Wu Caiping, Shi Mingli, Jiang Naiqianand Wang Dechang

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

Abstract: Warping (coarse sediment diversion) at Xiaobeiganliu reach in the Middle Yellow River is another new strategic attempt in handing sediment in the Yellow River after the regulation of both sediment and flow through the Xioalangdi Reservoir and the other reservoirs. The experimental investigation of the warping indicates that proper operation of the diversion sluice and exit sluice can realize the objective of trapping the coarse sediment and discharging the fine sediment back to the river. The paper shows our research results, one of which, among the others, is that about much coarse sediment can be reduced if six warping works of the kind are completed at the Xiaobeiganliu reach, and warping at the Xiaobeiganliu reach will have an apparent role in mitigating the deposition in the Lower Yellow River.

Key words: Xiaobeiganliu Reach, sediment diversion, trapping coarse sediment and discharging the fine sediment, mitigating deposition in river channel, the Yellow River

1.      Introduction

Warping (Coarse sediment Diversion) at Xiaobeiganliu reach in the Middle Yellow River (Fig.1) is another strategic attempt after the regulation of both flow and sediment through the reservoirs in the course of exploring the ways for training the Yellow River.

The objective of the warping project is to trap the coarse sediment and discharge the fine sediment back to the river in order to reduce the amount of sediment entering the Lower Yellow River so that the sedimentation in the Sanmenxia reservoir，the Xiaolangdi reservoir and in the Lower Yellow River could be mitigated.

Based on analysis of the field data and the data measured on the physical model for the coarse sediment diversion (warping) at Xiaobeiganliu reach, the paper shows the research results about the hazards done by coarse sediment on the lower Yellow River, and the role of the coarse sediment diversion in the deposition mitigation in the lower Yellow River.

Fig.1     Location of the Pilot Project of Warping

1.      Characteristics of Sediment in the Yellow River and Effect of Coarse Sediment on Sedimentation in the Yellow River

1.1  Characteristics of Runoff and Sediment

There are great differences in geological, topographical, and rainfall features over the Yellow River Basin. The striking features of the basin, amongst the others, are that flow and sediment originate from different sources, and that they are not evenly distributed yearly, seasonally or monthly. The Middle Yellow River, which ranges from Hekouzhen to Tongguan, covers 39.3% of the whole basin in area. The mean annual flow amount originating from the reach is 38% of the total in the basin, which is compatible with the area, while the amount of sediment originating from the reach is as much as 90% of the total in the basin. So the Middle reach is the major sediment source.

Geomorphologic features and distributions of yellow loess are different over the Yellow Loess Plateau in the area of the Middle Yellow River. Mean particle size of the yellow loess  is bigger than 0.045 mm in the northwestern region, and smaller than 0.015mm in the southeastern region. Under the attack of heavy rainfalls in the flood seasons, much sediment is eroded from the plateau and is brought to the Yellow River, which is the reason why the sediment load in the Yellow River is much large, and the sediment particles are much finer than those in the other large rivers. In the flood seasons, the mean particle size of the sediment entering the Sanmenxia Reservoir downstream of Tongguan is 0.026 mm in diameter, and the mean particle size of the sediment entering the Lower Yellow River is 0.02mm at Huayuankou. The fine particles move mainly in suspension. The coarse particles of the suspended exchange with the sediment particles on the riverbed at any time and the exchanging is subject to the hydraulic factors, consequently causing the morphological changes of the channel.

Having slim chances to stay on the river bed, the fine particles in suspension sediment can keep in suspension, and run far away way without stopping. The fluid of the fine particles are often in sub-saturation, and when the concentration of the fine particles reaches to some extent, the fine-sediment-loaded flow will change its fluid nature and form into a kind of quasi-homogeneous fluid, which is of a higher density. The newly formed fluid is favorable for transporting the sediment of medium and coarse sizes.

1.2  Hazards done by coarse sediment to the Lower Yellow River

Sediment-loaded floods which are originating from different sediment sources in the Yellow River Basin, have different effect on the Lower Yellow River.

In 1960-1990, there happened 14 floods originating from the coarse sediment source. The floods carried 25.8 billion m³ of water and 4.482 billion t of sediment. The mean discharge of the 14 floods was 1775 m³/s，the mean sediment concentration was 174kg/m³ and mean particle size was 0.032mm. The 14 floods made 2.795 billion t of sediment deposited in the Lower Yellow River, i.e., the deposition amount was 62.4% of the incoming sediment load. The coarse fraction of sediment, which is 8% of all the sediment fractions, was completely deposited in the Yellow River. Namely, if 0.1 billion t of coarse sediment can be trapped in the Middle Yellow River, deposition of the coarse particles in the lower Yellow River will reduce by 0.1 billion t. The coarse fraction (D=0.05 ~ 0.10mm), which is 24.3% of all the fractions, has as a high deposition ratio as 85.48%.

As a contrast, the floods which originate from the fine sediment source, have less severe hazards to the lower Yellow River. In the same 30-year period, there happened 108 floods of the kind, which carried 275.4 billion m³ of water and 17.48 billion t of sediment. The mean discharge of the floods was about 2300 m³/s with mean concentration 63.5 kg/m³. The mean D₅₀ was 0.022mm. The 108 floods caused 4.783 billion t of sediment deposited in the Lower Yellow River. The deposition ratio of all the fractions is 27.36%. 51.6% of the sediment with D>0.05mm is transported all the way through the channel to the Bo Sea.

In the same 30-year period, there were 76 floods which originated from slim sediment yielding source. The floods carried 27.9 billion m³ of water and 6.1 billion t of sediment with mean concentration of 21.8kg/m³. The floods served to dilute the flow-sediment mixture, so decreasing sediment concentration in the lower Yellow River, and so eroding 1.59 billion t of sediment out of the river bed , instead of depositing.

Summarily, all the data indicate that floods originating from different sediment yielding sources have different effect on sedimentation in the Lower Yellow River. It is recommended that measures must be taken to prevent the coarse fraction of sediment (D>0.05mm) from entering the Lower Yellow River so as to reduce sediment deposition there, so making the Lower Yellow River life healthy.

2 Role of Coarse Sediment Diversion at the Xiaobeiganliu Reach in Mitigating Deposition in the Lower Yellow River

2.1 Effect of Fine Sediment on Mitigating Deposition in the Lower Yellow River

Section 1 shows the hazards by the coarse sediment to the Lower Yellow River. The current section focuses on the role of fine sediment in mitigating deposition in the Lower Yellow River. Table 1 and Fig.2 shows three kinds of combinations of floods and sediment processes, which have nearly same quantities of flood volumes, mean flood discharge, and mean concentration. The suspended sediment carried by the floods has the mean particle sizes as 0.018mm、0.022mm and 0.028mm, respectively.

The table 1 shows that when the mean particle sizes of the suspended sediment become

Table 1  Ratios of Deposition of fractions of sediment particles

Fig 2  Ratio of deposition fractions in the Lower Yellow River

smaller, ratios of deposition of each or all the fraction(s) of particles become smaller, and that when the mean particle sizes become as small as 0.018mm, ratios of deposition of each or all the fraction(s) of particles become very small and approach to a same value, 11.5%. In other words, when the mean particle size become smaller than 0.018mm, sediment-laden flow becomes a homogeneous fluid. Coarse Sediment Diversion at the Xiaobeiganliu Reach will impove the sediment grading and reduce the deposition in the Lower Yellow River.

2.2 Performance of the Coarse Sediment Diversion at the Xiaobeiganliu Reach

The principle for the Coarse Sediment Diversion at Xiaobeiganliu Reach is that when the river discharge is greater than 500m³/s, and sediment concentration is greater than 50kg/m³, the diver sluice must be lifted to draw as much sediment, especially coarse sediment, as possible. The river flow and sediment hydrographs in 1998 are taken as the design flow and sediment conditions.

The statistics of the designed flow and sediment conditions are given here. The river discharge is greater than 500m³/s, and sediment concentration is greater than 50kg/m³. There are 16 days available for the coarse sediment diversion. In the period 94 million m³ of water and 13.39 million t of sediment are drawn，the concentration is 142kg/m³，the mean particle size of the suspended sediment is 0.025mm. In the 16-day diversion period, river flow volume is 1.617 billion m³ with river sediment load of 0.279 billion t and mean sediment concentration of 172 kg/m³.

Following the design, we did an experimental investigation on a physical model. The model ran for 37 days. The research results are summarized as follows.

Analysis of the data indicates that there is 21.46 million t of sediment deposited in the design trapping area. The deposition of coarse sediment (D>0.05mm) accounts for25.18% of all the sediment fractions.

The deposition of coarse sediment (D>0.025mm) accounts for 59.31% of the whole sediment composition.

For the whole trapping area, the ratio of deposition of the whole sediment composition is 66% and the ratio of deposition of the coarse fraction (D>0.05mm) is 83%. In other wards, four fifths of the sediment entering the trapping area is deposited in the area. The ratio of discharging sediment at the exit sluice is 44%, and the ratio of discharging the fine sediment (D<0.025mm) is 46%. Namely, half of the fine sediment entering the trapping area is successfully discharged into the river.

The data above indicate the design scheme can fundamentally realize the objective of the coarse sediment diversion, i.e., trapping the coarse sediment and discharging the fine sediment back to the Yellow River.

When the project was implemented on the site at Xiaobeiganliu, the performance will be better than that on the model because some engineering measures can be implemented in the field, but not on the physical model.

2.3 Estimation of Performance of the Coarse Sediment Diversion in Mitigating Deposition

The experiment of coarse sediment diversion at Xiaobeiganliu reach, which ran for 16 days in the flood season with the flow and sediment hydrographs in 1998 as the input conditions, indicates that the drawn flow and sediment account for 5.8% and 4.8% of the river flow and sediment load, respectively. Because there was only one single coarse sediment diversion works implemented in 2004, which was at Lianbotan, the diverted flow and sediment amounts were too small to produce apparent effect on the disposition in the Lower Yellow River.

If the coarse sediment diversion is done in as a large scale as possible, say, six diversion works of the kind on the banks of the Xioabeiganliu Reach, the drawn flow and sediment amounts will account for 34.8% and 28.8% of the river flow and sediment load, respectively. Further, if the deposition ratios and the discharging ratios of various fractions of sediment are taken as those obtained from the diversion at Lianbotan in 2004, and if the interaction between the diversion sluices are ignored in the estimated, the amounts of fractions of sediment entering the lower Yellow River will be given in the Table 2.

Table 2 shows that for the typical 1998 flow and sediment conditions, mean discharge at Longmen is 1170m³/s with mean concentration of 172kg/m³，and with mean suspended particle size of 0.025mm. It is assumed that after the flow and sediment goes through the coarse sediment diversion works, all the drawn water will go back to the river, so making the total river flow volume same as before. With the diversion, the river concentration will be reduced from 172 kg/m³ at Longmen, downstream, to 145kg/m³ at Tongguan, i.e, 15% less. Correspondingly, the mean suspended particle size will be reduced from 0.025mm to 0.023 mm.

Table 2 shows clearly that the coarse sediment diversion will have a good performance, which can be summarized as

l        trapping coarse sediment will reduce the river concentration by 15% and reduce the coarse sediment load (D>0.05mm) by 18%；

l        discharging the fine sediment will reduce the size of the particles entering the downstream river to 0.023mm.

The decrease in concentration of sediment, especially the coarse sediment (D>0.05mm), will, without doubts, mitigate the deposition in the Xiaolangdi reservoir and in the lower Yellow River. In addition, the decrease in the mean suspended particle size will improve the composition of the sediment carried by floods, which will be favorable for sediment transport, consequently contributing to deposition mitigation, too. Under the circumstances without more field data available, we have to do with the data in Table 1 and Fig.2 to give a rough estimation of the deposition with the six diversions at the Xiaobeiganliu.

Due to the decrease in mean particle size from 0.025mm (without coarse sediment diversion) to 0.023mm with the six diversions, if the interaction of sedimentation in the Sanmenxia reservoir and the Xiaolangdi reservoir are ignored, the deposition ratio in the lower Yellow River will decrease from 37% without the diversion to 30% with the diversion. Further, the deposition ratio of coarse sediment (D>0.05mm) will decreases from 61% to 53%.

The above estimation is, more or less, rough, but it is based on a clearly-explained mechanism for deposition mitigation, so the estimation remains quite convincing. The results indicate the large-scale coarse sediment diversion will have an apparent role in mitigating deposition downstream.

Table 2  Sediment and deposition with all the six sediment diversion works

3.Coarse Sediment Diversion, A New Promising Strategic Measures in Keeping the Yellow River Life Healthy

In the course of solving sediment-caused problems, YRCC has developed a system of integrated strategies for handling the sediment problems, which are sediment detention, sediment delivery, sediment diversion, regulation of both flow and sediment, and dredging.

About sediment detention, there will slim chances to construct large storage reservoirs on the stem Yellow River in the near future. for soil conservation in the Middle Yellow River, it has been found to have flat-footed paces in reducing sediment yield.

As a contrast, the analysis of the data measured both in the field at Lianbotan and on the physical model indicates that coarse sediment diversion has apparently good performance in modifying the sediment composition and in mitigating deposition.

Considering the circumstances for the local social and economic development in the Middle Yellow River, we think that only coarse sediment diversion is practically feasible. It has little inverse effect on the integrated use of sediment and flow sources in the Yellow River, and it can be easily implemented. In particular, in mitigating sedimentation in the Xiaolangdi Reservoir, coarse sediment diversion at the middle Yellow River is a kind of strategy that cannot be replaced by the others. So it will be a new strategy for keeping the Yellow River life healthy.

References

Zhao Yean, Zhou Wen Hao, etc, the Fundamental Law of the Fluvial Process in the Lower Yellow River, Yellow River Press, 1998.

Chen Ji Wei, Xu Ming Quan , etc, Training the Yellow River and Developing the Water Resources in the Yellow River Basin, Yellow River Press, Dec.1998.

Yellow River Institute of Hydraulic Research, Physical Experimental Investigation of Trapping the Coarse Sediment in the Liantan Floodplain at the Middle Yellow River (Research Report), 2004.5.

Yellow River Institute of Hydraulic Research, Analysis of the Performance of the on-site Experiment of Trapping the Coarse Sediment in the Lianbotan Floodplain at the Middle Yellow River ( Research Report), 2004.3