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Chapter 25. The effect of south-to-north water transfer on saltwater intrusion in the Chang Jiang estuary

Shen Huanting, Mao Zhichang and Gu Cuachuan
Estuarial and Coastal Institute, East China Normal University

Xu Pengling
Shanghai Water Works Corporation

PUBLIC WATER SUPPLY standards for chloride content vary according to the type of usage. Drinking water should not exceed 250 ppm. Industrial water at the Baoshan Iron and Steel Plant in Shanghai has a monthly average target of 50 ppm with a maximum limit of 200 ppm. Low chloride contents are also required in irrigation. For example, rice seedlings require under 600 ppm and general irrigation less than 1,100 ppm. The maximum concentration for safe water use of ordinary crops is about 3,150 ppm.

At present, during dry seasons, saltwater intrusion is quite serious in the Chang Jiang estuary. This poses a serious danger to industrial, agricultural and domestic water use in Shanghai Municipality and in some areas along the river in Jiangsu Province, as well as to the drainage of pollution in the Huangpu Jiang. In addition it has had a major impact on the hydrological regime, channel evolution and the ecological environment in the estuary. The proposed northward transfer of Chang Jiang water would unavoidably aggravate these effects. Therefore, exploring the patterns of saltwater intrusion into the Chang Jiang and estimating possible impacts after the water transfer would be helpful in considering the southto-north water transfer schemes.

SALTWATER INTRUSION IN THE CHANG JIANG ESTUARY

In general, saltwater intrusion is most serious during the dry period from December to April and has reached the point where it can no longer be taken lightly. Conditions vary between the four branches of the estuary (North Branch, North Passage, North Channel and South Channel) because of their different discharges and channel configurations (Figure 1).

Discharge into the North Branch has decreased from year to year. The average discharge curing the 16 tidal periods of 13-21 August 1959 was 0.7 per cent of the total of the north and south branches; during the two tidal periods of 89 September 1971 it was -7.5 per cent of the total, i.e., during spring tide periods a large amount of tidal water flowed into the North Branch. The reduction in discharge has led to a stronger tidal intrusion, so that the entire North Branch is governed year round by saltwater (C 1>1,100 ppm) and gates along the river can only be used for draining water, not for drawing it. Irrigation water for Qidong and Haimen counties must be transported over a long distance by canal from near Nantong, and water for Chongming Island can only be drawn from the South Branch.

Figure 1. The Chang Jiang Estuary

The South Branch is the main route for Chang Jiang discharge. Although saltwater intrusion is not as serious there as in the North Branch, it has an effect on industrial and agricultural water use which cannot be ignored. During the dry period of recent years, areas upstream from Wusong have been affected. Four years of data at the Laoshidong station show a chloride content in excess of 100 ppm 36.8 per cent of the time, with a maximum reaching 2,460 ppm.

The Huangpu Jiang is the main source of Shanghai's industrial and agricultural water. There are eight waterworks along the river. During the dry period the tidal incursion has caused the intake to become more saline. Data from the four years from 1973 to 1976 show an average of 65 days at the Wusong waterworks and 51 days at the Zhabei waterworks when the chloride content was greater than 100 ppm.

The average flow at Datong station in 1978 was 21,400 m³/sec, the lowest since 1928. In addition, a large amount of water was pumped from the river by adjoining districts (about 30 km³ were diverted in the provinces of Jiangsu and Anhui alone), causing saltwater intrusion to become the most severe in decades. On the mainstream of the Chang Jiang, saltwater reached the mouth of the Wangyu He in Changshu County, 120 km from the outlet of the Chang Jiang (the Changshu waterworks obtained a reading of 400 ppm on 4 February 1979). Nearly all the waterworks along the Huangpu Jiang were affected. In the 243 days from I1October 1978 to 31 May 1979, Wusong was affected by salinity for 215 days; Yangshupu for 151 days; and Minhang for 73 days, while Chongming Island was surrounded by saltwater (C1>1,100 ppm) for five months. Many places registered their highest chloride values on record: e.g., 3,950 ppm at Wusong, 3,820 ppm at Zhabei and 1,127 ppm below the Liu He gate. The strengthened tidal effect following the decline in discharge not only allowed saltwater intrusion to expand but also blocked the flow of pollution out of the Huangpu Jiang and worsened its murkiness and malodorousness. The water was murky and stank for 10⁶ days in 1978, an historical record.

From the winter of 1978 to the spring of 1979, numerous industries were affected, including textiles, foodstuffs, medicine, metallurgy, chemicals and electronics. Incomplete statistics indicate a direct economic loss to industry in excess of 14 million yuan. Agricultural losses were also considerable. Chongming Island alone reduced early rice plantings by over 1,300 ha. This was a small portion of the damage wrought by saltwater intrusion.

PATTERNS OF VARIATION IN THE SALINITY OF THE CHANG JIANG ESTUARY

The combined effect of river discharge, tidal flows, waves, saltwater density currents and ocean currents creates complex temporal and spatial changes in the salinity of the Chang Jiang estuary.

Temporal Variation

Daily variation. Time curves of daily variation in salinity are identical to those for the level of the tide, with two peaks and two troughs each day and marked differences from day to day. Maximum salinity generally occurs sometime around the flood slack; the minimum is usually near the ebb slack.

Semimonthly variation. In the Chang Jiang estuary there is a spring tide and a neap tide every half month. Within this period average daily salinity also has one peak and one trough. The effect of the tidal range on salinity is related to the discharge from upstream. When the monthly average discharge exceeds 30,000 m³/sec at Datong station, the tidal range has a minimal effect on the chloride content at Wusong waterworks.

Seasonal variation. There is a marked seasonal variation in the runoff of the Chang Jiang. The monthly average salinity at Yinshuichuan station has a strong negative correlation with the monthly average discharge at Datong station, with a correlation coefficient of 0.91. In general, the highest level of salinity occurs in February (2.09 per cent) and the lowest in July (0.885 per cent) (Table 1). The period of low salinity is from June to October and the high period is from December to April.

Yearly variation. Year-to-year variation in the salinity of the Chang Jiang estuary is closely related to the annual average discharge at Datong station. In high flow years the salinity is lower and in low flow years it is higher. Over the past several decades there has been a gradual tendency for the number of low flow years to increase. Data collected at the Hankou station between 1956 and 1974 show nine low flow years and only two high flow years. Saltwater intrusion in the estuary has worsened. Because of consecutive years of excessively low flows, salinity at the Yinshuichuan station in 1978 was about 0.20 per cent greater than the long-term average.

Spatial Distribution

In the Chang Jiang estuary freshwater mixes with saltwater within the roughly 400 km from the Liu He estuary to 125° E longitude. Salinity increases gradually downstream. Beyond the river mouth it increases rapidly. Salinity in the vicinity of Qiyakou in the south branch is higher than either upstream or downstream. The same situation exists in the Xinqiao channel. This is the result of the backing up of saltwater from the north branch into the south branch. Blocking up the north branch would have the great advantage of improving the navigation channel in the south branch and the drawing of freshwater for Chongming Island.

Salinity depends mainly upon the distribution of runoff. Lower discharges mean lower salinities and vice versa. In a multiple channel, the salinity of a given cross-section is higher in flood tide than in ebb. At the beginning of the flood tide within one of the channels, the salinity is higher at the sides than in midstream, while at full flood and flood slack, midstream salinity is higher than along the banks. At full ebb, side salinity is again higher than at midstream. Salinity along the north bank is higher than along the south bank. Vertical distribution of salinity depends chiefly on the types of mixture between fresh and saltwater. The major factors affecting salinity distribution in the Chang Jiang estuary are the tides beyond the river mouth and the freshwater discharge from upstream.

Table 1. Monthly Average Salinity at Yinshuichuan Station and Monthly Average Discharge at Datong Station

Notes: The data in the table are from 1960 and 1962-1975. Salinity (S) = 1.8065 Cl.

ESTIMATE OF THE EFFECT OF SOUTH-TO-NORTH TRANSFER ON SALTWATER INTRUSION IN THE CHANG JIANG ESTUARY

In the short term, 1,000 m³/sec is to be transferred from the Chang Jiang via the East Route. This is 1/29 of the 29,200 m³/sec average annual discharge at Datong station, 1/6 of the minimum dry season discharge (6,020 m³/sec), 1/50 of the July average (49,500 m³/sec) and 1/10 of the January average (10,100 m³/sec). We have used frequency and correlation analyses of discharge and salinity measurements to obtain a rough estimate of the effect of south-to-north transfer on saltwater intrusion in the Chang Jiang estuary.

Estimate of the Number of Days with Different Chlorine Levels at the Wusong Station After Water Transfer

According to data for the six years from 1974 to 1979, when discharge at the Datong station is greater than 20,000 m³/sec, there is an extremely low probability of a chloride content over 100 ppm at Wusong station. When discharge is more than 18,000 m³/sec, the chloride content is rarely over 250 ppm. When discharge is more than 16,000 m³/sec, the probability of a chloride content over 250 ppm increases markedly, but a value over 600 ppm is extremely rare.

When monthly average discharge at Datong station is less than 8,000 m³/sec, chloride at Wusong station is over 500 ppm for the entire month. When discharge is less than 10,000 m³/sec, chloride at Wusong station exceeds 100 ppm for nearly the whole month and is more than 500 ppm for about half the month. With the transfer of 1,000 m³/sec, within the statistical range of discharge, the number of hours with a chloride content over 500 ppm will increase by about 35 per cent and the number of hours with chloride over 100 ppm will increase by about 15 per cent. That is, in a normal dry season, chloride will exceed 100 ppm for an additional 3 days and will surpass 500 ppm for an additional 4 to 5 days.

The Maximum Possible Chloride Value at Wusong Station After Water Transfer

Chloride content observations are available at Wusong station for about eight years (1972-1979). The maximum of 3,950 ppm occurred in February 1979, when the average discharge at Datong station was 7,010 m³/sec. We have calculated that a transfer of 1,000 m³/sec might increase the chloride maximum at Wusong station by 55 per cent to 6,130 ppm.

During the flood period of ordinary years, the 1,000 m³/sec transfer would have little effect on saltwater intrusion in the Chang Jiang estuary but during the dry season, especially in low water years, the impact would be greater. Because the relative curves between chloride and discharge are nearly exponential, a 1,000 m³/sec transfer when discharge exceeds 16,000 m³/sec would make little change in the chloride values at Wusong Station, but when discharge is less than this amount, the same transfer would have a marked effect on chloride. Therefore, the lower the discharge, the more prudent we must be in transferring water. In addition, the same rate of transfer along the Middle Route and the East Route would have different effects on the Chang Jiang estuary. The Middle Route would have a minimal effect because the water transferred from the Danjiangkou Reservoir is basically flood water stored during the high water season. In addition, the series of lakes along the Chang Jiang, including the Dongting and the Poyang lakes, regulate the river's flow. The East Route would transfer water directly from estuarine areas, so its impact would undoubtedly be much more serious.

CONCLUSIONS

(1) There are many factors affecting saltwater intrusion in the Chang Jiang estuary, of which the primary one is the amount of water flowing from upstream. Although no water has yet been transferred north, saltwater intrusion has become quite serious in the past ten years and directly affects and threatens the water used for industry, agricultural and daily life in Shanghai and some coastal areas of Jiangsu Province. To develop water transportation in China, the channel to the ocean in the Chang Jiang estuary is continually being deepened, further allowing the saltwater to move upstream. If water is transferred at the rate of 1,000 m³/sec along the East Route, saltwater intrusion will definitely be worsened in the Chang Jiang estuary. This will also alter the ecological environment of fishery resources. Therefore, serious attention must be paid to these effects in the planning of the south-to-north transfer.
(2) The transfer of 1,000 m³/sec in the short term from the Chang Jiang will have little effect on the estuary during the flood season of years with average flows, but it would have a definite impact during the dry season. When the discharge at Datong station drops to less than 16,000 m³/sec, water should be transferred prudently or, even better, not at all. Transferring water directly from the estuarine areas would have a larger effect on the estuary than diversions from the upper and middle reaches of the River.

From an overall viewpoint, each of the East Route and Middle Route proposals has its advantages and limitations. Henan Province would not benefit from the proposed East Route, while neither northern Jiangsu and Anhui nor southern Shandong would derive any gain from the proposed Middle Route. These two plans should complement each other. The East

Route should tackle the problems of northern Jiangsu, northern Anhui and southern Shandong, but not cross the Huang He. For the Middle Route, priority should be given to providing water for industry and agriculture in the Tangbai He region and eastern Henan. Regions to the north of the Huang He should solve their problems by making the most of their own water sources. If water is still not sufficient, then it should be transferred along the Middle Route.

(3) The interbasin transfer of water closely affects both the importing and the exporting basins. The fact that water is plentiful in the south while insufficient in the north is one of the reasons for the transfer but not the only one. We must not only consider the water sources in north China, and how much water needs to be transferred from outside, but also analyze how much transferable surplus water there will be in the Chang Jiang basin with the development there of the four modernizations (of industry, agriculture, national defense and science and technology). Until this is made clear, as little water as possible should be transferred. The south-to-north water transfer must be joined with the drawing up of comprehensive long-term plans for the Huang He and Chang Jiang basins.

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