RESEARCHES OF ENVIRONMENTAL HYDRAULICS FOR THE YANGTZE THREE GORGES PROJECT

Fang Ziyun and Ye Min

Yangtze Water Resources Protection Bureau, Wuhan 430051, China

Abstract: The paper studies the impacts of TGP on hydraulics and water quality and introduces the work done for justification, EIA and recent researches in environmental hydraulics of the project.

1    INTRODUCTION

The Three Gorges Project (TGP) is a key project in the development and harnessing of the Yangtze River. The dam site is situated in Sandouping of Yichang City, Hubei Province, with a distance of 40 km upstream from the Gezhouba dam. The TGP is a multipurpose hydro-project mainly for flood control. Power generation, navigation and water supply improvement.

The main characteristic of the Three Gorges reservoir is:

l        The relative volume of the reservoir is small and its regulation capacity for runoff is low. It is a seasonal regulation reservoir.

l        The reservoir is of gorge type, with a length of 660 km and an average width of 1.1 km.

l        The operation program for reservoir regulation considers various requirements including the environmental aspect.

Now the total wastewater and sewage discharged into the Reservoir reach amounts to 1.5 billion ton annually. The water quality, however, remains good in general due to huge quantity of stream flow, except for pollution belts along the banks near cities. Impoundment of TGP will aggravate shoreline pollution.

2    IMPACT OF TGP ON HYDRAULICS AND WATER QUALITY

After regulation of the reservoir, the annual quantity of discharge downstream of the dam and at the estuary will remain unchanged. Only the distribution pattern of discharge among seasons will be changed. Without the project the highest stage within the reservoir occurs in the summer season, the lowest stage occurs in the winter season. However, with the project in place, the reverse often applies, it would make oncomalania to breed difficulty in the reservoir area.

In the reservoir area the reduced velocity of flow will decrease the ability of reoxygenation and dispersion of pollutants, thereby threatening to worsen existing local near shore pollution belts. Therefore, it is necessary to control strictly the pollution of the reservoir by industrial effluent, agricultural runoff and domestic sewage, and to mitigate the pollution by wastewater treatment and pollution prevention measures.

The thermal stratification of the reservoir water body begins around April and end in May. It would take 20 more days for the temperature of downstream water to rise spawning temperature of 18 ℃.

The reservoir will regulate the flood flow and reduce the catastrophic flood damage. It has the significant effect of reducing the flood peak. At the beginning of operation period of the reservoir, around 70% of the sediment of the previous 0.5 bill tons would be settled to the bottom of the reservoir and the fluctuating area of backwater. The discharge of clear water from the reservoir would erode the downstream bank as well as the bottom of the river, but the live span of Dongting Lake would be extended.

Due to the fluctuation of daily output of the powerplant, the flow discharge passing the turbines would have a big fluctuation within a day during the dry season, especially in the dry period of dry year. It would cause the daily max. fluctuation of the water stage in the Gezhouba Reservoir of 3 m and 1.4 m at Yichang, but the water surface of unsteady flow would be smoothed down very rapidly and have no significant effect below Zhicheng station.

3    SIMPLIFIED 2-D MODEL PREDICTION OF THE RESERVOIR POLLUTION TENDENCY IN EIA

The theoretical concentration of a pollutant increases in the reservoir downstream from a point source because of reduction in flow velocity. The basic equation is,

where y means transverse direction, E_y is transverse diffusion coefficient. The approximate solution for spreading from a maintained point source in two dimensions is:

Where m =q₀*C₀/H, q₀ is initial effluent flow (m³/sec). C₀ is initial pollutant concentration (mg/l). H is mean depth of receiving water body (m). u is river mean velocity (m/sec). x is longitudinal distance (from effluent)(m), y is lateral distance (m), and E_y is lateral diffusion co-efficient (m²/sec).

The following table shows the result obtained by considering a natural velocity of 2 m/sec and by setting y at 1 m.

Table 1  Relationship of pollutant concentration with reduction of flow vel. in reservoir

    The effect of TGP on dispersion of Chongqing and Wanxian point sources of pollution would be as table 2.

Table 2  Effect of TGP on dispersion of Chongqing and Wanxian pt. Sources

Table 2 shows the results of calculations for post impoundment conditions. First it gives the distance downstream necessary to dilute an effluent sufficient so that water quality standards are respected, assuming unchanged effluent concentrations, and predicted post impoundment flow velocities. Second, it gives the effluent concentrations necessary to maintain water quality standards at 1000 m downstream. In this table Pb has been chosen as a pollutant example for which the water quality standard is 0.1 mg/l and the current concentrations in the river are usually below the detection limit of 0.001 mg/l. The results show that downstream distance required obtaining third class surface water quality increase considerably with flow velocity reductions caused by the project. For example, under the NPL 180 scenario at Chongqing their standards would be met 50 km downstream instead of l km under natural conditions. This effect can nonetheless be mitigated by better water treatment. For the same example, water quality standards could be met 1 km downstream if effluent concentration was reduced from 8.9 mg/l to l.3 mg/l.

The model as above was used to investigate the Pollution of Main Stem of Natural River. For E_y in Wuhan reach. It was found that E_y is proportional to the discharge of the river. And also E_y is proportional to the side slope of the bank within the pollution belt as shown in Table 3.

Table 3  Relationship of E_z with River Discharge and riverbank Side Slope

4    RECENT RESEARCHES IN ENVIRONMENTAL HYDRAULICS ASPECT

Water quality in the Three Gorges Reservoir is a focus of public concern. A lot of industries waste and domestic discharge will be formed threateningly for water quality of the reservoir.

The data from the investigation on pollution sources in the reservoir area and the assessment on standard equivalent pollution loads showed that industrial wastewater and agricultural return flow, domestic sewage, urban runoff, and moving ships, etc., are the main pollution sources in the reservoir area. Most of the industrial and domestic pollution comes basically from cities along the Yangtze River, such as Fuling, Wanxian, and especially Chongqing. At present, the area's annual discharge of wastewater has reached about 15×10⁸t, the industry waste is 11.2×10⁸t and the domestic sewage is 3.8×10⁸t. Most of which is directly discharged into the Yangtze River causing pollution in local sections. The main pollutants include biochemical oxygen demand (BOD), chemical oxygen demand (COD), volatile phenols, total phosphorus, total nitrogen, oils, ammonia, etc.

The monitoring data of water quality section shows that the mean water quality of the reservoir is good. Most sections satisfy class II standard of GB. Only in a few sections exist pollution of different certain degree. The pollution belt near bank has become quite serious in Chongqi, Fulin, Wanzhou, Changshou and Yichang reaches of the Yangtze River, and is still worsening. Besides, the pollution from non-point source, oil, solid etc. must be paid attention to. These are also the potential pollution sources of the Three Gorges Reservoir.

After inundation, pollutant concentrations will be increased in some local regions near shores due to the decrease in the flow velocity and, as a result, in the turbulent diffusion ability. So the numerical simulation of 2-D flow field and water quality is applied for the study of water pollution control.

A depth-averaged turbulence model in boundary-fitted coordinate systems is used. Its character is to compute velocity and pollution concentration separately.

The  turbulence model equations of the depth-averaged two-dimensional river flow can be written as shown,

where  is the diffusion coefficient，and S is the source term.  Stands for different variables mentioned below. Different differential equations are obtained from different  in continuity equation, momentum equation, k equation, εequation and concentration equation etc._[3].

As an example, the Changshou reach of the Yangtze River is chosen to simulate the effect of the industrial and domestic pollution sources with the assumption that the wastewater load treated or no treated respectively.

According to the Three Gorges Reservoir operating rule and hydrological condition in dry season in Yangtze River, it is the most unfavorable period for water quality in the reservoir, because it is of low velocity and slow diffusion in the period. The input of flow was chosen by 7Q10 feature flow.

The input values of model are shown in table 4.

Table 4    The input value of hydrological factors

The result of the simulation is shown in Fig.1 and Fig. 2. The influence ranges of water quality shown in table 5.

Table 5  The influence range of waste in changshou reach    Unit: m

The above results show clearly the influence range of wastewater. The pollution sources will have significant effects on the water quality of reservoir when the wastewater is not treated; the length of pollution belt is about 7000m long. While the effect of pollution belt will be decreased obviously when the wastewater is treated, it can satisfy water quality demands.

The method used above has many advantages, as

(1) To solve the velocity and concentration field separately, the concentration has been solved in a smaller area. So that the values of the concentration computed could be more accurate, and the compute time has been saved.

(2) The results of simulation are showed by graphs. The pollution effect can be related the length of pollution zone or water quality standard. It can be more directly perceived through the senses.

(3) This model can be a calculated multilateral load simultaneously. Therefore it can better simulating natural and provide scientific basis for water pollution control.

Fig.1  The effect range of water quality without new treatment plants after inundation

s

Fig.2  The effect range of water quality by treatment after inundation

5    CONCLUSIONS

The environmental hydraulics applied and developed for TGP is step and step and following the procedures as follows:

(1) To identify the change of hydraulic regimes after impoundment of the reservoir;

(2) To investigate the peculiar pattern of pollution in Yangtze and in the Three Gorges  Reservoir- pollution zone;

(3) To examine the pollution condition aggravated after impoundment by simplified 2-D model;

(4) To study of the water quality and water pollution influence range, a numerical simulation of 2-D flow field and water quality is applied;

(5) Basing on the above studies, to formulate the integrated plan for protecting the quality of water both in the reservoir including entrance part and the downstream sections.

References

[1] Fang, Z.Y. (1993), “Integrated Approach To Water Quality Management of Yangtze”, Proceedings of Stockholm Water Symposium, Stockholm Water Company, p379-383.

[2] Fang, Z.Y. (1996), “Environmental Perspective: Beneficial and Adverse Effects”, International .L Sediment Research, IRTCES, p1-2l.

[3] Hou G.X., Li Qi and Ye Min etc. 1(2000) “A Numerical Simulation of Far-field COD Concentration Distribution for Natural Rivers”, Journal of Hydrodynamics, ser.B, p96-107.