Friday and Saturday, December 6-7, 1996
Common Room, University House,
and J G Crawford Building, National Centre for Development Studies,
Australian National University
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Seminar on Environment and Development in Vietnam
Friday and Saturday, December 6-7, 1996 Common Room, University House,
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Salinity Intrusion in the Red River Delta
Vu Thanh Ca
Department of Civil and Environmental Engineering
Saitama University, Urawa, Saitama 338 Japan
Tel: 81-48-858-3567
Fax: 81-48-855-9361
Email: vuca@env.civil.saitama-u.ac.jp
Abstract
Characteristics of salinity intrusion into estuaries of the Red River System, Vietnam are studied based on many year measurement data of salinity concentration at stations along the estuaries and on a numerical model. Computed results of salinity concentration at measurement stations along rivers and tributaries by the numerical model agree satisfactorily with observed data. It was found that in the dry season, the salinity intrusion length is as long as 20 km in the main river and more than 20 km for some tributaries. In the main river and tributaries with high freshwater discharge, the maximum salinity concentration is observed in January while for the tributaries with low freshwater discharge, the maximum salinity concentration is observed in March.
1 Introduction
The Lower Red River Delta (Fig. 1) is the most important part of the Bacbo Delta. the main agricultural production supplier for the North of Vietnam. There are two distinguished seasons in this area: the rainy season from May to October and the dry season from November to April of the next year. The difference in freshwater discharge between the two seasons is very large. The largest monthly average freshwater discharge in August is about ten times larger than the smallest one in March (Vu, 1992). In the rainy season, due to large freshwater discharge from upstream, the salinity intrusion problem does not present. However, the dry season. With small freshwater discharge, the salinity intrusion into river estuaries has been a main environmental problem. With relatively large slope of the river bed in this area. the salinity intrusion problem for this river system is not as serious as that in the Mekong River Delta in the South of Vietnam. However, the Lower Red River Delta is the most densely populated area in Vietnam, and consequently salinity intrusion can cause large damage to local economy.
Vi (1985) and Ngo (1991) based on the recorded data of salinity concentration at stations along estuaries of the Red River System has drawn some primary remarks on the characteristics of salinity intrusion there. Details of salinity intrusion in each tributary of the river network were not investigated.
The aim of this study is to give a detailed view on the characteristics of salinity intrusion in the estuaries of the Red River System. This report is based mainly on two previous publications by Vu (1990) and Vu et al (1991). Using many year recorded data of salinity concentration at stations along the estuaries, monthly-average salinity concentration at each station is computed. The salinity intrusion length in each estuary are also estimated. Details of salinity concentration distributions along the estuaries are studied using a numerical model of the transport and dispersion of salinity.
2 Regime of Salinity Intrusion in the Estuaries of the System
Fig. 1 depicts the estuaries of the Red River System together With locations of salinity measurement stations. The data of the salinity concentration at 10 stations have been recorded in the dry season for the period of 33 years (from 1963 up to now). However, in this study, the salinity concentration data for the period of 29 years (from 1963 to 1992) have been used for the evaluation of the regime of salinity intrusion into the estuaries of the Red River System since data for the last four years are not available.
Figure 1. Red Rivers system and salinity observation stations.
Figs. 2(a-d) depict the monthly average salinity concentrations at the stations along the estuaries of the Red River System in the dry season. In the Day river (Fig. 2a) and the Red River, the main river of the river system, (Fig. 2c), due to relatively large slope of the river bed, and consequently large freshwater discharge, the decrease of salinity concentration with the length from the estuaries is faster than that in the Ninhco and Traly Rivers (2b and 2d). The maximum salinity concentration at stations along the Red River and Day Rivers is observed in January while that in the Ninhco and Traly Branches is observed in March.
Figure 2. Monthly average salinity concentration at stations in the dry season.
In this study, the salinity intrusion length is defined as the length from the river mouth along the river channel to a point where the salinity concentration is l ppt (part per thousand). Fig. 3 depicts a map of contour lines of many-year-average salinity concentration and many--year-average maximum salinity concentration of 4 ppt and 1 ppt. This map is drawn based on the data collected in some special projects (Vi, 1985) and the recorded data at stations. As seen in this figure, the salinity concentration at a place in the river system depends mainly on the distance from the place to the shore-line, not on the distance from the place to the river mouth along the river channel. Thus, even relatively long, almost whole Ninheo river is suffered from salinity intrusion. It has been observed that at the Lieude station, which is about 23 km from the sea along the river channel, the many-year-average salinity in January is 1.976 ppt.
As in Fig. 3. the whole area surrounded by Red river and Ninheo river (Haihau and Xuanthuy districts, Namdinh province) is suffered from salinity intrusion. The main agricultural product of this area is rice and the affect of salinity intrusion to rice fields must be prevented.
Figure 3. Contourlines of average salinity concentration in the dry season in the lower Red River Delta.
To protect rice fields from flood and salinity intrusion, dikes have been constructed at both banks along the rivers. However, the construction of dikes can only partly improve the situation. since intake of freshwater from the estuaries is the main source of freshwater for irrigation, and during the dry season. the intake of water from the estuaries is not possible for almost whole region. Also, at places near the coast, the transport of salinity from deep soil layers to the surface in the dry season can pollute the surface soil layer if there is not enough freshwater to wash the salt out. Thus some proper measures should be proposed to reduce effects of salinity intrusion on agricultural activities in the area.
3 Numerical Model
3.1 Governing Equations
A numerical model has been developed to study in detail the salinity intrusion problem in the estuaries of the river system. This model is a modification of the flow and salinity intrusion model for a river network (Nguyen, 1985 and Vu, 1990). The governing equations of he model are:
Continuity equation
Momentum equation
Salt transport and dispersion equation
Where H is the water level, Q the river discharge. b the river width at the water surface, A is the cross-sectional area of the river. C the Chezy coefficient, g gravitational acceleration, R the average hydraulic radius of the river at the cross-section, q the lateral flow into the river, S the salinity concentration, Dx the apparent longitudinal dispersion coefficient of salinity and G(S) the source or sink of salinity due to lateral flow.
Strictly speaking, the system of Equations (1-3) can be applied only for well mixed estuaries. Study results (Vu, 1990) indicated that the estuarine number, defined as the ratio between the volume of seaward freshwater flow in a tidal cycle and volume of tidal prism, for estuaries in the system during dry season varies from 0.099 to 0.1l. Thus, it can be remark that all estuaries can be considered nearly well mixed. And! it is save to apply the system of Equation (1-3) to the river estuaries.
3.2 Numerical Scheme
The numerical scheme for the integration of the system of Equations (1-3) is the improvement of that of Nguyen (1985) and Vu (1990). The river network is divided into branches, linked at river junctions. A control volume, staggered grid scheme (Patankar. 1980) is applied for the integration of the system of Equations (1-3) in an individual branch. The branch is divided into segments with finite length [[Delta]]x. The water discharge Q and salinity concentration S are computed at the centre of the segment and the water elevation is computed at its ends. A first order upwind scheme is used for the connection terms in Equations (2) and (3). Central difference formulas are used for the salinity dispersion-diffusion term in the Equations (3). Since control volume scheme is used, there is no solution for the water discharge and salinity concentration at the end of the branch. Thus, additional segments with zero thickness (Patankar, 1980) is added to the ends of the branch and the solutions of water discharge and salinity concentration can be determined there. At the river junction, the condition of mass conservation requires that the flux of water towards the junction must equal the ones leaving it; and the solutions for water level an salinity concentration must have unique values there disregarding they are computed from the discretised equations for any of the branches joining the junction.
An implicit scheme with the implicit factor theta = 0.667 is used for the time.
The above mentioned scheme can produced a tri-diagonal matrix for the unknowns for each branch. By the well known double-sweep algorithm, the unknowns at intermediate points inside the branch can be eliminated to get a system of equations for unknowns of water level, water discharge and salinity at junctions. This system of equations is solved using a standard Gauss elimination scheme. Having, obtained the values of unknowns at junction points, the values of unknowns at nodal points inside a branch can be readily obtained by the Thomas algorithm.
Since Equation (2) is nonlinear in term of the unknown water discharge Q, an iterative procedure is used for solving the system of Equations (1) and (2). The solutions of Equations (1) and (2) then can be used for the integration of Equation (3).
The boundary conditions for the computation are given water discharge at the upstream boundary points, say, Hanoi station on the Red river and Trieuduong stations on the Luoc river. At Ninhbinh station on the Day river, since there's no available data of water discharge. the boundary conditions at this point and all estuarine boundary points: Nhutan station on the Day river, Phule station on the Ninhco river, Balat station on the Red river and Dinhcu station on the Traly river are given water elevation. The boundary conditions for salinity computations are given salinity concentration at the estuarine boundary points and zero value of salinity at the upstream boundary points.
3.3 Results of Computations and Discussions
The numerical model is used to simulate salinity intrusion into the river estuaries in March, 1980. The recorded data of water level, discharge and salinity at stations inside the river system was used to calibrate and verify the model. The Chezy coefficient C and apparent longitudinal dispersion coefficient Dx are determined from the calibration.
Figs. 4 depict examples of the comparison between the computed and observed salinity concentration at Lieude station on the Ninhco river. As seen, the model can simulate the real salinity intrusion picture with satisfactory agreement. Results of the computations for other days (not shown) also confirm this remark. Thus, the results of the computations reveal that the assumption of well mixed estuaries can be applied with satisfactory accuracy.
Figure 4. Calibration and verification of the model.
Using the model, details of salinity intrusion into the estuaries of the river system can be investigated. It is found that during the time of flood tide, the contour line of 1 ppt salinity concentration can cover all the Ninhco river while during the time of ebb tide, this line retreats farther than Lieude station. However. during the ebb tide, due to low water level in the rivers, the intake of water for irrigation is difficult.
The salinity intrusion length is shortest for the Day river, where the maximum salinity intrusion length is less than 20 km, while for the Red river and Traly river, the salinity intrusion length is about 20 km.
From the pattern of daily variation of salinity concentration in Figs. 1 it is remarked that the salinity intrusion has a strong relationship with tide. To study the effect of tide on the salinity intrusion, the numerical model had been used to generate the data at a number of points along the estuaries where the measurement data of salinity are not available. The generated data of salinity concentration were then used for harmonic analysis to obtain the values of harmonic constants of the four main tidal components M2, S2, Ol and Kl for salinity. The computed results (not shown) showed that daily variation of salinity at points along the estuaries can be predicted with reasonable accuracy using harmonic constants of the four tidal components.
References
Dac N.T. (1985) Mathematical Model for Flow and Salinity Simulation in Deltaic Areas. Report of The Institute of Mechanics, Hanoi, Vietnam.
Patankar S.V. (1980) Numerical Heat Transfer and Fluid Flow. Hemisphere Pub. Corp., l91 pp.
Ngo T.T. (1991) Salinity Intrusion in the Bac Bo Delta. Proc. 3rd Vietnam National Conference on Marine Science. Vol. 2, Vietnam Natational Centre for Scientific Research (in Vietnamese) 191-197.
Vi V.V. (1985) Some Primary Remarks on Salinity Intrusion in the Red River Delta, Proc. 3rd Scientific Conference of The Hydrometeorological Research Institute, Hanoi, Vietnam (in Vietnamese). 177-183.
Vu 'T.C. (1990) Characteristics of the Flow and Salinity Intrusion in the Red River Delta. MA Thesis, Asian Institute of Technology, Bangkok, Thailand 101 pp.
Vu T.C., V. Suphat and T. Asaeda (1991) Study on Salinity Intrusion in the Red River Delta. Environmental System Research, JSCE, Vol. 22, 213-218.
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