Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển

115

ĐÁNH GIÁ SỰ TƯƠNG TÁC GIỮA KHỐI NƯỚC VÙNG CỬA SÔNG
MÊ KÔNG VÀ VÙNG NƯỚC TRỒI NAM TRUNG BỘ

Lê Đình Mầu
1
, Bùi Hồng Long
1
, Thomas Pohlmann
2
, Bùi Xuân Thông
3
, Hartmut Hein
5
,
Đoàn Văn Bộ
4
, Nguyễn Kim Vinh
1
, Birte Hein
2,5
, Nguyễn Văn Tuân
1
, Phạm Sỹ Hoàn
1

và Nguyễn Chí Công
1

masses. Using water mass analysis, remote-sensing technique and modeling
methods, study results show that the features of nearshore bathymetry, tide,
river discharge and monsoonal circulation along the west coast of the South
China Sea are the main causes of the interactive processes between Mekong
River mouth and upwelling water masses. Preliminary studies show that the
interactive processes extend to the area between Binhthuan and Camau and
from the coast to Phyquy and Condao Islands.
I. INTRODUCTION
Mekong River mouth and upwelling areas are the most important regions for economical
development and environmental protection of Vietnam. The governing processes, structure
and variation of the upwelling area, the hydro-litho-dynamical processes in the Mekong River
mouth area, the interactive processes between Mekong River mouth water and upwelling
Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển 116
water masses and their ecology-environmental effects are of most concern for oceanographers
in Vietnam and internationally. To solve these scientific questions the co-operation project
between Vietnam and Germany on oceanography was established. Phase–I (2003-2006):
study on the forming processes, structure, variation and ecology-environmental effects of
upwelling area. Phase-II (2007-2010): study on the hydro-litho-dynamic processes, ecology-
environmental features in the Mekong River mouth area and the interactive processes between
Mekong River mouth and upwelling water masses. The co-operative project was carried out
based on most advanced measuring instruments, data analysis and modeling techniques. The
co-operative project results form an important scientific base for marine economical
development and environmental protection of Vietnam. The content of this paper is as
follows.

1. General scientific results of the co-operative project.
2. Assessment of the interactive processes between Mekong River mouth and upwelling

Models: ECOMSED (HydroQual, Holland) and
HAMSOM (Module: SPM)
5
Hydro-ecology-environmental
processes
Model: ROMS (USA)
6
Water exchange rate at a section
Ekman formula [3], [7]
7
Stability of water column
Model of Potential Energy Anomaly (Hamburg
University, Germany)

In there, to solve the problem: “Assessment of the interactive processes between Mekong
River mouth and upwelling water masses” the following data and methods were used:
 Wind data were collected from Condao station (1989-2008) and Phuquy Island (1987-
2007). Mekong River discharge data were collected from Cantho and Mythuan
stations (1997-2007). Hourly water level data were collected from Vungtau station
(1987-2006). Bottom topography in the study area was taken from hydrographic map
with scale of 1/50.000 (Published in 2004). Sediment characteristics were taken from
the cruise (9/2009) which was carried out by the Institute of Oceanography. Also, the
other related data have been collected from different projects.
 Criteria to classify the brackish and sea water masses was 32 psu [8].
 The method of data analysis and modeling were used are as follows (table 1).
Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển

117

III. STUDY RESULTS

investigate the upwelling phenomenon. Modeled results show that under the influence of SW
monsoon along the southern Central Vietnamese Coast the upwelling phenomenon occurred
especially from June to August (with an offshore directed velocity V ≈ 60cm/s in the surface
layer). a. Nghien Cuu Bien
b. NCB-95
c. HQ638
d. Sonne
Figure 1: Vessels participating in project Phase-I (2003-2006)
Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển 118

Figure 2: Some of the instruments which were deployed

+ Measured stations are shown in Fig.3
a. Location of measured stations
b. Location of Biogeochemical flux traps
Figure 3: System of measuring stations during Phase-I (2003-2006)
- Method of empirical orthogonal function (EOF) was used to indicate the main structure of
SST in the South China Sea under the influence of monsoonal winds and the effects of global
climate change.
- The region of strongest upwelling along southern Central Vietnamese coast was identified
by using the wind stress data from Pacific Center for Fishery.

- Highest density of marine algae (spherical lime and silic) occurred during upwelling
period along southern Central Vietnamese Coast.
- Publication of a monography about the upwelling area and its related processes.
Project results obtained in Phase-I play an important role as they are of high scientific and
practical values, because they contribute to the understanding of the upwelling phenomenon
off southern Vietnam. However, the remaining problems which have to be tackled in further
studies are:
- Seasonal variations of the interactive processes between the circulation at the west coast of
the South China Sea with Mekong River water and upwelling water masses.
- The processes forming the hydrological fronts and their interaction with involved water
masses.
- The question whether the upwelling area has received the material from Mekong River
system.
- Enlargement of the data base for the upwelling region including interactive processes
between upwelling and Mekong River mouth water masses.
c. Main study results of Phase-II (2007-2010)
The joint project in Phase-II (2007-2010) continued with the study of the remained
problems of Phase-I (2003-2006) with the title: “Study on the coastal processes in the Mekong
River mouth area and the interactive processes between Mekong River and the upwelling
water masses”. The main objectives of joint project are as follows.
1. To assess the interactive processes between Mekong River and upwelling water
masses;
2. To assess the frontal features: structures and variations;
3. To construct a scientific base for the assessment and prediction of the marine
resources for sustainable development of coastal economic;
4. To improve the potential of Vietnamese Oceanographers to perform studies,
applications and international cooperations.
Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển
characteristics
B. German side
1
Institute of Oceanography, Hamburg
University, Germany
Survey and modeling of meteo-hydro-
dynamical processes
2
Institute of Bio-geo-chemical, Hamburg
University, Germany
Survey and study on sedimentation
processes
3
Institute of Marine Science Baltic
Warnemünde, University of Rostock,
Germany
Survey and study on nutrients, primary
production, biology, and hydro-
chemical features
- Field investigation: 4 cruises were carried out during low (4/2007, 4/2010) and high
(9/2008, 9/2009) river discharge periods, sampling the following parameters:
1. Meteo-hydro-dynamical: Wind, wave, current, water temperature-salinity, water
transparency, etc.);
2. Nutrients: Total P (mg/l), Nitrate (NO
3
-N mg/l), Nitrite (N-NH
4
, N-NO
2
), Total N

/s at
Mythuan Station (3/2005) and

≈ 760 m
3
/s at Cantho Station (4/2004). The tidal regime is
irregular semidiurnal with a maximum value of the SWL of 490 cm occurring in November.
Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển

121

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

Condao Island
: measured stations
: continuous stations
: depth contours (m)
Baclieu
0
Phuquy Island
Tp.HCM

Figure 4a: System of measured stations during Phase-II: 2007-2010 (4/2007)
106.0 106.5 107.0 107.5 108.0
106.0 106.5 107.0 107.5 108.0
9.0
9.5
10.0
10.5
11.0
9.0
9.5
10.0
10.5
11.0
1
2
8
12
15
17
19
21

Phanthiet
Vungtau
Bentre
T
r
a
v
i
n
h
Soctrang
0
0
0
: measured stations
(anchorage)
: continuous stations
: depth contours (m)
: measured stations
(floating)
Tp.HCM

Figure 4b: System of measured stations during Phase-II: 2007-2010 (9/2008)

Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển 122
105 105.5 106 106.5 107 107.5 108
8.5

i
v
e
r
T
i
e
n

R
i
v
e
r
Tp. HCM
Dinhan Mouth
Cunghau Mouth
Hamluong Mouth
Dai Mouth
S
o
a
ø
i
r
a
p

M
o

an average value of 6.92 ± 5.6 mg/l. This SPM concentration is less than 50 mg/l (permission
threshold for critical nearshore water quality). During high river discharge period, the
concentration of chlorophyll-a and primary production rates are increasing from south to
north in the study area. During low river discharge period, the study area is a poor-nutrients
region. Concentration of plankton is 10 times higher during high river discharge periods
compared to low river discharge periods. Plankton is distributed mostly in the northern part
during SW monsoon and high river discharge, whereas, during low river discharge periods it
is distributed only around the river mouths.
- Mekong River mouth area is a region where water masses of the northern and southern
South China Sea interact with upwelling water, fresh water from river run-off. The interaction
of the above mentioned water masses induced the formation and distribution of seasonal
hydrological fronts. The density of total phytoplankton was mainly determined by silic algae,
whereas the density of toxic algae was low.
- Mekong River mouth area can be divided in to 3 regions depending on the affect of river
water. They are river water, intermediate and sea water dominated regions. Each region has its
specific ecology-environmental feature and bio-resources.
- The features of upwelling and Mekong River mouth water masses and their interaction are
depended on the western boundary current regime of the South China Sea and Mekong River
discharge.
Hội nghị Khoa học và Công nghệ biển toàn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển

123

- During NE monsoonal period (low river discharge), the water mass from the northern part of
the South China Sea with its characteristics of low temperature and high salinity follows the
western boundary current to the Mekong River estuary with characteristics of low salinity and
high temperature. Due to the interaction processes, it formed a water mass with characteristics
of T ≈ 25
0
C, S ≈ 34 psu. This water mass covered the area from Phanthiet to Camau, and

intrusion as well as coastal erosion. These problems will form the main challenge for a future
sustainable development in the Mekong River delta.
d. Some comments for the joint project:
- The joint project carried out was necessary to obtain the concerned objectives from both
parties. In general, the main objectives of the joint project have been achieved as they are: a)
Preliminary the understanding of the features related to the upwelling phenomenon and its
interaction with adjacent regions, b) Estimation of the main hydro-litho-dynamical processes
and features of ecology-environment in the Mekong River mouth area, and c) qualitative
assessment of the interactive processes between Mekong River mouth and upwelling water
masses.
- The joint project carried out was an enhancement for the bilateral co-operation between
Vietnam and Germany on marine research and upgraded the marine research potential of
Vietnam, especially the application of modern equipment and study methods, like numerical
models [4].
However, some shortcomings have occurred during the execution of the joint project, namely:
+ Since the joint project proposal was approved from each country separately, unfortunately
the financial support was not available for the same time. As an example, the joint project
Phase-II was approved in Germany starting from 2007, but for Vietnamese side it took until
2009 before Phase II was approved. One more example: during 2000-2003 the joint project
between Vietnam and India was also approved at different times (in the year of 2000 the
Vietnamese side was approved, but the Indian side was approved in 2001). These problems
Hội nghị Khoa học và Cơng nghệ biển tồn quốc lần thứ V - Tiểu ban Khí tượng, Thủy văn và Động lực học biển 124
seriously affected the bilateral co-operation works. In fact, this situation led to independent
works of each side, consequently the effectiveness of the joint project with regard to scientific
and financial aspects was not optimal.
+ In general, the collaboration had worked nicely during field surveys, whereas, the
Vietnamese role in data analysis, professional works and publications was rather limited and

S
o
â
n
g

T
i
e
à
n
Tp. HCM
Bà Ròa-Vũng Tầu
0
50 Km
Côn Đảo

Figure 5a: Distribution of the thickness (m) of Mekong River mouth water in April 2010
(S ≤ 32 psu)
104.5 105 105.5 106 106.5 107 107.5 108 108.5 109 109.5
8
8.5
9
9.5
10
10.5
11
11.5
12
22

105 105.5 106 106.5 107 107.5 108
8.5
9
9.5
10
10.5
Cµ Mau
Bạc Liêu
Trà Vinh
Sóc Trăng
Bến Tr e
Tiền Giang
Vòng Tµu
Kiªn Giang
S
o
â
n
g

H
a
ä
u
S
o
â
n
g


-30
-20
-10
0
Tr.1
Tr.2
Tr.3
Tr.4
Tr.5
Tr.6
Tr.7
Tr.8
Tr.9
Tr.10
m
107.0 107.2 107.4 107.6 107.8 N
o
W
E

Figure 6c: Distribution of salinity (S psu) in the transect Dai Mouth – Lagi (8/2009)

Figure 6d: SPM concentration on 24/9/2009
(MODIS Image)

Figure 6e: SPM
concentration in August

sufficient to quantify the total volume of Mekong River water dispersion into the upwelling
region.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge all colleagues for their kind help and encouragement
throughout the preparation of this paper.

REFERENCES
1. Bui Hong Long, “Study on the structure, variation and ecology-environmental effects of
Vietnamese upwelling area”. Final report of the joint project between Vietnam-Germany,
Phase-I: 2003-2006. Nhatrang, 2006, 223 ps.
2. CEM, Coastal Engineering Manual, 2006 (replacement to Shore Protection Manual
1984). Publication Number: EM 1110-2-1100. U.S. Army Corps of Engineers (USACE).
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and Training School Publisher, Hanoi.
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China Sea and its consequences on the Vietnamese upwelling area, 2011. Submitted to:
Global and Planetary Change.
5. Hein, H., B. Hein, T. Pohlmann, Recent sediment dynamics in the Mekong ROFI, 2011.
Submitted to: Global and Planetary Change.
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the Mekong River mouth area and the interactive processes between Mekong River mouth
and upwelling water masses”. Final report of the joint project between Vietnam-
Germany, 2011, Phase-II: 2007-2010. Nhatrang, 180 ps.
7. Pickard, G. L. and W. J. Emery, Descriptive Physical Oceanography. An Introduction,
Fifth enlarged Edition (in SI units), 1993. Pergamon Press, 320ps.
8. Robinson, M. K, The physical oceanography of the Gulf of Thailand, NAGA Expedition.
La Jolla, California: NAGA Report, 1974, v. 3, P. 1, p. 5-110.