salinity intrusion trend in river and canal systems and some ...

Vietnam-Japan Estuary Workshop 2006
August 22 nd -24 th , Hanoi, Vietnam
SALINITY INTRUSION TREND IN RIVER AND CANAL SYSTEMS
AND SOME PREVENTION METHODS IN BEN TRE PROVINCE
Abstract
TRAN THI XUAN MY 1 AND NGUYEN DINH VUONG 2
1) Southern Institute of water Resources Research
2A Nguyen Bieu street, District N o 5, Ho Chi Minh city, VietNam
daotao-siwrr@hcm.vnn.vn
2) Southern Institute of water Resources Research
2A Nguyen Bieu street, District N o 5, Ho Chi Minh city, VietNam
daotao-siwrr@hcm.vnn.vn
Ben Tre province locating in the Mekong delta has been affected mainly by tide of the East Sea;
saline water resources have intruded through 3 river mouths: Dai , Ham Luong and Co Chien. Due to
the small quantity of flow, salinity intrusion occurs through estuaries from from 2, 3 months to the
whole year with higher and higher concentration by the end of the dry season. From March to May,
salinity concentration of 4g/l can intrude 50km further inland and cause many difficulties for
production and people life. Salinity prevention and effective water sources utilization of the Mekong
River is an essential problem for the Mekong river basin. The paper has presented the salinity
intrusion trend and suggested some methods to prevent in Ben Tre province.
Key word: coastal areas, estuary, salinity intrusion, salinity concentration, structural production
1. BACKGROUND
Ben Tre province locating in the Mekong delta has been affected mainly by tide in the East Sea ,
saline water resources have intruded through 3 river mouths : Dai , Ham Luong and Co Chien.
Surface water is very abundant but over three fourth of the provincial surface water areas (water in
rivers, canals, lowland areas) have been affected by salinity from 2, 3 months to the whole year (see
figure 1).
In the dry season, discharge from Tien River (directly from the Mekong River) is the only source
that flows to the river systems in Ben Tre province. Discharge is fairly small every year from January
to April and gets smallest value in April. Small river slope, flat topography are factors to make salinity
intrude further to rivers and inland.
In recent years, salinity intrusion in terms of concentration and length in the Mekong delta
generally and in Ben Tre province in particular is a hot problem; it is necessary to have a research
about salinity intrusion as well as prevention methods for water usage and production.
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CHO LACH DISTRICT
VINH LONG PROVINCE
Salinity Intrusion Way
GHI CHUÙ
Provincial line
Boundery of District
Communication route
Rivers, canals
Provinces, towns
Dam
Sluice, Dyke
TIEN GIANG PROVINCE
CHAU THANH DISTRICT
BEN TRE TOWN
MO CAY DISTRICT
TRA VINH PROVINCE
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BA LAI RIVER
GIONG TROM DISTRICT
THANH PHU DISTRICT
BA TRI DISTRICT
R. Baøng Cung
Co Chien Estuary
BINH DAI DISTRICT
Ba Lai Sluice Dam
Ham Luong Estuary
N
Dai Estuary
SOUTH CHINA
Figure 1 Salinity intrusion sources to the Mekong River in Ben Tre province
2. FACTORS AFFECTED TO SALINITY INTRUSION
Salinity intrusion has a tight relationship with flow regime in the Tien River, when the flow
discharge is high, the salinity concentration is small and vice versa. Besides, salinity has a close
relationship with tide and northeast wind. Salinity in rivers and canals increases in the strong northeast
wind time.
Tran and Nguyen (2004) generalised some factors affected to salinity intrusion in the dry season in
Ben Tre province as follows:
• Flow discharge: depending on upstream flow .
• Tide: type, magnitude and period , nearshore current, saline concentration of sea water.
• Water level: water level in estuary in the dry season is lower than in the flood season.
• Internal factors: structure of river topography and canal systems; Hydraulic work systems,
transportation affected to flow, especially freshened hydraulic works; water regime for
production, domestic supply; water exchange with other areas as adjacent provinces Tien
Giang, Tra Vinh and Vinh Long .
• Climate and weather: rainfall, evaporation, wind ( especially northeast wind in the East Sea
and the coastal areas).
• Structural cultivation change: require a quantity of saline water from the sea for shrimp
growing.
3. SALINITY INTRUSION TREND IN THE DRY SEASON IN THE PERIOD 2002-2005
3.1 Salinity intrusion formation process:
There is no rainfall from January to April in the dry season. The only fresh water source is the
flow discharge of Tien River from upstream of the Mekong River.
The interactive impact of sea and river dynamics has affected directly to coastal areas. In the dry
season (the beginning of March and April), the requirement of water for agriculture is high. Therefore,

it causes shortage of water in the dry season and salinity intrusion has almost affected the provincial
areas . The problem becomes seriously when the discharge flowed toward sea is not strong enough to
prevent sea water to intrude. In addition, the high tide water level can also push saline water back
from estuaries further to inland and cause large areas affected with different saline concentration.
3.2 Salinity change in 2002-2005
In Ben Tre, the river mouth areas have the sea-river hydrology regime, seasonal interfere and
tidal fluctuation and salinity intrusion. The estuaries have been enlarged, the bed river has raised and
the erosion and siltation process occurs seasonally and yearly. Salinity change has often varied yearly
especially in the dry season.
3.2.1 Observed data
According to Le et al. (2002, 2005), the survey presented that the salinity spreads in 3 main flow
sources for 4 years (2002-2005) from February to May at 5 observed stations: Loc Thuan, Binh Dai
stations in Dai estuary; An Thuan, Son Doc stations in Ham Luong river and Ben Trai station in Co
Chien river.
In 2002, based on the observed data of SIWRR at Loc Thuan, Son Doc, Binh Dai stations, salinity
change in 5 months increased from February to April, it decreased slightly in May. In June, it
decreased rapidly and its highest salinity concentration was smaller than one in February. Salinity in
April is highest; in Loc Thuan station, the maximum monthly salinity is 15.9g/l, and in Son Doc
station 9g/l. when the construction of Ba Lai dam made, some areas in Binh Dai, Ba Tri, Giong Trom
were prevented salinity intrusion. Salinity intrusion trend in the areas is: the maximum salinity
concentration occurred in the end of March and the beginning of April and some days of the beginning
of May.
Table 1 Maximum monthly salinity concentration (g/l) in some stations (2002-2005)
TT Stations Year February March April May Location
2002 23.0 27.5 23.9 21.1
1 Binh Dai
2003
2004
18.1
25.6
27.0
27.4
20.8
24.3
21.7
25.6
Dai estuary
2 Loc Thuan
3 An Thuan
4 Son Doc
5 Ben Trai
2005 27.5 29.1 29.4 21.0
2002 10.2 14.1 15.9 16.4
2003 8.5 12.7 15.2 10.2
2004 8.3 16.8 17.5 17.5
2005 14.2 17.9 21.2 11.3
2002 19.3 21.0 22.7 23.0
2003 21.9 29.0 26.1 21.8
2004 21.3 25.6 28.0 26.8
2005 20.8 26.2 24.1 19.5
2002 4.0 7.0 9.0 8.7
2003 5.0 10.3 9.2 7.2
2004 11.6 16.7 17.2 15.2
2005 10.6 18.3 24.1 15.6
2002 21.1 19.1 23.5 17.0
2003 22.5 25.2 23.1 23.6
2004 26.1 29.1 25.4 26.4
2005 24.4 24.6 27.3 16.4
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Dai estuary
Ham Luong
estuary
Ham Luong
estuary
Co Chien
estuary
Source: SIWRR,2005
In 2003, generally the salinity intrusion decreased compared to one in 2002, especially in Ham
Luong estuary, salinity concentration was higher. In the flood season of 2003, the water level is very

low especially in the transition time from the end of the flood to the dry season in 2004, the flow
discharge is very small, in the contrary, the irrigation water requirement for rice, crop... , water supply
for households and the evaporation are very high. Salinity had the highest value in March and
decreased in April, May. The maximum salinity in March at An Dinh station is 6.6g/l .
In 2004, salinity intrusion had a special trait, salinity is higher than ones in 2002-2003 at almost
observed locations in the coastal areas. The maintaining time of high salinity was longer and the
maximum salinity occurred earlier than one month. Besides, 2004 is the leap lunar year so the dry time
was longer because of the leap month is in the dry season.
In the dry season 2005, salinity intruded seriously with high concentration and further to inland
than forecast because the flow discharge is smaller than the average one from 10-20%, the water level
is lower from 0.25-0.35m. The salinity is highest in March, April and tends to increase compared to
other previous years. The saline isograph 4g/l intruded 40-50km further to inland compared to 30-
35km last some years.The salinity 1g/l occurred in almost the Ben Tre province (see figure 2). The
maximum monthly salinity concentration from February to May is shown in table 1.
3.2.2 Salinity change
From February to April: the driest months in the year, salinity increased from February to
April; the maximum salinity occurred in March or April had a maximum concentration in estuary
location and decreased gradually in rivers. Observing Binh Dai station (near the sea) and Loc Thuan
station (in Cua Dai river), it can be seen in the observed data in the dry season 2005.
In May: May is the seasonal change month with rainfall inland, salinity in river and canals
decreased slightly compared to April.
In June: It is the beginning of rainy season with much rainfall than in May, flow discharge
upstream supplied to river and canals; salinity concentration decreased rapidly in the whole river and
canal systems of Ben Tre province.
CHO LACH DISTRICT
VINH LONG PROVINCE
GHI CHUÙ
Provincial line
Boundery of District
Communication route
Rivers, canals
Provinces, towns
Dam
Sluice, Dyke
CHAU THANH DISTRICT
MO CAY DISTRICT
2g/l
BEN TRE TOWN
4g/l
TRA VINH PROVINCE
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TIEN GIANG PROVINCE
BA LAI RIVER
GIONG TROM DISTRICT
16g/l
THANH PHU DISTRICT
BA TRI DISTRICT
R. Baøng Cung
Co Chien Estuary
BINH DAI DISTRICT
Ba Lai Sluice Dam
Ham Luong Estuary
N
Dai Estuary
SOUTH CHINA
Source: SIWRR,2005
Figure 2 Salinity isogragh 2g/l; 4g/l and 16g/l in April 2005 in Ben Tre province
3.3 Salinity intrusion rate

Salinity intrusion rate in the dry season has the essential relation with two components of
water sources (upstream flow toward Tien River and local areas rainfall) from February to
April. Rainfall is almost negligible (based on the statistical data in 2002-2004 in precipitation
gauging station Ben Trai and Ba Tri); In 2002,2003 rainfall is 0.3mm, 24mm respectively,
there is no rainfall in 2004-2005.
Based on Pham (2005), the statistical data from Southern Meteorological and hydrological
station have been shown that discharge of Tien and Hau Rivers decreases significantly in
March, April compared to other months of the year. Besides, in the transition time from
October to December (before the dry season of next year) the inflow change has affected to the
current flow in rivers for the dry season of next year and has governed significantly salinity intrusion
to river systems. Based on the observed data at Tan Chau, Chau Doc from October to December
(2002-2004) (Table 2), the average monthly discharge change shown the flow discharge to Tien and
Hau river in 2003, 2004 that had 78% and 80% of the average monthly discharge (October to
December 2002) respectively.
Table 2 Total monthly average discharge Q average (m 3 /s) at Tan Chau, Chau Doc stations from
October to December in 2002-2004
Year Q average Tan Chau + Chau Doc
( m 3 /s)
Percentage % on year 2002
2002 59220 100
2003 46260 78
2004 47810 80
The salinity intrusion rate ( from February to April) in 4 years (2002-2005) at Binh Dai station (Cua
Dai river) shown the salinity rate (Figure 3). To rank the maximum salinity , we see the salinity has
in descending order by 2005, 2004, 2002 and had the smallest salinity in 2003; it has a close
relationship with the flow before the beginning of the dry season every year.
Salinity (g/l)
30
27
24
21
18
Figure 3 Maximum monthly salinity concentration in Binh Dai station
In 2002-2005
15
2 3 4
Month
4. SALINITY PREVENTION BY THE YEAR 2010
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Monthly Smax in 2002 (g/l)
Monthly Smax in 2003 (g/l)
Monthly Smax in 2004 (g/l)
Monthly Smax in 2005 (g/l)

Based on dense river systems, the salinity intrusion in the dry season in Ben Tre province occurs
frequently every year. It formed 3 distinguished ecological areas: fresh water ecological , brackish
water and saline water areas with 37%, 27% and 36% of the total natural areas respectively.
From the forecast by the year 2010 and more, the Mekong delta will face the water lackage and
drought due to high increased requirement of operation and resources usage of upstream countries
(causes downstream flow discharge decrease). This phenomenon makes the risk of salinity intrusion
more seriously.
In the face of the complicate salinity intrusion, the local authority has some prevention methods by
the year 2010 as follows:
a) Structural methods
• Set up sufficient water sources to push saline water and enhance the water transmission
capacity by dredging main canals systems , hydraulic works, sluices rehabilitation and
upgradation .
• Construct completely hydraulic works as dikes, dams like Ham Luong river dike, Ba Tri sea
dike and othe small dikes .
• Establish salinity survey stations at main sluices for both sides .
• Establish procedure for sluice operation, schedule for close and open gates publicy to media.
b) Rural water supply
• Ensure Bac Ben Tre Hydraulic work supply sufficient clean and fresh water .
• Utilize supplement fresh water from deep underground water.
c) Non structral methods
• Propagandize people about fresh water storage, water use saving, environment protection and
water waste limitation to the inland rivers.
• Recommend farmers to choose kind of plants or animals suitable to each area for production .
• Have suitable cultivation periods.
• Diversify hydraulic purposes and modernize hydraulic works.
5. CONCLUSION
Coastal areas in Ben Tre province especially in estuaries are the place where the potentials of
agriculture- fishery development are abundant and diversified. Saline water is an important source to
ensure the production of rice and aquaculture for the sustainable development here, however, salinity
intrusion affects seriously to people life spacely and timely. Therefore, the water resource operation
and management in estuaries and salinity change monitoring in the future is very important and need
to have further study in order to prevent salinity intrusion more effectively .
REFERENCES
Le, S., Nguyen, V. L., and Nguyen, V. S. (2002). Litterature review about salinity intrusion for sociocological
development in the coastal areas in the Mekong delta-Science and Technological papers-
Southern Institute of Water Resources - Vietnam Agriculture publishers, pp. 12-15.
Le S., Nguyen, V. L., Nguyen, V.S., and Nguyen, D.V. (2005). National project: Survey on salinity
and acidity process in the Mekong delta in 1991 to 2005, pp. 165-166.
Pham, V. D. (2005). Observed hydrological data of the Mekong delta in 2000-2005.
Tran, T. X. M., and Nguyen, D. V. (2004). Salinity change before and after Balai dam construction in
Ben Tre province-Science and Technological papers- Southern Institute of Water Resources
Research, Vietnam Agriculture publishers, pp.72-79.
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