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

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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 - 129 - 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 - 130 - 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
Page 1: Vietnam-Japan Estuary Workshop 2006
Page 5 and 6: Salinity intrusion rate in the dry

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