Hou Suzhen,Wang Ping And Chu Weibin

 Yellow River Institute of Hydraulic Research, YRCC

Abstract: The water resource exploitation and utilization of the upper reach of Yellow River promoted the economy development of the basin and also had impact on the river itself. Based on a great deal of field data, in this paper it is analysed that the characteristics of the run off regulations of Longyangxia and Liujiaxia reservoir and researched the impacts they have on water-sediment and the adjusting of erosion and deposition of the riverbed, and furthermore discussed the relationship between maintaining the healthy life of Yellow River and the regulations of the reservoirs. The mature water-sediment regulating and controlling system and the reasonable reservoir regulations will have positive effect on decreasing channel deposition and maintaining the health of Yellow River.

Key words:Longyangxia reservoir, Liujiaxia reservoir, run off regulation, adjusting of the riverbed

1 Introduction

The upper reach of Yellow River, from the riverhead to Toudaoguai station, is 3472km long, controlling 386 thousand km² drainage area which is the 51.3% of the total drainage area of Yellow River. The even annual runoff in 1952~1990 is 3504 billion m³ which is the 57.3% of the total runoff of the Yellow River and the even annual sediment transportation is 134 million tons which is only the 9.6% of the total sediment transportation of the Yellow River. It is the main area of clear water source and water energy of the drainage.

With dams as Longyangxia and Liujiaxia were built and put into operation, the massive water resource utilization produced great benefit in agriculture and industry for the up stream drainage, but also caused great changes in drainage environment. In recent year the water-sediment condition of the up stream changed much. The factors causing these changes include reservoir regulations, irrigative diversion, branch administration, adjusting of the erosion and deposition of the riverbed and climate changes etc. The run off regulations of Longyangxia reservoir and Liujiaxia reservoir have the most effect on water and sediment. This paper stresses that trying to provide scientific basis for the water and sediment regulations of the whole Yellow River drainage.

2 Water-sediment changes in the upper reach of Yellow river

Since 1960s there have built 6 large and medium scale dams such as Longyangxia and Liujiaxia which have a total storage capacity of 31.4 billion m³ and have powerful ability in run off regulation. Table 1 lists the amount of run off and sediment of the main controlling stations of the upper reach of Yellow River in deferent periods. Anningdu station locating in the down stream of Liujiaxia reservoir is the controlling station of the Ningxia and Inner Mongolia reach of Yellow River. Table 1 shows that the even annual amount of run off and sediment of Toudaoguai station in 1987~1998 have separately decreased 10.05 billion m³ and 127.9 million tons compared to that in 1950~1968, 9.8 billion m³ and 111 million tons in flood season. The same changes took place in Aningdu station in the same periods with decreasing values 8.33 billion m³ and 117.8 million tons, 10.2 billion m³ and 110 million tons in flood season. These changes indicate that the decrease of the run off and sediment in 1987~1998 mainly take place in flood season and in the up stream of Aningdu station. In non-flood season, the run off increased a little in Anningdu but little in Toudaoguai. It indicates that reservoir regulations of Longyangxia and Liujiaxia have dominant effect on the water-sediment changes in Toudaoguai station.

Table 1  Variation of the runoff and sediment transportation

unit：runoff，billion m³；sediment transportation，million tons

3 Water and sediment regulation by Longyangxia reservoir and Liujiaxia reservoir

3.1 Regulating the course of the flow

The even annual coming run off of Longyangxia reservoir is 2.16 billion m³ and that between Longyangxia and Liujiaxia is 7.8 billion m³. The Longyangxia reservoir has a large storage capacity and regulates multi-year. The Liujiaxia reservoir has a small storage capacity and mainly assist Longyangxia reservoir to regulate.

After Longyangxia reservoir’s being put into operation, the flood peak discharge was cut down and small flow increased. The proportion that the flood peak discharge was cut  is 30%~70%, sometimes more than 80%, a average proportion is 55%. Table 2 shows, in flood season in1987~1997, the proportion of the days when the coming discharge is more than 1000 m³/s is 32.2%, but that of let out is 5.9%; Quantity of days of the coming discharge is more than 500 m³/s increases from 7.6 to 32.1, taking a proportion from 6.2% to 26%; The proportion of the days that the coming discharge is less than 1000 m³/s increases from 67.8% to 94.1%. Regulated by the reservoirs, the course of the flood became a relative stable course of medium and small discharge.

Table 2 Quantity of the days of discharge into and out of Longyangxia reservoir in flood season

While the reservoirs stored water to cut down the flood peak, they changed the distribution of water in one year. Figure 1 shows the average annual storing and sluicing course of the two reservoirs. When there’s only Liujiaxia reservoir in operation before 1986, it stored water from June to October and sluiced from November to next May. The run off of each month changed little. In 1987~2004, Longyangxia reservoir evenly stored water 4.34 billion m³ from June to October with a max storage in July. It added to sluiced 3.57 billion m³ water from November to next May with a max sluicing from December to next March. The Liujiaxia reservoir changed the previous operation mode by re-regulating the flow course regulated by Longyangxia reservoir, storing water from July to September, adding to sluice in October, storing water again when Longyangxia reservoir increase to sluice in ice season, sluicing intensively in May and November.

Fig. 1 Even monthly Runoff stored or sluiced by Longyangxia reseavoir and Liujiaxia reservoir in 1987~2004

Reservoir regulations changed the water distribution in each month in the down stream of them. Figure 2 compares the run off before 1968 with that after 1987. It shows that after 1987 the run off of Anningdu station decreased remarkable from July to October, decreased a little in June and November and increased from December to next May. The ratio of the run off of flood season to non-flood season became from 62:38 to 42:58 and the run off of months went evenly.

3.2 Regulating the sediment transportation

Table 1 shows that in 1968~1986 when Liujiaxia reservoir was operated singly, the sediment transportation of Anningdu station decreased 103.2 million tons compared to that in 1950~1968, 94.2 billion tons decreased in flood season and 9 billion tons decreased in non-flood season. The decrease in flood season was mainly caused by the deposition in the reservoir. The decrease in non-flood season owed to the erosion down stream of the reservoir. The Longyangxia reservoir controlling the clear water source area has a little effect on sediment storage. The even annual amount of sediment of Anningdu in 1987~1998 only decreased 14.6 million tons, 15.8 million tons decreased in flood season and a little increased in non-flood season. Figure 3 shows the variations of the annual amount of the sediment of Anningdu, Shizuishan and Toudaoguai. The three stations have approximate variation courses of sediment. After Liujiaxia reservoir’s being put into operation in 1968, the coming sediment of the three stations decreased obviously. After Longyangxia reservoir’s

Fig. 2 Variation of the monthly runoff of Anningdu station

Fig.3 Variation of the yearly runoff of Anningdu station, Shizuishan station and Toudaoguai station

being put into operation in 1986, the coming sediment of the Anningdu and Shizuishan changed a little, but the Toudaoguai’s decreased more. I t indicates that the reservoirs not only stored the sediment but also changed the capacity of the flow for sediment transportation.

4 Effect on sediment transportation of Toudaoguai by operations of Longyangxia reservoir and Liujiaxia reservoir

   The riverbed of Ningxia and Inner Mongolia reach was weakly deposited before the dams were built. With the reservoirs were put into operation, most of the sediment was stored in the reservoirs, thus the riverbed of Ningxia and Inner Mongolia reach was eroded. After the Longyangxia reservoir was put into operation, it changed the distribution of the run off in the year, cut down the flood peak, decreased the run off in flood season and impaired the capacity of sediment transportation. The reach deposited much more than before the dams were built. The amount of the coming sediment of Toudaoguai station decreased.

Based on the flied data from 1950 to 1967, correlative analysis between sediment transport rate and discharge and between run off and sediment transportation were done. The outcome of the analysis indicates the sediment concentration rose with the discharge rose, sediment transportation rose in deferent degrees with the run off rose. The correlation coefficients are more than 0.9.  Experiential formulas for sediment transport rate and discharge in flood season and for runoff and sediment transportation in non-flood season were established. Through these formulas the sediment transportation of Toudaoguai station was calculated in condition if there are no the two reservoirs, shown in table 3. Affected by reservoir regulation, the even sediment transportation of Toudaoguai station in flood season decreased 57 million tons and decreased more in years the reservoirs stored more water, decreased a little in years the reservoirs stored less water. If there are no the reservoirs, the sediment transportation will be 90 million tons in flood season.

If there are no the reservoirs, the runoff and the sediment transportation of Toudaoguai station in non-flood season will be 658 million m³ and 14.1 million tons. The reservoir regulation made the even sediment transportation of Toudaoguai station in non-flood season decreased 3.3 million tons.

The reservoir regulations made the even annual sediment transportation of Toudaoguai station decreased 54 million tons, increased the deposition in up stream of Toudaoguai station.

Table 3 Variation of the runoff and sediment transportation of Toudaoguai station

5 The view on runoff regulation by reservoirs

The water resource exploitation and utilization produced great economic benefit and promoted the economy development. At the same time, runoff and sediment regulation by the reservoirs of the up reach of Yellow River decreased the sediment of the down reach of the reservoirs, changed the runoff distribution in the year, decreased the runoff in flood season, cut down the flood peak, impaired the sediment transport capacity of the channel, increased the deposition in Ningxia and Inner Mongolia reach. It so that had negative effect on the health of the channel.

Longyangxia reservoir is a controlling project regulating multi-yearly located in the up reach of the Yellow River. Since the runoff of the whole Yellow River began to be centralized regulated, Longyangxia reservoir stored in flood season and sluiced in non-flood season, played an important role in maintaining the flow of the down reach of Yellow River. To maintain the healthy life of Yellow River, YRCC put out the “1493” system and three modes of flood conservancy (flood controlling, flood utilization and flood producing). Depending on the large-scale key reservoirs to construct a perfect water and sediment regulating and controlling system is an effective way to produce man-made flood, increase the sediment transport capacity of the channel and decrease the deposition. The mature water-sediment regulating and controlling system and the reasonable reservoir regulations will have positive effect on decreasing channel deposition and maintaining the health of Yellow River. The construction of the planned Daliushu reservoir will provide a chance to decrease the deposition of Ningxia and Inner Mongolia reach.   

Source:  www.yellowriver.gov.cn   Editor:HuangFeng