Lin Xiuzhi, Wu Caiping, Tianyongand Guo Humin

 Institute of Hydraulic Research，YRCC，Zhengzhou，450003

 

Abstract：According to water and sediment condition designed, a physical scale model was built to simulate the warping process which arrived at some important conclusions, some of which are given as follows:

(1) In the initial warping stage, sediment loaded flow begins to scatter all over the warping area, sediment tends to fall just near the inlet, flow surface becomes steeper, the flow velocity sees bigger extremes, due to the influence of the initial topography , sediment concentration decreases rapidly, the suspended sediment tends to become fine.

(2) In the middle and final stages, with the deposition extending downstream and the bed at the return flow gate rising, a mainstream gully is gradually formed in the warping area, the area undergoes accretion mainly due to overflowing over floodplains. The flow surface becomes mildly sloped, and the flow in the mainstream gully goes faster, along the cofferdamand longitudinal dike in most cases. The sediment concentration decreases downstream without obvious changes in the particle sizes.

The experimental result has provided a sound technical support for the design and the operation of the prototype warping project.

Keywords：the xiaobeiganliu of Yellow River;warping experiment ; physical scale model;Water and sediment movement.

1 Introduction

A river has its life .The moving water in it is the blood to maintain its life. Sediment is the impurities contained in the blood. To maintain the healthy life of the Yellow River makes it necessary not only to keep the water moving as usual, but also to treat the sediment in a proper way. The sediment related problems are the hard nuts to crack in the training the Yellow River. Experience gained in the past decades indicates that the sediment should be treated as in an integrated way, i.e., by Detaining, Transporting, Warping, Regulating, and Dredging. Warping is done by utilizing favorable topography of the two sides in the middle and lower Yellow River, and diverting sediment contained flow from the main channel into the object area and depositing coarse sediment as much as possible . By means of warping in the Xiaobeiganliu reach of Yellow River to detain the coarse sediment, the sediment conveyed the lower Yellow River can be decreased effectively, and the deposition of the Sanmenxia reservoir can be relieved as well as Xiaoliangdi reservoir. Which is new-step and complicated systematic-project in treating the sediment of the Yellow River .According to water and sediment condition ,the physical scale model was built to study the warping process ,which has provided not only scientific basis for the design and the operation of the prototype warping project but also technical support for the extensive warping later .

2 Experimental conditions

2.1 Topographic condition

        The Lianbo-Beach, which lies in the left bank of the reach between Huangyu-67 to Huangyu-65, was selected as the area of prototype warping project including inlet sluice, channel of transporting sediment, cofferdam and outlet sluice, i.e.. The inlet sluice was situated in the first dike of the Xiaoshizhui-Engineeringand the outlet sluice was located the end of work in the mouth of Fenhe River. The channel designed is 2.5 km long and 20 m wide, two bends of different radius lie in it’s middle reach ,and it’s longitudinal slope from the intake gate to the first bend is 4⁰/₀₀₀ then turn to 5⁰/₀₀₀  .The average width of the silting basin is 0.64 km , the length is 8.6km , the area is 5.5 km² which provides 13.95 million m³ of volume for sediment to deposit .In order to detaining the coarse sediment and transporting the fines and contrasting the different effect , the silting basin was divided by a lateral dike and a longitudinal dike .The longitudinal dike separate the left region from the right region ,the right one is area② .The left region was divided into two parts by a lateral dike ,the upper is region① and the lower is area③ .The warping order designed is from area① to area② to area③（Fig.1）.

Fig.1 Indicated figure of warping in the Xiaobeiganliu reach of Yellow River

The horizontal scale and the vertical scale of the physical model is 120 and 20, the initial topography of the warping area was made as well as the channel .The test was took following the order designed ,the water stage of the outlet was controlled by way of sediment delivery percentage between 30﹪ to 50﹪ till it increases to the designed .

2.2 Water and sediment condition

When the discharge of Yellow River is between 500 m³/s to 4000 m³/s , in the same tine, the sediment concentration is over 50 kg/ m³ in the prototype warping project designed , warping will begin. In the model experiment, the water and sediment series designed is warping according to the typical year of 1998 (table1), the series was cycled twice, total 32 days in the prototype experiment .On the basis of the average grading of the prototype, the medium particle diameter of the suspended load is 0.025 mm , the scale of sediment amount more than 0.025 mm is 50%.

Table 1  Process of water and sediment diversion in typical year of 1998 and

condition of water and sediment after overflow

 

3 The moving rule of the water and sediment in the warping area

3.1 Variation of water slope and flow velocity in the warping area

        It was shown in fig.2 that the variety of the water slope during warping in the area① . It can be seen form the fig.1 that the water slope is gentle between the cross-0+000 to cross-1+200 ,at the first day ,the average slope of the whole area is 3.52⁰/₀₀₀ affected by the initial topography .With the deposition thickened ,the water level rises rapidly .The discharge and the water level controlled in outlet is the same as that of the first day ,but contrasting with the first day ,the water level in the inlet has risen 0.7m and the average water slope increased to 5.27⁰/₀₀₀ .The water slope is great till to the third day ,but with the lasher heightened and the water level rose in the outlet ,it fell to 3.7⁰/₀₀₀ to the fifth day .

Fig.2  Variation of water levels along distance in the area①

The variety of the water surface slope in the region② is shown in fig.3 .It can be seen from the fig that the water slope of the lower reach is gentle while the upper is steep .The water level rose from 370.05m to 373.31m in front of the outlet and the water slope became gentler gradually when the lasher was heightened .The average slope fell from 5.2⁰/₀₀₀ to 3.2⁰/₀₀₀.

Fig.3  Variation of water levels along distance in the area②

The current flow through the region① during warping in the region-③ .The water level fell suddenly near the crevasse of the transverse dike and the water margin was about 0.8m, which result in the retrogressive erosion in the region① . The retrogressive erosion was developing up continuous till to the inlet and partial sediment of deposited area① was conveyed to the region③ .The variety of the water line in the region③ is showed in Fig.4.It can be seen from the figure that the initial slope, the water line of the twenty-fourth day’s, is about 8.9⁰/₀₀₀ in the upriver(the reach between 0km to 4.4km) while approximated to 3.4⁰/₀₀₀ in the lower reach . The water slope descend 6.5⁰/₀₀₀ following the developing of the retrogressive erosion, the water line of the twenty-eighth day’s in fig.4 . Eroding shifted to depositing because of the gentler water slope caused by the continual rising of the alluvium in the silting basin .The water slope in the upriver kept about 5.3⁰/₀₀₀ finally while ,in the lower reach ,it became very gentle as the lasher was heightened .

Fig.4  Variation of water levels along distance in the ①and ③ area

It can be seen from the model experiment that the range of the variety of the longitudinal velocity is wide because of effect of the initial topography while narrow down gradually following the formation of the main channel and the adjustment of the deposition . It is about 2.0m/s that the velocity in main channel of the upriver since the main channel formed ,then the aim of detaining the coarse sand and transporting the fines could be achieved easily .In the initial warping stage ,being gentle water slope and slow velocity(near 0.5m/s) ,both the coarse and fines sediment deposited together before the main channel formed in the lower reach ,but in the middle and final warping stages ,with the adjustment of the deposition ,part of fines could transported back into the Yellow River because the water slope became steeper and the flow went faster .

3.2 Longitudinal distribution of the sediment concentration

The longitudinal variety of the sediment concentration in the warping process in the region①(fig.5), as shown in the fig.5, the sediment concentration decreased along the stream and 90% of the sediment deposited in the region① in the first day . The deposition thickening in the warping area ,the mainstream gully is gradually formed .To the third day ,the sediment delivery percentage rose in the outlet and the sediment concentration changes little along the stream .It also can be seen from the fig that the deposition didn’t take place in the return water channel during the warping process in the region① .

Fig.5 Change of sediment concentrations in the area① during warping

Fig.6 shows the longitudinal variety of the sediment concentration in the warping process in the region② .It indicates that the rule of the variety of the sediment concentration in the region② is similar to that in the region① . In the initial warping stage, sediment tends to fall just near the inlet .To the second day warped in the region② ,namely the seventh day of the experiment , sediment concentration decreased rapidly along the stream in the reach between 0 km to 4.4 km .With the deposition extending downstream and the mainstream gully formed ,the decreasing pace of the sediment concentration became slow along the stream .To the seventh day(the twelfth day of the experiment) ,heightened the lasher ,the water level rose in front of the waste gate and the sediment concentration decreased along the stream ,which caused the deposition level to rise time by time .During the warping process in the region③, there is a little difference between the rule of the deposition in the region③ and that in the other two regions for water has to flow through the region① before it enter the region③ .In the early stage ,as shown in fig.7 ,the twenty-seventh curve and the twenty-ninth curve indicate that the longitudinal concentration tend to rise from 4.4m up the stream (namely region①) .That is to say ,erosion gained the upper hand in this region ,but the longitudinal concentration decrease rapidly in the region③ .In the final stage ,the thirty-first curve ,as shown in fig.7,indicates that the longitudinal concentration decrease in the region① ,which make it clear that there is depositing in this reach .

Fig.6  Changes of sediment concentration in the area ② during warping

Fig.7  Changes of sediment concentration in the area③ during warping

Fig.8 and fig.9 show the variation of the suspended-load grading .It may be seen from the fig.8 that the selecting of the suspended-load grading isn’t remarkable in the reach from 3.4km up the stream because of the formation of the mainstream gully .While it is obvious in the reach from 3.4 to 4.4 km where the mainstream gully hasn’t formed ,and the median particle diameter became fine along the stream .In region① ,suspended-load composition of the flow changed little along the stream because of the overflow and the obvious mainstream gully ,but in region③, it became fine .The median particle diameter is 0.028mm and 45% of the grain size is smaller than 0.025mm , while the value is 0.012mm and 70% in outlet .Which indicate that the sediment deposited obviously .

 

Fig.8  Variation of size grade of suspended sediment in the area ① on the third day

Fig.9  Variation of size grade of suspended sediment in the ① and③area on the thirtieth

3.3 Evolution of the water current form in warping area

In the early stage ,the overflow filled the dikes in each region(fig.10-a) .The deposition extend downstream slowly following warping ,while , in region① ,the deposition in the ending extend upstream affected by the backwater for transverse dike ,but the latter is slower .The mainstream gully formed when the deposition thicken to a certain extent ,then the beach and the channel became stable .Solid bed was taken and the mainstream gully along the cofferdam and longitudinal dike in most cases , therefore eroding function was restrained in the model warping experiment(fig.10-b) .

The width of the mainstream gully is between 50m to 80m under the condition of water and sediment designed. The flow velocity in the mainstream gully is fast in general and it’s maximum value reached 2.5 m/s .When the discharge is low , the flow current is in mainstream gully and the elevation of the beach and the channel is different obviously .Overflow took place when the discharge is heavy or the sediment concentration is high .Plentiful sediment deposited both in main channel and on beach ,but the depositing

 Fig.10-a   Evolution of water current form

 

Fig.10-b   Evolution of current form

 

 Fig.10-c   Evolution of current form