To understand the physical procedures of saline invasion in Red river and Thai Binh river systems, we carried out numerical simulations utilizing unidimensional theoretical account, MIKE 11. The information for standardization and proof are observed in local Stationss and the recent field informations measured in nine feeders. We found important differences in the physical procedure of the saline invasion between the two river systems owing to the tidal stages, river discharges and geometrical features of the river oral cavity.
Shown in Figure 1, the Red river basin that is typical of geographics in South East Asia. A huge spread of land is from 20o00 to 25o30 North and from 100o00 to 107o10 East. The catchment country is 169,000 km2 and its 51.35 % ( 86,660 km2 ) belongs to Vietnam. In Vietnam, the Red river basin consists of two river systems: the Red river system in the Southwest, and the Thai Binh river system in the East. Both river systems cover the Red river delta ( RRD ) that is located in the land nearby the South China Sea ( Tonkin Gulf ) . These two river systems are connected each other by the Duong river and the Luoc river in the upper portion and the lower portion of RRD, severally. Before making the Tonkin Gulf, the chief Red river branches out into nine feeders.
We named it “ Red river ” for the H2O coloring material ( red-brown ) having to its high deposit concentration. The one-year discharge of the Red river is 137 km3/year. At present, the entire sum of H2O in the Red river is diminishing significantly as a consequence of big addition in the population in the basin. In 2006, approximately 18.2 million people lived in the RRD 16,644 km2, while 9.51 million lived in the several major metropoliss such as Hanoi, Hai Phong, Quang Ninh, Hai Duong and Hung Yen. In fact, the population in the river basin has grown farther since 2006. The velocity of urbanisation and industrialisation is faster than that in other Vietnamese countries. It is no uncertainty that there is a great demand for H2O in the close hereafter. Therefore the deficit of the H2O supply will do serious jobs in the growing of this country.
In the Red river basin, the hydrological government is characterized by the rainy season ( May-Octorber ) and the dry season ( November-April ) . The one-year rainfall varies spatially and seasonally from 700 to 4,800 mm/year. Here we have 80 % of the entire rainfall in the rainy season ( MONRE, 1996-2006 ) . The difference of the H2O discharge between two seasons is rather big ( van Maren, 2007 ) . As a consequence, the river basin causes a deficit of H2O during the dry season and inundations in the rainy season. The lessening of river discharge consequences in the invasion of seawater from the river oral cavities doing the saline harm to the next metropoliss. The low flatter land and widening estuary accelerate the incursion of tidal H2O into the rivers. Therefore, longer drouths will do non merely deficient H2O supply but besides serious harm to the H2O environment.
Since the planetary clime alteration may impact the hydrological government, it is of import to look into the salt distribution due to the changing of the river discharge and the sea degree in the Red river and Thai Binh river systems. In this survey, we used a simulation theoretical account MIKE 11 to look into the spacial distributions of salt in the Red river and Thai Binh river systems. First, the standardization of the theoretical account parametric quantities was carried out utilizing the information of tidal degree, H2O discharge and salt in 2000 and 2002. Second, we used the graduated parametric quantities to formalize the field informations obtained in 2003 and 2004. Third, we reviewed the computed consequences and discussed the consequence of the increasing of H2O supply from the upstream of the Red river on salt invasion.
The truth attained by numerical computation is straight related to the quality of hydrological, meteoric, tidal, bathymetric and socio-economical datasets. A big portion of the whole hydrological informations used in this survey is dependent on the aggregation by the Vietnamese Institute of Meteo-Hydrology ( IMH ) . The H2O degree, discharge, and salt are of import parametric quantities to graduate and formalize the numerical theoretical account. The H2O degree and discharge have been obtained since 1970 at 64 Stationss: 8 at the upstream boundaries, 9 at the downstream boundaries and 47 in the basin country. The salt was measured at 12 Stationss during the dry season in 2002, 2003 and 2004. Additionally a hydrological study was conducted in Red river and Thai Binh river systems from January 3 to 20 in 2006. The measuring was carried out at 17 locations for salt concentration and at 9 major river oral cavities for H2O degree.
The rainfall and vaporization informations used in this survey were besides the aggregation of the IMH: the rainfall record is available at 118 Stationss from 1970 to 2004 and the vaporization is at 68 Stationss from 1996 to 2006. Figure 2 shows the monthly norm rainfall and vaporization in the Red river basin and the average discharge at the upper and lower watercourse. Both graphs demonstrate that July, August and September are in the rainy season while January, February and March are in the dry season.
The present numerical theoretical account requires a series of 2-dimentional points depicting each cross-sectional profile along the river. These plumbing informations was available from the Red River Investigation Company. Most Vietnamese research institutes have depended on this database.
MOdel apparatus and standardization
The unidimensional hydrodynamic theoretical account MIKE 11 by the Danish Hydraulic Institute will be applied to the Red river and Thai Binh river systems for gauging the spacial distributions of hydrodynamic measures and salt. The simulation theoretical account will be calibrated utilizing the unmoved H2O degree, discharge and salt informations measured in 2000 and 2002. The MIKE 11 together with the parametric quantities acquired by the preliminary computation will depict the flow and salt Fieldss during the dry season in 2003 and 2004.
Numeric theoretical account
MIKE 11 is a package bundle affecting several faculties and labs for the hydrokineticss, sediment conveyance and H2O quality in estuaries, rivers, irrigation systems and other H2O fortunes ( DHI, 2003 ) . It is being improved and going an effectual design tool for applied scientists, hydrologists, ecologists and contrivers. The hydrodynamic faculty is the karyon of the MIKE 11 and controls other faculties. In the present simulation, we use the hydrodynamic and advection-dispersion faculties. The former faculty treats the mass preservation and impulse preservation utilizing the Saint Venant equation. They are solved numerically in a infinite staggered computational grid utilizing a 6-point Abbott strategy ( Abbott, 1979 ) . The latter faculty trades with the advection-dispersion equation for dissolved stuff. It requires the discharge, H2O degree and flow speed informations as the input values that are obtained from the calculation utilizing the hydrodynamic faculty in progress. The advection-dispersion equation is solved numerically utilizing an inexplicit finite difference technique, which are devised unconditionally stable and less numerical scattering.
River web and theoretical account standardization
The computational web is established for the Red river and Thai Binh river systems. It consists of 27 rivers with 653 cross subdivisions. The boundary conditions are given at 8 discharge Stationss in the upstream and at 9 tidal water-level Stationss in the downstream. We set the salt nothing at all upstream boundaries and the existent clip fluctuations at all downstream boundaries. The river web besides includes several penstock Gatess and pump Stationss.
The undertaking of standardization is executed in two stairss. First, the theoretical account parametric quantities such as a Manning coefficient and an initial H2O degree at each cross subdivision are adjusted to obtain the best tantrum between the ascertained and computed values for both discharge and H2O degree at the prescribed station. Second, the scattering coefficient and initial salt concentration at each river section is determined for the standardization of salt.
Figure 3 shows the comparing of the ascertained ( ruddy line ) and computed ( bluish line ) information for the H2O degree in the dry season of 2000 at the Hung Yen station ( about 130 kilometers from the Red river oral cavity ) and for the discharge in the dry season of 2002 at the Hanoi station ( about 153 kilometer ) . The standardization consequence of the river-system theoretical account proves that the deliberate H2O degree agrees good with the ascertained one although the unmoved informations are strongly influenced by the tidal fluctuation. We besides attribute the inclination of larger gap and shallower deepness of the river oral cavity to the curious characteristic of the fluctuation. The simulation describes reasonably the discharge measured at Hanoi station.
As the saline invasion along a river is really sensitive to the initial conditions, boundary conditions and scattering coefficients, the standardization of salt is by and large more complicated when compared with that of discharge or H2O degree. Figure 4 shows the ascertained ( red-cross points ) and computed ( bluish line ) salt concentrations at the Ba Lat station of the Red river ( in the Red river system ) and the Cua Cam station of the Cam river ( in the Thai Binh river system ) . Although there are several disagreements between two lower salt concentrations, the theoretical account predicts the salt fluctuation moderately good. Therefore, from Figures 3 and 4, the parametric quantities obtained from the standardization are acceptable.
consequence and treatment
Spatial distribution of salt invasion during the dry season
Upon the mold, among many rivers, we select six paths that represent the Day, Tra Ly and Red rivers for the group of the Red river system, and the Thai Binh, Van Uc and Da Bac rivers for the group of Thai Binh river system. The calculation is carried out up to the extent of 55 kilometer from the river oral cavity in all paths. Figure 5 ( a ) represents the longitudinal salt profiles given as a consequence of a simulation in the dry season. Each curve describes the spacial fluctuation of the maximal salt due to the computation of the biennial informations and the norm of the maximal values of H2O degree at all spring tides. The consequence of the analysis demonstrates that the tendency of saline invasion is divided into two types. Within the distance of 10 kilometer from the river oral cavity, the salt is comparatively high in the group of the Red river system. This is likely due to the wider and shallower river oral cavity. Change the H2O maintains higher salt value in the first 15 kilometer, it reduces the salt denseness rapidly. In add-on, the narrow breadth and high bed degree together with the larger discharge from the sea contribute much to high denseness in the downstream. The salt of 1 psu ( practical salt unit E? ppt ) reaches about 25-27 kilometer from the river oral cavity, although it depends upon the path.
In contrast, the breadth of river oral cavity in the Thai Binh river system is comparatively smaller. Furthermore, owing to the river meander, the discharge of this system is instead smaller than that of the Red river system: accordingly, the saline penetrates further into the upper watercourse. Figure 5 ( B ) is the corresponding salt distributions obtained from the study ( 2006 ) . This consequence supports the two types of tendencies observed from the simulation.
To further look into the spacial fluctuations of salt, we made a practical map of saline invasion in the RRD after the analysis of the 2003 and 2004-year informations. Figure 6 shows a map of the mean saline invasion in January ( the driest month ) . The Vietnamese authorities specifies the salt of 1psu as the upper threshold value for imbibing H2O and that of 4psu as the border of safety for agribusiness usage. The mean salt concentration is indicated by the contour lines ( 1, 2, 4, 10, 20 and 25psu ) . It can be seen that the lower country of the Thai Binh river system is to a great extent affected by the salty breezes H2O although most coastal parts suffer from the saline H2O.
Tidal degree and salt invasion
The RRD is about level, and the river bed is lower than the average sea degree. The H2O incline varies by and large within the scope of 1/2000 – 1/5000. Therefore, the enlargement of salt invasion to the upper watercourse strongly depends on the tide of the Tonkin Gulf. During the high tide, when the river discharge is little, the salt H2O reaches much further. In the undermentioned computation, we evaluated the salt alterations at three different Moon stages for both Red and Thai Binh rivers. Figure 7 shows the maximal salt profiles at three stages ( spring, mean, and neap tides ) during the dry season in 2003 and 2004.
One can happen that at the high H2O in the spring tide, the H2O keeps the high saline denseness from the river oral cavity to the distance of 10 kilometers, and decreases the denseness quickly to the following 10 kilometer and easy to the terminal of computation section for the Red river. In contrast, the denseness profile decreases bit by bit from the river oral cavity to the upper watercourse in the same status for the
Thai Binh river. The other tendencies of salt invasion look like although these forms are non curious. The extent of salt of 1psu at the spring tide in the Red river is smaller than that in the Thai Binh river: the former is 26.8 kilometer, and the latter is 48.2 kilometer. The invasion value of 1psu depends upon the Moon stage ; it varies from 14.9 to 26.8 kilometers in the Red river, and from 20.0 to 48.2 kilometers in the Thai Binh river. By and large, the consequences illustrate the strong tidal consequence to the fluctuation of salt. As the river discharge is instead little in the dry season, the tidal degree chiefly affect the harm from salt H2O. The computed consequences give us the necessary information for the H2O supply at a location during the dry season.
Effectss of increasing of H2O supply from reservoirs on saline invasion
In the upper portion of the Red river basin, the Vietnamese authorities has built several reservoirs: i.e. Hoa Binh, Thac Ba, Na Hang and Tuyen Quang reservoirs. Among them, the Hoa Binh and Thac Ba reservoirs are the biggest and 2nd biggest one, severally. The former reservoir ( the storage capacity E? 9.5 billion M3s ) started the operation in 1990, while the latter reservoir ( 2.5 billion M3s ) in 1971. The entire capacity of these two reservoirs corresponds to 30 % of the entire H2O on the Red river basin in the dry season. The downstream of the Red river and Thai Binh river systems therefore receives more H2O supply owing to the presence of the reservoirs ( Kees, 2009 ) .
Therefore, the chief aim of the simulation is how the H2O supply from these reservoirs eases the salt invasion in the downstream. In order to analyze the influence of the increasing H2O we performed a numerical survey harmonizing to three scenarios that depend on the discharge from the Hoa Binh and Thac Ba reservoirs. In the scenario 1, the discharge and H2O degree in 2003 were selected at the upstream and downstream boundaries. Harmonizing to the dependable beginnings, the discharge Q2003 is the comparably low value in the basin. Then we use it as a base discharge in the dry season for the present survey. We assume 1.20Q2003 in the scenario 2 and 1.30Q2003A in the scenario 3. By and large, the maximal capacity of these two reservoirs is defined as 0.3Q2003. In the scenarios 2 and 3, the H2O degree was the same as that in the scenario 1.
Figure 8 represents the salt profiles obtained by the simulations for the Red and Thai Binh rivers. In both rivers, the extent of salt invasion decreases with the addition of H2O supply from the reservoirs. In the Red river, the location of nothing salt reduces 10 % for the scenario 2 and 25 % for the scenario 3 from that for the scenario 1, while in the Thai Binh river it reduces 8 % and 14 % in the several instances. Although the salt value of 1psu is changing from 22.1 to 26.8 kilometers, in the most portion of the Red river, it goes back toward the downstream conspicuously due to the addition of the discharge. In contrast, the salt profile does non demo a noticeable alteration in the Thai Binh river. A closer expression at these profiles reveals that the deliberate consequence is non sensitive to the addition of H2O discharge.
Unidimensional MIKE 11 theoretical account was applied to imitate the salt invasion in the Red river and Thai Binh river systems. The theoretical account predicted the H2O degree, discharge and salt at a location in these systems moderately good. These standardization consequences proved the dependability of the theoretical account accommodation in the hydrokineticss and advection-dispersion faculties.
The fluctuation of salt invasion was examined with reacting the alterations of tidal stage and river discharge. Harmonizing to the computed consequences, it could be concluded that all the rivers in the Red river and Thai Binh river systems are affected by the saline invasion. In the status of spring tides and low river discharges, the saline invasion reaches the location runing from 25 to 27 kilometers in the Red river system, and from 40 to 50 kilometers in the Thai Binh river system. In add-on, the salt invasion depends on the geometric characteristic of the river oral cavity. This paper provides a map of salt invasion in the driest month. As can be seen in the map, most portion of the RRD suffer from salt harm. However, the survey suggests that the addition of the H2O supply from the Hoa Binh and Thac Ba reservoirs will ease the saline invasion.
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- MONRE, 1996-2006. Vietnamese Ministry of Natural Resources and Environment. Annual Report on Hydrological Observation in Vietnam.
- Kees, S. , 2009. Strengthening Water Management and Irrigation Systems Rehabilitation Project, TA No7107-VIE.
- Van-Maren, D.S. , 2007. Water and deposit kineticss in the Red River oral cavity and next coastal zone. Journal of Asian Earth Sciences 29, 508-522.