Reservoir operation schemes for water pollution accidents in Yangtze River

Xiao-kang XIN*, Wei YIN, Meng WANG

Changjiang Water Resources Protection Institute, Changjiang Water Resources Commission, Wuhan 430051, P. R. China

Reservoir operation schemes for water pollution accidents in Yangtze River

Xiao-kang XIN*, Wei YIN, Meng WANG

Changjiang Water Resources Protection Institute, Changjiang Water Resources Commission, Wuhan 430051, P. R. China

After the Three Gorges Reservoir starts running, it can not only take into consideration the interest of departments such as flood control, power generation, water supply, and shipping, but also reduce or eliminate the adverse effects of pollutants by discharge regulation. The evolution of pollutant plumes under different operation schemes of the Three Gorges Reservoir and three kinds of pollutant discharge types were calculated with the MIKE 21 AD software. The feasibility and effectiveness of the reservoir emergency operation when pollution accidents occur were investigated. The results indicate that the emergency operation produces significant effects on the instantaneous discharge type with lesser effects on the constant discharge type, the impact time is shortened, and the concentration of pollutant is reduced. Meanwhile, the results show that the larger the discharge is and the shorter the operation duration is, the more favorable the result is.

water pollution accident; emergency operation; water environment model; Three Gorges Reservoir

1 Introduction

With the development of the economy and natural science research capabilities, the water environment and river health have drawn more and more attention (Moyle and Mount 2007). In the Yangtze River Basin of China, dozens of water control projects play important roles in power generation, water supply, water transport, and so on. The operation scheme of the Three Gorges Project, the largest hydraulic project in China, attracts the attention of scientists and the public. The optimal operation scheme of the Three Gorges Reservoir has been studied by taking the maximum flood control benefit as the main target (Tan 1996) or by taking the maximum power output as the main target (Yao et al. 2009; Soares and Carneiro 2001; Chen et al. 2009). The optimal reservoir discharge from the point of view of ecological protection has also been studied (Xu et al. 2009; Vehanen et al. 2005; Cheng and Chen 2007).

However, there have been few studies on the optimal operation scheme of the Three Gorges Reservoir from the point of view of eliminating the adverse effects of water pollutionaccidents. Fu et al. (2008) used a numerical model to study the pollution spill in the Songhua River, but they did not address methods to eliminate the adverse effects. Samuel and Bahadur (2006) developed an integrated water quality security system for emergency response, and suggested using a water regulation method to dilute the pollutants. Kuang et al. (2010) established a two-dimensional hydrodynamic and pollution transport model to study the influences of three different discharges from the Taipu Gate and proposed an optimal water regulation scheme. This study took advantage of a two-dimensional mathematical water environment model and carried out some research on the emergency operation of the Three Gorges Reservoir when water pollution accidents occurred at the Yichang section of the Yangtze River.

2 Two-dimensional water environment model

2.1 Control equations

The water environment model is composed of the hydrodynamic model and the advection-dispersion model. The control equations can be written as

Eq. (1) is the continuity equation, Eqs. (2) and (3) are the momentum equations in thexandydirections, respectively, and Eq. (4) is the advection-dispersion equation. In the equations,uandvare the velocities andFuandFvare the resistances in thexandydirections, respectively;Scis the source term;gis the acceleration of gravity;pais the pressure on the cross-section;tis time;ζis the water level;fis the coefficient of Coriolis force;ρ0is the water density;υtis the water dispersion coefficient;Cis the pollutant concentration; andDxandDyare the transverse and longitudinal diffusion coefficients, respectively.

These partial differential equations cannot be analytically solved, and a lot of mathematical solution methods have been developed, such as the finite difference method (Xing and Shu 2005), finite volume method (Begnudelli and Sanders 2006), finite element method (Yu et al. 2004), and finite analytic method. We take advantage of the simulation software MIKE 21 AD, which was developed by the Demark Hydraulic Institute with the finite volume method (Euler schedule), to solve these equations. It must be pointed out that the Courant number should be less than 1.0 in order to ensure the stability of the model (DHI 2005).

2.2 Model mesh and parameters

The study area is an 80 km-long river section of the Yangtze River between Yichang City and Zhijiang City, named the Yichang section, and shown in Fig. 1. It was meshed with quadrilateral and triangular grid cells, where the maximum size of the quadrilateral grid cells was about 200 m × 200 m, and the maximum area of the triangular grid cells was about 5 000 m2. The mesh of the Yichang section is shown in Fig. 2.

Fig. 1 Sketch of Yichang section of Yangtze River

Fig. 2 Sketch of model mesh of Yichang section of Yangtze River

The bathymetric data measured in 2007 were used to interpolate the elevation information to the mesh shown above. The data of the upstream boundary condition were from the Yichang gauging station and the data of the downstream boundary condition were from theChangmenxi gauging station. Taking the worst condition into account, the discharge of the upstream boundary was 5300 m3/s, and the corresponding water level of the downstream boundary was 33.62 m.

The main parameters of the hydrodynamic model are Manning’s coefficient and the eddy viscosity coefficient (Smagorinsky constant). The value of Manning’s coefficient used in the case study was 0.031 and the eddy viscosity was 0.28. The calibration of the model using the field data at Zhicheng Station shows that the calculated velocity was about 0.85 m/s and the monitoring velocity was about 0.92 m/s, which indicates that the precision of this hydrodynamic model was acceptable and it could be used in the case study.

3 Conditions of calculation cases

As the monitoring data of water pollution are insufficient, the pollution accidents were simplified as leakage quantities and leakage time of permanganate index (CODMn). Most attention was paid to the accidents occurring at Yiling and Xiaoting districts. The calculation start time was 2009-02-01T00:00:00 and the end time was 2009-02-02T23:53:20, and the pollution accident was imagined to have occurred at 2009-02-01T02:00:00. At that time, the Three Gorges Cascade Dispatching Center received emergency operation instructions, and the water discharged for regulation reached the Yichang cross-section at 2009-02-01T02:30:00. Here, the effects of river section between the Three Gorges Dam and Gezhou Dam were ignored.

It is assumed that there were three leakage modes of the pollutant: (1) the pollutant with a total amount of 180 t leaked into the Yangtze River channel center within 30 min (CCST); (2) the pollutant with a total amount of 18 t leaked from the bank side within 3 min (BST); and (3) the pollutant leaked from the bank side constantly with a rate of 10 kg/s (BCL).

There were five operation cases at the Yichang cross-section for each leakage type: (1) the operational discharge was 5 300 m3/s and lasted 1 h (contrast case); (2) the operational discharge was 15 900 m3/s and lasted 1 h (case 1); (3) the operational discharge was 15 900 m3/s and lasted 2 h (case 2); (4) the operational discharge was 15 900 m3/s and lasted 3 h (case 3); and (5) the operational discharge was 26 500 m3/s and lasted 1 h (case 4). In summary, the conditions of the calculation cases are shown in Table 1.

4 Results and discussion

4.1 Water velocity

Based on the results of calculated velocities of the operation cases using the hydrodynamic model, we extracted the point velocities at the Yiling and Xiaoting cross-sections (Fig. 3). As we can see, the velocities at these two sections increased significantly when the Three Gorges Reservoir started emergency operation, indicating that the reservoir emergency operation facilitated the convection and dispersion of the pollutant. Meanwhile, the effects of reservoir operation on the Xiaoting cross-section were delayed anddecayed. The reason is that the Xiaoting cross-section is 17 km downstream of the Yiling cross-section: within this distance, the instantaneous discharge was cut by channel storage and the conveyance time was lengthened.

Fig. 3 Sketch of velocity profiles at Yiling and Xiaoting cross-sections

The largest increment degrees of velocities for each calculation cases are listed in Table 2. As can be seen, the velocity increment of operation case 4 is the largest due to the largest instantaneous discharge, while those of the other three operation cases are almost the same.

Table 2 Velocity increment degree for each case

4.2 Increase of water volume

The main method of reservoir emergency operation to eliminate the adverse effects of sudden water pollution is to increase the discharge, because a large discharge is good for pollutant advection and dispersion. However, a large discharge may produce a large volume of water, which produces adverse effects on power generation or water supply. Therefore, the increment of water volume is an important indicator for optimizing the operation scheme. Based on the calculated results, the increment data of water volume are listed in Table 3. We can see from Table 3 that the increment of water volume of operation case 3 is the largest, since its operation duration is the longest, and case 2 and case 4 are the second largest while case 1 is the smallest.

Table 3 Increment of water volume from Three Gorges Reservoir under different cases 104m3

4.3 Pollutant concentration evolution

The evolution of the pollutant concentration can be obtained from the results of the environment model, and it can reflect the pollution plume’s position at different times. Taking the first leakage type (CCST) as an example, the sketches of the contrast case, case 2, and case 4 are shown in Fig. 4. Fig. 4 shows that the pollution plume’s position of case 4 is furthest while that of the contrast case is nearest after the emergency operation lasts 28 min, and that the high-concentration (larger than 0.01 kg/m3) polluted water mass has disappeared after the emergency operation lasts 6 h and 28 min for case 4, which demonstrates that the larger the discharge is, the shorter the pollution duration is. The characteristic numbers for each case are listed in Table 4. If the pollution accident happened at the Yiling cross-section, as it can be seen from the table, the cases with larger discharge spent less time in eliminating the high-concentration plume and reducing the impact scope accordingly. And similar results can be obtained in the case of pollution accidents occurring at the Xiaoting cross-section.

Table 4 Impact time and scope of pollutant under different cases

5 Conclusions

This study shows that water control projects not only supply power and fresh water, but also play positive roles in mitigating the adverse effects of sudden water pollution accidents under optimal operation schemes. From this study, three important conclusions can be drawn:

(1) Reservoir emergency operation can shorten the pollution duration and scope significantly. Taking case 3 as an example, the pollution duration is shortened by 7 h, and the pollution scope is shortened by 5 km.

(2) With regard to the discharge increment and pollution mitigation effects, case 4 is better than cases 3, 2, and 1, which indicates that the larger the instantaneous discharge is, thebetter the effect is.

Fig. 4 Profiles of pollutant concentration under different reservoir operation schemes

(3) As the Xiaoting cross-section is 17 km downstream of the Yiling cross-section, and the water discharged for regulation needs time to reach the accident location, the pollution duration is comparably long, while the pollution scope is shortened.

Meanwhile, since this is preliminary research, a few improvements should be made in further studies:

(1) The computation model simplified the water transport in the section between the Three Gorges Dam and Gezhou Dam, which may cause significant error. A model taking the dam, gate, and water channel into account together should be developed in future research.

(2) Although 12 cases have been studied here, they are insufficient to describe the pollution accidents because of their uncertainty.

(3) Since the emergency monitoring data are insufficient, the validation of model is inadequate.

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This work was supported by the Nonprofit Scientific Research Project of the Ministry of Water Resources of China (Grant No. 20081035) and the National Fund for Major Projects of Water Pollution Control (Grant No. 2009ZX07104-006).

*Corresponding author (e-mail:xin.xiaokang@163.com)

Received Apr. 27, 2011; accepted Oct. 9, 2011