue to emphasize afforestation”. So, instead of focusing on restoring ecosystems, they are aimed at establishing plantations that can be exploited for fuelwood, charcoal and wood chips for rayon production (Ellison, 2000). These plantations can not be seen as successful rehabilitation of mangroves, since they do not restore biodiversity and the characteristics of the mangrove ecosystem. To achieve successful mangrove restoration Lewis and Marshall (1997, in Lewis, 2005) have identified five critical steps:

1. Understand the autecology (individual species ecology) of the mangrove species at the site, in particular the patterns of reproduction, propagule distribution and successful seedling establishment.

2. Understand the normal hydrologic patterns that control the distribution and successful establishment and growth of targeted mangrove species.

3. Assess the modifications of the previous mangrove environment that occurred that currently prevents natural secondary succession.

4. Design a restoration program to initially restore the appropriate hydrology and utilise natural volunteer mangrove propagule recruitment for plant establishment.

5. Only utilise actual planting of propagules, collected seedlings or cultivated seedlings after determining that natural recruitment will not provide the quantity of successfully established seedlings, rate of stabilisation, or rate of growth of saplings established as goals for the restoration project.

However, many rehabilitation and restoration projects do not take these steps into account and especially ignore hydrological characteristics of sites. Most important in the rehabilitation is adequate site selection and investigating the sites characteristics. The reason for the initial degradation of site has to be understood before rehabilitation (Field, 1998). Before planting of seedlings or propagules, the hydrology of the site has to be known and, if needed, restored. Turner and Lewis (1997) give several examples of projects in which hydrologic restoration of rehabilitation was successful in reversing negative effects of earlier changes.


11

This is also indicated by Brockmeyer et al. (1997), who observed a rapid recovery to more natural conditions of a wetland after restoring tidal exchange. Restoration of the normal tidal flooding regimeis especially important for rehabilitation of disused shrimp ponds, because tidal regime is usually blocked by dikes in these areas (Lewis et al., 2003). An example of inadequate site selection is given by Lewis (1999, in Lewis et al., 2003); large plantations of Rhizophora spp. on existing unvegetated natural mudflats resulted in failures and a waste of funds, since natural tidal conditions of the mudflat are too wet for these species to establish. Besides, it is arguable whether changing existing mudflats into plantations is desirable. Due to monospecific afforestation the ecological and social values of the intertidal mudflats are lost and it results in habitat conversion rather than restoration (Erftemeijer and Lewis, 1999). It might also be a loss of economical value, since mudflats may be used for cockle fishery, as in Can Gio. After adequate site selection and hydrologic restoration, planting might be needed. There are two different approaches for mangrove planting; natural regeneration and artificial regeneration. Natural regeneration makes use of naturally occurring propagules or seeds of mangroves as the source for regeneration, resulting in a mix of locally present species. When there is insufficient natural regeneration, artificial regeneration is needed. Seeds, propagules or seedlings can be planted directly on the site or first be raised under nursery conditions and then planted (Figure 2.4). (Field, 1998) The first choice for rehabilitation should be natural regeneration, according to Field (1998) and Lewis and Marshall (1997, in Lewis, 2005). Overall, the most important lesson learnt from failed rehabilitation and restoration projects is the importance of accurately determining site characteristics, especially hydrology, before starting with planting of mangrove trees.

Figure 2.4 Nursery of mangrove seedlings in Thailand (picture by Roel Dijksma).

2.5 Hydrological classifications The relation between tidal flooding and vegetation in mangrove forests is still relatively unknown. In 1928, Watson gave a first classification indicating five inundation classes and related mangrove species. De Haan (1931, in Chapman, 1976) proposed six different inundation classes based on the number of floodings per year, while he also examined salinity tolerances and requirements of species.

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The allocation of the vegetation in the inundation classes between these two classifications differed (Chapman, 1976). After these mangrove schemes based on tidal flooding, Davis (1940, in Knight et al., 2007) and Macnae (1966, in Knight et al., 2007) indicated zonation schemes within the mangrove forest. As a supplement to these zonation schemes Lugo and Snedaker (1974) divided the forest in five major community types. They indicated that “the formation and physiognomy of these types appear to be strongly controlled by local patterns of tides and terrestrial surface drainage and they are distinguishable on these bases”. The five major community types are: fringe forest, riverine forest, overwash forest, basin forest and dwarf forest (Lugo and Snedaker, 1974). This hydrogeomorphic classification takes the importance of surface hydrology and tidal dynamics into account. However, these hydrologic characteristics are not quantified as in Watson’s classification. The extended classification of Van Loon et al. (2007) is based on the classification of Watson (1928). Both these classifications will be discussed in more detail in the following chapters.

2.5.1 Watson hydrological classification In 1928 Watson made a classification based on the nature of the tide. He distinguished five different inundation classes. The limits of the classes are highly arbitrary and only valid for the area of Port Swettenham, Malaysia, where Watson carried out his research (Watson, 1928). The tidal regime in this area is extremely regular and elevation in the forest is gradually rising from the coast. Despite these limitations, discussed by Watson, the classification is still used in current research and forest management projects, since it is regarded as the best hydrological tool available. The classification developed by Watson is given in Table 2.1. After dividing the tidal regime in five inundation classes, Watson indicated which vegetation can develop in each class. The distribution of the different species over the inundation classes is based on the ability of the species to regenerate itself under the given conditions (Watson, 1928). So species can exist in adjacent inundation classes, but will not find optimum requirements to regenerate in that case. Table 2.1 Watson’s hydrological classification (Watson, 1928).

Inundation class

Tidal regime

flooded by

Elevation

above admiralty datum

Flooding frequency

times per month

Vegetation

species 1 2 3 4 5

all high tides

medium high tides normal high tides

spring high tides

equinoctial tides

below 244 cm 244 to 335 cm 335 to 396 cm

396 to 457 cm

457 cm and above

56 to 62 45 to 59 20 to 45

2 to 20

- to 2

none

Avicennia spp., Sonneratia Rhizophora spp., Ceriops,

Bruguiera Lumnitzera, Bruguiera, Acrostichum aureum Ceriops spp., Phoenix

paludosa

2.5.2 Disadvantages of the Watson classification During the research of Van Loon et al. (2007) several disadvantages of the classification of Watson were found. The hydrological classification of Watson is developed for an area with a regularly rising elevation profile and an extremely regular tidal regime. In most mangrove forests ridges and basin structures are found, thereby creating an irregular elevation profile. These ridges impede overland flow, so water has to be discharged through sub-creeks or the soil, which are longer flow paths than overland flow. This affects the wetness of the soil and the duration of inundation. Within the classification of Watson this is not taken into account (Van Loon et al., 2007). An important factor in the classification is the frequency of inundation. An irregular semi-diurnal tidal regime has varying tidal amplitudes. This causes large variations in the high and low water levels and leads to inundation frequencies that also vary over time. The inundation frequency and duration of inundation of a site is determined by its elevation and the tidal regime. This is illustrated for an irregular semi-diurnal tidal regime in Figure 2.5. A site with elevation “1” mostly experiences one

2.5 Hydrological classifications

13

long inundation per day, so its inundation frequency is low. Only the highest high water level can reach a site with elevation “3”, so this site also has a low inundation frequency. A site with elevation “2” is reached by all high waters, but also falls dry between the high waters at a semi-diurnal tide. So this site has a higher inundation frequency than site “1” and “3”. (Van Loon, 2007) Therefore the parameter “frequency of inundation” gives unrealistic results for an irregular tidal regime, as a site that stays inundated during a longer period (“1”) gets a higher (drier) inundation class than a site with more but shorter inundations (“2”). As mangrove forests often have an irregular elevation profile and an irregular tidal regime, the classification of Watson might not be applicable in all cases.

Figure 2.5 Tidal prediction for the port of Vung Tau for the period 28 April to 3 May 2004 with three imaginary

surface levels (1 = -50 cm +MVT, 2 = 25 cm +MVT, 3 = 75 cm +MVT) (Van Loon et al., 2007).

2.5.3 Extended hydrological classification Van Loon et al. (2007) developed an extended hydrological classification based on the classification of Watson to improve the applicability for regions with an irregular elevation profile and irregular tidal regime. This extended hydrological classification is displayed in Table 2.2. The parameter “tidal regime” is not changed in the extended classification, but should only be used for a rough comparison and when no other data of the hydrological conditions are available. The parameter “elevation” is referred to Mean Sea Level instead of admiralty datum, because this is more used in practical situations and it is less arbitrary. The parameter “flooding frequency” is unchanged. For both these parameters the limits of the classes are changed for a more realistic prediction (Van Loon et al., 2007). Since the parameters elevation and flooding frequency were not suitable for irregular elevation profiles and/or an irregular tidal regime, Van Loon et al. have added the parameter “duration of inundation” expressed in minutes per day as well as minutes per inundation. At sites in a regular elevation profile the parameter elevation can be used to classify the sites, otherwise the duration of inundation should be used.

2 T

heo

ry

14

Table 2.2 Extended hydrological classification (Van Loon et al., 2007).

Inundation class

Tidal regime

Elevation

cm +MSL

Flooding frequency times per

month

Duration of inundation

min per day


min per inundation

Vegetation

species 1 2 3 4 5

all high tides

medium high tides normal high tides spring high tides equinoctial tides

< 0

0 - 90 90 - 150 150 - 210

>210

56 - 62 45 - 56 20 - 45 2 - 20 < 2

> 800

400 - 800 100 - 400 10 - 100

< 10

>400

200 - 400 100 - 200 50 - 100

< 50

none

Avicennia spp., Sonneratia Rhizophora spp., Ceriops, Bruguiera

Lumnitzera, Bruguiera, Acrostichum aureum Ceriops spp., Phoenix paludosa

15

3 Site description The study areas for this research are situated in Ho Chi Minh City and Ca Mau provinces in southern Vietnam (Figure 3.1). The study area in Ho Chi Minh City province is located in the Saigon-Dong Nai river delta in Can Gio. Can Gio is a suburban, marine district 65 km southeast of Ho Chi Minh City. The Biosphere reserve in Can Gio district, which includes the study area, covers 76 000 ha of land and measures 35 km from north to south and 30 km from east to west. The coordinates are 10°22’-10°40’N and 106°46’-107°00’E. The second study area is located at the Ca Mau peninsula, the southern tip of Vietnam, which is bordered by the Gulf of Thailand on the west and the South China Sea on the east. The south western tip of the peninsula forms Mui Ca Mau National Park (Cape Ca Mau). Mui Ca Mau National Park has a total area of 42 000 ha, and is located at 8°32’-8°49’N, 104°40’-104°55’E in Ngoc Hien and Nam Can district. Both areas consist mainly of planted mangrove forest, but naturally regenerated parts are also present. In these parts the research sites were located. The climatic conditions in southern Vietnam are dominated by the seasonally reversing monsoon circulation, resulting in two prevailing winds; the dry north-easterly and the rainy south-westerly monsoon2. This results in a moist tropical climate with a dry season from approximately December to April and a rainy season from about May to November (Tong et al., 2004). The mean annual temperature at sea level is about 27°C with little annual variation and precipitation is high. March, April and May have the highest monthly average temperatures, December and January the lowest (Van Loon, 2005). Mean annual precipitation in Can Gio is approximately 1 336 mm, September has the highest rainfall amount of on average 300-400 mm (MAB Vietnam National Committee, 1998). Ca Mau receives on average approximately 2 200 mm per year, in 120-150 rainy days (Hong and San, 1993).

3.1 Can Gio

3.1.1 History Within the Can Gio district mangrove forests account for 53% of the total natural area, about 40 000 ha (Tri et al., 2000). During the Second Indochina war (1962-1971) Can Gio was heavily sprayed with herbicides and defoliants, killing almost all vegetation (Tri et al., 2000). The most used defoliant was Agent Orange. After the war some natural regeneration of mangrove species occurred, but this new vegetation was destroyed by local people that used the wood as fuel. From 1978 the government started investing in reforestation programmes. This reforestation consisted mainly of monoculture of R. apiculata, although the mangrove species Nypa fruticans, Ceriops and R. mucronata were also planted on smaller scale (Hong, 2001). According to Hong (2001) an area of 35 000 ha was replanted with mangrove trees by 1996 and “the mangrove flora is now fairly similar to that before the herbicide spraying, although the amounts and distribution are not the same”. The Can Gio district has about 58 000 inhabitants. The activities of these people form a big threat to the natural environment. They cut down trees for timber and fuel and use parts of the mangrove for shrimp cultivation. The demand for fuel and timber remains larger than the supply from thinning. The management of R. apiculata plantations in Can Gio includes one thinning after 6 to 7 years, a second thinning after 9 to 10 years and a third thinning after 15 years. The final felling is carried out if plantations are 20 years of age (Hong, 1996). To prevent destruction of the mangroves, parts of the forest have been allocated to households, which protect their part for 30 years. In return for protecting the allocated forest, they can use a small part of it for aquaculture or salt production (Tri et al., 2000). Besides the households, forestry experts and rangers also protect the forest.

2 The term monsoon is used in literature both for the circulation of surface winds in tropical regions and for the prevailing wind which lasts for several months, thereby determining the climatic conditions in a season.

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Figure 3.1 Location of Can Gio and Ca Mau; zooming in from the world to southeast Asia and to southern Vietnam (Center for Sustainability and the Global Environment, 2007).

3.1 Can Gio

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3.1.2 Can Gio Man-and-the-Biosphere reserve In 2000 the MAB/UNESCO Committee appointed Can Gio as the first International Man-and-the-Biosphere Reserve in Vietnam. Authorities in charge of the reserve are the management board for protected forests from the Department of Agriculture and Rural Development, Ho Chi Minh City and the Peoples Committee of Can Gio district (Tri et al., 2000). The Can Gio reserve is divided in a core area, buffer zone and transition area. The core area consists of the forestry units 4b, 6, 11, 12 and 13 (dark green areas in Figure 3.2), and measures 4 700 ha (Tri et al., 2000). Management in this area is aimed at preserving the ecosystem and species diversity (Van Loon, 2005). There are some villages, with a population of around 300 people, but inhabitants are only allowed to carry out fishery activities and selective timber cutting at a sustainable level (Tri et al., 2000).The buffer zone comprises the other 18 forestry units (lighter green areas in Figure 3.2) and includes about 37 000 ha land as well as 3 800 ha marine environment (Tri et al., 2000). Within this zone a moderate level of habitation and economic development is allowed. Important activities are sustainable exploitation, scientific research and tourism (Van Loon, 2005). Ecotourism is becoming increasingly important in Can Gio since this generates income for local people. The remaining part of the biosphere reserve is transition area, which holds 29 000 ha land and 600 ha marine environment (yellow areas in Figure 3.2). Within this zone some land has been converted to agricultural land and the main crops produced are rice, coconut and pineapple. However productivity is low due to irrigation problems and salt intrusion. Urban areas, abandoned land and roads are also part of the transition area. (Tri et al., 2000)

3.1.3 Tidal regime The Can Gio area has an irregular semi-diurnal tidal regime, so most of the time high and low tide situations occur twice a day, except for some periods when only one high and low tide in 24 hours occur. The amplitude of the tidal regime is high: 3.3 to 4.1 m. In October and November maximum high tide water level is reached, in May the minimum. (MAB Vietnam National Committee, 1998)

3.1.4 Hydrology The mangrove ecosystem in Can Gio is not only influenced by the tides, but also by Dong Nai river. The main channel of this river has a length of 628 km and has several important tributaries, including the Saigon river (Ringler et al., 2002). The Dong Nai river flows from Cambodia through Vietnam to the South China Sea and its annual average discharge varies between 970 m3/s and 1 600 m3/s (Van Loon, 2005). The tropical climate causes a large variation between discharge in the dry season and wet season. During the rainy season, the river basin receives on average 87% of the total annual precipitation (Ringler et al., 2002). The maximum monthly discharge is 3 890 million m3 and the minimum 145 million m3 (Van Loon, 2005). Besides this difference, there are also large variations in flow between different years. The Dong Nai river basin includes several hydropower projects and reservoirs. It is the second largest river basin in hydropower potential in Vietnam. Although three major reservoirs are constructed in the basin, the large variation in flow between the seasons still exists. However, the reservoirs can prevent water shortages in the catchment during the dry season (Ringler et al., 2006). The increase in water flow in the dry season has decreased the salinity in the Can Gio area. Salinity in the mangrove forest is highly dependent on the seasons. During the rainy season salinity is only 4 to 8 ppt, while during dry months salinity can increase up to 19 to 20 ppt in the north and 26 to 30 ppt near the sea. The average monthly salinity is 18 ppt (Hong, 1996).

3.1.5 Topography Since Can Gio is part of the Saigon and Dong Nai river delta it has a low-lying, relatively flat and dynamic topography (Van Loon, 2005). The highest elevation found in the area is 10 m above sea level (MAB Vietnam National Committee, 1998), but almost the entire area has an elevation between 0 and 2 m above sea level (Tri et al., 2000). The area consists of a system of unstable alluvial islands with a dense network of rivers, channels, creeks and gullies. The fresh sediment beds are eroded by the swift river currents and wave action (Van Loon, 2005).

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Four main soil types in Can Gio are: saline mangrove soil, acid sulphate soil with a pH from 4.5 to 6.5, marine sandy soil and sand dune soil. All these soils were developed from young marine and fluviatile deposits in the Quaternary period (Chien et al., 2003), in which the Holocene was probably the main contributing period.

Figure 3.2 Map of the Can Gio Biosphere reserve (Centre for tropical marine ecology, 2007).

3.1.6 Vegetation Since most of the mangrove area in Can Gio has been replanted after the war, the area is dominated by R. apiculata. However, natural vegetation has also regenerated, mostly along rivers and creeks. These natural zones have a high biodiversity compared with the planted areas and main species are Avicennia spp., Sonneratia alba, Xylocarpus granatum, Kandelia candel, Ceriops spp., Xylocarpus moluccensis, Rhizophora spp., Lumnitzera littorea, Phoenix paludosa, Excoecaria agallocha and Acrosticum aureum (Van Loon, 2005). Hong (2001) has found 72 flora species, 30 of which are true mangroves and 42 are associate mangrove species, during his research in Can Gio.

3.2 Ca Mau

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3.2 Ca Mau

3.2.1 History It is estimated that before 1943 mangrove covered about 150 000 ha in the entire Ca Mau province (Maurand 1943, Moquillon 1950, in Tong et al., 2004). Like in Can Gio, the area was sprayed with herbicides and defoliants during the second Indochina war. According to Hong and San (1993) the tip of Ca Mau peninsula was sprayed heavily between 1966 and 1970, leading to an irreversible destruction of 52% of dense mangroves (45 000 ha) in the current study area. Of these mangroves 80% was natural Rhizophora forest. After the war, in 1975, natural regeneration and planting programs led to partial recovery of the mangrove vegetation. Natural regeneration at the tip of Ca Mau peninsula was very slow however, due to large quantities of dead trunks and wood damaging young seedlings or even hampering propagules from reaching favourable sites. This phenomenon was still one of the reasons for failure of natural regeneration 10 years after spraying (Hong and San, 1993). Extensive replanting was done mainly with monocultures of R. apiculata (Clough et al., 2002). Despite reforestation efforts, population pressure and conversion of mangrove forest to agriculture land, shrimp farms and fish ponds hampered the rehabilitation of mangroves. Relatively to other areas in Vietnam, the Ca Mau peninsula still had extensive areas of mangrove forest, which attracted people from other provinces to make a profit of these natural resources. In the early 1980’s the Vietnamese government encouraged shrimp farming for export, because over-fishing in coastal waters had led to a rapid decline of shrimp capture. Especially in the western provinces of southern Vietnam shrimp farming became a wide-spread activity (Hong and San, 1993). Shrimp farming appeared to be highly profitable leading to both expansion of existing farms and establishment of new farms, partly by migrants from other provinces. In the period 1983-1992 the population in Ngoc Hien district, where the main part of Mui Ca Mau National Park is situated, nearly doubled due to unauthorized influx of people from other provinces (Hong and San, 1993). Both overexploitation of mangrove resources due to population growth and expansions of shrimp farming contributed largely to the loss of mangrove forests in Ca Mau. Shrimp farming caused major changes in drainage patterns and tidal flooding frequency of the area (Tong et al., 2004). In the short period November 1987-July 1988, the area of mangroves in Ngoc Hien district decreased by 13 992 ha. In an attempt to increase mangrove area, mixed farming systems, where levees within the ponds are vegetated by mangroves, arose the last decade (Populus et al., 2003).

3.2.2 Mui Ca Mau National Park In 2003 Mui Ca Mau National Park was established by merging Dat Mui Nature Reserve, Bai Boi Coastal Protection Forest and some adjacent natural mangroves. Dat Mui and Bai Boi reserves consisted respectively of the southern and northern part of the current area of Mui Ca Mau National Park (Figure 3.3). Dat Mui Nature Reserve was already established in 1983, while Bai Boi Coastal Protection Forest was only set up just before the establishment of the national park. Mui Ca Mau National Park is, like the Can Gio Biosphere Reserve, divided in 3 management zones: core zone, buffer zone and transition zone (National Political Publishing House Vietnam, 2006). The core zone consists mainly of regions with strict protection, both forest land and coastal surface water. Hardly any human activity is allowed in this zone, except for forest management and scientific research. No people live in this zone and any form of aquaculture is prohibited, although fishing in natural open waters is allowed (Nam, 2007). The core zone’s function is to protect natural processes and ecology of mangrove forests, provide living environment for water birds and aquatic species, protect coastal areas and minimize natural calamities (National Political Publishing House Vietnam, 2006). The buffer zone forms an area with both possibilities for settlement and forest development. In this zone no aqua- or agriculture is allowed, but local people can live within it and use natural resources in a sustainable way. The transition zone forms the landward edge of the national park and is the main area of settlement and aquacultural activities. It borders the land not belonging to the national park.

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Figure 3.3 Map of the southern part of Ca Mau province. Dat Mui and Bai Bo, parts of Mui Ca Mau National

Park, are indicated (Asian Development Bank, 2007).

3.2.3 Tidal regime Although Mui Ca Mau National Park is situated at the south western tip of the Ca Mau peninsula, its tidal regime is determined by both the Gulf of Thailand and the South China Sea due to Cua Lon river. This river bisects the southern part of the peninsula from east to west, thereby connecting both seas. In the South China Sea the tidal regime is irregular semi-diurnal with an amplitude of 2.5-3.8 m. In the Gulf of Thailand a diurnal tidal regime predominates with relatively small amplitudes; 0.5-1.0 m (Nguyen et al., 2000). The combination of both tidal regimes and the extensive intertwined creek system in Ca Mau peninsula causes complex water interactions that are not fully understood (Tong et al., 2004). It results in an irregular semi-diurnal regime with amplitudes of 0.8-1.5 m at Cape Ca Mau where Cua Lon river discharges in the Gulf of Thailand.

3.2.4 Hydrology The Ca Mau peninsula is situated in the Mekong delta, but its hydrological conditions are not directly influenced by the Mekong River (Hong and San, 1993). The water in channels and rivers is saline; no fresh water is present in the area. Mean salinity is 22-26 ppt, which is a favourable range for many mangrove species. Salinity is rather constant throughout the year, since rainfall in the rainy season mixes with abundant seawater or it takes up accumulated salt from tree canopy (Hong and San, 1993, Populus et al., 2003).

3.2.5 Topography The geomorphology of Ca Mau peninsula is highly determined by the Mekong river. In rainy periods large volumes of sediment are transported by the Mekong river to the South China Sea. Coastal currents carry the sediments south-westwards, resulting in formation of sandy beach ridges at the south eastern coastal plain and deposits of finer sediment at the southwest coast of Ca Mau peninsula.

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Resulting landforms are beach-ridge and spit systems3 around the present channels as well as a deltaic margin in the southwest (Nguyen et al., 2005). At Mui Ca Mau the mudflats are extending rapidly westwards; extension rates up to 80 m/y are reported by Hong and San (1993). The east side of the peninsula however is subjected to considerable erosion. Topography at Ca Mau peninsula is relatively even and low-lying, with most of the land lying within the intertidal zone between about +1 m and -1 m with regard to MSL (Clough et al., 2002). Soil texture is for 95% of the soils clayey or loamy (Tong et al., 2004), but reports about acidity are conflicting. Tong et al.(2004) state that acid sulphate soils are uncommon, while Hoanh et al. (2006) report deep acid soils in the southern part of Ca Mau peninsula. According to Van Mensvoort (2007) potential acid sulphate soils were present, but drainage after construction of channels and shrimp ponds revealed a minor pyrite layer only. Shells, lime and saline water that were present created a sufficient buffer capacity to prevent large scale and severe acidity.

3.2.6 Vegetation The mangrove forests of Mui Ca Mau National Park are dominated by Avicennia spp. and Rhizophora spp., but Ca Mau peninsula has an abundant supply of propagules and seedlings of many mangrove species. Due to an extensive network of canals, good conditions for dispersion of seeds and propagules and the tropical monsoon climate, the area is highly suitable for mangrove development. Mangroves in the Ca Mau peninsula are the best in Vietnam in terms of number of species and tree sizes. At the eastern coast large stands of A. marina and A. officinalis are found, although erosion is diminishing the area covered by these forests. On newly accreted land with a substrate of deep, soft mud and affected by low-tide, a pure and pioneer population of A. alba is found growing along the coast and river banks. Also mixed communities of R. apiculata-B. parviflora and A. alba-R. apiculata occur. (Hong and San, 1993)

3 Spit: A long narrow accumulation of sediment lying generally in line with the coast, with one end attached to the land and the other projecting into the sea or across the mouth of an estuary (Voigt, 1998).

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4 Methodology

4.1 Site selection Within both study areas two plots were selected to investigate water levels, topography and vegetation in an area stretching from open water into the mangrove forest. Within these plots, sites were selected to conduct discharge and permeability measurements. The actual research locations are referred to in this report as ‘plots’, while for Can Gio and Ca Mau the indication ‘study areas’ is used.

4.1.1 Selection criteria of the measurement plots This study is a follow up of earlier research in Can Gio. Consequently one of the objectives was to verify the results and to extend the data series of that research. Therefore one measurement plot was set up that was consistent with the main transect, transect A, of the research of Van Loon et al. (2007). Setting up a measuring network at exactly the same location however, was believed to be less profitable than selecting a closely related measuring network, both with respect to distance and properties. Measurements at a new location could either reveal new insights, or turn out to be a verification of earlier observations. Since the latter was believed to be the case, newly gathered data was expected to support observations by Van Loon et al. (2007), thereby strengthening the proposition of an extended hydrological classification.

Since the objective of this research is to study processes in natural mangrove systems, potential research plots are restricted to places without recent human influences. This means that large parts of the study areas are not suitable because agri- and aquaculture are abundantly present, or have been present recently. Large parts of the mangrove forest in Mui Ca Mau National Park are regenerated after they have been used as shrimp farms. Remaining dikes and canals of these farms still play a significant role in distribution of water and vegetation. Besides this, constructions like roads can disturb hydrological conditions and consequently mangrove vegetation at a site. Human influence (mainly reforestation) in mangrove forests in Can Gio and Ca Mau was largest from about 1972 (just after the Second Indochina war) until 1980-1990. No recent activities took place in the study areas, but the human influence is still highly significant due to plantation of mainly monocultures of R. apiculata. Within Can Gio and Ca Mau locations are preferred where planted forest is not dominating. The criterion holds that natural regeneration of R. apiculata and other species should be present. To investigate tidal characteristics in a plot and their influence on vegetation, measuring plots are preferably situated at locations with variation in vegetation and topography. Variations in tidal characteristics in mangrove forest are mainly caused by (micro-)topography, which consequently is expected to influence vegetation occurrence. The variation within a plot itself should be representative for the entire study area. From a practical point of view the selected transects should be relatively easily accessible, although human presence should be limited, primarily to avoid any damage to or theft of installed equipment in the forest.

4.1.2 Locations of the measurement plots Locations of the plots within Can Gio and Ca Mau were selected based on the criteria described above. In Figure 4.1 the locations of the plots in Can Gio (plot A and B) are displayed and in Figure 4.2 the locations in Ca Mau (plot C and D). Firstly a rough selection was made by studying satellite images and aerial photographs which were obtained from earlier research (Van Loon, 2005) and online databases (Landsat.org, 2007). While studying these images, additional information was obtained by personal communication with Dr. V.N. Nam and local staff of the Forestry Service in Can Gio and Mui Ca Mau National Park in Ca Mau. After this, the suitability of some selected locations was investigated in the field in cooperation with local experts. Consistent with the research by Van Loon et al. (2007) plot A was located close to the sea, alongside Dong Tranh river (Figure 4.1). The plot is situated in compartment number 17 in Figure 3.2. The site has a high sedimentation rate and consequently a broad natural vegetation zone, although most of the inland part consists of R. apiculata plantation. The natural vegetation zone consists of a

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wide fringe of A. alba, further inland blending into a transition zone of predominately A. alba, A. officinalis, C. tagal, R. mucronata and R. apiculata. Creeks in the study area are expected to cause serious side-effects on tidal characteristics. To avoid this, the measuring plot had to be located at a satisfactory distance from these creeks. The new plot is chosen somewhat north of the transect along which Van Loon (2005) carried out water level and elevation measurements, although in this research an elevation measurement is done along the same transect as well (Figure 4.8 and Figure 5.7). The relative distance chosen is small enough to be able to extend earlier data series with reasonable reliability, but large enough to extend knowledge on spatial variation of topography and vegetation at this location. Plot B is also situated in Can Gio, somewhat further inside the creek system relatively to plot A. The site consists of a dendritical creek system along a main channel. Within this system levees and gullies are present, which account for variation in topography. Vegetation is dominated by planted R. apiculata, although other species are present along the creeks. Although variations are small, the plot is expected to be useful for verification of the extended hydrological classification of Van Loon et al. (2007). Plot selection in Ca Mau was hampered by several reasons. Purely naturally generated mangroves without noticeable former human activity are scarce in Mui Ca Mau National Park. On many sites hydrological conditions or vegetation are largely influenced by illegal cutting activities and management acts to prevent this, like damming of entrance creeks. Still local people often go into the forest in search of food and other articles of use. For security of the research equipment installed in the forest, both plots C and D were set up close to national park rangers’ posts. Plot C was located at a natural regenerated forest site of mainly A. alba and R. apiculata, formerly influenced by human activity (Nam, 2007). The extent of the former human activities is unknown. The site is situated at the mouth of Rang Ong Linh river, where extensive tidal flats form the transition between mangrove forest and the Gulf of Thailand. It is close to the national park rangers’ post Cai Bat (Figure 4.2 and Figure 4.10). In the middle of the mouth of Cua Lon river, near the Gulf of Thailand, two small islands with completely natural generated vegetation are situated (Nam, 2007). The islands are relatively recently formed by accretion of land; on satellite images of 1990 the islands were not present yet. Plot D was located at the western of the two, Con Ngoai (Figure 4.2 and Figure 4.11).

Figure 4.1 Locations of the measuring plots in Can Gio (Landsat.org, 2007).

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Figure 4.2 Locations of the measuring plots in Ca Mau (Google Earth, 2007).

4.2 Tidal predictions The Marine Hydrometeorological Center (MHC) is part of the Hydrometeorological Service of Vietnam and provides data on climate, weather and tides. At 7 gauge stations, hourly sea level measurements are carried out and tidal predictions are calculated. The most extensive sea level record is measured from 1918 onwards at Vung Tau meteorological station (Chuong, 2006). With these data, tides are predicted for the port of Vung Tau (10°20’N, 107°04’E, Figure 4.3). In the predictions, water levels are given for every hour in m above the bottom of Vung Tau port with an accuracy of 0.1 m. The distance between Vung Tau and Can Gio is approximately 10 km, but the Vung Tau tidal predictions can very well be used for the calculation of a reference level (Van Loon, 2005). This is necessary since the height of the bottom of the port of Vung Tau with respect to Mean Sea Level is not known. From the available predictions of 2004 and 2007 the average predicted level was calculated and used as a reference for the water level and elevation measurements in Can Gio. This average level, referred to as Mean Vung Tau (MVT), is regarded as Mean Sea Level (MSL) for our study in Can Gio. For the study area in Ca Mau predictions for Dinh An and the port of Ha Tien (Figure 4.3) were investigated, for the period January to June 2007 and the entire year 2007 respectively. Dinh An is located at the east coast of the Ca Mau peninsula (9°32’N, 106°22’E), where semi-diurnal tides with high amplitudes dominate. Ha Tien is located at the northwest coast of the Ca Mau peninsula, close to the Cambodian border (10°22’N, 104°28’E). Tides at this location are fully diurnal, with very small amplitudes.

4.3 Meteorological data Rainfall can have a significant influence on water level measurements during low tide. To be able to determine whether inundation events were caused by rain or tidal movement, daily rainfall data for the period 1 March to 22 May 2007 were obtained via the Southern Regional Hydrometeorological Center (SRHMC) in Ho Chi Minh City. Data were available for the station of Can Gio (10°38’N, 106°39’E, Figure 4.3) and for the station of Nam Can (8°46’N, 105°4’E, Figure 4.3), close to the study area in Ca Mau. Besides, rainfall events were recorded in the study area, but no rainfall amounts could be measured. This is described as a “binary rainfall analysis” by Van Loon (2005). Atmospheric pressure and temperature measurements were carried out by means of a BARO-diver (section 4.4.1).

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Figure 4.3 Locations of the tidal stations, meteorological stations and study areas (Google Maps, 2007).

4.4 Water level

4.4.1 Divers Water levels in the mangrove were measured with divers (Figure 4.4). A diver is specially designed for automatic measuring and recording of water levels. Divers consist of a pressure sensor, temperature sensor, data logger and battery (Van Essen Instruments BV, 2006).

Figure 4.4 A standard diver (Van Walt Limited, 2007). A diver measures the total pressure, which consists of air pressure (p air) and water pressure (p water) in case of immersion (Figure 4.7). A BARO-diver only measures the air pressure, which is used to compensate data obtained by a diver. To calculate the water pressure, air pressure is subtracted from the measured total pressure of the divers. From the water pressure, the water level above the diver sensor can be calculated. Divers also measure the water temperature for correction of the pressure. They have an accuracy of 0.1% for pressure measurements and 0.1ºC for temperature measurements (Van Essen Instruments BV, 2006). During the research 2 BARO-divers and 15 standard divers were used. The measuring interval of the divers was 5 minutes in order to obtain accurate data of the tidal movements. Standard divers are designed for use in fresh water only, because of corrosion when used in brackish and salt water. To prevent damage to the pressure sensors, each diver was covered with a small plastic bag filled with fresh water. This may have slightly affected the measured temperature, but laboratory tests showed that pressure measurements are not influenced by this technique (Dijksma, 2007). To create a stilling well, the divers were placed in a PVC-tube of 2 metres length (Figure 4.5 and Figure 4.6). A stilling well allows water to flow in and out freely, but small oscillations in water level are avoided. Some holes were made in the tubes to enable interaction with surface or groundwater.

Ca Mau Nam Can weather station

Ha Tien

Dinh An

Vung Tau

Can Gio Can Gio weather station

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According to Van Loon (2005) the optimum amount of holes above the surface, to prevent oscillations and follow only the tidal movement, is two holes of 6 mm. These holes were placed just above surface level and a cloth was placed around them to prevent sediment from flowing in. In the part of the PVC-tube that was placed in the upper 30 cm of the soil, many holes were made, because it was expected that most of the groundwater flow would occur in this zone. In the remaining part below the surface (approximately 70 cm) a limited amount of holes was made. Due to this construction the PVC-tubes served as both stilling wells and piezometers.

Figure 4.5 Piezometer in the forest at flood tide.

4.4.2 Analysis A diver measures the total pressure above its sensor. Water height that contributes to this pressure is given by E in Figure 4.7. To calculate the height of the water level above surface (F in Figure 4.7) or above a reference level (G in Figure 4.7) some additional measurements are necessary. The height of the top of the tube (TT) above surface (A in Figure 4.6) and the depth of the diver sensor below TT (B in Figure 4.6) have to be known. The water level above the surface is determined using equation 4.1 (Van Loon, 2005): )( ABEF −−= (4.1) where all variables are in [L] The water level above reference level (G) is calculated with equation 4.2 (Van Loon, 2005): CEG −= (4.2) where all variables are in [L] and in which: C : depth of diver under reference level [L] The depth of the diver under reference level (C in Figure 4.6) has to be determined before equation 4.2 can be used. To be able to calculate C it is assumed that the water at high tide in the mangrove forest is a horizontal plane over all divers in the plot. Water level data were plotted for each diver and these graphs were visually compared. Each tidal peak in the graphs of the different divers was fitted to be of

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equal height for every diver. The depth of the diver sensor under reference level was determined from these graphs. The computer program Logger Data Manager (LDM) (Van Essen Instruments BV, 2006) was used to obtain the data from the logger in the divers. The calculations and analysis of the data were done using the computer program MATLAB (The MathWorks, 2005). With this program the data from each diver was converted to water level above the surface and water level above reference level Mean Sea Level (MSL). From the water levels above surface and MSL the different inundation characteristics, that are needed for the Watson and the extended classification proposed by Van Loon et al. (2007) (chapter 2.5) can be calculated. The exact method for the determination of these characteristics is given in chapter 5.4.1.3. Manual measurements of the water level in the PVC-tubes were done with a dipper to verify the diver data. In each tube several measurements were done and the average difference between the manually and automatically measured water level was used as a correction for the diver.

Figure 4.6 Diver in a stilling well (Van Loon, 2005).

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Figure 4.7 Water level measurement by a diver (Van Loon, 2005).

4.4.3 Piezometer locations The data from the divers are used to investigate hydrological characteristics in mangrove forests. Therefore the specific location for each piezometer within the plots was based on visual observations of water flow and topography. Since no clear vegetation zones were present in plot B, C and D, the locations of the piezometers there did not depend on the vegetation. An overview of all piezometer locations can be found in Table 4.1 and the coordinates are given in appendix B. The piezometers in plot A were placed at different distances from the main channel, but not in a completely straight line (Figure 4.8). One piezometer (A0) was attached to a fisherman’s house on stilts in Dong Tranh river to measure complete tidal cycles. In plot B measurements were not done along a transect, but divided over the plot based on topography (Figure 4.9, no suitable picture was available from Google Earth (2007) due to the small distances). The first piezometer, B1, was placed on a levee that surrounded the plot. Piezometer B2 was situated in the lower lying part in the middle of the plot and piezometer B3 stood next to a very small creek entering the plot. The BARO-diver for plot A and B was placed between these plots to be valid for both of them. In Ca Mau the piezometers were placed along transects again, although these transects were not completely straight lines (Figure 4.10 and Figure 4.11). In plot C piezometer C0 was attached to the landing-stage of rangers’ post Cai Bat such that it was submerged continuously. Therefore it could be used for measuring complete tidal cycles. Because the tidal regime in Ca Mau is more complex than in Can Gio (Hong and San, 1993), the measured tides at C0 were thought not to be valid for the entire study area in Ca Mau. Besides, no tidal predictions of the exact location were available. Therefore, piezometer D0 was placed in the open water near transect D to investigate the tidal cycle. The piezometers at plot D were installed at Con Ngoai at different distances from the main channel (Figure 4.11). Piezometer D3 was only used in plot D from 21 to 24 April, after this period the piezometer was moved to site C3. In Ca Mau the BARO-diver was placed near plot C, but its measurements are assumed to be valid for plot D as well.

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Table 4.1 Piezometer locations, distances to main channel and measuring period.

1 Distance between piezometer A3 and A4 is 80 m. 2 Distance between B2 and B3 is 25 m. 3 Piezometer C2 was moved on 28 April. 4 Distance from southern border. 5 Distance from northern border.

Figure 4.8 Locations of the piezometers in plot A (Google Earth, 2007).

Piezometer Location Distance to main channel (m) Measuring period

A0 at fisherman’s house in Dong Tranh river 3 March – 25 May 2007 A1 Avicennia zone 18 3 March – 25 May 2007 A2 transition zone 205 17 March – 14 April 2007

A3 transition zone 3301 3 March – 25 May 2007 A4 transition zone 330 17 March – 14 April 2007

A5 transition zone 450 3 March – 25 May 2007 A6 Rhizophora plantation 680 3 March – 25 May 2007 B1 on levee - 6 March – 26 May 2007 B2 in basin 20 m (from B1) 6 March – 26 May 2007 B3 at small creek 27 m (from B1)2 6 March – 26 May 2007 C0 at landing stage in Rang Ong Linh river 20 April – 20 May 2007 C1 mixed zone 68 20 April – 20 May 2007 C2a mixed zone 175 20 April – 28 April 20073

C2b mixed zone 190 28 April – 20 May 2007 C3 mixed zone 350 24 April – 20 April 2007

D0 at landing stage in Cua Lon river 21 April – 20 May 2007 D1 mixed zone 504 21 April – 20 May 2007 D2 mixed zone 2554 21 April – 20 May 2007 D3 mixed zone 1805 21 April – 24 April 2007

4.4 Water level

31

Figure 4.9 Locations of the piezometer in plot B (after: Garmin, 1999).

Figure 4.10 Locations of the piezometers in plot C (Google Earth, 2007).

4 Methodology

32

Figure 4.11 Locations of the piezometers in plot D (Google Earth, 2007).

4.5 Elevation The topography of the land strongly influences the characteristics needed for hydrological classification of sites, like inundation frequency and duration of inundation. Therefore, getting insight in the topography of the mangrove forest was crucial for our research. Next to this, an overview of elevation profiles at different locations was needed for investigating the necessity of a classification that is developed for irregular elevation profiles. Relative elevation was measured using laser levelling and the obtained data were combined with water level data in order to get a reference level.

4.5.1 Laser levelling With laser levelling the topography between two points can be measured. The method is explained by Van Loon (2005) and is shown in Figure 4.12. The used equipment is shown in Figure 4.13. The laser was placed on a tripod and had to be levelled. The laser beam was projected on a tree (2) in the desired direction. The height of the laser point on the tree above the soil surface (2a) and the distance from the starting point to the tree (D2) were measured with a measuring tape. Then the equipment was moved to the other side of the tree, levelled and projected on the same tree (2). The height of the laser point above the surface (2b) was measured again. The laser was then turned and projected on the next tree (3). The same measurements (3a and D3) were done again at this tree. In this way the topography along a transect can be measured in sections of about 20 m. One of the errors of this method is the neglect of the possible elevation difference under a tree. To prevent this, the height of the first laser point was also measured on the other side of the tree, so the elevation difference between both sides of the tree is measured. Since no spots with known elevation were present, the elevation was referred to MSL by use of the water level measurements. The error in accuracy of the laser levelling is estimated at about 10% by Van Loon (2005), due to the local circumstances of unstable soils, dense vegetation and irregular soil surface.

4.5 Elevation

33

Figure 4.12 Concept of the laser levelling method (Van Loon, 2005).

Figure 4.13 Photograph of the laser levelling equipment on a tripod.

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34

4.5.2 Combination of laser levelling and water level data From the diver data the depth of the diver below a fixed reference level (C in Figure 4.6) was determined (section 4.4.2). The elevation of the soil surface above the reference level (D in Figure 4.6) can be calculated with the equation (Van Loon, 2005): CABD −−= (4.3) where: B : depth diver under TT [L] A : height of TT above surface [L] The results of the elevation at the piezometer location calculated from the water level data and the laser levelling gave different values. At high tide, when there is little flow, the water levels at the measuring sites with respect to the same reference level should be similar. However, differences in water levels at high tide, with respect to the surface levels found by laser levelling, between the piezometer locations were too high to be realistic. Because the error of the laser levelling is much larger than the error of the divers, the elevation from the diver data is assumed to be correct. However, only the elevation at the piezometer locations itself can be determined with this method. For this reason the methods were combined; the exact elevation of the piezometer locations was known and the profile measured with the laser levelling was fitted between them. This was possible in transects that included a piezometer, when this was not the case, only the laser levelling data were used.

4.5.3 Locations Because the elevation of the measurement sites can be calculated more accurately with the diver data than by laser levelling, most transects were situated over the piezometer locations to include at least one piezometer per transect, but this was not always possible. In plot A five transects perpendicular and two transects parallel to the main channel were measured. For the research the elevation in areas with significant micro-topography was most interesting. In plot A the Avicennia zone had a regular elevation profile, therefore only one perpendicular transect included the Avicennia zone. The other transects started in the transition zone. Van Loon (2005) also measured topography in the Avicennia zone. Laser levelling in plot B was used to find the differences in elevation between the piezometer locations and to get an overview of the topography in the plot. Four different transects were measured; three transects from one piezometer to another and one transect over B2 across the entire plot. One transect parallel and two transects perpendicular to the main channel were measured in plot C, in plot D one transect over the width of the entire island.

4.6 Vegetation In order to relate calculated inundation characteristics to vegetation patterns, vegetation analyses were carried out along -and in the vicinity of- the laser levelling transects. In this way a spatial investigation was done, with the possibility to locate vegetation zones precisely. The extent of vegetation patterns was measured, but no measurements at individual trees or plants were carried out. In Can Gio information about plantations and natural regeneration was obtained during field trips and conversations with staff of the Forestry Service. In Ca Mau this information was provided by Dr. V.N. Nam and staff of Mui Ca Mau National Park.

4.7 Creek flow

4.7.1 Locations and tidal regime Creek flow measurements were carried out to investigate the differences between in- and outflow in the small creeks during a tidal cycle. The measurements were also used to find a relation between the discharge and tidal regime. Locations for the discharge measurements were chosen in creeks close to the piezometers to investigate the flow pattern within the study areas. Because discharge depends on the tides, measuring

4.7 Creek flow

35

was necessary during diurnal as well as semi-diurnal components of the tidal cycle. From the tidal prediction of Vung Tau days with different tides were selected for discharge measurements in plot A and B. For several creeks within these plots, measurements were done at the same location for diurnal and semi-diurnal tides. Tidal predictions for the study area Ca Mau were not available, so it was not possible to select days with different tides. Therefore discharge was only measured during one day at each location in this area. Since the discharges were determined by a method based on the velocity-area method, measurements on cross-sectional area and flow velocity were necessary. Flow velocity was measured with a surface float. For measurements with a surface float a site has to comply with a couple of conditions. The flow has to be as uniform as possible over the measurement reach in the creek, so a straight reach with a uniform cross-section is needed. The water depth should be sufficient to provide effective immersion of the float and the flow direction in the reach is preferable perpendicular to the prevailing wind direction (Boiten, 2003). At each location a measurement reach as straight as possible was chosen.

4.7.2 Cross-section At each discharge measuring location, 2 or 3 cross-sections were measured. The distance between the cross-sections depended on the size of the creek; in larger creeks a larger distance was taken. To measure the creek profile, the total width was divided in several verticals with a variable interval between them (Figure 4.14). The width and shape of the profile determine the number of verticals that is sufficient to acquire an accurate cross-section. At steep parts of the creek profile the distance between verticals was kept smaller than in flatter parts, in order to obtain a reliable approximation of the profile. At each vertical the depth with regard to a chosen reference level was measured with a measuring tape. The total area of the cross-section is the summation of the areas per panel, which can be calculated with the following equation (Boiten, 2003):

bdd

A ∗+

=2

21 (4.4)

where: A : total area of the panel in [L2] d : depth measured at the vertical in [L] b : width between the verticals in [L]

Figure 4.14 Calculation of the cross sectional area A. Note that in this example a standard width is used for the

panels. (Adapted from: Boiten, 2003)

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4.7.3 Measurements To determine the discharge, flow velocity, flow direction and water level in the creeks were measured. The time interval between the measurements depended on the tide. Near the moment of transition from inflow to outflow or vice versa the frequency of measurements was higher. Flow direction was determined visually. For the velocity measurements a surface float was used. The time needed to travel a known distance in the creek was measured. With this method the velocity at the water surface was estimated. As some of the measured creeks were very small and had a relatively rough bed, the float was not always suitable. In these cases a small, round piece of fruit was used for the measurements. The water level was determined with a measuring tape at a fixed point in different cross-sections of each creek. Water level data was needed to calculate the area of the cross-section belonging to each velocity measurement.

4.7.4 Calculations The following equation is used to acquire the discharge from the measurements (Boiten, 2003): AvQ ∗= (4.5) where: Q : discharge in [L3T-1] v : average flow velocity in [LT-1] A : cross-sectional area in [L2] For the flow velocity in the cross-section a parabolic distribution in the vertical is assumed. The average velocity in a cross-section partly depends on the bed-roughness. Because velocity is measured only at the water surface the following assumption is used to approximate the average velocity: 09.0 vv ∗≈ (4.6)

where: 0v : flow velocity at water surface in [L/T]

4.8 Hydraulic conductivity Groundwater is expected to influence the development of mangroves. Both groundwater level and groundwater flow are assumed to be important factors. While investigating the relationship between inundation characteristics and vegetation, water logging is one of the main phenomena of interest. In cases of insufficient overland and creek discharge at ebb tide, water logging can occur due to micro- topography and flow resistance. The only discharge possibility in these cases might be groundwater flow, which thereby affects duration of inundation. Besides, for mangrove vegetation not only the inundation with surface water is relevant, but also the aeration of the root zone and consequently groundwater level (Van Loon et al., 2006). Groundwater flow is important for vegetation since it has been reported as a mechanism in removing accumulated salt from mangrove soils (Susilo and Ridd, 2005, Wolanski and Gardiner, 1981). In order to get an impression of the order of magnitude of groundwater flow, an estimation of the permeability of the soil should be obtained. In mangrove soils the permeability (or saturated hydraulic conductivity) largely depends on total pore volume, size of pores and the rate of interconnection of the pores. Total permeability (or bulk hydraulic conductivity) of a soil is divided in a primary or matrix permeability and a secondary permeability. Matrix permeability of a soil is the conductivity as a result of pores within the soils matrix, leading to intergranular flow. Secondary permeability is the conductivity caused by the presence of biopores, like crab burrows and root canals, which results in bypass flow.

4.8 Hydraulic conductivity

37

To determine effective permeability of the soil simple pumping and recovery tests were carried out. From the obtained data, transmissivity4 and consequently total permeability could be estimated as described by Van der Schaaf (1999). Although Van der Schaaf described the methods for determination of transmissivity in raised bogs in Ireland, they were assumed to be applicable in this research.

4.8.1 Description of the tests Square pits of approximately 10×10 cm and 30 cm deep were dug in the soil, mainly near the piezometer locations in the mangrove forest. These locations were chosen to measure at representative sites. After digging, the pits were left to allow re-establishment of the water level. At each pumping test, water was removed from the pit using a cup with a volume of 200 ml, every 10 to 60 seconds at a constant rate. So actually bailing tests were carried out, but to avoid confusion with the applied calculation methods, the tests are called pumping tests here. The constant “pumping” rate depended on expected transmissivity and drawdown in the pit. Drawdown is the lowering of the water level in the pit and this was measured with a ruler placed in the pit while pumping (Figure 4.15). Pumping was continued until no visible further drawdown occurred or pumping-time exceeded 15 minutes. After pumping had been stopped, recovery of the water level in the pit was measured at the same interval as pumping had been done. To analyze both the pumping and the recovery test, two sets of equations were necessary to calculate conductivity.

Figure 4.15 Photograph of the set up used to measure hydraulic conductivity.

4.8.2 Equations Complete derivations of the relevant equations for both tests were reported by Van der Schaaf (1999, 2004). Here, only a limited overview will be given of the underlying equations and assumptions.

4 Transmissivity: the integral of the hydraulic conductivities over the saturated depth of the aquifer (Rushton, 2003).

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38

The steady-state Thiem-equation was used as a basis for the pumping test. This equation is approximately valid if the following conditions are satisfied (Kruseman and de Ridder, 1990):

a) the lateral extent of the aquifer is much larger than the distance to which the phreatic level is noticeably affected by the drawdown in the well;

b) the aquifer properties do not show any variation over the area in which the phreatic level is influenced by the drawdown in the well;

c) the phreatic level was approximately horizontal immediately before the test; d) the discharge rate was constant during the test; e) the well fully penetrates the aquifer and thus flow is horizontal; f) the saturated depth of the aquifer is constant over the area in which the phreatic level is

noticeably affected by the drawdown in the well. Conditions a), c), and d) are normally satisfied. Condition b) is assumed to be satisfied for all locations, since the sites are expected to be properly chosen and the area of influence is expected to be small. While condition e) is probably not met in all cases, the assumption of horizontal flow seems reasonable in cases of a small drawdown in the well and if secondary permeability is of major influence in the upper part of the soil. A small drawdown can also secure that condition f) is approximately satisfied. (Van der Schaaf, 2004) Van der Schaaf (1999, 2004) adapted Thiem’s equation to make it applicable for semi-steady state situations, yielding:

w

w

y

r

rQ

Tπ2

ln 2

= (4.7)

where: T : transmissivity [L2T-1] Q : pumping discharge [L3T-1] r2 : distance to which the effect of pumping has extended [L] rw : well radius [L] yw : drawdown in the well [L] For square wells rw ≈ 0.6L, where L is the length of one side of the square pit. Since there are two unknowns in equation 4.7, namely T and r2, a second equation is needed to be able to solve it. Van der Schaaf (1999, 2004) reports the derivation of this equation, which is based on an estimation of the radius of the depression cone that can be calculated from the volume of water removed from the well. This volume is the product of pumping discharge Q and pumping time t. This yields:

( )

−−+=

w

wwww

n

nn

Q

yrt

ln2

1ln21

22 µπ (4.8)

where: t : pumping time [T] µ : storage coefficient [-] nw : ratio r2/rw [-]


39

From equation 4.8 the value of nw can be found either graphically by plotting it versus t for a known Q, µ and yw or by iteration. From the design of the tests t and Q are known and yw is calculated from the tests data. A value for µ is obtained from literature. De Klein et al. (2002) report a value for µ of 0.02 for very dense clay. This value is used in the calculations in this research. The applied recovery test is based on the so-called pit bailing method5. The basic equation has also been derived from Thiem’s equation. Van der Schaaf (1999) again describes the derivation yielding equation 4.9, for which change in nw during the recovery test is assumed to be relatively small:

w

w

r

ww

y

y

t

nrT 0

2

ln2

ln≈ (4.9)

where: tr : recovery time [T] yw0 : drawdown in the well at the beginning of the recovery test [L]

4.8.3 Aquifer thickness With this set of equations Van der Schaaf (1999, 2004) is putting forward a method to calculate transmissivity T. Since transmissivity T is a product of hydraulic conductivity and aquifer thickness, an estimation of the depth of the aquifer is needed to obtain a value of the hydraulic conductivity k. The upper part of the mangrove soil in which animal burrows, biopores and roots are abundantly present is assumed to be far more permeable than the underlying sediments. Several authors report the use of this assumption and an estimated depth of about 1 m (Mazda and Ikeda, 2006, Susilo and Ridd, 2005). This estimation is based on reported depths of animal burrows and mangrove roots. Therefore an aquifer thickness of 1 m is used in all calculations of k-values in this research. This thickness has also been used by Van Loon (2005) in her research in Can Gio.

5 N.B. Do not confuse with the pumping tests, which were actually bailing tests. The recovery tests in this research are based on the pit bailing method as described by Van der Schaaf, but called recovery tests from here onward.

4 Methodology

40

41

5 Results

5.1 Tidal regime

5.1.1 Can Gio The tidal regime in Can Gio was analyzed by investigation of tidal predictions of Vung Tau and by measurements of water levels in open water. The tidal predictions for the port of Vung Tau were expected to be representative for the Can Gio area, despite the distance between the sites.

5.1.1.1 Tidal predictions Vung Tau The prediction of the tidal regime at Vung Tau for the year 2007 is shown in appendix C1. In these tidal predictions the two-weekly (neap and spring tides) and yearly variations in tidal range, as discussed in chapter 2.2.1, are clearly observed. Figure 5.1 shows the same tidal prediction zoomed in at the fieldwork period from March to May 2007. Water levels are referred to Mean Vung Tau (MVT). MVT is the average predicted water level calculated from tidal prediction of two periods; 22 March to 31 July 2004 and the year 2007. The monthly cycle of neap and spring tides is displayed again. Figure 5.1b shows a neap tide period, with a lower than average tidal range around 3 April. It is followed by the transition to a spring tide period, which is at its maximum around 20 April (Figure 5.1c). Besides this, Figure 5.1 reveals specific features of the tide at Vung Tau. The two-weekly variation between diurnal and semi-diurnal tides can be observed clearly. In Figure 5.1b two moments of fully diurnal tides are present, from 27 March to 28 March and at 12 April, with only one high tide in 24 hours. Between these diurnal tides, there is a shift to a fully semi-diurnal tide which is reached around 4 April. Here two high tides and two low tides per day are present, with approximately the same tidal amplitude. In between the fully diurnal and fully semi-diurnal moments, the tides are dominantly semi-diurnal with a large difference in amplitudes within 24 hours; therefore the tidal regime at Vung Tau is irregular semi-diurnal. In May and June fully diurnal tides are completely absent, but tides with either two high tides and one low tide, or one high tide and two low tides occur over the entire year. The moments of diurnal tides are situated around a shift between neap and spring tide periods, while fully semi-diurnal tides often occur within 5 or 6 days after fully diurnal tides in March and April.

5.1.1.2 Open water measurements The tidal levels in Can Gio, measured by piezometer A0 in Dong Tranh river, follow the same pattern as the predicted water levels for Vung Tau (Figure 5.2). Since surface level at location A0 is -151 cm +MVT, tidal movements below this level could not be measured. The main differences between the graphs are around each maximum and minimum water level, but no consistent vertical shift between the measurements and the predicted water levels can be observed. This is supported by Nauta (1994) who states tidal amplitudes in Can Gio are between 0.9 and 1.1 times the tidal amplitude of the prediction of Vung Tau. Amplitudes of the predicted tides in the measuring period range from 2 m to 3.8 m. This range is larger than the range reported by MAB Vietnam National Committee (MAB Vietnam National Committee, 1998), who reports tidal ranges from 3.3 to 4.1 m. The correlation coefficient (R2) between the predicted and measured hourly data is calculated to be 0.943. This is in accordance with the value of 0.945 found between predicted and measured water level data of 2004 by Van Loon (2005). Mean and standard deviation for the predicted water level are 4.08 cm and 82.7 cm, for the measured data 4.4 cm and 83.4 cm. So the measured tidal regime in Can Gio resembles the tidal regime predicted for Vung Tau, both in pattern as in magnitude. However a difference in timing of the tides is present and clearly visible at the extremes in Figure 5.2. The average time lag between high tides at Vung Tau and Can Gio is in this research calculated at 35 minutes, which is the average propagation time of tides between the sites. Earlier research revealed time lags of 40 minutes (Van Loon, 2005) and 24 to 30 minutes (Nauta, 1994).

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Figure 5.1 Predicted water levels at the port of Vung Tau; a) 3 to 24 March 2007, b) 24 March to 14 April 2007,

c) 14 April to 5 May 2007, d) 5 May to 27 May 2007 (after: Marine Hydrometeorological Centre, Vietnam (unknown)).

5.1 Tidal regime

43

Figure 5.2 Comparison of predicted water levels for Vung Tau and measured water levels in Dong Tranh river

for the period 11 April to 18 April 2007.

5.1.2 Ca Mau Mui Ca Mau was selected as a study area, because the tidal regime was expected to be different from the tidal regime at Can Gio based on literature (Hong and San, 1993, Nguyen et al., 2000). Like for Can Gio the tidal regime was measured in the open water. Since tidal stations are situated far from Mui Ca Mau, it was not expected that the predictions of these stations would be representative. To test this hypothesis tidal predictions of the two closest stations, Ha Tien and Dinh An, are compared with the measurements of the tidal regime at Ca Mau. The tides at Ca Mau were measured at two sites; C0 and D0.

5.1.2.1 Tidal predictions Ha Tien and Dinh An The locations of the tidal stations of Ha Tien and Dinh An are described in chapter 4.2 and shown in Figure 4.3. The tidal predictions of these stations differ; at Ha Tien a diurnal tidal regime prevails while at Dinh An irregular semi-diurnal tides dominate. This can be seen in Figure 5.3, where the predicted water levels are plotted for the measuring period at Mui Ca Mau (21 April to 20 May). In this figure the predicted water level at Ha Tien and Dinh An are referred to their average level of 2007 and of the period January to June 2007 respectively. These tidal predictions have an accuracy of 10 cm, therefore the line is not smooth in the figure of Ha Tien. Tidal predictions for longer periods for Ha Tien and Dinh An are displayed in appendix C2 and C3. As shown in Figure 5.3a the tidal regime at Ha Tien is almost fully diurnal, with only one high and one low tide in 24 hours most of the time. In the plotted period only three non-diurnal days are present, but it should be mentioned that in total nine non-diurnal days occur in April. In May however only one non-diurnal day occurs. The amplitudes are fairly low compared to Can Gio, maximum 1.0 m in the measuring period. The two-weekly and yearly variation due to the lunar and solar cycles (chapter 2.2.1) can be seen as well. Since the average tidal range at Ha Tien is small compared to Vung Tau, the absolute variations within a cycle are less clear. Overall the variation in tides is small at all time-scales (day, month, year).

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Figure 5.3 Predicted water levels at a) Ha Tien and b) Dinh An, from 21 April to 20 May 2007. Water levels are referred to local mean sea level, here indicated as MHT (Mean Ha Tien) and MDA (Mean Dinh An). Note the graphs have different scales for the y-axis. (After: Marine Hydrometeorological Centre, Vietnam (unknown))

5.1 Tidal regime

45

At Dinh An (Figure 5.3b) the variation is much more pronounced. First of all the tidal regime is irregular semi-diurnal instead of diurnal, with amplitudes ranging from 2 to 3.9 m in the measuring period. This range is somewhat larger than reported by Nguyen et al. (2000), who report amplitudes between 2.5 and 3.8 m. For the measuring period only 4 days are predicted without semi-diurnal conditions. In the tidal predictions in Figure 5.3b, variations due to the lunar and the solar cycle are also observed. The alteration between spring and neap tide periods is clearly seen, with large tidal ranges at the beginning and the end of the graph and a period with smaller tidal ranges in the middle part. It is also observed that predicted tidal ranges at Dinh An are larger than average around 2 January and lower than average towards 2 July (Appendix C3). The tidal regime is highly comparable to that at Vung Tau. The correlation coefficient of the predicted water levels of January to June 2007 of the two stations is calculated to be 0.875.

5.1.2.2 Open water measurements at C0 and D0 Open water measurements at sites C0 and D0 are shown in Figure 5.4. The locations of the measuring sites are described in chapter 4.4.3. The distance between the locations is approximately 8 km. The reference height for all measured water levels is the average water level in the open water over the measuring period. This reference height is called Mean Cai Bat (MCB) for plot C and Mean Con Ngoai (MCN) for plot D, after the locations names. It is assumed that these reference heights are comparable to local Mean Sea Level, therefore the indication MSL is used in Figure 5.4. Measuring site D0 falls dry every now and then, since it was installed at the side of a river. C0 did not fall dry, because it stayed inundated by the base flow of the river at low tides. High tides are not registered very accurately in all cases due to wave action causing oscillations. Apparently, there was too much interaction between the water in the stilling well and the water in the river, so the number of holes in the tube was too large. The patterns in Figure 5.4 reveal an irregular semi-diurnal tidal regime for both measurement sites. There is however a strong diurnal component present and consequently the semi-diurnal component is only poorly pronounced. In the measuring period of 29 days, 4 completely diurnal days are observed at C0 and D0. Still the tidal regime is approximating a diurnal regime, especially when the tidal amplitude is relatively large. In these cases the difference between higher low tide and lower high tide is often not larger than 10 cm. Tides at both sites follow the same pattern but some subtle differences can be observed. In periods of relatively low tidal ranges, the amplitude at D0 seems somewhat larger than at C0, although this is hardly visible at the scale of Figure 5.4. In the measuring period amplitudes at D0 range from 70 cm to about 170 cm. In Figure 5.4 a time lag between C0 and D0 can be seen. High tides arrive 30 minutes earlier at D0 than at C0. The correlation coefficient between the measured water levels at C0 and D0 is high though; 0.886. Figure 5.5 shows a comparison of the measured water levels at D0 and the predicted water levels at Ha Tien and Dinh An. For clarity and because D0 and C0 are highly similar, only the water levels at D0 are plotted and data from C0 is omitted. As expected, this reveals a very different picture than the comparison of the predicted data for Vung Tau and the measurements at site A0. Correlation coefficients between the measured water levels at D0 and the predicted water levels at Dinh An and Ha Tien are respectively 0.576 and 0.484. This indicates the tides at D0 reveal more similarity with the tidal regime at Dinh An than at Ha Tien. This is probably due to the semi-diurnal tides at both D0 and Dinh An. From the correlation coefficients it is also clear that tidal regimes in Ha Tien and Dinh An are not representative for the tidal regime at Mui Ca Mau. Figure 5.5 shows a transitional situation between a diurnal and a semi-diurnal period at site D0. In this period the tides at Ha Tien are fully diurnal, while the tides at Dinh An are semi-diurnal. It seems that the joined effort of tides at Dinh An and Ha Tien raises the water levels at D0 considerably. If high tides at Dinh An and Ha Tien occur within several hours from each other, water levels at D0 are raised to above average levels. This might be the reason for the difference in occurrence of high tides between Dinh An and D0 in semi-diurnal periods as well. For 25 and 26 April it can be seen that both the tidal regime at Dinh An and D0 are semi-diurnal, but at the moment of the higher high tide at Dinh An, D0 experiences its lower high tide and vice versa. At the moment of higher high tide at Dinh An the water level at Ha Tien is low, while at the moment of higher high tide at D0, water level at Ha Tien is raised. High tide occurs somewhat later at D0 than at Dinh An; the magnitude of this time differentiation is about 2 hours. Propagation time of high tides between Mui Ca Mau and Ha Tien is

5 Results

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reported to be 5 hours, with a difference in amplitude of 40 cm (Marine Hydrometeorological Center, unknown). Apart from the difference in tidal regime, these numbers do not fit to the measurements entirely. Comparison between the predicted tides for Ha Tien and the measured tides at D0 reveals that high tides arrive about 3 hours earlier at D0 than at Ha Tien. The difference in amplitude is somewhat larger as well.

Figure 5.4 Open water measurements at site C0 and D0 from 21 April to 20 May 2007.

Figure 5.5 Detail of predicted water levels at Ha Tien and Dinh An and measured water levels at site D0 for the

period 23 April to 28 April 2007. Water levels are referred to local mean sea level.

5.2 Meteorology

47

5.2 Meteorology

5.2.1 Air temperature and pressure Obtained meteorological data are air temperature, air pressure and precipitation. Air temperature and pressure are measured by the BARO-divers in Can Gio and Ca Mau, at an interval of 5 minutes. The average temperatures, temperature ranges and measuring periods for both study areas are given in Table 5.1. Mean temperature in Ca Mau was slightly lower than in Can Gio. The measurements in Ca Mau were carried out during the rainy season (section 5.2.2). Air pressure data are used to calculate water levels as described in chapter 4.4.1. Table 5.1 Measured temperature data in Can Gio and Ca Mau.

5.2.2 Precipitation Precipitation data from 1 March to 22 May are shown in Figure 5.6. Next to the measured rainfall amounts at Can Gio and Nam Can weather stations, observed rainfall events in the study area Ca Mau are displayed. The events are plotted as days with a rainfall amount of 10 mm. At Can Gio no rainfall events were observed, since observers were not present in the plotted part of the rainy season. It can be seen that rainfall at Nam Can is higher than at Can Gio. The cumulative amount of rainfall in the plotted period is 368 mm at Nam Can and 148 mm at Can Gio. Already at the end of March considerable amounts of rainfall are recorded at Nam Can. However, this seems not to be the start of the rainy season, since no rain is recorded between 26 March and 16 April. From 19 April to 12 May and on 20 May observations of rainfall events could be done at Ca Mau, since at these dates observers were present. The observed events compare well with the measured events at Nam Can, although Mui Ca Mau and Nam Can weather station are situated about 30 km from each other. This is also the approximated distance between Can Gio weather station and measuring plot A.

Figure 5.6 Measured precipitation at Can Gio and Nam Can weather stations from 1 March to 22 May 2007 (after: Southern Regional Hydrometeorological Center, 2007) and observed rainfall events at Mui Ca Mau.

Temperature Measuring period Average (°C) Range (°C)

Can Gio 29.7 25.1 - 34.8 3 March - 25 May 2007 Ca Mau 29.0 23.8 - 35.8 19 April - 20 May 2007

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5.3 Elevation

5.3.1 Elevation measurement sites The elevation of all piezometers with regard to the local mean sea level is calculated by using water level measurements (see chapter 4.4.2) and presented in Table 5.2. The topography of the zone between these known locations was measured with the laser levelling method. Table 5.2 Elevation of all measurement sites.

5.3.2 Elevation profiles Can Gio The location of the laser levelling transects in plot A is presented in Figure 5.7, while Figure 5.9 displays the measured elevation profiles along these transects. In each figure the small rise of the elevation is somewhat misrepresented due to exaggeration of the y-axis, which is given in cm, while the x-axis is displayed in m. Perpendicular transects have a gradually rising elevation profile close to the main channel which changes to a more horizontal profile after approximately 300 m. The first part of the transects is not measured except in transect 1 in this research and in transect 4 by Van Loon in 2004. It was visually observed that no major topographical features occur within this part of the other transects, so because of time-efficiency it was not measured there. For the sake of completeness the first part of the elevation profile of transect 1 or of the transect measured by Van Loon has been added to the other profiles. In the more horizontal part of the elevation profiles, micro-topography appears in ridge-like structures and slightly lower basins behind such ridges. Some of the piezometer sites (indicated as black dots in the figures showing measured elevation profiles) are placed behind these ridges. Although it was suggested by Van Loon et al. (2007), in this research one large ridge parallel to the main channel over the entire plot A is not found. The ridges in the various transects are situated at different distances from the main channel. In transect 3 a ridge is measured at 280 m from the main channel, while in transect 2 the first ridge is found at 370 m. In the graphs of transects parallel to the main channel the creeks are visible at each side of the transect by the abrupt lowering in elevation. Between the creeks micro-topography is measured and ridges are visible. Especially in transect 7 the levees next to the creeks are higher than the surroundings. Piezometer A3 is placed in a lower lying part of this transect, while in the perpendicular transect it is situated just behind a basin, so in the vicinity of A3 micro-topography is abundantly present. The locations of the laser levelling transects in plot B are presented in Figure 5.8. The transects in this plot were mostly chosen from one piezometer to another. In Figure 5.10 obtained elevation profiles are displayed. Plot B is surrounded by creek IV and VII and a dike. Two transects (8 and 9) intersect this entire area. In the elevation profiles of transect 9 and 10 the levees of the surrounding creek are visible on the sides of the profiles by the higher elevation next to the abrupt lowering that indicates the creek. Piezometer B1 and B2 are placed behind the levee in lower lying parts of the plot, with B2 situated more inland. Piezometer B3 is placed next to a small creek with a depth of approximately 30 cm, which intersects the levee. Transect 8 ends at the dike that borders a part of the plot, this dike has a higher elevation than the levee on the other side of the plot.

Plot A Elevation (cm +MVT)

Plot B Elevation (cm +MVT)

Plot C Elevation (cm +MCB)

Plot D Elevation (cm +MCN)

A0 -151 B1 83 C0 -87 D0 -70 A1 12 B2 81 C1 32 D1 46 A2 92 B3 60 C2a 30 D2 35 A3 104 C2b 24 D3 35 A4 114 C3 28 A5 116 A6 121

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49

Figure 5.7 Locations of the laser levelling transects in plot A.

Figure 5.8 Locations of the laser levelling transects in plot B.

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Figure 5.9 Elevation profiles in plot A; a) profiles perpendicular to the main channel (top), b) profiles parallel

to the main channel (bottom). Note that the y-axes are in cm, while the x-axes are in m.

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Figure 5.10 Elevation profiles in plot B. Note that the y-axes are in cm, while the x-axes are in m.

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5.3.3 Elevation profiles Ca Mau In plot C two elevation profiles were measured perpendicular (transect 12 and 13) and one profile parallel to the main channel (transect 14). The exact locations of the transects are displayed in Figure 5.11 and the elevation profiles in Figure 5.13. The perpendicular transects start at a mudflat in front of the forest (distance from coast is 0 m). From this mudflat the surface level rises gradually, but from about 120 m inland it starts descending towards creek X. Therefore both measurement site C2a and C2b have a lower elevation than site C1. So, a broad ridge is present parallel to the main channel. Creek X has a clear levee on the southern bank while creek XI is bordered on the north by a less pronounced, but broader levee. Levees are also found in the parallel transect; each creek at the border of the profile has a levee next to it. Creek VIII has a much higher levee than creek IX. In plot D only one transect over the entire width of the island was measured. The exact location is shown in Figure 5.12 and the elevation profile in Figure 5.14. No extensive mudflats are present on either side of the transect. Both sides of the island have a gradual rise in elevation until the highest point of a ridge is reached, behind the ridges a lower lying basin is situated. The levee on the southern side of the island has a higher elevation than the northern levee. Piezometer D1 is situated just before the highest point of the ridge, while D2 and D3 are placed in the lower basin in the middle of the island. The elevation difference between D1 and D2 is over 10 cm (Table 5.2).

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Figure 5.11 Locations of the laser levelling transects in plot C.

Figure 5.12 Location of the laser levelling transect in plot D.

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Figure 5.13 Elevation profiles in plot C; a) profiles perpendicular to the main channel (top), b) profile parallel to the main channel (bottom). Note that the y-axes are in cm, while the x-axes are in m.

5.4 Water level measurements

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Figure 5.14 Elevation profile in plot D perpendicular to main channel. Note that the y-axis is in cm, while the x-axis is in m.


5.4.1 Can Gio

5.4.1.1 Plot A A detail of water level data in plot A is displayed in Figure 5.15. The water level data of all piezometers of the entire fieldwork period are displayed in appendix D. The relatively horizontal parts in Figure 5.15 indicate that there is hardly any change in water levels in these periods. Water level has then dropped to or below surface level. At all locations an abrupt reaction to the income of flood can be seen, but there is a time lag between the subsequent reactions. It could be expected that a response is seen in the water levels of a particular site at the moment that the water level at A1 is as high as the elevation of this site. The time lag is the time between the moment that the water level at A1 has reached the elevation of this particular site and the actual response occurs in the water level at the site. The average time lags for the piezometer sites in plot A are shown in Table 5.3. The measurement site A4 has a smaller time lag than A3, even though they are located at the same distance inland. There is no time lag between measurement sites A0 and A1, and A1 and A2.

Table 5.3 Average time lag and distance to Dong Tranh river for piezometer sites in plot A.

Site Distance to river (m) Time lag (min) A1 18 0 A2 205 0 A3 330 7 A4 330 10 A5 450 30 A6 680 67

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The lowering of the water level at ebb tide is not that abrupt for most sites and differs between locations. When surface water can discharge without barriers, it is expected that water levels at the measuring sites will follow the tidal movement. When the water level reaches surface level, hardly any drop of water level is observed below that point (site A1, Figure 5.15). When overland flow is impeded by micro-topography, the water has to be discharged through gullies, creeks or by means of groundwater flow through biopores. This causes a delay in the discharge and thus a slower decrease of the water level, as can be seen for sites A3, A4, A5 and A6 in Figure 5.15. Piezometers A3 and A4 are located at the same distance from Dong Tranh river, but do not have the same elevation. Site A3 remains inundated for a longer period than site A4, while the rise of water level at both sites starts at almost the same time.

Figure 5.15 Detail of water levels in plot A from 21 March 13:00 to 22 March 13:00, 2007.

5.4.1.2 Plot B The piezometers in plot B are located at small distances from each other, compared to the piezometers in the other plots. However, there is still a small time lag between the piezometers (Figure 5.16). Between B1 and B3 the difference between expected and actual arrival of flood is around 8 minutes, between B2 and B3 on average 7 minutes. The time lag between site A0 and the sites in B is for all sites on average around 20 minutes. The water level at B3 rises faster than the water level at A0 when the rising tide just reaches the site, which can be seen from the steeper rise of the graph of B3 in Figure 5.16. An abrupt change in speed of descent of the water level at B3 can be seen at a height of about 60 cm +MVT, which is the elevation of the surface at this site. The water level clearly drops lower beneath surface level compared to the other sites. At site B1 and B2 no delay in surface water outflow is measured; there is an abrupt change from inundation to levels below or just on the surface and no obvious tailing is observed like at site A3, A4, A5 and A6. Water levels at B1 and B2 gradually lowered between two inundations, but at site B1 the level declines faster than at site B2. This can be seen in Figure 5.16 from the intersection of the blue and red line and the difference in water levels before and after the peak.


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Figure 5.16 Detail of water levels in plot B and at A0 from 16 April 21:00 to 17 April 6:00, 2007.

5.4.1.3 Inundation characteristics In order to be able to classify sites based on hydrological conditions, inundation characteristics are determined as suggested by Van Loon et al. (2007). These inundation characteristics are ‘frequency of inundation’ and ‘duration of inundation’, the last one being expressed both in minutes per inundation and minutes per day. These inundation characteristics are determined from the water level data of all piezometer sites. The frequency of inundation is counted in the data series of each piezometer; every peak above the surface level of the site represents an inundation. If the water level does not drop under surface level between two tidal peaks, these peaks count as only one inundation. Duration of inundation is difficult to determine since the exact position of the soil surface above the diver sensor is not known. This is due to the correction needed for data from each diver and measuring errors. Therefore, a line above the measured small-scale variations in the water level data (see red line in Figure 5.17) is taken as the artificial surface level for each piezometer. It is assumed that these variations are mainly below surface level and that they should not be counted as inundations. The total duration of inundation at each piezometer site is the total time that the water level exceeds the artificial surface level. From this, the inundation duration per day and per inundation are calculated. An overview of the calculated characteristics for the measurement sites in plot A and B is given in Table 5.4. Measuring period, total inundation frequency and cumulative period of inundation are also given in this table, since these data are needed to calculate the duration of inundation in minutes per day and minutes per inundation. To calculate the number of inundations per month, 30.5 days per month are used. Since the surface level in plot A rises land inward, the inundation frequency and inundation duration are expected to decrease land inward. Inundation frequency can best be compared if expressed as ‘times per month’. Piezometer A0 does not follow these expectations; the frequency determined is lower at A0 than at locations land inward. A0 is placed in the open water, therefore the site often stays inundated at ebb tide and multiple tidal movements are counted as one inundation. It is noteworthy that the inundation frequencies in plot B are comparable to the inundation frequencies of site A1 and A2. The elevations of B1, B2 and B3 are in between the elevations of A1 and A2. The inundation frequency at B3 is even higher than the frequency at A1, while the elevation of site A1 is almost 50 cm lower than the elevation of B3.

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In plot A the duration of inundation expressed in minutes per inundation does decrease land inward, up to site A6. Site A6 is longer inundated per event than A2, A3, A4 and A5. The duration in minutes per day however does decrease inland; due to the higher elevation, micro-topography and consequently longer flow routes, the measurement sites inland are inundated less frequent. A6 thus is inundated less frequent and consequently the number of minutes per day is smaller. If inundation occurs however the site stays wet for a longer time than most of the other sites. Although measurement sites B1 and B2 have almost the same elevation, the duration of inundation per inundation as well as per day is considerably longer for piezometer B2. In general the durations of inundation in plot B are, like the inundation frequency, comparable to some sites between A1 and A2, which is expected based on the elevation of the sites in B.

Figure 5.17 Detail of water levels at site C1 and the artificial surface level line on 2 cm +surface.

Table 5.4 Inundation characteristics for plot A and B.

Duration of inundation Site Measuring period (days)

Inundation frequency

total

Inundation frequency (times

per month)

Total duration of inundation

(min) min per

inundation min

per day A0 83 43 16 117 625 2 735 1 419 A1 83 128 47 66 570 520 802 A2 29 39 41 8 575 220 296 A3 83 96 35 20 990 219 254 A4 28 20 22 4 170 209 149 A5 79 45 17 9 330 207 118 A6 77 28 11 7 710 275 100 B1 85 119 43 25 890 218 304 B2 81 116 44 37 560 324 463 B3 81 135 51 40 550 300 500

Figure 5.18 displays a detail of the water level at site A3 with respect to calculated surface level. The more or less horizontal parts between the peaks indicate that the water dropped to below or just at soil surface. It is clear from Figure 5.18 that the apparent surface level (indicated by the red line) could not be used to determine durations of inundation, since many minutes of inundation would be omitted. However in this graph, a straight artificial surface level can hardly be determined either, because the relatively horizontal parts do not have the same elevation over the entire period. Apparently the surface level does not have the same elevation during the measuring period and this makes it difficult to determine the duration of inundation.


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Figure 5.18 Detail of water level and the apparent soil surface at site A3.

5.4.2 Ca Mau

5.4.2.1 Plot C A detail of the water levels measured in plot C is displayed in Figure 5.19. The data in this figure appear to have more small variations than the data from piezometers in other plots, but this is just due to the difference in scale of the y-axes. There is a time lag between the moment the water level at C0 reaches the level of the soil surface of the measuring sites and the moment flooding is actually observed at these sites. The time lags and distances from Rang Ong Linh river are shown in Table 5.5. Table 5.5 Average time lag and distance to Rang Ong Linh river for piezometer sites in plot C.

Site Distance to river (m) Time lag (min) C1 68 15 C2a 175 15 C2b 190 15 C3 350 54

Measurement sites C2a and C2b both have a lower elevation than piezometer C1 (Table 5.2). Due to the ridge at the beginning of the transect, site C2a and C2b were not expected to be inundated earlier than site C1 since the water would first have to pass this ridge. However, often site C2b is inundated somewhat earlier than site C1, which is observed to be caused by water entering via creek X. This phenomenon also causes the inundation frequency at C2b to be higher than at C1 (Table 5.7). The water level at site C3 starts rising last, since it is located between several ridges which impede overland flow and cause large time lags. In plot C the delay in discharge is clearly visible as tailing in the graphs. At location C3 the decrease in water level is the most gradual, but even C1 does not follow the tidal movement. Water level at C2a decreases very gradually and does not follow the tidal movement at any time.

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Figure 5.19 Detail of water levels in plot C from 17 May 9:00 to 18 May 9:00, 2007.

5.4.2.2 Plot D Time lags and distances to Cua Lon river for each piezometer site in plot D are shown in Table 5.6 and a detail of the water levels in Figure 5.20. For each piezometer site the distance to the closest border of the island is given, since the river can inundate the island from both sides. The time lag in plot D is largest between D0 and D2, which is seen in both Table 5.6 and Figure 5.20. D1 is flooded first, although it has the highest elevation.

Table 5.6 Average time lag and distances to Cua Lon river for piezometer sites in plot D. The distance to the

closest border of the island is given per site and direction is indicated by S (southern) and N (northern).

Site Distance to river (m) Time lag (min) D1 50 (S) 15 D2 255 (S) 60 D3 180 (N) 40

Water levels at D1 follow the tidal movement without any delay, until surface level is reached. The water levels at sites D2 and D3 rise faster than the levels at D0 and D1; the water levels at D2 and D3 start rising later, but maximum water level is reached at the same moment at all sites. Also at ebb tide the water levels at D2 and D3 do not completely follow the measured tides at D0, but they show a delay in discharge. As D2 and D3 have almost the same elevation, the water level drops further below surface level at piezometer D3 than D2 which can be seen from the lower level at D2 just before the tidal peak.


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Figure 5.20 Detail of water levels in plot D from 22 April 12:00 to 23 April 6:00, 2007

5.4.2.3 Inundation characteristics The inundation characteristics for plot C and D are calculated in the same way as for plot A and B. An overview is given in Table 5.7. In plot C and D the elevation does not increase land inward, but it decreases in plot D and is rather irregular in plot C. Therefore the frequency and duration of inundation do not decrease inland. The piezometers in open water (C0 and D0) again have a low inundation frequency, since the locations did not fall dry at every ebb tide. In plot D the differences in frequency and duration between the sites correspond with the elevation difference (Table 5.2). From this can be concluded that the water also reaches site D2 through a small creek or from the northern border of the island instead of only from the closer southern border, because the frequency of inundation of site D1 and D2 should have been similar in the latter case. Table 5.7 Inundation characteristics for plot C en D.

Duration of inundation Site Measuring period (days)

Inundation frequency

total

Inundation frequency (times per

month)

Total inundation

duration (min) min per

inundation min per

day

C0 29 1 1 41 420 41 420 1 440 C1 29 21 22 6 125 292 213 C2a 8 5 19 2 630 526 333 C2b 17 16 28 6 685 418 386 C3 26 18 21 9 640 536 376 D0 29 13 14 39 270 3 021 1 364 D1 29 17 18 3 765 221 131 D2 29 22 23 6 180 281 214 D3 3 3 30 1 300 433 421

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5.5 Vegetation Species composition in the four plots was investigated. In this paragraph an overview of the species composition is given. No attempts have been done to study vegetation density and characteristics of individual trees. For plot A the species composition is described rather detailed, since vegetation diversity was high there. In the other plots this was not the case and consequently the descriptions are limited. In appendix E a list of observed vegetation species and their optimal requirements with regard to inundation and soil type can be found.

5.5.1 Can Gio

5.5.1.1 Plot A In plot A species composition was investigated along the laser levelling transects (chapter 5.3.2). An overview of observed vegetation zones is given in Figure 5.21.

Figure 5.21 Overview of observed vegetation zones in plot A (Av: Avicennia, Rh: Rhizophora apiculata). Dashed

lines at the end of transects indicate that the zone continues for an unknown distance. As described in chapter 4.1.2, the plot has a natural developed A. alba zone and a planted R. apiculata zone, with a mixed transition zone in between. During the fieldwork obvious variation within this transition zone is observed, therefore a sub-division between the mixed zones has been made. In total 5 vegetation zones are defined: Avicennia alba zone: The first zone is 150 to 200 m broad consisting purely of naturally

established A. alba. The transition to a zone with more vegetation variation is quite abrupt. Just before this transition Acanthus ilicifolius has been observed on levees along creek II and creek III.

Mixed Avicennia and Rhizophora zone: In all transects perpendicular to the main channel this

zone is situated behind the A. alba zone. The zone is dominated by A. alba and R. apiculata, but also A. officinalis, A. marina, R. mucronata and Ceriops spp. are abundantly present. Besides, Nypa fruticans is observed at the transition from the A. alba zone to the mixed zone in transect 1.

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Rhizophora dominated mixed zone: The Rhizophora dominated mixed zone is defined to emphasize the difference with the mixed Avicennia and Rhizophora zone. In the Rhizophora dominated mixed zone R. apiculata is present with only a few different species, while A. alba is absent. At transect 4 and 5 this zone consists of A. marina, C. tagal and Acanthus ilicifolius next to R. apiculata. In transect 1, R. apiculata, A. officinalis and Acanthus ilicifolius are observed within this zone.

Rhizophora plantation: The Rhizophora dominated zones are a former part of the monospecific

R. apiculata plantations, which are planted around 1980 (Kiet, 2007). At the border with a mixed zone some other species might be present, mainly A. officinalis or A. marina. At transect 2, in the vicinity of piezometer site A6 and near creek III at around 550 m from the main channel, small areas of Acrostichum aureum have been observed in relatively open parts of the forest.

Mixed zone with shrub layer: In general there is a sequence with pure A. alba stands at the river

side to pure R. apiculata stands inland. In between several species mix with these species, but A. alba and R. apiculata are dominant. Between creek II and creek III however, another pattern is observed. Mainly in the vicinity of the creeks a mixed zone where shrubby species and smaller tree species are abundantly present is observed. R. apiculata and A. alba are present, but not dominant in this mixed zone with shrub layer. Observed species in this zone are: A. officinalis, A. marina, A. alba, Ceriops spp., R. apiculata, Acanthus ilicifolius, Sonneratia ovata, Sonneratia alba, Excoecaria agallocha, Lumnitzera racemosa and a liana type plant which could not be identified.

Examples of the different vegetation zones can be found in Figure 5.22 and Figure 5.23.

Figure 5.22 Mixed zone with shrub layer at the north border of transect 6.

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Figure 5.23 A. alba and R. apiculata dominated mixed zone with C. tagal.

5.5.1.2 Plot B Plot B consists of a R. apiculata plantation, in which hardly any variation in vegetation is observed. However, some solitary A. alba are found between the R. apiculata trees (Figure 5.24). On the banks of the creek leading to plot B, Acanthus ilicifolius is present and on artificial dikes near the plot large amounts of Acrostichum aureum are found.

Figure 5.24 Solitary A. alba in the dense R. apiculata plantation in plot B.

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5.5.2 Ca Mau

5.5.2.1 Plot C The forest at plot C is naturally regenerated after it had been used by local people (Nam, 2007). Since the former human activity at the plot is unknown, it is not exactly clear how vegetation was affected. Nowadays the plot consists mainly of A. alba, A. officinalis, R. apiculata and Bruguiera parviflora. A. alba is observed all over the plot, but at the river side it is more dominant than in the rest of the plot. R. apiculata is also found in the entire plot, but several monospecific R. apiculata strips of about 25 m width are found parallel to the main channel at different distances inland. The strips have very sharp borders and are densely populated with many small trees. Between piezometer site C3 and creek XI, a large strip with cut R. apiculata trees is present (Figure 5.25). It seems the strips were planted in this area. At the created open parts young R. apiculata trees are establishing. B. parviflora is also found in plot C, but not very frequent.

Figure 5.25 Cut R. apiculata strip in plot C. Young trees can be seen at the open spots.

5.5.2.2 Plot D Plot D is located at a recently formed island in Cua Lon River. From satellite images it can be seen that the island was not fully formed yet by 1990. This implies the vegetation is fairly young. The species composition is similar to plot C, although the pattern is different. At a small fringe at the borders of the island A. alba is the dominant species, but further inland R. apiculata is dominant. Also large shares of B. parviflora and A. marina trees are observed here, compared to plot C. At the southern border of the island some Sonneratia alba is found. The R. apiculata trees in plot C and D are in general rather small in comparison with plot A and B. The average size, both height and diameter, of A. alba trees is also estimated to be smaller than in Can Gio, but the difference is less clear.

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Figure 5.26 Vegetation in plot D with A. alba, R. apiculata and B. parviflora.

5.6 Creek flow Discharge measurements were performed in creeks in all plots. The measuring and calculation methods are discussed in chapter 4.7. Maps with the discharge measurement locations in all plots are displayed in appendix F. Seaward directed flow is given as a positive velocity and discharge, the landward direction is negative. Creek discharges could only be calculated as long as the discharge was contained within the banks, since overland flow could not be quantified.

5.6.1 Can Gio Discharge measurements in plot A were carried out at three different locations. The first location was situated in creek II at about 300 m inland, somewhat before piezometer A4 with respect to the main channel (Appendix F). The second measurement site was located at a bifurcation in creek III, also at about 300 m inland, between piezometer A2 and A3. Here discharge measurements were carried out in both creeks at the same time. Creek III at this point is called location 2a, the smaller side-branch is location 2b. The third location in plot A was located in creek III as well, but further inland at about 400 m, between piezometer A3 and A5. In plot B discharge measurements were also carried out at three locations. In creek IV, the large creek that surrounds a part of plot B, location 4 was situated. Measuring location 5 was at the mouth of creek V, the small creek besides which piezometer B3 is placed, where it discharges into creek IV. There is another small gully intersecting the levee of creek IV, called creek VI, which is the third location of the discharge measurements. To investigate the effect of different tides on the discharges, the measurements were repeated at some locations at several days. An overview of all these measurements is given in Table 5.8.

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Table 5.8 Overview of discharge measurements in Can Gio.

Plot Location Date Plot Location Date A 1 (creek II) 18-3 B 4 (creek IV) 19-3 28-3 5 (creek V) 31-3 2 (creek III) 17-3 26-5 27-3 6 (creek VI) 2-4 29-3 25-5 3 (creek III) 16-3 30-3 24-5

5.6.1.1 Creek profiles To determine the discharge, the velocity area method was used. Therefore at each location three creek profiles were used to calculate the cross sectional area. In Figure 5.27 creek profiles at different locations in plot A are displayed. For clarity only one of the cross-sections per location is shown. Each creek has a rather steep profile. The profiles of location 1 and 2a, respectively creek II and creek III, are fairly similar in shape and size. Location 2b is smaller and has a width of only 1.5 m. Upstream of location 2, creek III decreases in size; the cross-section of the creek at location 3 is smaller than at location 2a. The creeks at locations 1, 2a and 3 do not fall dry at ebb tide, but retain a small base flow, while location 2b does fall dry at ebb tide. The measured creek profiles at plot B are presented in Figure 5.27, again with only one profile per location. Creek IV is comparable in size with creeks II and III. The profile is somewhat different since creek IV has a floodplain at the right side of the creek. At the other side the profile is steep between the levee and the deepest point in the profile. The cross-sections at the other measurement locations are clearly smaller. These creeks fall dry at ebb tide, while creek IV retains a small base flow. Some pictures of measurements creeks in Can Gio are given in Figure 5.28.

Figure 5.27 Creek profiles in Can Gio: plot A (left) and plot B (right).

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Figure 5.28 Examples of the variety of discharge measurement locations; creek II, location 1 (left) and creek VI, location 6 (right).

5.6.1.2 Flow velocity and water level In Can Gio water levels and flow velocities in creeks are measured during days with a variety of tides. Measurements at location 1 were done on 18 March and 28 March. The water levels and velocities found are displayed in Figure 5.29. To compare the measured water levels in the creek with the tides, the water levels measured at site A0 are also plotted in this figure. Water levels at A0 are referred to MVT, while water levels in the creeks are referred to the creek bottom. The semi-diurnal component is more pronounced and the tide has a larger amplitude on 18 March than on 28 March. On 18 March the water level at site A0 starts rising at 9:25 and maximum water level is reached at 14:25. The change in water level over this period is 2.74 m. On 28 March the differences between minimum and maximum tide was 2.12 m, while flood began at 5:45 and maximum water level was reached at 13:00. This difference in tidal regime causes a large difference in velocities in the creek. The maximum velocity on 18 March is 0.5 m/s, while on 28 March it is only 0.045 m/s. The water level at location 1 follows the same pattern as the water level at site A0 (Figure 5.29). At this location the direction of the flow changes at about the same time as the water level in the creek starts to increase. At other locations water levels often increased already during the last period of seaward flow. At the transition from landward to seaward flow, the moments of zero velocity correspond very well to the maximum water level in the creek. So, flow velocities generally increase and decrease again within one ebb or flood tide period.

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69

Figure 5.29 Velocity and water levels at location 1 and water levels at site A0; 18 March 2007 (left), 28 March

2007 (right). Note that the velocity axes have different scales. The maximum velocities found at each location are shown in Table 5.9. At most locations the maximum velocity was reached during seaward flow (positive). According to Van Loon (2005) peak ebb currents are on average about two times higher than peak flood currents in Can Gio. This can not be concluded from this study, because no complete 24 hour tidal cycle was measured. Seaward flow velocities are however higher than landward velocities. In general, the velocities found in this research are of the same magnitude as the velocities found by Van Loon (2005) and Nauta (1994). However the maximum velocities are lower; maximum velocities found by Van Loon (2005) and Nauta (1994) were 0.67 m/s and about 0.7 m/s respectively.

Table 5.9 Maximum flow velocities in creeks in Can Gio.

Location Maximum velocity (m/s) 1 0.5 2a 0.25 2b 0.23 3 0.18 4 -0.23 5 -0.09 6 0.3

5.6.1.3 Discharge Calculated discharge predominantly follows the same pattern as the measured velocity. The tides, and consequently the open water levels, determine the amount of discharge. Since the velocity at location 1 was much higher on 18 March than on 28 March, discharge was also larger. The maximum discharge measured on 18 March is 112 m3/min, while on 28 March the maximum was only 4.6 m3/min. Net discharge could not be calculated, since not the complete tidal cycle was measured and high water levels resulted in overland flow. Higher seaward than landward velocities might be an indication for larger outflow at ebb tides than inflow at flood tide.

5.6.2 Ca Mau In Ca Mau the available time for the discharge measurements was limited and tidal predictions were not available. Therefore the number of locations is small and measurements at each location are only carried out at one day. In plot C discharge was measured in creek X (location 1) and in creek XI (location 2). These were both small creeks parallel to the main channel, which are connected to creek IX. In both creeks the measurements were carried out near the outlet of the creeks, at

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approximately 80 m inland from creek IX (Appendix F). In plot D the locations were situated at the bifurcation of creeks XII and creek XIII (Appendix F). The measurements were done at the same time in both creeks, location 3 and location 4 respectively. An overview of the dates and locations of the measurements is given in Table 5.10.

Table 5.10 Overview of discharge measurements in Ca Mau.

Plot Location Date Plot Location Date C 1 (creek X) 30-4 D 3 (creek XII) 3-5 2 (creek XI) 1-5 4 (creek XIII) 3-5

5.6.2.1 Creek profiles In Figure 5.30 the creek profiles for plot C and D are displayed, again only one profile for each location while more profiles per location were measured. In plot C both creeks have about the same width, but creek X (location 1) is deeper than creek XI (location 2). The sides of creek X are very steep, while creek XI has more gentle sloping sides. At ebb tide these creeks retain a small base flow. In plot D measurements were done at a bifurcation, where creek XII is clearly the main branch (Figure 5.30). Both creeks have very steep banks. Again at neither of the creeks dry-fall at ebb tide was observed. However, due to the limited measurements this is not verified for very low tides. Given the size of creek XII a base flow is expected there. For creek XIII this is less certain, since upstream of the measurement site the creek decreases in size fast and does fall dry at ebb tide. The upstream part of creek XIII is located close to piezometer site D2, where its dimensions are comparable to that of the gully at location 6 in plot B (Figure 5.28).

Figure 5.30 Creek profiles in Ca Mau: plot C (left) and plot D (right).

5.6.2.2 Flow velocity and water level In plot C the discharge measurements were done at location 1 on 30 April and at location 2 on 1 May. For a comparison of water levels in the creek with the tidal regime, the measured water levels at site C0 are displayed in Figure 5.31. The tidal regime is almost similar for both measuring days and on both days no complete tidal cycle is measured, but only the transition from ebb to flood tide. In Figure 5.32 the measured flow velocities and water levels in the creek for both locations at the different days are shown. The maximum velocity measured was 0.24 m/s at location 1 and 0.35 m/s at location 2, both in seaward direction. The maximum velocities found are in the same range as in Can Gio. The water levels in the creeks at both locations stay rather constant in the beginning and start to rise later than the water levels at site C0 due to elevation difference. However the water levels are already rising before the direction of the flow changes, so even at seaward flow the water levels react to the tide. Between 14:00 and 15:00 on 1 May the water level at C0 drops a bit, on which the water level and flow direction at location 2 react immediately. The flow direction varies from landward to seaward and back to landward and the water level in the creek completely follows the water levels at

5.6 Creek flow

71

site C0 during this period. However, even in this small time period the water levels are again rising before the transition of the flow direction. The increase in velocity at the beginning of the measuring period at location 1 is remarkable. The velocity measurement at 10:00 might be an error, since no indication is found in the open water level for the rapid increase in velocity between 10:00 and 10:30. The measurements in plot D were done on 3 May. The measured water levels at site D0 during the measuring period are also given in Figure 5.31. Again only flood tide is measured and not a complete tidal cycle. The measured water levels and flow velocities at both locations are displayed in Figure 5.33. The maximum velocities are 0.41 m/s at location 3 and 0.35 m/s at location 4, both directed seaward. So, although the creeks have different sizes, the maximum velocities are almost similar. The changes in velocities also follow the same pattern in both creeks, where location 4 (creek XIII) reacts somewhat later during the transition from ebb to flood than location 3 (creek XII). However the maximum velocity in both creeks is reached at the same time and the transition from flood to ebb also takes place at the same time, around 15:15. From that moment the measured velocities are completely similar. The water level at location 3 is rising from the beginning of the period. It follows the open water level as measured by D0, although this level starts rising somewhat later (about 10:45). Again water levels are rising at seaward flow; the flow direction at location 3 changes around 11:00. At the transition from flood to ebb however, the water levels descend at almost the same time as the change in direction of the flow occurs. This also fits with the decline of the open water level.

Figure 5.31 Water levels at site C0 on 30 April and 1 May 2007 (left) and at site D0 on 3 May 2007 (right).

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Figure 5.32 Flow velocity and water levels at location 1 on 30 April 2007 and location 2 on 1 May 2007.

Figure 5.33 Flow velocity and water levels at location 3 and 4 on 3 May 2007.

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5.6.2.3 Discharge Like in plot A and B, for plot C and D no net discharge could be calculated. Only the actual discharges for the measured period are calculated to give an indication of the changes in discharge during tidal transitions. For both plots the discharges at all locations are displayed in Figure 5.34. It can be seen that the amount of creek flow (absolute discharge) at location 1 is larger than at location 2. This is caused by the larger cross-sectional area, which compensates for the smaller velocities compared to location 2. In plot D the cross-section at location 3 is much larger than at location 4 and flow velocities are comparable. Therefore the (negative) discharge at location 3 is also larger (Figure 5.34). In all creeks in plot D the discharge pattern is highly comparable to the pattern of the velocities which is described earlier. In plot C the discharge at location 1 has more variation than the discharge at location 2. Here the range of calculated discharges is fairly small.

Figure 5.34 Discharge at location 1 (30 April), location 2 (1 May), location 3 (3 May) and location 4 (3 May).

5.7 Hydraulic conductivity In this research groundwater levels are measured at ebb tide and permeability tests have been carried out like described in chapter 4.8. Calculated k-values for all locations and both methods are given in Table 5.11, graphs of the permeability tests are displayed in appendix G. In this chapter no division between the study areas is made. The obtained k-values per location are mostly an average of multiple tests. At some locations the tests were carried out in only one pit or the data of only one pit was suitable for calculation of k-values. At location C2 and C3, no k-value from a recovery test could be obtained. In Table 5.11 the number of measured pits per location is given. The pits were in almost all cases located next to a piezometer or along a laser levelling transect. This is indicated by the distance from the main channel, accompanied by a site number if relevant. In plot B the pits were located at the small levee that surrounds the plot, near piezometer B3 and in the somewhat lower basin near piezometer B2. In plot C and D two locations were situated near a creek.

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Table 5.11 Calculated k-values for all plots.

Site Pumping test Recovery test Plot Number Location1 k (m/d) pits k (m/d) pits

1 330 m (site A3) 6.5 2 1.4 2 2 400 m 18.0 2 1.0 2 3 450 m (site A5) 52.2 2 3.8 2

A

4 680 m (site A6) 12.0 2 0.7 2 1 levee (site B3) 0.9 1 0.3 1 B 2 basin (site B2) 8.1 3 1.8 3 1 70 m (site C1) 9.6 2 7.3 1 2 80 m 1.3 1 - - 3 195 m (site C2b) 9.6 2 - - 4 210 m (creek X) 18.7 3 2.6 3 5 250 m 15.0 2 0.5 1

C

6 350 m (site C3) 7.1 2 1.1 2 1 50 m (site D1) 9.0 1 2.0 1 2 180 m 6.6 2 2.1 2 3 250 m (site D2) 35.5 2 5.1 2

D

4 270 m (creek XIII) 9.1 3 3.5 2 1Distances are given in meter from main channel. Distances in plot D are from the southern border.

By carrying out in situ pumping and recovery tests the effective permeability of the soil is measured. It can be seen from Table 5.11 that k-values obtained from the pumping tests are in all cases higher than those from the recovery tests. Standard deviations from the averaged k-values are, partly consequently, also larger for the pumping test: this average standard deviation is 7.0 m/d, while for k-values from the recovery test it is 1.2 m/d. Apparently, there is a difference in dominant flow types during the tests, that results in the systematic difference in k-value. Thus, flow to the pit during the recovery test is slower than during the pumping test. During the pumping test biopores intersecting the pit are emptied, so at the beginning of the recovery test these biopores do not contribute to the flow to the pit. This implies that the extent of the system of connected pores is limited. Although the range of the obtained k-values is somewhat large, they are in line with findings of other studies. Mazda et al. (2006) estimated the bulk hydraulic conductivity in the Maira-gawa mangrove area on Iriomoto Island in Japan to be 12.96 m/d. The soil at the research sites was reported to be intersected by animal burrows and roots. This was also the case at research sites investigated by Susilo et al. (2005) in Australia, who report k-values ranging from 0.8 to 9.9 m/d. The k-values obtained using the (inversed) auger hole method by Van Loon (2005) in Can Gio, have an even larger range than the k-values found in this research. Values between 0.09 and 3000 m/d are reported, but they were concentrated between 15 and 70 m/d (Van Loon et al., 2006). These values are slightly higher than the values obtained in this research. A value of 0.5 m/d was calculated by Van Loon (2005) using piezometer data. This was reported to be the effective permeability of the upper 1 meter of the soil.

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6 Discussion

6.1 Tidal regime

6.1.1 Amplitude The main difference in tidal regime between Can Gio and Ca Mau is the tidal amplitude. Tidal amplitude at Can Gio ranges between 2 and 3.8 m, while at Ca Mau (D0) the amplitude ranges between 0.70 and 1.70 m. Measured amplitudes at C0 are even less, since this measuring site stays inundated by base flow in the river at low tide. The influence of Cua Lon River, at the bank of which site D0 is situated, could explain the difference in amplitude between C0 and D0. It is reported by Hong and San (1993) that high tides from the South China Sea propagate through Cua Lon River towards the Gulf of Thailand. So, water levels at D0 are influenced by the tides from the South China Sea. In D0 this effect is probably larger than at C0, since C0 is connected less directly to the South China Sea compared to D0. The effect of the tides of the South China Sea is restrained by the extensive creek system that lies between the sea and site C0. This might also explain why there is a time lag of 30 minutes between C0 and D0.

6.1.2 Diurnal vs. semi-diurnal Besides the difference in amplitude another difference between the tidal regimes in both study areas is relevant. The tidal regime at Mui Ca Mau tends towards a diurnal regime, although most of the days show a semi-diurnal tide. Since the difference between higher low water and lower high water is often very small (often less than 10 cm and 40 cm maximum), a lot of these semi-diurnal tides might have the same influence as a diurnal tide. The measuring sites in plot C and D are in the measuring period never inundated twice at one day. Only the very first part of the forest bordering the mud flat without any vegetation, and some areas direct next to creeks, have a lower elevation than site C2b. So, the majority of the forest is only inundated once per day at maximum. The same applies to plot D. At Can Gio, even site A6 experiences two inundations per day regularly.

6.2 Elevation From all the laser levelling transects measured during this research it is clear that the soil surface of mangroves contains a lot of variation. Based on measurements by Van Loon (2005) and general sedimentation patterns of rivers, ridges parallel to the main channels were expected to be found. In this research the elevation profiles and patterns in the inner mangrove forests turned out to be much more complex. In the gradually rising part of the elevation profile in plot A micro-topography is negligible. The profile enables water at ebb tide to discharge overland. Further inland in plot A and in all measuring plots in general, the surface slope is smaller and the easiest way to discharge is not always straight to the main channel. If water depth in the forest is still high just after maximum water level has been reached, the water will discharge straight to the main channel. Friction of the bottom does not play a role at that moment. If water depth gets shallower however, bottom friction and obstructions can cause the water to take a longer route to discharge. Obstructions in shallow water can be the observed micro-topography, but also trees and their pneumatophores. Micro-topography can become more pronounced due to scouring of the surface by ebb-currents. If water is forced to flow the same route during every ebb tide, preferential flow paths can be created which will evolve into gullies. In this way lower and higher parts of the soil surface are more or less randomly orientated in the forest. Consequently, the small ridges and basins that are observed occur both perpendicular and parallel to the main channel. The initial micro-topography can be caused by irregular sedimentation of the main channel and creeks, due to variations in flow velocity. Irregular sedimentation can be amplified by obstructions like trees and pneumatophores. These obstructions exert friction on water flow, thereby causing flow velocities to drop and eventually triggering suspended sediment to settle (Erftenmeijer and Lewis, 1999). Small variations in the surface level are observed around trees, with the trees standing on parts that are only a few centimetres elevated above their surroundings. Animal burrows are another factor causing small-scale topography.

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The most pronounced ridges arise next to the creeks in all plots. The levees of the creeks are somewhat higher than the surroundings, due to sedimentation by the creeks. Sedimentation close to the creeks results in firmer sediment. This is clearly observed along several creeks; levees were best suitable to walk on, while away from the creeks the mud was too soft for walking. The soil at creek banks from about 400 m inland in plot A is quite firm. The elevation of the banks and the good discharge possibilities to the creeks (for both overland flow and groundwater flow) causes the duration of inundation to be short and the soil to dry up relatively well. Therefore, many species are present here that grow well on relatively dry sites with clay and firm mud, like Acanthus ilicifolius, Acrostichum aureum, Excoecaria agallocha, Lumnitzera racemosa and Sonneratia ovata. Along the main channels in Ca Mau levees are also observed. In both plot C and D there is a broad ridge present at the border of the forest. Van Loon (2005) also found a parallel ridge in Can Gio, however this was less broad and situated further inland. The ridges in plot C and D have large effects on the water movement, since they block the main channel from the basins further inland. Plot B, C and D are basins with ridges at all sides (Figure 5.10, Figure 5.13 and Figure 5.14). In plot A, a basin is present as well, but it is lying further inland. In laser levelling transects 3 and 7 the basin is clearly visible around measurement site A3, which is situated just away from the deepest point of the basin (Figure 5.9). The basin is bordered by higher land on three sides. From Figure 5.9 it seems that the basin is also bordered just behind A3, but the basin extended further inland towards measurement site A5, although this is not visualized by the measured transects. Further inland the borders of the basin have a rather variable orientation, with parts parallel and perpendicular to the main channel. Just in front of measurement site A3 a small gully is situated which partly drains the basin at ebb tide. In the front, the basin is bordered by a small ridge parallel to the main channel.


6.3.1 Time lag between measurement sites From the water levels data, several time lags were observed. Time lags between the open water sites and the sites in the forest are a consequence of the distance between the sites and the retardation in travel time that occurs due to indirect flow paths, friction of the soil surface and obstructions in the forest. In case of a gradually rising elevation profile, the time lag is only caused by the distance between the measurement sites. No time lag is observed between site A0 and A1, and A1 and A2, since the profile up to A2 is gradually rising and A1 and A2 are situated closer to the sea than site A06. The water level at the forest border rises earlier than at A0, but due to the slower velocities in the forest, comparable water levels are measured at site A0, A1 and A2 at the same time. Further inland the surface gets more uneven and measuring sites have a higher elevation. Here the water has to reach the sites through gullies and creeks or by overland flow with more surface friction, and thus is delayed. For this reason a time lag is observed between A1 and A3, A4, A5 and A6. The time lag between A1 and A4 is slightly smaller than the time lag between A1 and A3, although they are located at the same distance from the main channel. Apparently the flow route to A3 is longer than to A4. In chapter 5.3.2 the elevation profiles within plot A are presented, which show piezometer A3 is placed behind a ridge seen from the direction of creek III. Measurement sites A5 and A6 are considerably harder to reach for high waters, according to the large average time lags of 30 and 67 minutes. The difference in time lag between these sites and site A3 is 20 minutes for A5 and 57 minutes for A6. These differences are not proportional to the distance from site A3, which is only 120 m for A5 and 350 m for A6. The differences in time lag are also not related to elevation of the sites, since A4 and A5 have comparable elevations but a large difference in time lag. From this can be concluded that the micro topography as observed in the elevation profiles in Figure 5.9 causes retardation of inflow. Influence of creeks is also relevant in inflow, but it does not determine the differences in time lag for A3, A4 and A5, since these sites are all situated at about the same distance from a creek. It seems reasonable that the location of A3, A4 and A5 with regard to creek III could reduce the time lag between A1 and these sites. However it is observed that the creek has a significant contribution in

6 N.B. Do not confuse with the main channel. A1 and A2 are located more south than A0 and therefore closer to the South China Sea.


77

inundating site A5, but the water from the creek does not reach A5 earlier than overland flow through the basin near A3. At both A5 and A6 observations have been done of water close to the site being blocked by a small ridge. When the water level became high enough, the basin behind the small ridge inundated and rapid increases in water level of up to 18 cm within 5 minutes were seen at the measurement sites. At the moment the water reached the site, contribution of multiple flow routes was observed. In plot D a similar phenomenon is observed; very fast increases in water level have been recorded by piezometer D2 and D3 at the moment of arrival of the water. The fast rise in this case can be caused by the ridges on both sides of the island. The water first has to flow over these ridges before reaching the basin in which D2 and D3 are located. Once the water level is high enough the basin can fill up very fast driven by the gradient between the ridges and the basin. Consequently water levels at D2 and D3 increase faster than the water level in the main channel. In plot C, creek X has a large influence on the time lag between sites C0 and C2b. The elevation of site C2b is about 8 cm lower than the elevation of site C1. The time lag of the sites is similar, but water from the main channel does not inundate site C1 and site C2b at that same time by flowing over the ridge. If this would have been the case the time lag between C0 and C2b would have been larger than between C0 and C1. The height of site C2b is always reached earlier at C0 than the height of site C1. So from the time lags it can be concluded that inundation of site C2b happens via a different flow route. It is observed that this flow route is via creek X, which causes inundation of C2b. This is supported by the discharge measurements in which is seen that the water level in creek X starts rising quite quickly after the water level at C0 has started rising. At ebb tide retardation of outflow is observed, which can be seen as tailing in the graphs of almost all piezometers. Like time lags at inflow, the tailing at outflow is an indication of obstructions influencing the duration of inundation of sites. Although in plot B the differences in time lag and in elevation between measurement sites B1 and B2 are negligible and tailing is only clearly observed at B3 (Figure 5.16), a considerable difference in duration of inundation between B1 and B2 is calculated (Table 5.4). This difference is caused by the direct discharge possibilities to creek IV at B1 (Figure 5.10), while B2 lies in a basin from which water can only be discharged along lengthy flow routes, like groundwater. An effect of topography on groundwater flow is illustrated by the drop of water levels below surface at site B3. This piezometer is placed very near to a creek (Figure 5.10) and the topographical gradient is large. Therefore, groundwater flow over this short distance causes fast discharge below surface level. This also explains the fast increase in water level at the site when flood is coming in; the low water level in the piezometer is replenished fast if flood reaches the surface level of the site. Groundwater flow does not have any effect at inflow. This effect of topography on the groundwater flow is investigated by Mazda and Ikeda (2006). In a riverine forest, which is composed of a tidal creek and fringing mangrove swamps, the presence of a creek bank strongly affected the groundwater flow, since the gradient of the groundwater depends on the slope of the bank. This gradient is larger close to the creek, therefore a portion of the swamp close to the creek bank drains rapidly, though the inner part of the swamp stays wet (Mazda and Ikeda, 2006).

6.3.2 Inundation characteristics In chapter 5.4.1.3 the method used to determine the duration of inundation and inundation frequency is explained. By choosing an artificial surface level all small-scale variations around surface level, originating from a high measuring frequency by both the normal diver and the BARO-diver, are omitted. As a result this method could cause an underestimation of the duration of inundation. However, since the exact determination of the surface level is very difficult and an overestimation would be made by taking into account all the small-scale variations, the error made by applying an artificial surface level is assumed acceptable. In Figure 5.18 the surface level change during a part of the measuring period is presented. A net change in surface level over the entire measuring period is not visible, but the level changes over smaller times during the period. This implies the process is reversible. After a dry period the surface level is lower than after several inundations. In literature more examples of changing surface levels in mangroves are found. Fityus and Welbourne (in Hughes et al., 1998) state that “frequent tidal and soil

6 Discussion

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moisture content fluctuations would be expected to induce a shrink–swell response of 10 to 20 mm in the highly porous surface muds, particularly in the less frequently inundated zones”. Whelan et al. (2005) have found that a mangrove peat-dominated soil profile was strongly influenced by groundwater. Their study indicates that “changing groundwater head pressure was driving the monthly shrink and swell of the soil surface elevation”. Whether shrink and swell is also the explanation for the change in surface level in this research is not known, since peat soils were not present and changes were larger than 10 to 20 mm, but it might partly explain the variations in soil surface. For the determination of inundation frequency, the peaks above surface level are counted. The differences between peaks caused by rainfall or by high tides are not taken into account. Rainfall data is available for both study plots, so the rainfall peaks could be omitted. However, the difference in effect of inundations caused by rain or by tides on vegetation is not known. Therefore in this research rainfall peaks are included in the inundation frequency. Further research of the effects of water type causing inundation on vegetation is needed to decide whether this is correct or not. In Table 5.4 and Table 5.7 the inundations characteristics of all measurement sites are given. Durations of inundations per event are rather high in plot C, compared to the other plots. Partly this can be caused by hampered discharge, since it occurs in zones dominated by micro-topography and in a basin shaped plot. The durations per inundation at C2a, C2b and C3 are comparable to the rather wet site A1, but durations per day are much lower. Site A1 has a low elevation and stays inundated frequently if higher low tide is not low enough to expose the site. Therefore A1 has a high duration per inundation. In plot C this might be due to the apparent diurnal tidal regime that only inundates the plot once a day, but for a prolonged period per inundation, or due to hampered discharge from the basins. Average duration per day is relatively low in plot C since the plot is not reached by all the high tides. The inundation characteristics give information about the contribution of creek flow to inundation of the measurement sites. On one hand creeks are indirectly hampering water to reach the areas because they create topographical variation, but on the other hand inundation of basins via creeks is also observed. This is why site C2b has a higher inundation frequency than site C1, located at the ridge in front of the basin (Table 5.7). Site C2b also has high durations of inundation. Site D2, located in a basin and in the vicinity of a creek, also has a higher inundation frequency and duration of inundation than site D1, but this is not only the influence of the creek. D2 can also be reached by flooding of the lower northern border of the island. The creeks have a share in inundation of the sites, but at ebb tide they can not discharge the basins entirely because micro-topography hampers the last shallow layer of water to flow towards the creeks.

6.4 Creek flow In most of the creeks where discharge measurements were carried out, a base flow was present during ebb tide. The size of the catchments of the creeks can explain this prolonged outflow. Creek II and III for example, have larger catchments than the sub-creek at location 2b, which falls dry at a certain moment during ebb tide. Within the larger catchments more water through the soil and gullies enters the creek with retardation and a larger spreading, thereby creating a base flow in the creek. The sub-creek at location 2b discharges the basin near piezometer A3. At this basin, water logging has been observed often, suggesting the sub-creek falls dry because the water can not reach it due to obstructions or friction. In that case the water in the basin can only be discharged by means of relatively slow groundwater flow, resulting in long inundation periods. Base flow influences the velocities occurring in the creeks. In accordance with observations by Van Loon (2005) and Nauta (1994), maximum velocities are mostly found during seaward flow. The base flow in all the creeks has a seaward direction. During ebb tide the direction of tidal currents is also seaward, while during flood tide the direction is landward. So at ebb tide, outward flow can be enhanced by base flow coming from higher parts of the forest that were flooded before. The water in the base flow can be retarded water from earlier inundations, rainwater or groundwater. The enhanced outward flow can result in higher velocities. During flood tide, the base flow exerts a force opposite to the flood wave, so smaller velocities occur. Tidal asymmetry in mangrove creeks, with ebb velocities being larger than flood velocities, was also found in mangrove creeks in Gazi Bay, Kenya (Kitheka, 1997). Besides this, tidal asymmetry also occurs inside the vegetated swamp (Wolanski, 1992).

6.4 Creek flow

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Flow retarding effects of the dense mangrove vegetation during spring high tide, when most parts of the mangrove forest are inundated, contribute to this tidal asymmetry (Kitheka, 1997). Vegetation also influences the currents in the creek, since a change in vegetation density modifies the Manning coefficient (Wolanski et al., 1980). Therefore Wolanski et al. (1980) state that “mangroves appear to be responsible for maintaining a drainage channel whose geometry (size and meanders) is related to vegetation density”. Another interaction between base flow and open water occurs when the base flow is blocked by incoming water at flood tide in downstream parts of creeks. Water is then accumulated upstream in the creek, like the backwater effect that occurs at weirs. This might be the explanation why at many discharge measurement locations the water level already started rising during seaward flow. At the transition from flood to ebb, water levels start declining at the same time as the change in flow direction, because both base flow and tide have a seaward direction in this case. At the transition from ebb to flood it might also be that flow direction still seemed to be outward, while in fact the water was already penetrating the creeks. Since flow direction is determined at the water surface, no observations are done of vertical differences in flow direction. Van Loon (2005) observed vertical differences in flow direction at both the transition from ebb to flood tide and vice versa. In open channel flow in general a vertical velocity gradient exists, with low near-bed velocities due to friction and maximum velocities near the surface (Figure 6.1).

Figure 6.1 Example of a parabolic vertical velocity profile in open channels (adapted from: Torfs, 2006).

Consequently, at the transition from ebb to flood tide the near-bed velocities are easier reversible, causing flow direction in the vertical to differ. At a certain moment net flow will be directed inward, causing water levels to rise. In Figure 6.2 this mechanism is illustrated. Indications in the figure suggest that the time-scale of the mechanism is about 30 minutes. This corresponds well with observations in this research, since the shift of flow direction at the surface is often observed between 30 and 60 minutes after water levels started rising.

6 Discussion

80

Figure 6.2 Shift in near bed and near-surface velocities at the transition from ebb to flood tide (adapted from:

Rijn, 1990 (in Van Loon, 2005)). Horizontal differences in flow direction are also observed. Near the transition of the tides, surface flow at the sides of the creek could be directed opposite to the flow in the middle of the creek. Like in the vertical, this is caused by velocity differences due to friction. The sides of the creeks and the small roots that are present exert friction on the flowing water and consequently velocities are low at the sides (Figure 6.3). Like in the vertical the lower velocities are easier reversible at the transition of tides and consequently flow directions can vary horizontally as well.

Figure 6.3 Example of a parabolic horizontal velocity profile in open channels (adapted from: Torfs, 2006).

Differences in maximum velocities are found between the discharge measurement locations. Larger creeks have more interaction with the open water; this interaction determines the force the tides exert on the water in the creeks (Van Loon, 2005). This explains the difference in maximum velocity between the different creeks; in large creeks the tides exert more inland pressure at flood tide and more outward forcing at ebb tide which can result in higher velocities. Because the measurement locations in the creeks in plot C and D are located closer to larger channels as measurement sites in plot A and B, the interaction with the open water is larger. This could explain why the maximum velocities found in plot C and D are of the same magnitude as the ones in plot A and B, although the tides exert less forcing on the water due to smaller gradients (less variation due to smaller amplitudes and a stronger diurnal component). The lower maximum velocities found in this research than the maximum velocities found by Van Loon (2005) and Nauta (1994) support the theory of interaction between creek water and open water, since in this research measurement locations were situated in smaller creeks than the ones in which Van Loon and Nauta measured the maximum velocities. The large variation in flow velocities in general and flow directions at the transition of tides indicate the complexity of flow in tidal creeks. The observations also indicate the limitations of the velocity measuring method used in this research. Since velocities are measured at the surface and in the middle of the creek only, an overestimation of the velocity, and consequently the discharge, is


81

expected. It is uncertain if the correction factor that is used to obtain the average velocity in the vertical is sufficient. It is however very difficult to perform simultaneous velocity measurements at several points in both the vertical and horizontal direction, due to the small sizes of the creeks. Performing velocity measurements at several points in succession to calculate discharge at one fixed moment is also impossible; velocities are highly variable and will change over the measurements. Incapability to measure complete tidal cycles and the restriction of discharge calculations at water levels lower than the creek banks only are other limitations. In order to determine net discharge, continuous measurements are needed during a complete tidal cycle, but still net discharge will be difficult to calculate due to overland flow. The amount of overland flow can not be neglected by assuming zero velocity over the banks. Though the velocity outside the creek profile is lower than in the creek, the amount of discharge is still considerable due to the large cross sectional area in cases of overland flow. Continuous measurements can avoid errors in velocities. The sampling interval between measurements has to be small. Reed (1987) indicated the problems of drawing conclusions about creek behaviour when collecting data at large time intervals. According to Reed, velocity pulses occur between the intervals, causing discharge to be measured incorrectly. He states that “sampling at 30-min intervals can underestimate discharge when velocity pulses on spring tides are not accurately monitored”. So, discharges measured during this research give an estimation of the order of magnitude of the creek flow at certain moments, but no net discharges.

6.5 Hydraulic conductivity Decreases in groundwater level as observed by the piezometers in the period between two inundations can be caused by both groundwater flow and evaporation (Susilo and Ridd, 2005). Van Loon (2005) found that the effect of evaporation on groundwater level in Can Gio was very small in the period March-May 2004. Nauta (1994) however states that this effect is considerable, “taking in account the lowering of the water level during the hottest hours of the day”. No quantification has been reported by Nauta. Other reports of the importance of evaporation and evapotranspiration on water transport in mangroves are conflicting as well. According to Nuttle and Harvey (1995) evapotranspiration is the dominant factor determining water removal from mangrove soils. Hughes et al. (1998) however found no indication of groundwater table sensitivity to evaporative forcing from well data inspection in the Hunter River estuary, Australia. Here, evaporation in the order of 5 to 10 mm/day was much smaller than tidally driven head fluctuations that occurred daily in the groundwater table. It is believed that also in this research, evaporation is negligible compared to groundwater flow. Wolanski and Gardiner (1981) found that groundwater flow is the main water transport process in a mangrove swamp, at least at depths of inundation of a few centimetres. Although hydraulic conductivity is not the only factor determining groundwater flow, the bulk hydraulic conductivities in the research plots are sufficient to support it. Whether groundwater flow does actually occur also depends on the hydraulic gradient. In plot A groundwater is assumed to flow mainly in the direction of the coastline, due to the gradient in piezometric head that is present at low tide. In plot B and D this gradient is directed towards the lower parts of the basin. From this can be concluded that groundwater flow out of these basins is impossible. In the seaward part of plot C the hydraulic gradient is directed towards the main channel, more inland it will be directed towards creek X or the creek west of the plot. The effect of creeks and gradients on groundwater flow is illustrated in chapter 6.3.1. The obtained k-values in this research are in line with the results of other studies. So it seems that the methods used in this research are suitable to determine bulk hydraulic conductivity in mangrove soils and that the conditions as listed in chapter 4.8.2 are satisfied. The representative depth of the calculated transmissivities is limited to the upper 30 cm of the soil. The obtained transmissivities thus are weighted averages of the upper 30 cm. However, k-values are calculated from these transmissivities assuming an aquifer thickness of 1 m. Over this depth a difference in conductivity may exist, since in the upper part (~20 cm) of the soil, biopores are more interconnected, resulting in a large secondary conductivity. Deeper in the profile biopores are still present, but their magnitude, extent and rate of interconnectivity is much smaller (Stieglitz et al., 2000, Susilo and Ridd, 2005).

6 Discussion

82

Consequently conductivities in the deeper soil are expected to be smaller, approximating the matrix conductivity in the order of 10-3-10-4 m/d. Therefore the deeper soil is expected to play a negligible role in the large-scale groundwater flow of the mangrove swamp (Van Loon et al., 2006) and k-values obtained in the tests are an over-estimation of the k-value for the entire aquifer. The hydraulic conductivity decrease with depth and the representative depth of obtained transmissivities are illustrated in Figure 6.4.

Figure 6.4 Expected hydraulic conductivity profile in mangrove soils intersected by biopores. The red box

indicates the measuring range in this research. The hydraulic conductivity thus depends highly on the rate of interconnectivity of the pores. The variation in this rate causes the large range of obtained k-values per measuring site. Although previously it is stated that in the upper part of the soil biopores are interconnected, from the pumping and recovery test data follows that the systems of connected pores do not extend over large areas. During the pumping test the water from the pores in the upper soil is removed, until no visible further drawdown is observed. In the meanwhile, water flow from connected pore systems intersecting the pit causes fast replenishment of the removed water. The conductivity resulting from the pumping test therefore is high. A comparable conductivity is expected to be obtained from the recovery test if the pore system has the same influence, when connected biopores extend over a large area. However, k-values obtained via the recovery test are in all cases smaller than k-values of the pumping test. Recovery of the water levels thus is driven by slower flow than the type of flow to the pit during pumping. This slower type of flow is matrix flow, while during the pumping test both matrix and bypass flow occur. The difference exists since the recovery test is carried out directly after the pumping test, so biopores are empty at the beginning of the recovery test and do not contribute to the flow. From this can be concluded that the extent of the system of connected pores is limited. Groundwater flow in the upper part of the soil therefore is a combination of bypass and matrix flow, which is still much faster than matrix flow only. The importance of crab burrows in enhancing groundwater flow is illustrated by Wolanski (1992) who reports observation of rainfall infiltrating easily in the soil and producing no local runoff in the presence of crab burrows, while direct runoff does occur in the absence of burrows. Wolanski (1992) and Mazda et al. (1990) have observed the first few mm of water flooding a mangrove swamp, to come from the ground through numerous biopores. In their observations, a thin sheet of advancing water was blocked by high vegetation density and did not flow over obstructions on the ground, but rather through the ground.


83

According to Kruseman and De Ridder (1990) recovery data (or residual drawdown data) are more reliable than pumping test data because recovery occurs at a constant rate, whereas a constant discharge during pumping is often difficult to achieve in the field. This is especially true if pit bailing is used instead of pumping and working conditions are hard. Mangrove trees can survive under unfavourable conditions, but still are adversely affected by anaerobic and reducing conditions, possibly resulting in anoxia, which can occur in saturated soils (McKee, 1993). Koch and Snedaker (1997) report negative effects of soil anoxia and positive effects of root aeration on root development and biomass. McKee (1993) states that seedlings may experience oxygen deficiencies during submergence by tides, since they lack aerial roots. Seedlings may suffer from oxygen deficiencies as well if the soil is saturated with water. Groundwater flow can occur in mangrove forests and influence the vegetation. However, groundwater processes are not explicitly taken into account in hydrological classifications.

6 Discussion

84

85

7 Hydrological classifications The most widely used hydrological tool in mangrove rehabilitation projects is the classification developed by Watson (1928), which recently is extended and improved by Van Loon et al. (2007). Both classifications are given in chapter 2.5. The current research was aimed at verifying the extended classification. Therefore the inundation characteristics found at the piezometer sites during this research are classified according to both the Watson classification and the extended classification. Based on the results of these classifications a new extended hydrological classification is developed.

7.1 Results existing hydrological classifications From the water level measurements the inundation characteristics were calculated for each piezometer site. These are given in Table 5.4 and Table 5.7. Based on these characteristics inundation classes can be assigned to the sites using both classifications. The resulting classes for the measurement sites according to Watson’s classification and the extended classification are given in Table 7.1 and Table 7.2. Based on the observed vegetation at the measuring sites expected inundation classes were defined. The obtained classes based on the inundation characteristics are compared with these expected classes in Table 7.3. Measurement sites where A. alba and R. apiculata are both observed or expected are given the class “2.5”, because Watson (1928) and Van Loon et al. (2007) have not defined a class including both these species. The different zones mentioned in the observed vegetation are described in chapter 5.5. The class boundaries of the parameter elevation in Watson’s classification are referred to admiralty datum. Since the admiralty datum is not known for the study areas, the depth of the river bottom at the open water sites is taken as reference point. This gives unrealistic outcomes, especially in plot C and D. In Watson’s classification, flooding frequency is one of the key parameters. In Table 7.1 it can be seen however that a large error will occur by using this parameter for sites in the open water (A0, C0 and D0). These sites do not fall dry during ebb tides, and therefore the inundation frequency is very low. This leads to a high inundation class for these open water sites baring any vegetation, while at sites with high inundation classes vegetation is expected to occur. Therefore, the parameter flooding frequency gives an unrealistic prediction for these sites. For other sites the error is less, but often the class obtained from the flooding frequency is higher than expected. When comparing the expected inundation classes with the inundation classes of Watson, all open water sites are classified too high. The sites with A. alba and R. apiculata mostly have inundation class 2, which implies only A. alba or S. alba would grow there. Overall, the Watson classification only gives a good result for site A1, A4 and A6. These disadvantages of the use of Watson’s classification have led to the extended classification of Van Loon et al. (2007). In this classification the parameter duration of inundation has been added. This parameter is important in areas with an irregular elevation profile and an irregular semi-diurnal tidal regime. When the elevation profile is regular, classification can be based on the parameter elevation only, independent of the tidal regime. When both elevation and tidal regime are irregular however, the inundation class has to be determined by averaging the classes of the duration of inundation in minutes per day and in minutes per inundation. The measurement sites that are placed in a regular elevation profile, in this case the open water sites A0, C0 and D0 and the sites A1 and A2, are therefore classified by their elevation only. The elevation of each site is determined with respect to local mean sea level, so MVT (plot A and B), MCB (plot C) and MCN (plot D). The open water sites now get the correct inundation class, 1. The extended classification results in the expected inundation classes for half of the measurement sites. When the classes are not similar, the expected classes are higher than the calculated classes, so the sites are classified too wet by the extended classification.

7 Hydrological classifications

86

Table 7.1 Inundation classes attributed to the measurement sites using the Watson classification.

Site Tidal regime Elevation Flooding frequency Average Class

Flooded by Class Above admiralty datum (cm)

Class Times per month

Class

A0 A1 A2 A3 A4 A5 A6

B1 B2 B3

C0 C1 C2a C2b C3

D0 D1 D2 D3

all high tides all high tides

medium high tides normal high tides normal high tides spring high tides spring high tides

medium high tides medium high tides medium high tides

all high tides

medium high tides medium high tides medium high tides medium high tides

all high tides

normal high tides normal high tides normal high tides

1 1 2 3 3 4 4 2 2 2 1 2 2 2 2 1 3 3 3

102 265 345 357 367 369 374

336 334 313

0

119 118 111 115

0

155 104 104

1 2 3 3 3 3 3 3 2 2 1 1 1 1 1 1 1 1 1

16 47 41 35 22 17 11

43 44 51 1 22 19 28 21

14 18 23 30

4 2 3 3 3 4 4 3 3 2 5 3 4 3 3 4 4 3 3

2 2 3 3 3 4 4 3 2 2 2 2 21

2 2 2 3 2 22

1 Measuring period only 8 days. 2 Measuring period only 3 days.

Table 7.2 Inundation classes attributed to the measurement sites using the extended classification of Van Loon et al. (2007).


Site Tidal regime Elevation Flooding frequency Duration of inundation Duration of inundation Average Class Class cm +MSL Class Times per month Class Min per day Class Min per inundation Class

A0 A1 A2 A3 A4 A5 A6

B1 B2 B3

C0 C1 C2a C2b C3

D0 D1 D2 D3


medium high tides normal high tides normal high tides spring high tides spring high tides

medium high tides medium high tides medium high tides

all high tides

medium high tides medium high tides medium high tides medium high tides

all high tides

normal high tides normal high tides normal high tides

1 1 2 3 3 4 4 2 2 2 1 2 2 2 2 1 3 3 3

-151 12 92 104 114 116 121

83 81 60

-87 32 30 24 28

-70 46 35 35

1 2 3 3 3 3 3 2 2 2 1 2 2 2 2 1 2 2 2

16 47 41 35 22 17 11

43 44 51 1 22 19 28 21

14 18 23 30

4 2 3 3 3 4 4 3 3 2 5 3 4 3 3 4 4 3 3

1 419 802 296 254 149 118 100

304 463 500

1 440 213 333 386 376

1 364 131 280 421

1 1 3 3 3 3 4 3 2 2 1 3 3 3 3 1 3 3 2

2 735 520 220 219 209 207 275

218 324 300

41 420

292 526 418 536

3 021 221 367 433

1 1 2 2 2 2 2 2 2 2

1 2 1 1 1 1 2 2 1

1 2 3

2.5 2.5 2.5 3

2.5 2 2 1

2.5 21

2 2 1

2.5 2.5 1.52


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Table 7.3 Differences between expected and calculated inundation classes (Av = Avicennia; Rh = Rhizophora apiculata).

Site Observed vegetation

Expected class

Classes Watson

Difference Expected and

Watson

Classes Van Loon

Difference Expected and Van

Loon

A0 A1 A2 A3 A4 A5 A6

B1 B2 B3

C0 C1 C2a C2b C3

D0 D1 D2 D3

None Av alba

Av and Rh zone Rh plantation Mixed zone Mixed zone

Rh plantation

Rh plantation Rh plantation Rh plantation

None

Mixed zone Mixed zone Mixed zone Mixed zone

None

Mixed zone Mixed zone Mixed zone

1 2

2.5 2.5 3 3 4

2.5 2.5 2.5

1

2.5 2.5 2.5 2.5

1

2.5 2.5 2.5

2 2 3 3 3 4 4 3 2 2 2 2 2 2 2 2 3 2 2

1 0

0.5 0.5 0 1 0

0.5 -0.5 -0.5

1

-0.5 -0.5 -0.5 -0.5

1

0.5 -0.5 -0.5

1 2 3

2.5 2.5 2.5 3

2.5 2 2 1

2.5 2 2 2 1

2.5 2.5 1.5

0 0

0.5 0

-0.5 -0.5 -1 0

-0.5 -0.5

0 0

-0.5 -0.5 -0.5

0 0 0 -1

7.2 Errors and uncertainties in existing classifications In both the Watson classification and the extended classification no inundation class is defined in which A. alba and R. apiculata are expected to grow side by side. In the classifications species are allocated to the inundation class representing optimum conditions for species to regenerate itself (Van Loon et al., 2007, Watson, 1928). Conflicting statements have been reported about the role of inundations for seedling establishment and mature trees. According to Lewis (2005) flooding depth and duration and frequency of inundation are critical factors in the survival of both mangrove seedlings and mature trees. This is supported by Brockmeyer et al. (1997) who report elimination of mangrove species with short pneumatophores due to permanent high water levels at impounded sites in Florida (USA). Chapman (1976) however states that water level and number of inundations are probably very important in establishment of seedlings, but of less importance for adult trees. Salinity is thought to be more important for mature plants, which is related to water level (and hence time available for freshwater influences), proximity of creeks and the efficiency of drainage according to Chapman. So, indirectly water level and inundation frequency also influence growth of mature trees. Probably mature trees are less sensitive than seedlings and therefore can survive under a broader range of conditions (Watson, 1928).

7.2 Errors and uncertainties in existing classifications

89

Several lists of optimum requirements of different species with regard to inundation are published by Watson (1928), Hong and San (1993) and the MAB Vietnam National Committee (1998). In appendix E a comparable list can be found, only including species that were observed during this research. Watson (1928) underlines that most species can exist within much wider extremes than the conditions in the class attributed to them. This is observed in this research. Although in vegetation investigations in this research the emphasis was mainly on species composition in general, an estimation has been made of the suitability of sites for regeneration of species based on the development of individuals and the occurrence of young trees. Mixed zones including both A. alba and R. apiculata are frequently observed, as seen in chapter 5.5. Although optimum conditions for these species are not in the same range, conditions in the zone where they both occur seem sufficient for natural establishment and development of both (Figure 7.1).

Figure 7.1 Relation between inundation class and growing conditions for 4 species. The figure is based on lists

of optimum requirements with respect to inundation (Watson, 1928, Hong and San, 1993 and MAB Vietnam National Committee, 1998) and field observations.

In Figure 7.1 can be seen that based on hydrological conditions there is a substantial reason to expect A. alba and R. apiculata in one zone. This zone lies around inundation class “2.5”. Based on the observed vegetation at the measurement sites, an expected inundation class for each site is determined (Table 7.3). The allocation of expected inundation classes led to a class “2.5” at several sites, because at these sites both A. alba and R. apiculata occurred. For all sites in plot B for example the expected class is “2.5”, since in the dense R. apiculata plantation some naturally regenerated A. alba were observed. This indicated that the sites are suitable for A. alba to settle and for R apiculata to grow very well. Classifying sites with the extended hydrological classification by Van Loon et al. (2007) also results in class “2.5” frequently. This class is the average class of the two components of the parameter duration of inundation (minutes per day and minutes per inundation). During the field study in this research, differences in hydrological conditions and vegetation were observed between sites classified in class “2.5”. The class boundaries as suggested by Van Loon et al. (2007) in the extended hydrological classification are relatively broad. This causes sites with a variety of conditions to end up in the same class. For example, based on the main parameter duration of inundation, site A3 to A5 were classified as “2.5”. However, inundation occurred in different ways and elevation and vegetation at the sites were different. Next to this, it is not desirable to assign numbers with a decimal component to sites, since this yields too much open space for interpretation.


90

7.3 New hydrological classification Based on the uncertainties in the extended classification and observations in this research, the definition of an extra class, 2*, between class 2 and 3 of that classification is suggested. This adaptation copes with the variations between class 2 and 3, emphasises the differences in hydrological conditions and vegetation and avoids the class “2.5”. The indication 2* for the additional class is chosen to make sure the other inundation classes retain their original indication. This ensures the possibility of comparison with the Watson and the extended classification and all the related works. Class boundaries of 2* are determined from the inundation characteristics of the measurement sites of the fieldwork of 2007, while validation of the classes is based upon the inundation characteristics determined by Van Loon in 2005 (appendix H). The new hydrological classification is presented in Table 7.4. The parameter frequency of inundation is omitted, since classification based on this parameter yields unrealistic results. The number of inundations is however still relevant in the new classification, since it is needed to calculated the duration per inundation. So, indirectly the inundation frequency is affecting the result of the classification, but its influence is far less than in Watson’s classification and it does not yield unrealistic results anymore. The parameter tidal regime is still included in the classification. When it is impossible to acquire data on the elevation and duration of inundation, this parameter can give an indication of the inundation class for a certain site. However this is only an indication and it is always preferable to use data on inundation characteristics for a better determination of the inundation classes. Besides this, the parameter can be interpreted in different ways. Tomlinson (1994, in Schmitz et al., 2006) suggests a less arbitrary scale for it. He states that inundation classes one, two, three and four in Watson’s classification correspond to an area being inundated by respectively 100-76, 75-51, 50-26 and 25-5% of the high tides. However, this scale is not known to be used in any hydrological classification. Therefore we suggest the use of piezometers and the calculation method used in this research. Lewis (2005) also suggests the installation of tide gauges to measure the tidal hydrology among his list of restoration principles for mangrove forests. The vegetation in each class of the new classification is highly comparable to the vegetation in the equivalent classes of Watson and the extended classification, since this vegetation division is directly adapted from these classifications. Some adjustments have been done, based on observed vegetation and supported by literature (Watson, 1928, Hong and San, 1993, the MAB Vietnam National Committee, 1998). Compared to the extended classification Avicennia spp. in class 2 is replaced by Avicennia alba, since optimum conditions for A. officinalis and A. marina are found in higher classes. The new class 2* mainly indicates hydrological conditions that are suitable for both Avicennia spp. and Rhizophora spp. Although in this research dominance of A. alba and R. apiculata has been observed, A. officinalis, A. marina and R. mucronata are also seen and expected to develop well in class 2*. The allocation of R. apiculata and R. mucronata in a lower class than Watson and Van Loon et al. did, seems justified by our observations and reports of other authors. Haron (in Khoon and Eong, 1995) for example reports mortality rates of R. apiculata seedlings to be higher than 50% in inundation class 1, 4 and 5, and considerably lower in class 2 and 3. Watson (1928) reports large stands of R. mucronata in inundation class 2. Optimum growing conditions for B. parviflora are reported by Hong and San (1993) to be found in class 2 and 3 and according to Chapman (1976) B. parviflora is “a species that seems peculiarly able to tolerate water-logging and hence is found in depressions”. This is in accordance with the observations of this species in plot C and D, where water logging occurred at sites in basins classified by class 2*. Therefore, addition of B. parviflora to the list of vegetation in this class seems justified as well. Like in the extended classification, Ceriops is listed in class 3 and Ceriops spp. in class 5, while neither one of the indications is present at class 4. This seems rather conflicting, but no adaptation is done here since no appropriate observations regarding this issue have been done in this research.

7.4 Application of the new hydrological classification

91

7.4 Application of the new hydrological classification To determine inundation classes for sites in mangrove forests, the parameters elevation and duration of inundation in minutes per day and in minutes per inundation are averaged. The average class of these three parameters can be rounded to obtain an integer. No division has to be made between a regular elevation profile and an irregular elevation profile; all sites are classified in the same way. This makes the classification easy in use and it leaves no open space for interpretation. The inundation classes of the measurement sites in this research according to the new classification are given in Table 7.5. The expected classes for each site are also displayed in this table. Comparison reveals that the expected and calculated inundation classes do not match at only four measurement sites (indicated in red colour in Table 7.5). These measurement sites include C2a and D3, where was measured for a very short period; therefore the data are less reliable. In Table 7.5 it can be seen that classification based on the number of minutes per inundation reveals little variation; class 2* is assigned to the majority of the sites. Only the duration of inundation expressed in minutes per day, does not give satisfactory results either. The calculated duration of inundation for each measurement site does represent the actual hydrological situation in the field however. In plot A for example, there is little variation between the sites in the duration of inundation after flooding, but there is a considerable difference in the frequency of inundations per month. So, duration per inundation does not decrease so much inland, while duration per day does. The latter is due to elevation differences and variations in tidal amplitude. As shown before, classification based on frequency however, does not give satisfactory results and does not represent the actual hydrological situation. Averaging of duration per inundation and duration per day does give good results, despite the fact that individual use of the parameters is not desirable. So, both parameters and the parameter elevation are needed to obtain realistic classes. The actual number of minutes of inundation also gives a good overview of the differences between sites and makes determination of the relevant processes at the sites possible. Micro-topography highly determines the duration of inundation and inundation frequency in Can Gio and Ca Mau. The relation between micro-topography and tidal influences in mangrove forest is also investigated by Knight et al. (2007). In their research the limitations of inundation classes based on flooding frequency are discussed for the area of Coombabah Lake, southeast Queensland, Australia. The classification of sites, based on the flooding frequency in this area experiencing semi-diurnal tides, resulted in only the classes 4 and 5. No typical zonation of species was found in the research area, since no regular elevation profile existed. Instead several types of basins were identified, including basins with impeded overland flow that stayed inundated for longer periods. (Knight et al., 2007) This emphasizes the need for a classification with parameter ‘duration of inundation’ and confirms that micro-topography is abundantly present and plays an important role in mangrove forests. The importance of duration of inundation was also mentioned by Chapman (1976). He indicates that “the most important aspect of the tidal phenomenon is the number of consecutive days at any one level when no tide covers the area.” So, instead of the duration of inundation he names the contrary duration of exposure as the factor determining the ability of species to regenerate in a certain area. Of course these factors are strongly related. The least inundated measurement site during this research is site A6. This site has a lower inundation class than expected; this could mean the classification is not suitable for drier parts of mangrove forests. The expected class on this site is based on observations of naturally generated Acrostichum aureum in gaps between planted R. apiculata. Because of the unnatural conditions in the plantation, the expected class might be inaccurate. More research is needed to test the classification in drier parts with natural vegetation. An adaptation of the boundaries of the higher inundation classes might be relevant, but a suggestion is not given here since no measurements have been done at a sufficient number of dry sites. The error made by using the method of calculating inundation characteristics as described in chapter 5.4.1.3 and discussed in chapter 6.3.2, is very small if used in combination with the new classification. The class boundaries in the new classification are based on values determined with this method, so by using the combination of the calculation method and the new classification the underestimation of duration of inundation influences the final result only slightly.

7 Hyd

rolog

ical classification

s

92

Table 7.4 New hydrological classification and the southeast Asian mangrove species attributed to each class.

Inundation class

Tidal regime Elevation

cm + MSL

Duration of inundation min per day


min per inundation

Vegetation

species

1 2 2* 3 4 5

all high tides lower medium high tides higher medium high tides

normal high tides spring high tides equinoctial tides

< 0 0 - 50

50 - 100 100 - 150 150 - 210

> 210

> 800 400 - 800 250 - 400 150 - 250 10 - 150

< 10

> 600 450 - 600 200 - 450 100 - 200 50 - 100

< 50

none Avicennia alba, Sonneratia

Avicennia spp., Rhizophora spp, Bruguiera parviflora Rhizophora spp., Ceriops, Bruguiera

Lumnitzera, Bruguiera, Acrostichum aureum Ceriops spp., Phoenix paludosa

Table 7.5 Inundation classes attributed to the measurement sites using the new classification of Table 7.4.


Site Tidal regime Elevation Duration of inundation Duration of inundation Average Class Expected Class Class cm +MSL Class Min per day Class Min per inundation Class

A0 A1 A2 A3 A4 A5 A6

B1 B2 B3

C0 C1 C2a C2b C3

D0 D1 D2 D3


higher medium high tides normal high tides normal high tides normal high tides spring high tides

higher medium high tides higher medium high tides lower medium high tides

all high tides

higher medium high tides lower medium high tides lower medium high tides higher medium high tides

all high tides

normal high tides higher medium high tides higher medium high tides

1 1 2* 3 3 3 4

2* 2* 2 1 2* 2 2 2* 1 3 3 3

-151 12 92 104 114 116 121

83 81 60

-87 32 30 24 28

-70 46 35 35

1 2 2* 3 3 3 3

2* 2* 2* 1 2 2 2 2 1 2 2 2

1 419 802 296 254 149 118 100

304 463 500

1 440 213 333 386 376

1 364 131 280 421

1 1 2* 2* 4 4 4

2* 2 2 1 3 2* 2* 2* 1 4 2* 2

2 735 520 220 219 209 207 275

218 324 300

41 420

292 526 418 536

3 021 221 367 433

1 2 2* 2* 2* 2* 2*

2* 2* 2*

1 2* 2 2* 2 1 2* 2* 2*

1 2 2* 2* 3 3 3

2* 2* 2* 1 2* 21 2* 2 1 2* 2* 22

1 2 2* 2* 3 3 4

2* 2* 2* 1 2* 2* 2* 2* 1 2* 2* 2*


94

95

8 Conclusions The conclusions from this research are divided over the two main parts. Firstly some conclusions about the general hydrologic characteristics based on the field measurements are given. Secondly conclusions about the hydrological classifications are given.

8.1 Conclusions characterization of mangrove hydrology - The tidal regime in both Can Gio and Ca Mau is irregular semi-diurnal. In Can Gio the amplitude

is much larger than in Ca Mau. The tidal regime at Mui Ca Mau has a strong diurnal component. The differences between the tides are caused by the different tidal regimes in the South China Sea and the Gulf of Thailand. The tides at Mui Ca Mau are highly influenced by the South China Sea through Cua Lon river. The predicted tidal regime in Vung Tau resembles the tidal regime in Can Gio, only with an average time lag between the high tides at both locations of 35 minutes. For Ca Mau the predicted tidal regimes of the closest tidal stations do not correspond with the tidal regime in the study area.

- Micro-topography is an important factor influencing water flow in the mangrove forest. Ridges and basins are present in several directions, parallel as well as perpendicular to the main channel. Ridges are mainly formed by creeks and gullies. This micro-topography impedes overland flow at small water depths and creates longer flow paths. Water then has to be discharged through the soil, gullies and creeks.

- Indirect flow paths, friction of the soil surface and obstructions in the mangrove forest cause a retardation of the water during flood tide.

- In Can Gio a high biodiversity is found in plot A within the mixed zones. The dominant species found in Ca Mau are A. alba, R. apiculata and B. parviflora.

- The creeks with larger catchments retain a base flow during ebb tide, consisting of retarded water from lengthy flow paths, like groundwater, rainwater or water from earlier inundations. This base flow is blocked by incoming flood, causing a backwater effect. Water levels rise before the direction of flow changes at the water surface. The base flow also contributes to higher flow velocities at ebb tide than at flood tide.

- Creek flow is an important factor determining the inundation frequency and duration of inundation. Some areas can not be reached by overland flow during periods of low tidal range, but only via sub-creeks. Indirect discharge through creeks is slower than direct overland flow.

- The hydraulic bulk conductivities found in this research indicate that groundwater flow can influence inundation dynamics of sites. The hydraulic conductivity decreases with depth, because it highly depends on density of biopores. The pumping and recovery tests are useful methods to determine the bulk hydraulic conductivity of the upper mangrove soil.

- Biopores in the upper soil are interconnected, although to a certain extent. Therefore groundwater flow in the upper mangrove soil is a combination of bypass flow and matrix flow, while in the deeper soil transport is dominated by matrix flow only.

8.2 Conclusions hydrological classification - Micro-topography has a considerable effect on duration of inundation, since it impedes overland

flow. This is not necessarily caused by one extensive ridge. Therefore the duration of inundation gives a good indication of the hydrological conditions at sites, because sites remain inundated longer than expected based on their elevation and a gradually rising profile.

- Observations of micro-topography in a mangrove forest are an indication of prolonged durations of inundation. Long durations of inundation at a site indicate the presence of micro-topography or a low elevation.

- The classification of Watson (1928) does not give satisfactory results in describing the hydrological characteristics at the measurement sites of this research. The parameter inundation frequency and the reference of elevation to admiralty datum result in unrealistic inundation classes for almost all sites.

8 Conclusions

96

- Assigning inundation classes to the measurement sites using the extended classification of Van Loon et al. (2007) yields better results than using Watson’s classification. Some sites are classified too wet however.

- At many sites a mixed zone with A. alba and R. apiculata was observed. In the existing classifications no inundation class with both species is distinguished. An extra class, 2*, is proposed in a new classification to describe both the variation in hydrology and vegetation in the wet parts of mangrove forests in more detail. Several mixed vegetation zones occur in the original classes 2 and 3, which do not all have the same hydrological conditions. The extra class emphasizes the differences in vegetation and hydrologic conditions in these zones.

- Frequency of inundation can not be used as a parameter in hydrological classifications in areas with an irregular tidal regime. Sites that remain inundated between tidal peaks have a low inundation frequency and are classified too dry by this parameter. Flooding frequency is omitted from the new classification.

- The new classification is suitable for different elevation profiles. In both regular and irregular elevation profiles the same parameters, elevation and duration of inundation, are used for classifying the sites. This makes the classification very practical in use and only requires relative simple measurements in the field.

- In both Can Gio and Ca Mau the tidal regime is irregular semi-diurnal. However in Ca Mau there is a strong diurnal component, in both cases the new classification gives correct results. So, it appears to be valid for different tidal regimes.

- The new hydrological classification gives promising results for mangrove rehabilitation projects. The sites can be classified more accurately with the extra class, especially in the mixed zones.

97

9 Recommendations Several recommendations for further research based on the results of this research are presented in this chapter: - Verification of the new hydrological classification is needed at different locations. Especially in

drier mangrove parts this is needed, since only one measurement site in this research was expected to have inundation class 4.

- More research is needed in areas with different tidal regimes to test if the classification is valid for e.g. regular diurnal tides. In a wider perspective, different mangrove systems, like in America and the west coast of Africa, give good opportunities for further research and development of hydrological classifications.

- Longer measuring periods are needed to include different seasons. This research was mostly carried out during the dry season; conditions could be very different during the rainy season.

- The behaviour of a small water layer on the surface should be investigated to find the importance of the small-scale variations in water level and effects of the inaccuracy in surface level. Surface level changes during the two-weekly tidal cycle, as observed in this research, may also be an interesting subject of research.

- The effects of prolonged inundations with shallow water levels, caused by micro-topography, on mangrove vegetation could be assessed in more detail. Further research should take into account the differences between the effect of water logging by tidal flooding or by rain water.

- To be able to test whether a difference between in- and outflow occurs through creeks, net discharges have to be calculated. Therefore continuous velocity and water level measurements are needed during complete tidal cycles. Also an appropriate estimation of overbank discharge should be made in this case.

- Vegetation has to be recorded in more detail. Information about the regeneration of species at different sites is desirable to determine the hydrological conditions the species require for regeneration.

- The allocation of different species in the inundation classes in the new classification is based on Watson’s classification and the observed vegetation during this research. More information is needed about the optimum requirements of mangrove species to improve the division of species in the inundation classes. This could also lead to improvement of the boundaries of the inundation classes. At present, conflicting reports are available in literature

9 Recommendations

98

99

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Appendix A Abstracts earlier research Abstract Van Loon (2005): “Water flow and tidal influence in a mangrove-delta system; Can Gio, Vietnam”.1 The mangrove ecosystem is a unique system, located along (sub-)tropical coasts. It is characterized by complex land-sea interactions. The composition and extent of the mangrove vegetation are dependent on the interaction between climate, soil and hydrology. The main objective of this exploratory research is to provide information on the hydrological functioning of a mangrove-delta system, and its interactions with the mangrove vegetation. It aims at theory-building based on actual field measurements in Can Gio, Vietnam. A delta system is characterized by some large rivers and a dense network of meandering creeks of various sizes. The river discharge is divided over the creek system, depending on the tidal regime and the amount of river discharge. Seawater penetrates into the creek system at flood and inundates a large part of the land surface. The combined effect of river and seawater causes continuous variation in surface and groundwater conditions (both physical and chemical). The tidal regime in Can Gio is irregular semi-diurnal. The tidal wave entering the creek system experiences a time lag and distortion due to the effects of friction and baseflow. The result is a tidal asymmetry, both in the vertical and horizontal tide. In Can Gio peak ebb velocities were a factor two higher than peak flood velocities. The penetrating seawater can also block fresh water discharge. In the dry season zones with low salt concentrations develop in the finer branches of the creek system. The mangrove swamp behaves like a reservoir, that empties through overland flow and groundwater flow during low tide and recharges when flood water reaches the site. In regions where the inland area is lower than the coastal zone, i.e. behind a creek levee, flood water can not be discharged via overland flow. Consequently, the discharge is slower and the inundation period longer than in regions with a gradually rising surface. Groundwater flow is a combination of matrix and bypass flow. The matrix permeability of the clay soil in Can Gio is in the order of 10−4 m/d, while secondary permeability can be as high as 103 m/d due to connected biopores in the upper part of the soil. The tidal inundation factors: duration, frequency and height of flooding, determine the composition and distribution of the mangrove vegetation. The different vegetation zones were classified according to the classification of Watson [1928], leading to the conclusion that in the transition zone the hydrological conditions are favorable for the establishment of various mangrove species. Due to changing environmental conditions, the borderline between the vegetation zones will shift and natural regeneration will result in a mixed zone bordering the mangrove plantation. More research is needed on the hydrological system of the delta area and its interaction with the soil characteristics and mangrove vegetation. The formulated theories require verification by longer and more detailed field measurements. Keywords: Delta system; Vietnam; Hydrology; Groundwater; Tides; Mangrove ecology

1 VAN LOON, A. F. (2005) Water flow and tidal influence in a mangrove-delta system Can Gio, Vietnam. Department of Environmental Sciences. Wageningen, Wageningen University.

Appendix A Abstracts earlier research

Abstract A.F. van Loon, R. Dijksma and M.E.F. van Mensvoort (2007): Hydrological classification in mangrove areas: A case study in Can Gio, Vietnam.2 The common hydrological classification for mangroves developed by Watson [Watson, J.G., 1928. Mangrove forests of the Malay Peninsula. Malayan Forest Records No. 6. Forest Department, Federated Malay States, Kuala Lumpur] is not sufficient in mangrove areas with an irregular tidal regime and elevation, such as Can Gio in Vietnam. Incorporating more detailed hydrological information in the Watson classification gave promising results for the measuring sites in Can Gio, but should be tested in other mangrove areas.

2 VAN LOON, A. F., DIJKSMA, R. & VAN MENSVOORT, M. E. F. (2007) Hydrological classification in mangrove areas: A case study in Can Gio, Vietnam. Aquatic Botany, 87, 80-82.

Appendix B Coordinates of the piezometer locations

Table B.1 Coordinates of the piezometer locations.

Coordinates Piezometer N E

A0 10 25.844’ 106 51.607’ A1 10 25.237’ 106 51.997’ A2 10 25.110’ 106 52.185’ A3 10 25.121’ 106 52.253’ A41 10 25.167’ 106 52.226’ A5 10 25.145’ 106 52.320’ A6 10 25.210’ 106 52.421’ B11 10 29.544’ 106 52.734’ C0 08 39.081’ 104 48.664’ C1 08 39.218’ 104 48.263’

C2a1 08 39.126’ 104 48.279’ C2b1 08 39.111’ 104 48.213’ C3 08 39.059’ 104 48.215’ D0 08 42.944’ 104 48.872’ D1 08 43.379’ 104 48.883’ D2 08 43.497’ 104 48.973’ D3 08 43.553’ 104 49.020’

1 The coordinates of these sites are an indication of the exact locations of the piezometers, since accurate determination of the coordinates was not possible at these sites due to the dense canopy.

Appendix B Coordinates of the piezometer locations

Appendix C Tidal predictions In this appendix graphs of the investigated tidal predictions are presented. In appendix C1 the tidal predictions for Vung Tau are shown, in appendix C2 the predictions for Dinh An and in appendix C3 the predictions for Ha Tien. The predictions for Vung Tau and Ha Tien are shown for the period from 1 January to 31 December 2007, while the predictions for Dinh An are only given for half a year; 1 January to 30 June 2007.

Appendix C Tidal predictions

Appendix C1 Vung Tau

Figure C.1 Predicted water levels at the port of Vung Tau; a) 1 January to 1 April 2007, b) 1 April to 1 July 2007 (after: Marine Hydrometeorological Centre, Vietnam (unknown)).

Appendix C1 Vung Tau

Figure C.2 Predicted water levels at the port of Vung Tau; a) 1 July to 1 October 2007, b) 1 October 2007 to

1 January 2008 (after: Marine Hydrometeorological Centre, Vietnam (unknown)).


Appendix C2 Ha Tien

Figure C.3 Predicted water levels Ha Tien; a) 1 January to 1 April 2007, b) 1 April to 1 July 2007

(after: Marine Hydrometeorological Centre, Vietnam (unknown)).

Appendix C2 Ha Tien

Figure C.4 Predicted water levels at Ha Tien; a) 1 July to 1 October 2007, b) 1 October 2007 to 1 January 2008 (after: Marine Hydrometeorological Centre, Vietnam (unknown)).


Appendix C3 Dinh An

Figure C.5 Predicted water levels Dinh An; a) 1 January to 1 April 2007, b) 1 April to 1 July 2007 (after: Marine Hydrometeorological Centre, Vietnam (unknown)).

Appendix D Graphs of the water levels In this appendix graphs can be found of the measured water levels in all piezometers in all plots. Appendix D1 contains graphs of water levels in plot A, appendix D2 of plot B, appendix D3 of plot C and in appendix D4 graphs of plot D are shown.

Appendix D1 Plot A

Figure D.1 Measured water levels in plot A from 3 March to 24 March 2007.

Appendix D Graphs of the water levels

Figure D.2 Measured water levels in plot A from 24 March to 14 April 2007.

Appendix D1 Plot A

Figure D.3 Measured water levels in plot A from 14 April to 5 May 2007.


Figure D.4 Measured water levels in plot A from 5 May to 26 May 2007.

Appendix D2 Plot B

Appendix D2 Plot B

Figure D.5 Measured water levels in plot B from 2 March to 24 March 2007.


Figure D.6 Measured water levels in plot B from 24 March to 15 April 2007.

Appendix D2 Plot B

Figure D.7 Measured water levels in plot B from 15 April to 7 May 2007.


Figure D.8 Measured water levels in plot B from 7 May to 27 May 2007.

Appendix D3 Plot C

Appendix D3 Plot C

Figure D.9 Measured water levels in plot C from 20 April to 21 May 2007.


Appendix D4 plot D

Figure D.10 Measured water levels in plot D from 21 April to 21 May 2007.

Appendix E Observed vegetation The table below gives a list of observed vegetation species and their optimal requirements with regard to inundation classes, soil type and position, according to Watson (1928) and MAB Vietnam National Committee (1998). Table E.1 Optimal requirements of observed vegetation species, according to Watson (1928) and MAB Vietnam

National Committee (1998).

Inundation classes Species Watson (1928) MAB Vietnam

National Committee

(1998)

Soil and position

Acanthus ilicifolius 4-5 2-3 Loam, silt-clay with fine sand on river banks and estuaries

Acrostichum aureum 3-4-5 4-5 Firm mud on river banks, in clearings Avicennia alba 2 1-2 Deep mud, sea face, river banks

Avicennia officinalis 3-4 2-3 Loam clay on river banks on degraded soil

Avicennia marina - 2 Deep sandy mud, sea face Bruguiera parviflora 3-4 - Soft mud, along river banks

Ceriops decandra - 3-4 Firm mud, river banks, land fringe Ceriops tagal 3-4 3-4-5 Quite firm mud, under Rh canopy

Excoecaria agallocha 4-5 4-5 Clay, firm mud on river banks Lumnitzera racemosa 4-5 4-5 Clay, sandy loam, firm mud on river

banks Nypa fruticans 3-4-5 2-3 River bank, low brackish water

Rhizophora apiculata 3-4 1-2 Deep soft mud, river banks Rhizophora mucronata 2-3 - Deep soft mud, river banks, sandy mud

Sonneratia alba 3-4 1-2 Deep soft mud, sea face Sonneratia ovata - 2-3 Soft mud, salty water, islands

Appendix E Observed vegetation

Location 4 Location 5

Location 6

Location 1

Location 2 Location 3

Appendix F Locations discharge measurements

Figure F.1 Locations of the discharge measurements in plot A.

Figure F.2 Locations of the discharge measurements in plot B.

Appendix F Locations discharge measurements

Figure F.4 Locations of the discharge measurements in plot D.

Location 3

Location 4

Figure F.3 Locations of the discharge measurements in plot C.

Location 1

Location 2

Appendix G Graphs of the permeability tests Below graphs of the water levels in the pits during the permeability tests are shown. Water level in every pit that is used for calculation of k-values is plotted and the figures are divided per measuring location. Site A1:

Site A2:

Site A3:

15

17

19

21

23

25

27

29

0 500 1000 1500 2000 2500 300012

13

14

15

16

17

18

19

20

21

22

0 500 1000 1500 2000 2500

25.5

26

26.5

27

27.5

28

28.5

29

0 200 400 600 800 1000 1200 1400

22.5

23

23.5

24

24.5

25

25.5

26

0 500 1000 1500 2000

25

25.2

25.4

25.6

25.8

26

26.2

26.4

26.6

26.8

27

0 200 400 600 800 1000 1200 1400 160021.5

22

22.5

23

23.5

24

0 500 1000 1500 2000 2500

Appendix G Graphs of the permeability tests

Site A4:

Site B1:

Site B2:

12

14

16

18

20

22

24

26

28

0 500 1000 1500 2000 2500 3000 3500 4000

12

13

14

15

16

17

18

19

20

21

22

0 500 1000 1500 2000 2500 300010

12

14

16

18

20

22

24

26

28

0 500 1000 1500 2000 2500

17

18

19

20

21

22

23

24

25

26

27

0 500 1000 1500 2000 2500 3000

19.8

20

20.2

20.4

20.6

20.8

21

21.2

21.4

21.6

0 500 1000 1500 2000 2500 300012

14

16

18

20

22

24

0 500 1000 1500 2000 2500


Site C1:

Site C2:

Site C3:

30

31

32

33

34

35

36

37

0 500 1000 1500 2000 2500 3000

30

31

32

33

34

35

36

0 200 400 600 800 1000 1200 1400 1600 1800

28.5

29

29.5

30

30.5

31

31.5

32

0 50 100 150 200 250 300

28.5

29

29.5

30

30.5

31

31.5

32

32.5

0 20 40 60 80 100 120 140 160

32.8

33

33.2

33.4

33.6

33.8

34

34.2

34.4

34.6

34.8

0 20 40 60 80 100 120 140


Site C4:

Site C5:

Site C6:

30

30.5

31

31.5

32

32.5

33

33.5

0 500 1000 1500 2000 2500 3000 350030

30.5

31

31.5

32

32.5

33

33.5

34

34.5

35

0 200 400 600 800 1000 1200 1400 1600

30

30.5

31

31.5

32

32.5

33

33.5

34

34.5

35

0 500 1000 1500 2000 2500

27

29

31

33

35

37

39

0 200 400 600 800 1000 1200

20

20.5

21

21.5

22

22.5

23

23.5

24

24.5

25

0 500 1000 1500 2000 2500

27

28

29

30

31

32

33

34

0 100 200 300 400 500 600 700 800

20

22

24

26

28

30

32

34

36

0 500 1000 1500 2000 2500 300017

19

21

23

25

27

29

31

0 500 1000 1500 2000 2500 3000 3500


Site D1:

Site D2:

Site D3:

22

24

26

28

30

32

34

0 200 400 600 800 1000 1200 1400 1600

15

17

19

21

23

25

27

29

31

33

35

0 500 1000 1500 2000 2500 3000 3500 4000 45009

11

13

15

17

19

21

23

25

0 500 1000 1500 2000 2500

25.1

25.2

25.3

25.4

25.5

25.6

25.7

25.8

25.9

26

26.1

0 10 20 30 40 50 60 7031

31.5

32

32.5

33

33.5

34

0 500 1000 1500 2000 2500 3000 3500


Site D4:

24

25

26

27

28

29

30

31

32

33

0 200 400 600 800 1000 1200 1400 1600 180020

22

24

26

28

30

32

34

0 100 200 300 400 500 600 700 800 900

20

22

24

26

28

30

32

34

0 200 400 600 800 1000 1200 1400

Appendix H Verification of the new classification The new classification was verified with the data of Van Loon (2005). In the table below the inundation classes for each site according to the Watson classification, the extended classification by Van Loon et al. (2007) and the proposed new classification are given. These classes are compared to the expected classes based on vegetation descriptions. From these comparisons it is concluded that the new classification gives good results for the selected sites. Table H.1 Comparison of inundation classes attributed to measurements sites of Van Loon (2005), according to

the Watson classification, extended classification and new classification.

Site Watson Extended New Expected Vegetation A0 2 1 1 1 Creek A1 2 1.5 2 2 Mudflat-Av zone A2 2 2 2* 2* Av- transition A3 3 3 3 3 transition A4 4 3 3 3 transition- Rh zone B0 2 1.5 2 2 Mudflat B1 4 2.5 3 3 Rh zone

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