Equilibrium sediment transport in lower Yellow River during later sediment-retaining period of Xiaolangdi Reservoir

Sho-leiGuo*，Dong-po SunEn-hui Jing，Peng LiCollege ofWter Conservny，North Chin University ofWter Resoures nd Eletri Power，Zhengzhou 450011，PRChinYellow River Institute of Hydruli Reserh，Zhengzhou 450003，PRChinYellow River Henn Bureu，Yellow River Conservny Commission，Zhengzhou 450000，PR Chin Reeived 27 Otoer 2013；epted 19 Novemer 2014 Aville online 29 Jnury 2015



Equilibrium sediment transport in lower Yellow River during later sediment-retaining period of Xiaolangdi Reservoir

Shao-leiGuoa，*，Dong-po Suna，En-hui Jiangb，Peng LicaCollege ofWater Conservancy，North China University ofWater Resources and Electric Power，Zhengzhou 450011，PRChinabYellow River Institute of Hydraulic Research，Zhengzhou 450003，PRChinacYellow River Henan Bureau，Yellow River Conservancy Commission，Zhengzhou 450000，PR China Received 27 October 2013；accepted 19 November 2014 Available online 29 January 2015

Abstract

The Xiaolangdi Reservoir has entered the later sediment-retaining period，and new sediment transport phenomena and channel re-establishing behaviors are appearing.A physicalmodel testw as used to forecast the scouring and silting trends of the lower Yellow River.Based on water and sediment data from the low er Yellow River during the period from 1960 to 2012，and using a statisticalmethod，this paper analyzed the sediment transport in sediment-laden flowswith different discharges and sediment concentrations in the lower Yellow River.The results show that rational water-sediment regulation is necessary to avoid silting in the later sediment-retaining period.The combination of 3000m3/s

©2015 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license（http:// creativecommons.org/licenses/by-nc-nd/4.0/）.

Lower Yellow River；Xiaolangdi Reservoir；Water-sediment regulation；Equilibrium sediment transport；Later sediment-retaining period

1.Introduction

The Yellow River is famous for its sedimentation problem，which is particularly severe in the lower reaches.After the Xiaolangdi Reservoirwas completed，water-sediment regulationwas carried out in the lower Yellow River.Ithasbeen seen that water-sedim ent regulation has huge potentialities（Chen etal.，2012；Han and Guan，2009；Han，2009；Li，2004；Liand Sheng，2011）.

Many scholars have studied the sedimentation problem of the lower Yellow River based on water-sediment regulation. Hu et al.（2012）concluded that，through water-sediment regulation，theshrinkage of channelscan be prevented，and the medium-sizedmain channel can be shaped andmaintained.Fu et al.（2010）found that an incom ing sediment coefficient of 0.003 and a sediment delivery ratio of 2 m ight be a threshold for distinguishing the characteristics of sediment transport in the lower Yellow River.Han（2004，2008）analyzed theerosion and siltation laws for the lower Yellow River in the watersediment regulation period.The critical conditions between erosion and siltation in the lower Yellow Riverwere provided by Liu et al.（2000），and Liang et al.（2003，2004，2005a，2005b）.Xu（2010）determined the erosion-siltation thresholds for the lower Yellow River in different periods.

Most of these studies were based on balanced sedim ent transport.However，it is difficult to achieve balanced sedim ent transport because of the complicated relationships among sediment transport，the amount of scouring and silting，theboundary condition of the riverbed，and water and sedim ent conditions.Moreover，balance is relative in nature，and imbalance is absolute.Thus，equilibrium sediment transport is more feasible than balanced sediment transport in the lower Yellow River（Han，2011；Peng，2011）.In the condition of equilibrium sediment transport，the variationsof scouring and silting are not significant（<10%）over a short period of time，and balanced sediment transport can be achieved over a long period of time.Han（2011）has proven that the annual sediment transport capacity of a braided reach and ameandering reach in the lower Yellow River is approximately at equilibrium under the condition of no overflow，w ithout considering hyper-concentrated flow and discharge decrease from upstream to downstream.

After an initial sediment-retaining period，the Xiaolangdi Reservoir has entered the later sediment-retaining period，and new sediment transport phenomena and channel re-establishing behaviorsareappearing.Thus，the importantquestionsare，underwhatwater-sediment conditions can the channel of the lower Yellow River develop toward an equilibrium state，and how can this condition be achieved by a water-sediment regulation system？The answers to these questions w ill significantly help to solve the problems of silting and flood control of the lower Yellow River.

In this study，a physical model test was conducted to forecast the scouring and silting trends of the lower Yellow River.Also，in order to find the controllable relation between runoff and sediment in which equilibrium sediment transport can be achieved in the lower Yellow River in the later sediment-retaining period，the statistical method was used to analyze the sediment transport process w ith different dischargesand sedimentconcentrationson the basisofwater and sediment data from the lower Yellow River during the period from 1960 to 2012.

Table 1 Mainmodel scales.

2.M aterialsand methods

2.1.Physicalmodel

A physical model from the Xiaolangdi Reservoir to the Taochengpu section was built to study the fluvial process in different sediment-retaining periods of the Xiaolangdi Reservoir.Its lengthwasabout 800m，and itsw idth wasabout 36 m.Themain model scales are shown in Table 1.

After themodelwasbuilt，verification testswere conducted using data from 1982，1963，and 2001.The correctness of all the scales and the similarity lawswas proven in the verification tests.The reliability of the physicalmodel was verified w ith the 2008 flood routing test in the lower Yellow River，the river regulation test of the braided reach of the lower Yellow River，and the fluvial process test in the initial sedimentretaining period of the XiaolangdiReservoir.

In order to forecast the evolution trend of the lower Yellow River in the later sediment-retaining period，the flow and sediment processes，which were designed using actual data from 1990 to 1999 and from 1956 to 1960，were used in the model.Compared w ith the flow and sedimentprocesses in the initial sediment-retaining period of the Xiaolangdi Reservoir，theactualdata from 1990 to 1999 and from 1956 to 1960 show a similar annual averagewater inflow，and a large increase in the incoming sediment amount.Thus，it is feasible to design flow and sediment processes using the actual data from 1990 to 1999 and from 1956 to 1960.

2.2.Measured data and methods

Measured data from the Huayuankou，Jiahetan，Gaocun，Aishan，and Lijin sectionsalong the lower Yellow Riverduring theperiod from 1960 to2012wereused in thisstudy.Allthedata were observed under the condition that the discharge and sediment concentration did not show a large change（<20%）over 3-5days，w ithnooverflow andnohyper-concentrated flow，and w ith an average dischargeofmore than 1000m3/s.

Based on channel pattern and length，the lower Yellow River was divided into four reaches:Huayuankou-Jiahetan，Jiahetan-Gaocun，Gaocun-Aishan，and Aishan-Lijin.The reaches from Huayuankou to Jiahetan and Jiahetan to Gaocun are braided reaches.There are broad beaches and the channel is w ide and shallow in the braided reaches.In these two reaches，seriouserosion and siltation occur in the channeland themain channel continually wanders.The reach from Gaocun to Aishan isa transitional reach，where thew idth and wandering ranges decrease，and the stability of the main channel increases.The reach from Aishan to Lijin isameandering reach，where the river regime is stable and the channel isnarrow and deep.Fig.1 is a sketch of the lower Yellow River.

During theanalysis，data from theHuayuankou sectionwere selected initially.Then，othersections'datawereselected based on the flow velocity.The cross-sectionmethod，which isbased on the cross-section data，wasused to calculate the amount of erosion and siltation.In order to study the erosion and siltation laws，the statisticalmethod wasused to analyze the sediment transport in sediment-laden flowsw ith differentdischargesand sedimentconcentrations in the lower Yellow River.

Table 2Amountof scouring and silting obtained from physicalmodel test.

3.Results and d iscussion

3.1.Forecast of scouring and silting trends in later sediment-retaining period of Xiaolangdi Reservoir based on physicalmodel experiment

The distance from Tiexie to Taochengpu is 430.5 km.The experimental results regarding the amount of scouring and silting obtained from the physical model test during the designed water and sediment processes are shown in Table 2 and Fig.2.It can be seen from Table 2 and Fig.2 that the amount of scouring increased from Tiexie to Huayuankou. There are two reasons:First，some river trainingworks，which decrease the wandering range of the channel，were built between Tiexie and Huayuankou.Second，the conveyance capacity of the main channel increased and the overflow decreased in the reach from Tiexie to Huayuankou in the later sediment-retaining period of the Xiaolangdi Reservoir，as a result of low-concentration sediment-laden flow scouring the m ain channel in the initial sediment-retaining period of the Xiaolangdi Reservoir.It also can be seen from Table 2 and Fig.2 that the amount of silting increased gradually from Huayuankou to Taochengpu.Between Huayuankou and Jiahetan，river training workswere also built，and overflow was not excessive.Thus，the silting in this reach was not severe. However，the silting was very severe in the reach from Jiahetan to Taochengpu.As shown in Table 2 and Fig.2，the amount of silting increased sharply in this reach.That was because overflow was excessive，there were three large floodplains，and large quantities of sediment thatwere carried by the overflow were deposited on the floodplains.

It can be seen from the analysis that the sediment transport capacity in the lower Yellow Rivershowed significantvariation upstream and downstream.If the regulation isnotappropriate，silting isinevitablebelow the Jiahetansection.Thedifferencein sedimenttransportcapacity between upstream and downstream should receiveattention in the latersediment-retainingperiodof theXiaolangdiReservoir.Rationalwater-sedimentregulation is necessary to avoid silting in thisperiod.

Fig.1.Sketch of lower Yellow River.

Fig.2.Cumulative amount of scouring and silting obtained from physicalmodel test.

3.2.Analysis of flow with different sediment concentrations

According to thestatisticsof the period from 1960 to 2012，when the sediment concentration was less than 20 kg/m3and the discharge wasmore than 1000 m3/s at the Huayuankou，Jiahetan，Gaocun，Aishan，and Lijin sections，the four reaches were mostly scoured，except for the reach from Aishan to Lijin，where there was little silting.When the sediment concentration at the five sectionswas between 20 and 30 kg/m3，the four reachesweremostly scoured，under the condition that thedischargeof each sectionwasmore than 2000m3/s.When thesediment concentration at the fivesectionswasbetween 30 and 40 kg/m3，the four reachesweremostly scoured，under the condition that the discharge of each section wasmore than 2500 m3/s.When the sediment concentration at the five sections was between 40 and 60 kg/m3，the four reaches were m ostly scoured，under the condition that the discharge of each section was more than 2 800 m3/s.W hen the sediment concentration at the five sections was between 60 and 80 kg/m3，the four reaches weremostly silted.When the sediment concentration at the five sectionswas greater than 80 kg/m3，the four reacheswere silted.

Fig.3.Relationship between scouring-silting rate and sediment concentration in lower Yellow River when dischargewas between 1 000 and 2000m3/s.

3.3.Analysis of flow with different discharges

3.3.1.Discharge between 1000 and 2000m3/s

In order to analyze the scouring-silting law in the channel w ith different discharges，the scouring-silting rate of each reach，which is the ratio of the scouring-silting amount to the incoming sedimentamount，was calculated.

Fig.3 shows the relationship between the scouring-silting rate and sediment concentration（S）when the discharge（Q） was between 1 000 and 2 000 m3/s in the years from 1960 to 2012.

As shown in Fig.3，the scouring-silting rate of the four reaches increased w ith the sediment concentration when the discharge was between 1000 and 2000 m3/s.The scouringsilting rate of the reach from the Huayuankou to Gaocun sectionswas close to 0when the sediment concentration at the Huayuankou and Jiahetan sections was about 22 kg/m3.The scouring-silting rate of the reach from the Gaocun to Lijin sectionswas close to 0when the sediment concentration at the Gaocun and Aishan sectionswas about 18 kg/m3.

3.3.2.Discharge between 2 000 and 3000 m3/s

Fig.4 shows the relationship between the scouring-silting rate and sediment concentration when the discharge was between 2000 and 3000m3/s in the years from 1960 to 2012.

As shown in Fig.4，the scouring-silting rate of the four reaches increased w ith the sediment concentration when the dischargewas between 2 000 and 3000 m3/s.The scouringsilting rate of the reach from the Huayuankou to Jiahetan sections was close to 0 when the sediment concentration at the Huayuankou section was about 33 kg/m3.The scouring-silting rate of the reach from the Jiahetan to Gaocun sections was close to 0 when the sediment concentration at the Jiahetan sectionwasabout35 kg/m3.The scouring-silting rate of the reach from the Gaocun to Aishan sections was close to 0 when the sediment concentration at the Gaocunsection was about 32 kg/m3.The scouring-silting rate of the reach from the Aishan to Lijin sectionswas close to 0 when the sediment concentration at the Aishan section was about 30 kg/m3.

Fig.4.Relationship between scouring-silting rate and sediment concentration in lower Yellow River when discharge was between 2 000 and 3000 m3/s.

3.3.3.Discharge between 3000 and 4 000 m3/s

Fig.5 shows the relationship between the scouring-silting rate and sediment concentration when the discharge was between 3000 and 4000 m3/s in the years from 1960 to 2012.

As shown in Fig.5，the scouring-silting rate of the four reaches increased w ith the sediment concentration when the dischargewas between 3000 and 4000 m3/s.All the reaches were being scoured when the sediment concentrationwas less than 45 kg/m3.

3.3.4.Discharge between 4000 and 5 000 m3/s

Fig.6 show s the relationship between the scouring-silting rate and sediment concentration when the discharge was between 4000 and 5000 m3/s in the years from 1960 to 2012.

As can been seen in Fig.6，all the reaches were mostly scoured when the dischargewasmore than 4000m3/s.

Itcan be seen in Figs.3 through 6 that，when the discharge was low，the ratio of the number of floods that scour the channel to the total number of statistical floodswas higher in the braided reach than that in the meandering reach.W ith increasing discharge，this ratio in the meandering reach increased gradually and finally exceeded that in the braided reach.This reflected the difference in sediment carrying capacity between the braided reach and themeandering reach. That ism ainly due to differences in the channel pattern and gradientbetween the braided reach andm eandering reach:the channel of the braided reach isw ide and shallow，the channel of themeandering reach is narrow and deep，and the channel gradient of the braided reach is larger than that of the meandering reach.Thus，when the discharge was low，the sediment carrying capacity of the braided reach was higher than thatof themeandering reach.

The principal problem in the lower Yellow River is low runoff，too much sediment，and an imbalance between the amountsof incoming water and sediment.It can be seen from the analysis that when the discharge and sediment concentration changed in the ranges of 2 000 m3/s

Fig.5.Relationship betw een scouring-silting rate and sediment concentration in low er Yellow River w hen discharge was between 3000 and 4000m3/s.

Fig.6.Relationship between scouring-silting rate and sediment concentration in lower Yellow River when discharge wasbetween 4000 and 5000m3/s.

The amounts of incom ing water and sediment at the Huayuankou section were used as the control condition in practice.Considering the decrease of the discharge from Huayuankou to Lijin，if there was enough water，the control discharge could be increased to 3000m3/s

When the Xiaolangdi Reservoir is used to regulate the discharge and sediment concentration，a single combination w ithoutmodification of the discharge and sediment concentration during a long period of tim e should be avoided，and the discharge and sediment concentration in the Huayuankou section should change within the ranges of 3000 m3/s

4.Conclusions

（1）Analysis of themodel test results shows that the sediment transport capacity difference between upstream and downstream reachesof the lower Yellow River should receive attention in the later sediment-retaining period of the Xiaolangdi Reservoir.Rational water-sedim ent regulation is necessary to avoid silting in this period.

（2）The findingsshow thatequilibrium sediment transport is feasible in the lower Yellow River.The combination of 3000m3/s

（3）Another way to prevent silting in the lower Yellow River is to use the overflow.If the overflow can be controlled and allowed to flow into the floodp lains at the right time，the sedim ent carried by the overflow can be deposited in the floodp lains and flow that carries little sedim entw ill flow back to the main channel.Thus，the main channel w ill have less silting and even scouring.This requires further study.

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Thiswork was supported by the National Natural Science Foundation of China（Grants No.51039004 and No.51079055），the High-Level Personnel Research Start-Up Funds of North China University ofWater Resources and Electric Power（GrantNo.201403），and the Science and Technology Research Project of the Education Department of Henan Province（Grant No. 14A 570001）.

*Corresponding author.

E-mail address:guoshaolei@ncwu.edu.cn（Shao-leiGuo）.

Peer review under responsibility of HohaiUniversity.

http://dx.doi.org/10.1016/j.wse.2015.01.006

1674-2370/©2015 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license（http:// creativecommons.org/licenses/by-nc-nd/4.0/）.