Liu Yuelan¹,Zhang Huaxing² and Chen Dongling³

1.Yellow River Institute of Hydraulic Research, Zhengzhou, China, 450003

2. Department of Cooperation Science and Technology, YRCC 3. Hydrology Bureau, YRCC

Abstract:Characteristics of incoming flow and sediment as well as the erosion and deposition in the Lower Weihe River are analyzed in this paper. Deposition alleviation impacts in the Lower Weihe River of transferring water from Xiaojiang River of the Three-Gorge Reservoir to Weihe River and regulating flow with control reservoirs are analyzed and demonstrated.

Key Words: the Lower Weihe River,Water and Sediment Relationship,Water Transfer from Xiaojiang,Regulation Reservoir; Deposition Alleviation

1 Water and sediment conditions in the Lower Weihe River

WeiheRiver is the largest tributary of the Yellow River and also a silt-loaded river. The main river is 818 km long and pours itself into the Yellow River in Tongguan. The lower reach is below Xianyang and is 208 km long.

The characteristics of incoming water and sediment in the Lower Weihe River are water and sediment coming from different sources and the disharmonious between them. Recently, the inflow has reduced significantly.

1.1 Water and sediment come from different sources

According to the analysis on 1960-1998, the amounts of water and sediment of Huaxian Station in the Lower Weihe River are 7.044×10⁹ m³ and 0.345×10⁹ t, respectively; the annual average inflow is 4.223×10⁹ m³ and the sediment is 0.118×10⁹ t in Xianyang Station of main river, which are 60% and 34.2% of the inflow and sediment in Huaxian Station, respectively; the annual average inflow is 1.397×10⁹ m³ and the sediment is 0.234×10⁹ t in Zhangjiashan Station on Jinghe River, which are 20% and 68% of the inflow and sediment in Huaxian Station, respectively. The inflow of Nanshan tributary below Xianyang Station is about 20% of Huaxian Station whereas the sediment is a little.

1.2 Disharmonious between water and sediment 

Fig.1 shows the daily average discharge and sediment transportation rate of Huaxian Station in 1974-1996. It is indicated that the sediment peaks of Weihe River mainly concentrate on July and August, whereas the flow peaks occur in September. The incoming sediment amounts are 35% and 37% of annual amount, respectively, whereas incoming water amounts are both about 15% of annual amount in July and August; in September, incoming water is 19% and sediment is 14％ of annual amount, respectively, as well as the values are 14% and 3.3% in October.

Hyperconcentrated low flows often occur in the Jinghe River, which is one of the tributaries of Weihe River. The daily average maximum peak flow is less than 1000 m³/s; the days of daily average discharge less than 100 m³/s occurred are over 50% and less than 200 m³/s are 80%; the highest sediment concentration is 600-700 kg/m³, which is shown in Figure 2. In Huaxian station, such Hyperconcentrated low flows occurred are once or twice every year on average before 1990, whereas 2.5 times during 1991-2001, 5 times in 1994 and 1995. Before 1990, 3%-4% of water amount of Hyperconcentrated low flows carried 23% of sediment amount of that year. In 1991-2001, 53% of sediment came from Hyperconcentrated low flows in Huaxian Station every year and 53% of yearly sediment amount was carried by 12% yearly water amount. The relationship between water and sediment was worse than ever.

1.3 Recently incoming water and flood peak have reduced apparently

The days of discharge levels occurred in different periods of Huaxian station are counted in Table 1. It is shown that water amounts and peak discharges were big in 1950s, 1960s and 1980s, but it was drier in 1970s and 1990s, especially in 1990s, when the days of discharge less than 50 m³/s increased whereas the days of discharge more than 200 m³/s decreased. Among them, the days of 1000-2000 m³/s appeared were about one fourths of previous and the discharge level of more than 3000 m³/s appeared only one day in 14 years.

Table 2 lists the variations of field measured sediment amounts in different periods of Huaxian station. It shows that the incoming sediment amount has reduced since 1980s. The annual average amount is 0.276×10⁹ t in 1980-1989 and 0.249×10⁹ t in 1990-2000. They are 66.2% and 60% of the annual average values in 1950-1979, respectively.

The above analysis shows that during 1990-2000 the field measured water amount reduced half of the yearly average nature runoff in Lower Weihe River, and the sediment reduced by 40% of 1950-1979 on average. The peak flow discharges reduced significantly and the Hyperconcentrated low flows increased.

Table 1. Comparisons of Days Appeared Yearly on Average

        of Discharges in different Periods of Huaxian Station

Table 2.  Incoming Sediment Amount in different Periods of Huaxian Station  (10⁹ t)

2  Analysis on Erosion and Deposition Characteristics in the Lower Weihe River

2.1  Erosion and Deposition Process of the Lower Weihe River

At the original operation period of Sanmenxia Reservoir, corresponding relationship obviously exists between the variations of erosion-deposition in the Lower Weihe River and the lift of Tongguan elevation. When tongguan elevation kept relative stability in 1973-1986, erosion and deposition kept balanceable in the Lower Weihe River. After 1986 lacking of water, Tongguan elevation lifted and accumulative deposition occurred in the Lower Weihe River. As shown in Table 3, during June of 1960 and October of 1973, 1.019×10⁹ m³ of sediment was deposited in the riverbed of the Lower Weihe River Lintong to the estuary, which was 81% of the total deposition of this reach in 1960-2000 and the yearly average was 0.079×10⁹ m³. In 1974-1990, the deposition was 0.019×10⁹ m³ and the yearly average was 0.001×10⁹ m³; in 1991-2000, the deposition of this reach was 0.241×10⁹ m³ and the yearly average was 0.024×10⁹ m³。These show after Sanmenxia Reservoir was operated, the deposition in the Lower Weihe River was affected mailnly by the lift of Tongguan elevation; after the rebuild of Sanmenxia multi-purpose project in 1973, the erosion and deposition in the Lower Weihe River were regulated mainly by the incoming flow and sediment conditions.

Table 3.  The Relationships between the Erosion-Deposition Process and Tongguan Elevation

and Inflow & Sediment conditions in the Lower Weihe River

2.2  The Relationship between Erosion-Deposition and Incoming Flow-Sediment Load in the Lower Weihe River

2.2.1  The Channel Regulation Process of Erosion & Deposition

As other alluvial rivers, riverbed erosion and deposition in the Lower Weihe River regulate with the variation of incoming water and sediment conditions. In less-water-heavy-sediment years, river channel deposits badly and the deposition amount in several years is always near or more than that in ten years. In big-flood years, river channel scours significantly, and the erosion in these years can remove the deposition in dry years. The annual erosion & deposition process line of the Lower Weihe River and the inflow process line of Huaxian have invert image relationship. Both the erosion magnitude in big-flood season and the deposition one in small-flow season are significant.   

Comparing the two periods of 1973-1990 and 1990-2000 in Table 3, under the conditions of 0.96 m and 0.73 m of Tongguan elevation lift, the yearly average water amount was 7.4×10⁹ m³ and sediment amount was 0.33×10⁹ t in Huaxian Station in the former period, and slight scouring occurred in the Lower Weihe River; Because of the inflow reduced dramatically in the latter period, big deposition occurred in the Lower Weihe River, which leaded to the river channel shrinking and the flood discharging capacity apparently decreasing. 

The Lower Weihe River channel kept slight depositing in 1974-1990. The reason one is the Hyperconcentrated low flows occurred not too many; the other is the scouring in the years of rich water, especially in 1975, 1983-1985 and 1988, which counteracted the deposition in dry years. In the Hyperconcentrated low flow years of 1994, 1995 and 1997, the annual average incoming water is 2.245×10⁹ m³ and sediment concentration is up to 124.72kg/m³. The annual average deposition is 0.176×10⁹ m³ and the total deposition in these three years is 73% of that in 1991-2000. Thus, less water with more sediment load and lacking of big floods scouring in rich water years cause the channel deposition and shrink in the Lower Weihe River.

2.2.2  The Relationship between Erosion & Deposition and Incoming Water & Sediment Conditions

Figure 3 shows the relationship of annual deposition in the Lower Weihe River and the annual water amount in Huaxian Station in 1974-2000 after the rebuilding of Sanmenxia Reservoir. It presents that the trend of annual deposition amount decreasing with the increase of annual water amount is obvious. When annual water amount is more than 8×10⁹ m³ in Huaxian Station, scouring would come out in the Lower Weihe River. The point data are the corresponding deposition values of different years and the annual average values, which are basically on the trend line.

Fig.3  The relationship between the annual erosion & deposition amounts

              in the Lower Weihe River and the annual water amounts of Huaxian Station

Figure 4 shows the relationship between erosion & deposition amount in the Lower Weihe River and inflow amount of Huaxian Station in flood seasons. It presents that when the incoming water amount in flood season is more than 5.5×10⁹ m³, the Lower Weihe River channel is scoured; With the increasing of the incoming water, the scouring amount rises very fast. Both Figure 3 and Figure 4 show that under the field water and sediment conditions, to keep the erosion and deposition balance in the Lower Weihe River, the annual and flood season water amount for sediment transport are 8×10⁹ m³ and 5.5×10⁹ m³, respectively.

Fig.4  The relationship between the erosion & deposition amounts in flood season

         of the Lower Weihe River and the incoming water amounts of Huaxian Station

Figure 5 shows the relationship between the annual incoming sediment coefficient in Huaxian station and the erosion & deposition amount in the reach below Huaxian. It indicates that deposition is great in overbank-flood years. In Hyperconcentrated low flow years, the incoming sediment coefficient is beyond 1 and the annual average deposition is more than 50 million m³; whereas in big flood years, the incoming sediment coefficient is less than 0.1 and the annual average erosion is 50-100 million m³. It reveals the different bed-forming functions of the two water and sediment load compositions in the Lower Weihe River.

Fig.5  Relationship between Annual Erosion & Deposition in the Lower     Wei River and the Incoming Sediment Coefficient of Lintong

2.2.3 The relationship between sediment transport rate and discharge of Hyperconcentrated low flow in Jinghe River

    Hyperconcentrated low flows often occur in Jinghe River, which is a tributary of Weihe River. So called Hyperconcentrated low flow, its discharge is less than 1000 m³/s and the sediment transport rate is commonly less than 100%. The relationship between sediment transport rate and inflow of Jinghe and Weihe River in Hyperconcentrated low flow is shown in Figure 6.

  Figure 6 shows，When the discharge in Jinghe River is less than 500 m³/s and the inflow from Weihe    River is also very small, the sediment transport rate is very low (normally 20-30%); when the inflow from Weihe River is more than 100 m³/s (peak value), the sediment transport rate is about 40%, and it is up to 80% when the inflow is over 200 m³/s. The Hyperconcentrated low flow of Huaxian Station transported 53% annual sediment amount with 12% annual flow amount in 1991-2001. It is the main reason to cause

2.2.4 Changes of bankful discharge in the Lower Weihe River

Bankful discharge reflects the flood discharging capacity of river channels. Only big flood can keep channel scouring and increase flood discharging capacity. As shown in Figure 7, the bankful discharge of Huaxian Station in the Lower Weihe River basically corresponds to the inflow process. At the beginning of 1980s, which are rich water years, the bankful discharges are up to 4000-4500m³/s. Both in 1988 and 1992, the bankful discharges are beyond 3000 m³/s. After the deposition caused by Hyperconcentrated low flow in 1994 and 1995, the bankful discharge reduced to below 1000 m³/s.

Figure 8 shows the relationship between the bankful discharge and the maximum 3 days discharges in the same year and the bankful discharge values in past years in Huaxian Station. It indicates that banful discharges are caused by flow and sediment load conditions in many years, and also relate to the incoming flow process in that year. Both in big flood years the maximum discharge scouring and in small flood years the low flow depositing cause the riverbed deforming significantly.

Fig.7  Changes of Bankful Discharge and Annual Water Amount in Huaxian Station

Fig.8  Relationship between the Bankful Discharges and the Maximum 3 days Discharge For a year and the bankful discharge values in past years in Huaxian Station.

3  Analyses of effects on erosion and deposition of the Lower Weihe River caused by transferring water from Xiaojiang River to Weihe River 

3.1  Water amount variations of Huaxian in different series years

The annual average inflow of Huaxian is only 3.7×10⁹ m³ during 1991-2000. After transferring water with 300m³/s and 600m³/s, the annual average water amounts of Huaxian are up to 6.87×10⁹ m³ and 10.03×10⁹ m³, respectively. When water was transferred with discharges of 300m³/s and 600 m³/s during low flow years of 1986-1997, the annual average water amounts of Huaxian are 8.1×10⁹ m³ and 11.26×10⁹ m³, respectively. They will be more in rich and even flood years.

3.2  Scenarios of reservoir regulation

Based on the two situations of daily water transfer with discharges of 300m³/s and 600m³/s (clear water) in flood season (June 1^st - September 30^th) from Xiaojiang River, and combining reservoir regulation mode (The storage capacity of the reservoir is around 0.3×10⁹ m³ and it is full with 300m³/s in 12 days, 600m³/s in 6 days, respectively.), calculations of 7 scenarios are carried out.

Scenario 0-0: no water transfer.

Scenario 1-1: Transfer water from Xiaojiang River with daily discharge of 300m³/s without reservoir regulation and directly put into Weihe River. That is to say, during the period of water transfer, the daily discharge in Xianyang station increases by 300m³/s. 

Scenario 1-2: The daily discharge of water transfer from Xiaojiang River is 300m³/s. Through the reservoir regulation, firstly make the reservoir storage full, and then empty it in three days. To the Xianyang Station, the period is 15 days. Discharge does not increase in the first 12 days and it increases by 1500m³/s in the last 3 days. The discharge increases by 300m³/s in 29^th and 30^th of September every year.

Scenario 1-3: The daily discharge of water transfer from Xiaojiang River is 300 m³/s. Through the reservoir regulation, considering the sediment load situations of Zhangjiashan in Jinghe River, release water when Jinghe River encounters hyperconcentrated flow (sediment concentration is more than 200 kg/m³). The specific regulation modes are:

A.    If hyperconcentrated flow does not occur in Jinghe River after June, impound water to full reservoir stage and add 300m³/s to Xianyang;

B.    If Jinghe River encounters hyperconcentrated flow, the reservoir releases water to Xianyang with discharge of 1500m³/s. If the stored water cannot make this discharge, emptying the reservoir is considered;

C.    If hyperconcentrated flow still appears in Jinghe River after emptying the reservoir, releasing 300m³/s is considered;

D.   If the reservoir has storage to impound water after hyperconcentrated flow, firstly store water in reservoir without releasing. After the reservoir is full, release water with discharge of 300m³/s;

E.    Empty the reservoir at the end of September and control the maximum releasing discharge into Xianyang within 1500m³/s.

Scenario 2-1, 2-2 and 2-3: The daily discharge of water transfer from Xiaojiang River is 600 m³/s and the reservoir regulation modes are the same as scenario 1-1, 1-2 and 1-3; but the regulation period is 9 days and water releasing to Xianyang are 600 m³/s and 1800 m³/s.

3.3 Comparisons of flow and sediment load processes in different regulation scenarios

Figure 9 shows the flow and sediment load processes of scenario 1-2 and 1-3 of Huaxian in 1995, with the daily water intake is 300 m³/s during June – September and after the reservoir regulation. In scenario 1-2, 8 times of flood with the peak discharge of over 1500m³/s are formed; in scenario 1-3, only 5 times flood peak are formed, but they match the sediment peaks well. In addition, small flood occurred during the period of sediment peak, thus, the flood peaks increase and the maximum one is above 2000m³/s.

	Fig.9  Discharge Processes in Huaxian Station for Different Regulation Scenarios in 1995

 

3.4  The day’s variation occurred before and after water transfer of different grade discharge

Table 4 lists the days of discharge levels appeared from June to October before and after water transfer during 1991-2000. Comparing all the scenarios after and before water transfer, the days of discharges beyond 500 m³/s appeared increase by more than one time. The days of discharges beyond 2000 m³/s appeared are 3.5-3.7 days on annual average for Scenario 1-2 and 1-3, which reach the level of 1950s-1980s in Table 1, but the days of discharges beyond 3000 m³/s appeared are below that level.

Table 4.  Days of discharge levels appeared before and after water transfer

                    in annual average for different scenarios in Huaxian station during 1991-2000

3.5  Prediction of erosion and deposition changes in the Lower Weihe River

As water transfer with the discharge of 300m³/s from Xiaojiang River during June-September every year, the annual water amount of Huaxian is 8.1×10⁹ m³ during the low flow years 1986-1997 and 5.92×10⁹ m³ in flood seasons on annual average. In Scenario 1-2 and 1-3, the days of discharge between 1000-3000 m³/s appeared are more than or near those in the periods before 1990. Based on above analyses, encountering the flow and sediment load series of year 1986-1997, the Lower Weihe River channels will come out erosion and deposition balance; while encountering lower flow series of year 1991-2000, the annual average water amount of Huaxian is 6.87×10⁹ m³, and slightly deposition will occur in the Lower Weihe River main channels.

3.6  Prediction of flood discharging capacity changes in the Lower Weihe River

If encountering low flow series of year 1990-2000 after water transfer from Xiaojiang River to Weihe River, as shown in Figure 8, calculated Scenario 1-3 basing on the relationship between annual maximum 3 days average discharge and bankful discharge, the bankful discharge of Huaxian can reach about 3000m³/s, which can not lead to the situation of channel shrinking badly (see Figure 10).

Fig.10  Bankful Discharge Changes before and after Water Transfer

4  Conclusions

（1）           Water and sediment come from different sources as well as disharmonious between water and sediment in the Lower Weihe River. After Sanmenxia Reservoir was built, tracing to the source deposition has occurred in the Lower Weihe River and the situation of suspended river has further worsened. In recent years, the demand of water resources has increased and the continuous low flow has come out, which cause the relationship between water and sediment worse and worse. Therefore, the river channels shrink by deposition and the flood discharging capacity decreases. So that the flood disasters occur frequently and it is necessary to be harnessed as soon as possible.

（2）           After the rebuilding and application of Sanmenxia Reservoir, erosion and deposition in the Lower Weihe River mainly changes with incoming water and sediment, especially in Hyperconcentrated low flow years, when river channels shrink and flood disasters are easily caused by median and small floods. This situation will get worsen in great water demand and low flow years. Therefore, transferring water to Weihe River to reduce the occurrence probability of Hyperconcentrated low flow and enlarge the peak discharge of scouring river channels is an effective approach for deposition alleviation in the low Weihe River.

（3）           The multi-year average incoming water is 7×10⁹ m³ and sediment is about 0.35×10⁹ t in the Lower Weihe River. Both before and after the operation of Sanmenxia Reservoir, slight deposition occurs in the Lower Weihe River channels.

（4）           After water transfer with the discharge of 300m³/s from Xiaojiang River flom June to September, the annual total water amount of Huaxian is 8.1×10⁹ m³ and incoming sediment is 0.286×10⁹ t during the low flow years of 1986-1997. Cooperating with sediment peak of Jinghe river to regulate discharge, the discharge in Weihe River will increase and the relationship between water and sediment will be harmonious. It is predicted that erosion and deposition generally reach balance and scouring is occurred in main channels.

（5）           After transferring water from Xiaojiang, the discharge will increase and main channel will be scoured. If encountering very low flow series such as in 1990s, the annual average water amount will reach 6.87×10⁹m³. After the reservoir regulation, the days appeared with discharge of 1000-1500 m³/s will be more than and 2000-3000 m³/s will be near those in 1950s or 1980s. It is predicted that balance between erosion and deposition will appear in the Lower Weihe River hannels，and the bankful discharge will keep 3000 m³/s or so and it will not lead to the river channels shrinking badly.

References

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Zhang Cuiping and Zhang Yuanfeng, Consultation Report on the initial design of river channel regulation controlling lines of the Lower Weihe River [R], Yellow River Institute of Hydraulic Research, 1997.

Zhang Hongwu, Zhao Lianjun, et al, Mathematical model calculation of the river reach between Xianyang and Sanmenxia for the Dongzhuang Hydraulic Project [R], Tsinghua University, 2003.12.

Cheng Longyuan, Liu Shuanming, Xiao Junfa, et al, Experimental Research on Hydrologic Sediment of Sanmenxia Reservoir Area, Yellow River Water Conservancy Press, 1999.

Li Guoying, The 3^rd Experiment on Water and Sediment Regulation in the Yellow River [J], People’s Yellow River, 2004, (10).

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