SALINITY INTRUSION TREND IN RIVER AND CANAL SYSTEMS AND SOME PREVENTION METHODS IN BEN TRE PROVINCE

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1 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 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 2) Southern Institute of water Resources Research 2A Nguyen Bieu street, District N o 5, Ho Chi Minh city, VietNam Abstract 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

2 TIEN GIANG PROVINCE N CHAU THANH DISTRICT CHO LACH DISTRICT BEN TRE TOWN BA LAI RIVER Dai Estuary VINH LONG PROVINCE GIONG TROM DISTRICT BINH DAI DISTRICT Ba Lai Sluice Dam MO CAY DISTRICT BA TRI DISTRICT Salinity Intrusion Way GHI CHUÙ Provincial line Boundery of District Communication route Rivers, canals Provinces, towns Dam Sluice, Dyke TRA VINH PROVINCE THANH PHU DISTRICT R. Baøng Cung Ham Luong Estuary SOUTH CHINA Co Chien Estuary 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 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,

3 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 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 Observed data According to Le et al. (2002, 2005), the survey presented that the salinity spreads in 3 main flow sources for 4 years ( ) 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 ( ) TT Stations Year February March April May Location Binh Dai Dai estuary Loc Thuan Dai estuary 3 An Thuan 4 Son Doc 5 Ben Trai 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

4 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 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 m. 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 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 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. TIEN GIANG PROVINCE N CHAU THANH DISTRICT CHO LACH DISTRICT BEN TRE TOWN BA LAI RIVER Dai Estuary VINH LONG PROVINCE GIONG TROM DISTRICT BINH DAI DISTRICT Ba Lai Sluice Dam MO CAY DISTRICT BA TRI DISTRICT GHI CHUÙ Provincial line Boundery of District Communication route Rivers, canals Provinces, towns Dam Sluice, Dyke 2g/l 4g/l TRA VINH PROVINCE 16g/l THANH PHU DISTRICT R. Baøng Cung Co Chien Estuary Ham Luong 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

5 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 in precipitation gauging station Ben Trai and Ba Tri); In 2002,2003 rainfall is 0.3mm, 24mm respectively, there is no rainfall in 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 ( ) (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 Year Q average Tan Chau + Chau Doc Percentage % on year 2002 ( m 3 /s) The salinity intrusion rate ( from February to April) in 4 years ( ) 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. Figure 3 Maximum monthly salinity concentration in Binh Dai station Salinity (g/l) In Monthly Smax in 2002 (g/l) Monthly Smax in 2003 (g/l) Monthly Smax in 2004 (g/l) Monthly Smax in 2005 (g/l) Month 4. SALINITY PREVENTION BY THE YEAR

6 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 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 Pham, V. D. (2005). Observed hydrological data of the Mekong delta in 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