Su Yunqi^1，2 Zhang Huaxing¹  and Chen Haichao²

1．Yellow River Institute of Hydraulic Research, Zhengzhou, Henan Province, 450003

2．Yellow River Sediment Key Laboratory of the Ministry of Water Resources, Zhengzhou, Henan Province, 450003

Abstract：One of the important signs of the Yellow River healthy life is the channel having functions for flood discharge and sediment transport. Based on the analysis of the floods characteristics and sediment transport characteristics of the Yellow River, the lower reaches should have a stable main channel with the bankful discharge of 4000 m³/s and the width of main channel is 800-1000 m.

Key Words∶Floods Discharge and Sediment Transport,River Channel, Confirmation, the Lower Yellow River

River channel is the main passage for floods and sediment. Under natural conditions, about 70%-80% floods discharge to the sea via river channel of the Yellow River. To the Yellow River, sediment transportation capacity of the river channel is one of the important signs of its healthy life. It affects floods discharge capacity of the river channel through changing riverbed configuration. Thus, it is very important for the flood control safety of the Yellow River to maintain a river channel with bigger floods discharge and sediment transportation capacity.

A healthy and natural river should be able to discharge all the possible floods occurred in the basin safely. Correspondingly, river channel is required to have big enough cross section area. However, it is not realistic because of the rising pressure on the land from population today. Therefore, people have to choose a management goal for both guaranteeing society stable and fitting real economic society conditions through analyzing the occurrence frequency of different levels floods in the future and balancing flood control investment and flood risk.

1  Flood characteristics in the lower Yellow River channel

Storm floods mainly occur in July and August in the Yellow River while the ice floods usually occur in February and March. Storm floods bring the most threat to the flood defense in the lower Yellow River, but the ice floods come fiercely with high water level and it is difficult to defend.

Storm floods mainly occur in the river course between Hekouzhen and Huayuankou of the middle and lower Yellow River. The upper stream flood only forms its base flow.

The annual maximum flows during 1946-1999 of Huayuankou Gauging station are summed up in Fig.1 It shows that the annual maximum flows less than 4000m³/s are only 4 years at Huayuankou gauging station in these 54 years while the years which discharges are more than 4000m³/s are 92.6%. Thus, before the Xiaolangdi reservoir put into operation, flows beyond 4000m³/s appeared more frequently. With the regulation of Xiaolangdi reservoir, the probability of occurring floods beyond 4000m³/s is bigger than ever. Therefore, it is necessary to maintain a main channel whose bankful discharge is 4000-5000 m³/s.

Fig. 1 The Annual Maximum Discharges at Huayuankou Gauging station during 1946-1999

2  The Impact of Cross Section Configuration to the Channel Flood Discharge and Sediment Transportation Capacity

The significant characteristic of water and sediment of the Yellow River is “insufficient water and excessive sediment as well as disequilibrium between water and sediment”, which caused the lower river channel depositing badly. Meanwhile, with the building of water and sediment controlling works on the main river, the water and sediment entering the lower reaches change significantly, which makes the cross sections regulating greatly and directly influences the capacity of flood discharge and sediment transportation in the lower Yellow River.

2.1 The variations of bankful discharge in the lower Yellow River

The cross sections of the lower Yellow River are normally double-step ones. There exist big differences among different areas of floodplain and main channel, and the latter is the main passage of flood discharge. Thus, the variations of bankful discharge reflect the capacity of flood discharge in some extent.

Before the Sanmenxia reservoir was built, the bankful discharge of the downstream channel was about 6000 m³/s and it changed with water and sediment conditions. Main channel scouring and floodplain depositing often occurred when big flow came. The elevation gap between floodplain and main channel enlarged and the bankful discharge increased. For example, in 1958, catastrophic flood of 22300m³/s came out at Huayuankou gauging station of the lower Yellow River. After the flood, the bankful discharge augments to 8000-10000 m³/s. During 1960-1964, when Sanmenxia reservoir released clear water, the downstream channels were scoured and the bankful discharges were further enlarged. And during 1965-1973, when Sanmenxia reservoir released sediment, the downstream main channels were deposited and the bankful discharge decreased. Before the flood season of 1973, the bankful discharge was reduced to 3200~3700m³/s. When Sanmenxia reservoir launched an operation of impounding clear water and discharging sediment flow in November of 1973, and with the impacts of the floods depositing floodplains and scouring main channels in 1975 and 1976, the bankful discharges of the downstream channels enlarged to 4300~5500m³/s before the flood season of 1980. In 1980, as the water and sediment didn’t match well, the bankful discharge reduced 330m³/s. The bankful discharges were further enlarged with good water and sediment conditions during 1981-1985. Before the flood season of 1985, the bankful discharge in the lower reaches was about 6000~7000m³/s, which was obvious bigger than that of pre-flood season of 1981.  

After 1986, with the operation of Longyangxia reservoir in upper stream and the increasing water use for agriculture and industry along the Yellow River, severe deposition occurred in the downstream channels. Especially in 1988, 1992 and 1994, when hyperconcentrated flow occurred frequently, the young floodplains also deposited severely and the cross section reduced apparently. The bankful discharge decreased year after year. Before the flood season of 1999, the bankful discharge reduced to 3000-4000m³/s, which was the lower value in history. After the operation of Xiaolangdi reservoir in 1999, with releasing clear water, the downstream channels were scoured and the bankful discharge increased slightly. But the water volume is small and the discharge is low, thus the bankful discharge increased mainly above Jiahetan gauging station. In pre-flood season of 2003, the bankful discharge was about 2500-4500m³/s of the lower reaches.   

2.2 The impact of cross section configuration regulation to flood level

Water level is a direct factor of reflecting the variations of channel cross sections and a key reference indicator for flood defense. To the lower Yellow River channels, discharge is not the unique factor for controlling water level; the rapid change of riverbed caused by strong scouring and depositing can influence the water level much. There are two parameters to denote water level characteristics: swelling water level (water level of flood swelling) and swelling rate (water level change of increasing certain discharge). The pre-exist channel scouring and deposition situations decide swelling water level, while the incoming water and sediment and channel cross section configuration (mainly channel width ) decide flood swelling rate. For the broad reach in the lower Yellow River, the impact of water and sediment conditions to flood swelling rate is realize by regulating channel cross sections in flood processes. Therefore, the variation of cross section is the decisive factor to water level swelling rate.

In Huayuankou gauging station for example (see Fig 2), swelling water level of the lower Yellow River changes with channel scouring and depositing process. After heavy scouring with big flood in 1958, water level declined near 1 m when the discharge was 3000 m³/s. When Sanmenxia reservoir was releasing clear water, the lower channel was experiencing a process of continuous erosion. After 1964, with the change of Sanmenxia reservoir operation mode and the unfavorable water and sediment conditions, the lower channel was experiencing a process of deposition. The main channel deposited badly and the swelling water level in 1975 was 0.74 m higher than that in 1958 and 1.29 m higher than that in 1964. After the heavy deposition caused by hyperconcentrated flow in 1977 and the erosion caused by low sediment load flow in 1981, the swelling water level of big flood in 1982 was 0.39 m higher than in 1976. Because of the most unfavorable incoming water and sediment conditions since 1986, the water levels have risen dramatically in the process of continuous depositing in the lower Yellow River.  

Fig. 2 The relationship between water level and discharge of the representative flood at

Huayuankou gauging station of the lower Yellow River 

For the variation of the water level swelling rate, based on Manning formula, the discharge can be expressed as 

Rewrite the formula with water depth H:

Water level swelling rate expression can be obtained through making partial differential coefficient to discharge Q in the above formula.

                       （2-1）

Formula (2-1) reflects that the water level swelling rate not only correlates to discharge and riverbed gradient, but also has close correlation with main channel width. Under the conditions of definite discharge and riverbed gradient, the bigger of the river width, the smaller of the water level swelling rate. 

The relationship between water level swelling rate and main channel width of discharge rising from 3000m³/s to 8000m³/s in a representative flood process is shown in Fig 3. It demonstrates that water level swelling rate and main channel width in the lower gauging stations have obvious relationship of declining function, i.e. swelling rate decreases with the increasing of channel width. When channel width is less than 800-1000 m, water level swelling rate increases apparently. Thus, main channel width should not be less than 800-1000 m.

Fig. 3 The relationship between water level swelling rates and main channel widths

in the gauging stations of the lower Yellow River

2.3 Variations of channel sediment transportation ratio

Channel sediment transportation ratio is the sediment transportation amount ratio of channel outlet to inlet. It is an indicator for directly reflecting channel sediment transportation capacity. Given sediment transportation of the lower Yellow River mainly concentrates in flood seasons, sediment transportation situations in July and August from Huayuankou to Gaocun broad reaches are emphatically analyzed.

Fig. 4 indicates the variations of sediment transportation ratio with flood discharges of the broad reach Huayuankou to Gaocun. Comparing with the variations of sediment transportation ratio before and after 1986, we can clearly find that not only the sediment transportation ratios of the same discharge universally decrease but also the corresponding discharges of the maximum sediment transportation ratio decrease (from 4500 m³/s to 2500 m³/s) after 1986. The lower channels shrinking badly since 1986 and the bankful discharges decreasing are the main reasons to cause the above changes. Also they imply the decrease of channel sediment transportation capacity.      

As in the process of hyperconcentrated flow, channel cross sections autoregulate rapidly, sediment transportation ratio can stand for the sediment transportation process. Fig. 4 shows that the most sediment transportation efficiency comes out at daily discharge about 4500 m³/s before 1986. That is to say, sediment transportation effect is the best when bankful discharge is about 4500 m³/s.

Fig. 4  Variations of sediment transportation ratio with flood discharges

of the broad reach Huayuankou to Gaocun

3  Main conclusions

In order to meet the requirements of flood discharge and sediment transportation, the lower Yellow River must have stable channels with better cross section configuration and bigger sediment transportation capacity to contain definite bankful discharge. The specific characteristics are as followed:

(1) Definite bankful discharge

Based on the analysis of bankful discharge variations in different historical periods, sediment transportation characteristics and flood characteristics in the lower Yellow River, the bankful discharge of the lower Yellow River channel should be about 4000 m³/s.

(2) Definite broad and stable main channel

Based on the impacts analysis of main channel variations to flood water level in different historical periods in the lower Yellow River, the main channel width of the broad reach above Taochengpu gauging station in the lower Yellow River should not be less than 800-1000m.